U.S. patent application number 12/746546 was filed with the patent office on 2010-10-21 for lash adjuster.
Invention is credited to Eiji Maeno, Katsuhisa Yamaguchi, Makoto Yasui.
Application Number | 20100263614 12/746546 |
Document ID | / |
Family ID | 40801165 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100263614 |
Kind Code |
A1 |
Yasui; Makoto ; et
al. |
October 21, 2010 |
LASH ADJUSTER
Abstract
A lash adjuster is provided in which the friction coefficient
between the pressure flanks is less likely to decrease over a long
time use. The lash adjuster 1 includes a nut member 10 having an
internal thread 17 on its inner periphery, an adjusting screw 11
having an external thread 18 on its outer periphery which is in
threaded engagement with the internal thread 17, and a return
spring 12 biasing the adjusting screw 11 in a direction to protrude
from the nut member 10, the external thread 18 and the internal
thread 17 having pressure flanks 19 and 21, respectively, for
receiving an axial load that tends to push the adjusting screw 11
into the nut member 10, wherein a satin finished surface is formed
on the pressure flank 19 of the external thread 18, and the satin
finished surface has a higher surface hardness than the pressure
flank 21 of the internal thread 17.
Inventors: |
Yasui; Makoto; ( Shizuoka,
JP) ; Maeno; Eiji; ( Shizuoka, JP) ;
Yamaguchi; Katsuhisa; ( Shizuoka, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
1030 15th Street, N.W.,, Suite 400 East
Washington
DC
20005-1503
US
|
Family ID: |
40801165 |
Appl. No.: |
12/746546 |
Filed: |
December 19, 2008 |
PCT Filed: |
December 19, 2008 |
PCT NO: |
PCT/JP2008/073202 |
371 Date: |
June 7, 2010 |
Current U.S.
Class: |
123/90.52 |
Current CPC
Class: |
F01L 1/22 20130101; F01L
1/0532 20130101; F01L 1/143 20130101 |
Class at
Publication: |
123/90.52 |
International
Class: |
F01L 1/14 20060101
F01L001/14 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2007 |
JP |
2007 331759 |
Mar 25, 2008 |
JP |
2008 077648 |
Oct 2, 2008 |
JP |
2008 257483 |
Claims
1. A lash adjuster comprising a nut member (10) having an internal
thread (17) on its inner periphery, an adjusting screw (11) having
an external thread (18) on its outer periphery which is in threaded
engagement with the internal thread (17), and a return spring (12)
biasing the adjusting screw (11) in a direction to protrude from
the nut member (10), said external thread (18) and said internal
thread (17) having pressure flanks (19 and 21), respectively, for
receiving an axial load that tends to push the adjusting screw (11)
into the nut member (10), characterized in that a satin finished
surface is formed on one of the pressure flank (19) of the external
thread (18) and the pressure flank (21) of the internal thread
(17), and that the satin finished pressure flank has a higher
surface hardness than the other of said pressure flanks (19 and
21).
2. The lash adjuster of claim 1 wherein said return spring (12) is
a compression coil spring axially biasing the adjusting screw (11),
said external thread (18) and said internal thread (17) having a
serration-shaped section, with said pressure flanks (19 and 21)
having a larger flank angle than clearance flanks (20 and 22) of
the external thread and said internal thread, respectively.
3. The lash adjuster of claim 1 wherein said return spring is a
torsion spring (76) for applying torque to the adjusting screw (11)
that tends to protrude the adjusting screw (11) from the nut member
(10).
4. The lash adjuster of claim 1 wherein said satin finished surface
has a surface roughness Ra of 1.6 to 12.5.
5. The lash adjuster of claim 1 wherein said satin finished surface
is formed by shot peening.
6. The lash adjuster of claim 1 wherein the satin finished pressure
flank (19) is coated with a hard film (27) and has a higher surface
hardness than the other pressure flank (21).
7. The lash adjuster of claim 6 wherein said hard film (27) is one
of a titanium nitride film, a chromium nitride film, a diamond-like
carbon film and a ceramic film.
8. The lash adjuster of claim 1 wherein the satin finished pressure
flank (19) is subjected to carbonitriding treatment so that the
satin finished surface flank (19) has a higher surface hardness
than the other pressure flank (21).
9. The lash adjuster of claim 1 wherein the satin finished pressure
flank (19) is subjected to WPC treatment so that the satin finished
surface flank (19) has a higher surface hardness than the other
pressure flank (21).
10. The lash adjuster of claim 1 wherein a backlash of 0.1 to 0.4
mm is defined between the external thread (18) and the internal
thread (17).
11. The lash adjuster of claim 1 further comprising a lifter body
(9) vertically slidably inserted in a guide hole (13) formed in a
cylinder head (2), wherein said nut member (10) is fixed to said
lifter body (9), and wherein said adjusting screw (11) has an end
protruding from the nut member and pressing a valve stem (5) of a
valve gear.
12. The lash adjuster of claim 1 wherein the nut member (35) is
inserted in a mounting hole (38) formed in a bottom surface of an
arm (33) that pivots as a cam (15) rotates, and wherein said
adjusting screw (36) has an end protruding from the nut member (35)
and pressing a valve stem (5) of a valve gear.
13. The lash adjuster of claim 1 wherein the nut member (53) is
inserted in a mounting hole (56) formed in a top surface of a
cylinder head (2), and wherein said adjusting screw (54) has an end
protruding from the nut member (53) and pivotally supporting an arm
(52) of a valve gear.
Description
TECHNICAL FIELD
[0001] This invention relates to a lash adjuster mounted in a valve
gear of an engine.
BACKGROUND ART
[0002] Known valve gears for moving a valve provided at an intake
port or an exhaust port of an engine include one comprising a valve
lifter vertically movably supported and adapted to be pushed down
by a cam, thereby pushing down a valve stem (direct type valve
gear), one comprising an arm pivotable about its central portion
and adapted to be pushed up by a cam at one end thereof, thereby
pushing down a valve stem at the other end (rocker arm type valve
gear), and one comprising an arm pivotable about one end thereof
and adapted to be pushed down at its central portion, thereby
pushing down a valve stem at the other end (swing arm type valve
gear).
[0003] In these valve gears, gaps between their component parts may
change due to differences in thermal expansion between component
parts, which may cause noise and compression leakage. Also, when
the sliding parts of the valve gear become worn too, gaps between
component parts of the valve gear change, which may also cause
noise.
[0004] In order to prevent such noise and compression leakage,
ordinary valve adjusters include a lash adjuster for absorbing gaps
between component parts of the valve gear.
[0005] One known lash adjuster used in a direct type valve gear
comprises a lifter body vertically slidably inserted in a guide
hole formed in a cylinder head, a nut member fixed to the lifter
body, an adjusting screw having an external thread on its outer
periphery which is in threaded engagement with an internal thread
formed on the inner periphery of the nut member, and a return
spring biasing the adjusting screw in the direction to protrude
downwardly from the nut member, the adjusting screw pressing the
valve stem of the valve gear with its end protruding from the nut
member (Patent document 1).
[0006] One known lash adjuster used in an arm type valve gear
comprises a nut member inserted in a mounting hole formed in the
bottom surface of the arm which pivots as the cam rotates, an
adjusting screw having an external thread on its outer periphery
which is in threaded engagement with an internal thread formed on
the inner periphery of the nut member, and a return spring biasing
the adjusting screw in the direction to protrude downwardly from
the nut member, the adjusting screw pressing the valve stem of the
valve gear with its end protruding from the nut member (Patent
document 2).
[0007] One known lash adjuster used in a swing arm type valve gear
comprises a nut member inserted in a mounting hole formed in a top
surface of a cylinder head, an adjusting screw having an external
thread on its outer periphery which is in threaded engagement with
an internal thread formed on the inner periphery of the nut member,
and a return spring biasing the adjusting screw in the direction to
protrude upwardly from the nut member, the adjusting screw
pivotally supporting the arm of the valve gear with its end
protruding from the nut member (Patent document 3).
[0008] In these lash adjusters, as a cam rotates and a load that
tends to push the adjusting screw into the nut member is applied to
the lash adjuster, the pressure flank of the external thread of the
adjusting screw is supported on the pressure flank of the internal
thread of the nut member, so that the adjusting screw is axially
fixed in position.
[0009] If the relative position between the arm and the valve stem
changes due e.g. to thermal expansion of the valve gear, according
to the degree of change in the relative position, the adjusting
screw axially moves in the nut member while rotating, thereby
absorbing gaps between component parts of the valve gear.
[0010] In these lash adjusters, when the pressure flank of the
external thread of the adjusting screw and the pressure flank of
the internal thread of the nut member become worn over a long time
of use, the surfaces of the pressure flanks become smooth, so that
the friction coefficient between the pressure flanks decreases.
This may cause slip between the pressure flanks when the cam
rotates and a load is applied to the lash adjuster that tends to
push in the adjusting screw, thus allowing the adjusting screw to
be pushed in, which in turn reduces the valve lift.
[0011] In order to suppress reduction in friction coefficient
between the pressure flanks over a long time use, a lash adjuster
is proposed in which a satin finished surface is formed on one of
the pressure flank of the external thread and the pressure flank of
the internal thread (Patent document 4).
[0012] With this arrangement, since the protrusions and the
recesses of the satin finished surface are sufficiently high
compared to the depth of wear, even when the pressure flank of the
external thread and the pressure flank of the internal thread
become worn over a long time use, the friction coefficient between
the pressure flanks is less likely to decrease.
Patent document 1: JP Patent Publication 2003-227318A Patent
document 2: JP Patent Publication 2006-132426A Patent document 3:
JP Patent Publication 2005-273510A Patent document 4: JP Patent
Publication 2005-127189A
DISCLOSURE OF THE INVENTION
Object of the Invention
[0013] But even when a satin finished surface is formed on one of
the pressure flanks, if its surface hardness is substantially equal
to or lower than that of the other pressure flank, the satin
finished surface tends to quickly become worn. Thus, it may be
difficult to sufficiently suppress reduction in friction
coefficient between the pressure flanks.
[0014] For these lash adjusters, it is required to reduce their
axial length in order to increase the freedom of design of the
engine. But if the axial length of the lash adjuster is reduced,
the meshing length between the threads also decreases, which in
turn increases the surface pressure that acts between the pressure
flanks of the external thread and the internal thread. This
quickens wear of the external thread and the internal thread.
[0015] The inventors of the invention wanted to develop a lash
adjuster in which the wear between the eternal thread and the
internal thread is small even if the meshing length between the
external thread and the internal thread is short, and for this
purpose, conducted experimental tests for many lash adjuster
samples of which the backlashes between the respective external and
internal threads are different in size from each other.
[0016] The object of the present invention is to provide a lash
adjuster of which the friction coefficient between the pressure
flanks of the external thread and the internal thread over a long
time use, and to reduce wear between the external thread and the
internal thread of the lash adjuster.
[0017] In order to achieve this object, the present invention
provides a lash adjuster comprising a nut member having an internal
thread on its inner periphery, an adjusting screw having an
external thread on its outer periphery which is in threaded
engagement with the internal thread, and a return spring biasing
the adjusting screw in a direction to protrude from the nut member,
the external thread and the internal thread having pressure flanks,
respectively, for receiving an axial load that tends to push the
adjusting screw into the nut member, wherein a satin finished
surface is formed on one of the pressure flank of the external
thread and the pressure flank of the internal thread, and wherein
the satin finished pressure flank has a higher surface hardness
than the other of the pressure flanks. As used herein, the satin
finished surface is a surface comprising irregularly arranged
protrusions and recesses.
[0018] The return spring may be a compression coil spring axially
biasing the adjusting screw, or a torsion spring for applying
torque to the adjusting screw that tends to protrude the adjusting
screw from the nut member. If a compression coil spring is used as
the return spring, the external thread and the internal thread may
have a serration-shaped section, with the pressure flanks having a
larger flank angle than clearance flanks of the external thread and
the internal thread, respectively.
[0019] The satin finished surface may have a surface roughness Ra
of 1.6 to 12.5. The satin finished surface can be formed by
electric discharge machining or laser beam machining. But by using
shot peening, it is possible to harden the pressure flank and thus
increase its durability. The surface hardness of the satin finished
pressure flank can be increased by e.g. coating it with a hard film
so as to be higher than the surface hardness of the other pressure
flank. The hard film may be a titanium nitride film, a chromium
nitride film, a diamond-like carbon film or a ceramic film.
Carbonitriding treatment or WPC treatment can also increase the
surface hardness of the satin finished pressure flank so as to be
higher than that of the other pressure flank.
[0020] The inventors of the present invention conducted
experimental tests for many lash adjuster samples of which the
backlashes between the respective external and internal threads are
different in size from each other, and as a result, discovered that
by setting the backlash between the external thread and the
internal thread within the range of 0.1 to 0.4 mm, the external
thread and the internal thread are extremely less likely to become
worn, compared to the case where this backlash is larger than 0.4
mm.
[0021] The concept of the present invention can be applied to any
of the following types of lash adjusters:
1) A lash adjuster for use in a direct type valve gear, the lash
adjuster including a lifter body vertically slidably inserted in a
guide hole formed in a cylinder head, wherein the nut member is
fixed to the lifter body, and wherein the adjusting screw has an
end protruding from the nut member and pressing the valve stem of
the valve gear. 2) A lash adjuster for use in a rocker arm type
valve gear, wherein the nut member is inserted in a mounting hole
formed in a bottom surface of an arm that pivots as a cam rotates,
and wherein the adjusting screw has an end protruding from the nut
member and pressing the valve stem of the valve gear. 3) A lash
adjuster for use in a swing arm type valve gear, wherein the nut
member is inserted in a mounting hole formed in a top surface of a
cylinder head, and wherein the adjusting screw has an end
protruding from the nut member and pivotally supporting the arm of
the valve gear.
ADVANTAGES OF THE INVENTION
[0022] With the lash adjuster according to the present invention,
even if the pressure flank of the external thread becomes worn
after a long-term use, since the protrusions and recesses of the
satin finished surface are sufficiently high compared to the depth
of wear, the surface of the pressure flank of the external thread
is less likely to become smooth, so that the friction coefficient
between the pressure flanks of the external thread and the internal
thread is less likely to decrease. Further, since the satin
finished pressure flank has a higher surface hardness than the
other pressure flank, the satin finished surface is less likely to
become worn. This makes it possible to effectively suppress
reduction of the friction coefficient between the pressure
flanks.
[0023] With this lash adjuster 1, since one of the pressure flank
of the external thread and the pressure flank of the internal
thread is satin finished, when the pressure flanks of the external
thread and the internal thread move close to each other, oil film
is less likely to form due to the squeeze effect. Thus, even while
the temperature is low and the viscosity of lubricating oil is
high, the internal thread of the nut member can quickly receive the
axial load applied to the adjusting screw.
[0024] By using titanium nitride film, chromium nitride film,
diamond-like carbon film or ceramic film as the hard film, even if
oil is used to which organic molybdenum is added, a lubricating
film containing molybdenum disulfide is less likely to form on the
hard film. This further reliably prevents slip between the pressure
flanks of the external thread and the internal thread.
[0025] By setting the backlash between the external thread the
internal thread to be 0.4 mm or less, compared to the arrangement
in which the backlash is larger than 0.4 mm, the external thread
and the internal thread are extremely less likely to become worn.
This makes it possible to shorten the meshing length between the
external thread and the internal thread, which in turn makes it
possible to reduce the axial length of the lash adjuster.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a front view of a valve gear including a lash
adjuster of a first embodiment according to this invention.
[0027] FIG. 2 is an enlarged sectional view of the lash adjuster
shown in FIG. 1.
[0028] FIG. 3 is an enlarged sectional view of the adjusting screw
shown in FIG. 2, showing its surface region.
[0029] FIG. 4 is an enlarged sectional view of a modified example
of FIG. 3 in which the internal thread of the nut member is coated
with the hard film shown in FIG. 3.
[0030] FIG. 5 is an enlarged sectional view of the lash adjuster of
FIG. 1, showing a backlash between its external and internal
threads.
[0031] FIG. 6 is a front view of a valve gear including a lash
adjuster of a second embodiment according to this invention.
[0032] FIG. 7 is an enlarged sectional view of the lash adjuster of
FIG. 6, showing a backlash between its external and internal
threads.
[0033] FIG. 8 is a front view of a valve gear including a lash
adjuster of a third embodiment according to this invention.
[0034] FIG. 9 is a graph showing the relationship between the
backlash values before and after an endurance test for a sample
with a thread meshing length that is substantially equal to that of
a conventional device.
[0035] FIG. 10 is a graph showing the relationship between the
backlash values before and after an endurance test for a sample
with a thread meshing length that is 1/2 that of a conventional
device.
[0036] FIG. 11 is a graph showing the relationship between the
backlash values before and after an endurance test for a sample
with a thread meshing length that is 1/3 that of a conventional
device.
[0037] FIG. 12 is a front view of a valve gear including a lash
adjuster of a fourth embodiment according to this invention.
[0038] FIG. 13 is an enlarged sectional view of the lash adjuster
shown in FIG. 12.
[0039] FIG. 14 is a front view of a valve gear including a lash
adjuster of a fifth embodiment according to this invention.
[0040] FIG. 15 is an enlarged sectional view of the lash adjuster
shown in FIG. 14.
[0041] FIG. 16 is a front view of a valve gear including a lash
adjuster of a sixth embodiment according to this invention.
[0042] FIG. 17 is an enlarged sectional view of the lash adjuster
shown in FIG. 16.
DESCRIPTION OF THE NUMERALS
[0043] 1. Lash adjuster [0044] 2. Cylinder head [0045] 5. Valve
stem [0046] 9. Lifter body [0047] 10. Nut member [0048] 11.
Adjusting screw [0049] 12. Return spring [0050] 13. Guide hole
[0051] 15. Cam [0052] 17. Internal thread [0053] 18. External
thread [0054] 19. Pressure flank [0055] 20. Clearance flank [0056]
21. Pressure flank [0057] 22. Clearance flank [0058] 27. Hard film
[0059] 31. Lash adjuster [0060] 33. Arm [0061] 35. Nut member
[0062] 36. Adjusting screw [0063] 37. Return spring [0064] 38.
Mounting hole [0065] 51. Lash adjuster [0066] 52. Arm [0067] 53.
Nut member [0068] 54. Adjusting screw [0069] 55. Return spring
[0070] 56. Mounting hole [0071] 76. Torsion coil spring [0072] b.
Backlash
BEST MODE FOR EMBODYING THE INVENTION
[0073] FIG. 1 shows a valve gear including a lash adjuster
according to a first embodiment of the present invention. This
valve gear includes a valve 4 provided at an intake port 3 of a
cylinder head 2, and a valve stem 5 connected to the valve 4. The
valve stem 5 extends upwardly from the valve 4. An annular spring
retainer 6 is fixed to the outer periphery of the valve stem 5 at
its top end portion. A valve spring 7 biases the spring retainer 6
upwardly, thereby seating the valve 4 on a valve seat 8.
[0074] The lash adjuster 1 comprises a lifter body 9, a nut member
10, an adjusting screw 11, and a return spring 12. The lifter body
9 is vertically slidably inserted in a guide hole 13 formed in the
cylinder head 2.
[0075] The lifter body 9 comprises a cylindrical portion 9a and an
end wall 9b closing the top end of the cylindrical portion 9a. Over
the lifter body 9, a cam 15 is located which is carried on a
camshaft 14. As the camshaft 14 rotates, a cam lobe 15b that
protrudes from the base circle 15a of the cam 15 presses the top
end of the end wall 9b, thus pushing down the lifter body 9.
[0076] As shown in FIG. 2, the nut member 10 is fixed to the bottom
surface of the end wall 9b by means of a snap ring 16. The member
10 has an internal thread 17 on its inner periphery which is in
threaded engagement with an external thread 18 formed on the outer
periphery of the adjusting screw 11. The external thread 18 has a
pressure flank 19 that receives pressure when a load is applied
that tends to push the adjusting screw 11 into the nut member 10.
The external thread 18 has a serration-shaped section with the
pressure flank 19 having a larger flank angle than its clearance
flank 20. The internal thread 17 has also a serration-shaped
section with its pressure flank 21 for receiving pressure when a
load is applied that tends to push the adjusting screw 11 into the
nut member 10 having a larger flank angle than its clearance flank
22.
[0077] The end of the adjusting screw 11 protruding from the nut
member 10 is in contact with a spacer 23. The spacer 23 is
rotationally fixed to the nut member 10 by means of a retainer 24
fixed in position by the snap ring 16 so as to be vertically
movable within a cutout 25 formed in the retainer 24.
[0078] An oil hole 26 vertically extends through the end wall 9b of
the lifter body 9 so that lubricating oil supplied onto the top
surface of the end wall 9b is introduced into the nut member 10
through this oil hole 26. Lubricating introduced into the nut
member 10 lubricates the external thread 18 and the internal thread
17.
[0079] As shown in FIG. 3, the external thread 18 of the adjusting
screw 11 is subjected to shot peening and then coated with a hard
film 27. Thus, the pressure flank 19 forms a satin finished surface
having a surface hardness higher than the surface hardness of the
pressure flank 17. Preferably, the satin finished surface has a
surface roughness Ra of 1.6 to 12.5 to ensure that the protrusions
and recesses of the satin finished surface are higher than the
depth of abrasion. The hard film 27 may be a titanium nitride (TiN)
film, chromium nitride (CrN) film, diamond-like carbon (DLC) film,
or ceramic film.
[0080] The pressure flank 21 of the internal thread 17 is smaller
in surface hardness than the pressure flank 19 of the external
thread 18. As shown in FIG. 5, between the external thread 18 and
the internal thread 17, a backlash b (axial clearance) of 0.1 to
0.4 mm is provided.
[0081] As shown in FIG. 2, the return spring 12 is a compression
spring mounted between the adjusting screw 11 and the end wall 9b
of the lifter body 9 in an axially compressed state. The return
spring 12 has its top end supported by the end wall 9b with its
bottom end axially pressing the adjusting screw 11, thereby biasing
the adjusting screw 11 in the direction to protrude downwardly from
the nut member 10. As shown in FIG. 1, the end of the adjusting
screw 11 protruding from the nut member 10 presses the top end of
the valve stem 5 through the spacer 23.
[0082] Now the operation of the lash adjuster 1 is described.
[0083] When the engine is started and the camshaft 14 rotates, the
end wall 9b of the lifter body 9 is pushed down by the cam lobe 15b
of the cam 15. The valve 4 thus separates from the valve seat 8,
opening the intake port 3. At this time, a load is applied to the
adjusting screw 11 that tends to push it into the nut member. But
since the pressure flank 19 of the external thread 18 is received
by the pressure flank 21 of the internal thread 21, the adjusting
screw 11 remains axially fixed in position.
[0084] As the camshaft 14 further rotates and the cam lobe 15b
moves past the end wall 9b, the valve stem 5 rises under the
biasing force of the valve spring 7 until the valve 4 is seated on
the valve seat 8 and the intake port 3 is closed.
[0085] In a strict sense, when the cam lobe 15b pushes down the
arm, slight slip occurs between the pressure flanks 19 and 21 of
the external thread 18 and the internal thread 17. But after the
cam lobe 15b has moved past the end wall 9b and by the time the cam
lobe 15b moves back to the position of the end wall 9b, the
adjusting screw 11 moves back to its original position under the
biasing force of the return spring 12, because the load on the
adjusting screw 11 that tends to push it into the nut member is
removed during this period.
[0086] When the distance between the cam 15 and the lifter body 9
increases while the engine is running, due to thermal expansion
differences between component parts of the valve gear, including
the cylinder head 2 and the valve stem 5, the adjusting screw 11
protrudes by a larger distance when the cam 15 further rotates
after pushing down the lifter body 9 and the load that tends to
push in the adjusting screw 11 has been removed than the distance
by which the adjusting screw 11 is pushed in when the lifter body 9
is pushed down by the cam lobe 15b of the cam 15. Thus in this
state, every time the cam 15 rotates once, the adjusting screw 11
gradually protrudes, thus preventing formation of a gap between the
base circle 15a of the cam 15 and the end wall 9b of the lifter
body 9.
[0087] Conversely, when the contact surfaces of the valve 4 and the
valve seat 8 become worn, even while the base circle 15a of the cam
15 faces the end wall 9b of the lifter body 9, the biasing force of
the valve spring 7 acts on the adjusting screw 11, so that the
adjusting screw 11 protrudes by a shorter distance when the cam 15
further rotates after pushing down the lifter body 9 and the load
that tends to push in the adjusting screw 11 has been removed than
the distance by which the adjusting screw 11 is pushed in when the
lifter body 9 is pushed down by the cam lobe 15b of the cam 15.
Thus in this state, every time the cam 15 rotates once, the
adjusting screw 11 is gradually pushed in. This prevents formation
of a gap between the contact surfaces of the valve 4 and the valve
seat 8.
[0088] With this lash adjuster 1, even if the pressure flank 19 of
the external thread 18 becomes worn after a long-term use, since
the protrusions and recesses of the satin finished surface are
sufficiently high compared to the depth of wear, the surface of the
pressure flank 19 of the external thread 18 is less likely to
become smooth, so that the friction coefficient between the
pressure flanks 19 and 21 of the external thread 18 and the
internal thread 17 is less likely to decrease. This in turn
prevents excessive slip between the pressure flanks 19 and 21 of
the external thread 18 and the internal thread 17 when a load acts
on the adjusting screw 11 that tends to push it into the nut
member.
[0089] With this lash adjuster 1, since the pressure flank 19 of
the external thread 18, which is a satin finished surface, has a
higher surface hardness than the pressure flank 21 of the internal
thread 17, the satin finished surface is less likely to become
worn. This makes it possible to effectively suppress reduction of
the friction coefficient between the pressure flanks 19 and 21.
[0090] With this lash adjuster 1, since the pressure flank 19 of
the external thread 18 is satin finished, when the pressure flanks
19 and 21 of the external thread 18 and the internal thread 17 move
close to each other, oil film is less likely to form due to the
squeeze effect. Thus, even while the temperature is low and the
viscosity of lubricating oil is high, the internal thread 17 of the
nut member 10 can quickly receive the axial load applied to the
adjusting screw 11.
[0091] With this lash adjuster 1, by using titanium nitride film,
chromium nitride film, diamond-like carbon film or ceramic film as
the hard film 27, even if oil is used to which molybdenum
dithiocarbamate (MoDTC) or molybdenum dithiophosphate (MoDTP) is
added (what is known as FM oil), a lubricating film containing
molybdenum disulfide (MoS.sub.2) is less likely to form on the hard
film 27. This further reliably prevents slip between the pressure
flank 19 of the external thread 18 and the pressure flank 21 of the
internal thread 17.
[0092] With this lash adjuster 1, since the backlash b between the
external thread 18 and the internal thread 17 is 0.4 mm or less,
compared to the arrangement in which the backlash b is larger than
0.4 mm, the external thread 18 and the internal thread 17 are
extremely less likely to become worn. This makes it possible to
shorten the meshing length between the external thread 18 and the
internal thread 17, which in turn makes it possible to reduce the
axial length of the lash adjuster 1.
[0093] With this lash adjuster 1, since the backlash b between the
external thread 18 and the internal thread 17 is 0.1 or over,
compared to the arrangement in which the backlash b is less than
0.1 mm, the adjusting screw 11 can rotate smoothly, so that the
lash adjuster can stably perform its expected function.
[0094] In this embodiment, the external thread 18 is coated with
the hard film 27 to increase the surface hardness of the pressure
flank 19 of the external thread 18 higher than the surface hardness
of the pressure flank 21 of the internal thread 17. But instead,
the external thread 18 may be subjected to carbonitriding treatment
to increase the surface hardness of the pressure flank 19 of the
external thread 18.
[0095] The satin-finished surface may be formed on the pressure
flank 19 of the external thread 18 of the adjusting screw 11 by
subjecting the external thread 18 to WPC treatment. WPC treatment
is a shot peening treatment for heating the surface of the external
thread 18 to a temperature equal to or higher than the A3
transformation point, e.g. a treatment in which shots of 40 to 200
.mu.m are thrown at the surface of the external thread 18 at 100
m/minute. By this treatment, the residual austenite on the surface
of the eternal thread 18 turns into martensite, and the surface
structure of the external thread 18 is recrystallized and refined,
so that it is possible to increase the surface hardness of the
external thread 18 higher than that of the pressure flank 21 of the
internal thread 17.
[0096] As shown in FIG. 3, the external thread 18 may be subjected
to shot peening and then coated with the hard film 27, over the
entire surface thereof. With this arrangement, a satin finished
surface is also formed on the clearance flank 20 of the external
thread 18, so that the satin finished clearance flank 20 also has a
surface hardness higher than that of the clearance flank 22 of the
internal thread 17. This makes it possible to maintain a
sufficiently high friction coefficient between the clearance flanks
20 and 22 of the external thread 18 and the internal thread 17,
even if the clearance flank 20 of the external thread 18 becomes
worn over a long time of use.
[0097] This satin finished surface can be formed by electric
discharge machining or laser beam machining. But by using shot
peening as in the above embodiment, it is possible to harden the
pressure flank 19 and thus increase its durability.
[0098] In the above embodiment, of the pressure flank 19 of the
external thread 18 and the pressure flank 21 of the internal thread
17, a satin finished surface is formed on the pressure flank 19 of
the external thread 18, and the satin finished pressure flank 19
has a higher surface hardness than the pressure flank 21 of
internal thread 17. But instead, as shown in FIG. 4, the internal
thread 17 may be subjected to shot peening to form a satin finished
surface on the pressure flank 21 of the internal thread 17, and its
surface hardness may be made higher than the pressure flank 19 of
the external thread 18. In this case too, the internal thread 17
may be subjected to shot peening and then coated with the hard film
27, over the entire surface thereof. This makes it possible to
maintain a sufficiently high friction coefficient between the
clearance flanks 20 and 22 of the external thread 18 and the
internal thread 17, even if the clearance flank 22 of the internal
thread 17 becomes worn over a long time of use.
[0099] FIG. 6 shows a valve gear including the lash adjuster 31
according to the second embodiment of the present invention.
Elements corresponding to those of the first embodiment are denoted
by identical numerals and their description is omitted.
[0100] This valve gear includes an arm 33 having its central
portion pivotally supported on a pivot shaft 32. The art 33 carries
a roller 34 at one end thereof, and carries the lash adjuster 31 at
the other end. The roller 34 is in contact with a cam 15 fixed to a
camshaft 14 so that when the camshaft 14 rotates, the arm 33
pivots.
[0101] The lash adjuster 31 includes a nut member 35, an adjusting
screw 36 and a return spring 37. The nut member 35 is mounted in a
mounting hole 38 extending vertically through the arm 33, and has
an internal thread 17 formed on the inner periphery thereof and in
threaded engagement with an external thread 18 formed on the outer
periphery of the adjusting screw 36.
[0102] As with the first embodiment, a satin finished surface is
formed on the pressure flank 19 of the external thread 18 by shot
peening, and the external thread 18 is coated with a hard film 27
so that the satin finished surface has a surface hardness higher
than the pressure flank 21 of the internal thread 17. A backlash b
is defined between the external thread 18 and the internal thread
17 (see FIG. 7), which measures within the range of 0.1 to 0.4
mm.
[0103] The nut member 35 has its top end protruding from the top
surface of the arm 33, and a cylindrical cap 39 having a bottom is
interference-fitted on and fixed to the protruding top end of the
nut member 35. The cap 39 engages the top edge of the mounting hole
38, thereby preventing the nut member 35 from downwardly separating
from the hole 38. The cap 39 is formed with an oil hole 40
extending vertically therethrough so that lubricating oil supplied
onto the top surface of the arm 33 is introduced into the nut
member 35 through the oil hole 40.
[0104] The nut member 35 has a flange 41 at its bottom end that
abuts the bottom surface of the arm 33, thereby supporting any
upward force applied to the nut member 35.
[0105] The return spring 37 is a compression coil spring mounted
between the cap 39 and the adjusting screw 36 in an axially
compressed state. The return spring 37 has its top end supported by
the cap 39 and axially presses the adjusting screw 36 at its bottom
end, thereby biasing the adjusting screw 36 in the direction to
protrude downwardly from the from the nut member 35. The end of the
adjusting screw 36 protruding from the nut member 35 presses the
top end of the valve stem 5.
[0106] With this lash adjuster, as with the first embodiment, even
if the pressure flank 19 of the external thread 18 becomes worn
after a long-term use, since the protrusions and recesses of the
satin finished surface are sufficiently high compared to the depth
of wear, the surface of the pressure flank 19 of the external
thread 18 is less likely to become smooth, so that the friction
coefficient between the pressure flanks 19 and 21 of the external
thread 18 and the internal thread 17 is less likely to decrease.
Also, since the pressure flank 19 of the external thread 18, which
is a satin finished surface, has a higher surface hardness than the
pressure flank 21 of the internal thread 17, the satin finished
surface is less likely to become worn. This makes it possible to
effectively suppress reduction of the friction coefficient between
the pressure flanks 19 and 21.
[0107] With this lash adjuster 31, since the pressure flank 19 of
the external thread 18 is satin finished, when the pressure flanks
19 and 21 of the external thread 18 and the internal thread 17 move
close to each other, oil film is less likely to form due to the
squeeze effect.
[0108] With this lash adjuster 31, since the backlash b between the
external thread 18 and the internal thread 17 is 0.4 mm or less,
compared to the arrangement in which the backlash b is larger than
0.4 mm, the external thread 18 and the internal thread 17 are
extremely less likely to become worn. This makes it possible to
shorten the meshing length between the external thread 18 and the
internal thread 17, which in turn makes it possible to reduce the
axial length of the lash adjuster 31.
[0109] With this lash adjuster 31, since the backlash b between the
external thread 18 and the internal thread 17 is 0.1 or over,
compared to the arrangement in which the backlash b is less than
0.1 mm, the adjusting screw 11 can rotate smoothly, so that the
lash adjuster can stably perform its expected function.
[0110] FIG. 8 shows a valve gear including the lash adjuster 51
according to the third embodiment of the present invention.
Elements corresponding to those of the first embodiment are denoted
by identical numerals and their description is omitted.
[0111] This valve gear includes an arm 52 having one end thereof
pivotally supported by the lash adjuster 51 and the other end in
contact with the top end of the valve stem 5. A cam 15 fixed to a
camshaft 14 is provided over the arm 52 so that when the camshaft
14 rotates, the arm 52 pivots about the lash adjuster 51.
[0112] The lash adjuster 51 includes a nut member 53, an adjusting
screw 54 and a return spring 55. The nut member 53 is received in a
mounting hole 56 formed in the top surface of a cylinder head 2,
and has an internal thread 17 formed on its inner periphery and in
threaded engagement with an external thread 18 formed on the outer
periphery of the adjusting screw 54.
[0113] As with the first embodiment, a satin finished surface is
formed on the pressure flank 19 of the external thread 18 by shot
peening, and the external thread 18 is coated with a hard film 27
so that the satin finished surface has a surface hardness higher
than the pressure flank 21 of the internal thread 17. A backlash is
defined between the external thread 18 and the internal thread 17,
which measures within the range of 0.1 to 0.4 mm.
[0114] A bottom member 57 is fixed to the bottom end of the nut
member 53. The return spring 55 is a compression coil spring
mounted between the adjusting screw 54 and the bottom member 57 in
an axially compressed state. The return spring 55 has its bottom
end supported on the bottom member 57 and axially presses the
adjusting screw 54 at its top end through a spring seat 58, thereby
biasing the adjusting screw 54 in the direction to protrude
upwardly from the from the nut member 53. The end of the adjusting
screw 54 protruding from the nut member 53 is engaged in a recess
59 formed in the bottom surface of the arm 52, thereby pivotally
supporting the arm 52.
[0115] With this lash adjuster, as with the first embodiment, even
if the pressure flank 19 of the external thread 18 becomes worn
after a long-term use, since the protrusions and recesses of the
satin finished surface are sufficiently high compared to the depth
of wear, the surface of the pressure flank 19 of the external
thread 18 is less likely to become smooth, so that the friction
coefficient between the pressure flanks 19 and 21 of the external
thread 18 and the internal thread 17 is less likely to decrease.
Also, since the pressure flank 19 of the external thread 18, which
is a satin finished surface, has a higher surface hardness than the
pressure flank 21 of the internal thread 17, the satin finished
surface is less likely to become worn. This makes it possible to
effectively suppress reduction of the friction coefficient between
the pressure flanks 19 and 21.
[0116] With this lash adjuster 51, since the pressure flank 19 of
the external thread 18 is satin finished, when the pressure flanks
19 and 21 of the external thread 18 and the internal thread 17 move
close to each other, oil film is less likely to form due to the
squeeze effect.
[0117] With this lash adjuster 51, since the backlash between the
external thread 18 and the internal thread 17 is 0.4 mm or less,
compared to the arrangement in which the backlash b is larger than
0.4 mm, the external thread 18 and the internal thread 17 are
extremely less likely to become worn. This makes it possible to
shorten the meshing length between the external thread 18 and the
internal thread 17, which in turn makes it possible to reduce the
axial length of the lash adjuster 31.
[0118] With this lash adjuster 51, since the backlash between the
external thread 18 and the internal thread 17 is 0.1 or over,
compared to the arrangement in which the backlash b is less than
0.1 mm, the adjusting screw 11 can rotate smoothly, so that the
lash adjuster can stably perform its expected function.
[0119] In order to confirm that wear can be reduced by setting the
backlash between the external thread 18 and the internal thread 17
of this lash adjuster 51 to 0.4 mm or less, many samples having
different backlashes between the external thread 18 and internal
thread 17 from each other were prepared for each of the following
three types 1) to 3) of the lash adjuster 51. Each sample was
mounted on an actual engine and subjected to an endurance test. The
relationship between the backlashes before and after the endurance
test was investigated.
1) Lash adjuster 51 of which the meshing length between the
external thread 18 and the internal thread 17 is substantially
equal to that of a conventional lash adjuster. 2) Lash adjuster 51
of which the meshing length between the external thread 18 and the
internal thread 17 is 1/2 that of a conventional lash adjuster. 3)
Lash adjuster 51 of which the meshing length between the external
thread 18 and the internal thread 17 is 1/3 that of a conventional
lash adjuster.
[0120] The test conditions are as follows:
[0121] Engine revolving speed: 6000 rpm
[0122] Engine displacement: 1500 cc
[0123] Engine oil: OW-20 (SAE viscosity)
[0124] Duration: 500 hours
[0125] As a result, it was confirmed that although for samples of
which the meshing length between threads is substantially equal to
that of a conventional lash adjuster, as shown in FIG. 9, the
backlash scarcely increased after the endurance test irrespective
of the size of the initial backlash, for samples of which the
meshing length between threads is 1/2 or 1/3 that of a conventional
lash adjuster, as shown in FIGS. 10 and 11, an increase in backlash
was extremely small after the test for any sample of which the
initial backlash was 0.4 mm or less, compared to samples of which
the initial backlash was larger than 0.4 mm.
[0126] The test results thus indicate that even if the meshing
length between the external thread 18 and the internal thread 17 is
made shorter than in conventional arrangements in order to reduce
the axial length of the lash adjuster 51, it is possible to
extremely effectively reduce wear between the external thread 18
and the internal thread 17.
[0127] For samples of which the initial backlash was 0.4 mm or
less, the maximum surface pressure that acts between the pressure
flanks 19 and 21 of the external thread 18 and the internal thread
17 was measured. For samples of which the meshing length between
threads is substantially equal to that of a conventional lash
adjuster, the maximum surface pressure was 10 MPa, for samples of
which the meshing length between threads is 1/2 that of a
conventional lash adjuster, the maximum surface pressure was 20
MPa, and for samples of which the meshing length between threads is
1/3 that of a conventional lash adjuster, the maximum surface
pressure was 30 MPa.
[0128] FIGS. 12 and 13 show a valve gear including the lash
adjuster of the fourth embodiment according to the present
invention. Elements corresponding to those of the first embodiment
are denoted by identical numerals, and their description is
omitted.
[0129] The adjusting screw 11 of this embodiment has an external
thread 73 on its outer periphery that is in threaded engagement
with an internal thread 72 formed on the inner periphery of nut
member 10. The external thread 73 and the internal thread 72 have a
vertically symmetrical trapezoidal section, and are configured to
receive axial loads that tend to push the adjusting screw 11 into
the nut member 10 on the pressure flanks 74 and 75 of the external
thread 73 and the internal thread 72.
[0130] As with the first embodiment, a satin finished surface
having a surface roughness Ra of 1.6 to 12.5 is formed on the
pressure flank 74 of the external thread 73 by shot peening, and
the external thread 73 is coated with a hard film so that the satin
finished surface has a surface hardness higher than the pressure
flank 75 of the internal thread 72. A backlash is defined between
the external thread 73 and the internal thread 72, which measures
within the range of 0.1 to 0.4 mm.
[0131] A torsion coil spring 76 is mounted between the adjusting
screw 11 and the end wall 9b of the lifter body 9. The torsion coil
spring 76 has its top end engaged in an engaging groove 77 formed
in the top surface of the nut member 10 and is thus rotationally
fixed to the nut member. Its bottom end is engaged in an engaging
hole 78 formed in the adjusting screw 11. Thus, when the coil
spring 76 is twisted, it applies torque to the adjusting screw 11
that tends to protrude the adjusting screw 11 downwardly from the
nut member 10. The end of the adjusting screw 11 protruding from
the nut member 10 presses the top end of the valve stem 5 through a
spacer 23.
[0132] With this lash adjuster 71, even if the pressure flank of
the external thread 73 becomes worn, since the protrusions and
recesses of the satin finished surface are sufficiently high
compared to the depth of wear, the surface of the pressure flank of
the external thread 73 is less likely to become smooth, so that the
friction coefficient between the pressure flanks 74 and 75 of the
external thread 73 and the internal thread 72 is less likely to
decrease. Also, since the satin finished pressure flank 74 of the
external thread 73 has a higher surface hardness than the pressure
flank 75 of the internal thread 72, it is possible to slow down the
progression of wear of the satin finished surface, which in turn
makes it possible to effectively suppress reduction in friction
coefficient between the pressure flanks 74 and 75.
[0133] With this lash adjuster 71, since the pressure flank of the
external thread 73 is satin finished, when the pressure flanks 74
and 75 of the external thread 73 and the internal thread 72 move
close to each other, oil film is less likely to form due to the
squeeze effect.
[0134] In this embodiment, the torsion spring for applying torque
to the adjusting screw 11 that tends to protrude the adjusting
screw 11 from the nut member 10 is the torsion coil spring 76. But
instead of the torsion coil spring 76, a spiral spring may be
used.
[0135] FIGS. 14 and 15 show a valve gear including the lash
adjuster 91 of the fifth embodiment according to this invention.
Elements corresponding to those of the second embodiment are
denoted by identical numerals and their description is omitted.
[0136] The nut member 35 is inserted in a mounting hole 92 formed
in the bottom surface of the arm 33, and has an internal thread 93
on its inner periphery which is in threaded engagement with an
external thread 94 formed on the outer periphery of the adjusting
screw 36. The external thread 94 and the internal thread 93 have a
vertically symmetrical triangular section, and are configured to
receive an axial load that tends to push the adjusting screw 36
into the nut member 35 on their respective pressure flanks 95 and
96.
[0137] As with the second embodiment, a satin finished surface
having a surface roughness Ra of 1.6 to 12.5 is formed on the
pressure flank 95 of the external thread 94 by shot peening, and
the external thread 94 is coated with a hard film so that the satin
finished surface has a surface hardness higher than the pressure
flank of the internal thread 93. A backlash is defined between the
external thread 94 and the internal thread 93, which measures
within the range of 0.1 to 0.4 mm.
[0138] A torsion coil spring 97 is mounted between the adjusting
screw 36 and the inner bottom surface of the mounting hole 92. The
torsion coil spring 97 has its top end engaged in a through hole 98
formed in the inner bottom surface of the mounting hole 92 and thus
is rotationally fixed to the arm 33. Its bottom end is engaged in
an engaging hole 99 formed in the adjusting screw 36. Thus, when
the coil spring 97 is twisted, it applies torque to the adjusting
screw 36 that tends to protrude the adjusting screw 36 downwardly
from the nut member 35. The end of the adjusting screw 36
protruding from the nut member 35 presses the top end of the valve
stem 5.
[0139] Engine oil splashed onto the top surface of the arm 33 is
introduced into the mounting hole 92 through the hole 98, thereby
lubricating between the external thread 94 and the internal thread
93.
[0140] With this lash adjuster 91, even if the pressure flank 95 of
the external thread 94 becomes worn, since the protrusions and
recesses of the satin finished surface are sufficiently high
compared to the depth of wear, the surface of the pressure flank 95
of the external thread 94 is less likely to become smooth, so that
the friction coefficient between the pressure flanks 95 and 96 of
the external thread 94 and the internal thread 93 is less likely to
decrease. Also, since the satin finished pressure flank 95 of the
external thread 94 has a higher surface hardness than the pressure
flank 96 of the internal thread 93, it is possible to slow down the
progression of wear of the satin finished surface, which in turn
makes it possible to effectively suppress reduction in friction
coefficient between the pressure flanks 95 and 96.
[0141] With this lash adjuster 91, since the pressure flank 95 of
the external thread 94 is satin finished, when the pressure flanks
95 and 96 of the external thread 94 and the internal thread 93 move
close to each other, oil film is less likely to form due to the
squeeze effect.
[0142] In this embodiment, the torsion spring for applying torque
to the adjusting screw 36 that tends to protrude the adjusting
screw 36 from the nut member 35 is the torsion coil spring 97. But
instead of the torsion coil spring 97, a spiral spring may be
used.
[0143] FIGS. 16 and 17 show a valve gear including the lash
adjuster 111 of the sixth embodiment according to the present
invention. Elements corresponding to those of the third embodiment
are denoted by identical numerals, and their description is
omitted.
[0144] The nut member 53 has an internal thread 17 on its inner
periphery at its lower portion. The adjusting screw 54 comprises an
externally threaded member 54A having an external thread 19 on its
outer periphery that is in threaded engagement with the internal
thread 17 on the nut member 53, and a pivot member 54B axially
slidably fitted in the nut member 53. A disk spring 112 is mounted
between the pivot member 54B and the externally threaded member
54A. The end of the pivot member 54B inserted in the nut member 53
is supported by the externally threaded member 54A through the disk
spring 112.
[0145] When the engine cools down and there appear differences in
shrinkage between component parts of the valve gear, the disk
spring 112 is configured to be compressed, thereby absorbing such
differences in shrinkage. This prevents a gap between the valve 4
and the valve seat 8 due to differences in shrinkage between
component parts of the valve gear, thereby preventing compression
leakage, when the engine is restarted.
[0146] As shown in FIG. 3, the external thread 18 is subjected to
shot peening and then coated with a hard film. Thus, the pressure
flank 19 of the external thread 18 forms a satin finished surface
having a surface roughness Ra of 1.6 to 12.5. By coating the
external thread 18 with the hard film, the satin finished surface
has a surface hardness higher than the pressure flank 21 of the
internal thread 17.
[0147] As shown in FIG. 17, a torsion coil spring 114 is mounted
between the externally threaded member 54A and a bottom portion 113
of the nut member 53. The torsion coil spring 114 has its bottom
end engaged in a through hole 115 formed in the bottom portion 113
of the nut member 53 and thus is rotationally fixed to the nut
member. Its top end is engaged in an engaging hole 116 formed in
the externally threaded member 54A. Thus, when the spring 114 is
twisted, it applies torque to the externally threaded member 54A
that tends to protrude the pivot member 54B from the nut member
53.
[0148] As shown in FIG. 16, the pivot member 54B has its end 117
that protrudes from the nut member 53 hemispherically shaped. The
protruding end 117 is engaged in a recess 59 formed in the bottom
surface of the arm 52 at its end, thereby pivotally supporting the
arm 52.
[0149] An oil discharge hole 118 is formed in the inner bottom
surface of the mounting hole 56 that communicates with the through
hole 115. Thus, engine oil flowing from the top end surface of the
nut member 53 into the nut member 53 through the gap between the
external thread 18 and the internal thread 17 is discharged from
the nut member 53 through the hole 115 and then through the oil
discharge hole 118.
[0150] With this lash adjuster 111, even if the pressure flank of
the external thread 18 becomes worn, since the protrusions and
recesses of the satin finished surface are sufficiently high
compared to the depth of wear, the surface of the pressure flank 19
of the external thread 19 is less likely to become smooth, so that
the friction coefficient between the pressure flanks 19 and 21 of
the external thread 18 and the internal thread 17 is less likely to
decrease. Also, since the satin finished pressure flank 19 of the
external thread 18 has a higher surface hardness than the pressure
flank 21 of the internal thread 17, it is possible to slow down the
progression of wear of the satin finished surface, which in turn
makes it possible to effectively suppress reduction in friction
coefficient between the pressure flanks 19 and 21.
[0151] With this lash adjuster, since the pressure flank 19 of the
external thread 18 is satin finished, when the pressure flanks 19
and 21 of the external thread 18 and the internal thread 17 move
close to each other, oil film is less likely to form due to the
squeeze effect.
[0152] In this embodiment, of the pressure flank 19 of the external
thread 18 and the pressure flank 21 of the internal thread 17, a
satin finished surface is formed on the pressure flank 19 of the
external thread 18. But instead, as shown in FIG. 4, the internal
thread 17 may be subjected to shot peening to form a satin finished
surface on the pressure flank 21 of the internal thread 17, and its
surface hardness may be made higher than the pressure flank 19 of
the external thread 18. In this case too, the internal thread 17
may be subjected to shot peening only on the pressure flank 21 of
the internal thread 17 or over the entire surface thereof.
[0153] In this embodiment, the torsion spring for applying torque
to the adjusting screw 54 that tends to protrude the adjusting
screw 54 from the nut member 53 is a torsion coil spring. But
instead of the torsion coil spring, a spiral spring may be
used.
* * * * *