U.S. patent number 3,636,995 [Application Number 05/051,988] was granted by the patent office on 1972-01-25 for tensioner for saw chain.
This patent grant is currently assigned to Textron, Inc.. Invention is credited to Albert K. Newman.
United States Patent |
3,636,995 |
Newman |
January 25, 1972 |
TENSIONER FOR SAW CHAIN
Abstract
A chain saw is provided with means for tensioning the cutting
chain to compensate for wear and thermal expansion during
operation. The chain guide bar is slidably mounted on the chain saw
frame so as to be movable longitudinally toward and away from the
chain-driving sprocket. A spring-operated cam rotatably mounted on
the frame engages a pin on the guide bar to move it in a direction
away from the driving sprocket and thereby tighten the chain. The
cam has a generally spiral cam contour with equally spaced cusps
and intervening concavities.
Inventors: |
Newman; Albert K. (Noroton,
CT) |
Assignee: |
Textron, Inc. (Providence,
RI)
|
Family
ID: |
21974674 |
Appl.
No.: |
05/051,988 |
Filed: |
July 2, 1970 |
Current U.S.
Class: |
30/386;
474/135 |
Current CPC
Class: |
B27B
17/14 (20130101) |
Current International
Class: |
B27B
17/14 (20060101); B27B 17/00 (20060101); B27b
017/14 () |
Field of
Search: |
;143/32J,32 ;144/72,73
;74/242.11,242.8,242.16 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schran; Donald R.
Claims
What I claim and desire to secure by Letters Patent is:
1. In a chain saw having a frame and a chain-driving sprocket, the
combination of a chain guide bar, means mounting said guide bar on
said frame in alignment with said sprocket for movement in a
longitudinal direction toward and away from said sprocket, an
endless chain running on said guide bar and sprocket and driven by
said sprocket, and means for automatically tensioning said chain to
proper operating tension, said tensioning means comprising an
abutment on said guide bar, a cam movably mounted on said frame and
engaging said abutment, said cam having a cam contour for moving
said abutment and hence said guide bar in a direction away from
said sprocket to tension said chain upon movement of said cam in a
forward direction, and spring means for moving said cam in a
forward direction to tighten said chain, said cam contour having
alternate high and low increments engageable with said abutment to
arrest forward movement of said cam when the chain is under proper
tension and to resist movement of said cam in a reverse
direction.
2. A combination according to claim 1, in which said cam contour of
said cam comprises a series of spaced cusps alternating with
concave curves between said cusps.
3. A combination according to claim 2, in which said abutment
comprises a round pin fixed on said guide bar and in which said
concave curves of said cam contour have a radius of curvature
slightly greater than that of said pin.
4. A combination according to claim 1, in which said guide bar is
provided with a plurality of spaced holes at different distances
from said sprocket and in which said abutment comprises a pin
removably fixed selectively in one of said holes.
5. A combination according to claim 1, in which said cam is
rotatably mounted and has a generally spiral contour, said spring
means comprising a spiral spring acting on said cam to rotate it
about its axis of rotation.
6. A combination according to claim 5, in which said cam contour
comprises a series of spaced cusps and concave arcs between said
cusps, the distance between said cusps and the axis of rotation of
said cam progressively increasing from one terminus of said cam
contour to the other, said cusps being substantially equally spaced
along said cam contour and the included angles between said cusps
with apices at said axis of rotation progressively decreasing with
increase of the distance between said cusps and said axis.
7. A combination according to claim 1, comprising manually operable
means for moving said cam in a reverse direction and thereby
tensioning said spring means.
8. A combination according to claim 7, in which said cam is fixed
on a rotatable shaft and in which said manually operable means
comprises a knob fixed on said shaft.
9. A combination according to claim 1, in which said guide bar has
a longitudinally extending slot in an end portion adjacent said
sprocket and in which said mounting means comprise an elongated
block received in said slot, said block having a width slightly
less than the width of said slot, a length less than the length of
said slot by an amount at least equal to the desired range of
longitudinal movement of said guide bar and a thickness slightly
greater than the thickness of said guide bar, guide plates on
opposite sides of said block and said guide bar, said block spacing
said plates apart a distance slightly greater than the thickness of
said guide bar and means securing said plates and said block
together, said block and plates firmly supporting said guide bar
for free longitudinal movement by said cam.
10. In a chain saw having a frame and a chain-driving sprocket, the
combination of a chain guide bar means mounting said guide bar on
said frame in alignment with said sprocket for free movement in a
longitudinal direction toward and away from said sprocket, an
endless chain running on said guide bar and sprocket and driven by
said sprocket, and means for tensioning said chain to proper
operating tension, said tensioning means comprising an abutment on
said guide bar, a cam movably mounted on said frame and engaging
said abutment, said cam having a cam contour for moving said
abutment and hence said guide bar in a direction away from said
sprocket to tension said chain upon movement of said cam in a
forward direction and means for progressively moving said cam in a
forward direction to tighten said chain and compensate for
progressive increase in length of said chain through wear and
increase of temperature.
11. A combination according to claim 10, in which said cam contour
comprises a plurality of spaced cusps with intervening concavities
between said cusps.
12. A combination according to claim 10, in which said cam is
rotatably mounted and said cam contour is generally spiral.
13. A combination according to claim 12, in which said cam is fixed
on a rotatable shaft and means for rotating said cam comprises a
manually operable knob fixed on said shaft.
14. In combination with an endless chain running over a driving
sprocket and guide means which is movable toward and away from said
driving sprocket to vary the tension of said chain,
chain-tensioning means comprising a support, an abutment on said
guide means, a cam movably mounted on said support and engaging
said abutment, said cam having a cam contour for moving said
abutment and hence said guide means in a direction to tension said
chain upon movement of said cam in a forward direction, and spring
means for moving said cam in a forward direction to tighten said
chain, said cam contour having alternate high and low increments
engageable with said abutment to arrest forward movement of said
cam when the chain is under proper tension and to resist movement
of said cam in a reverse direction.
15. Chain-tensioning means according to claim 14, in which said cam
contour comprises a series of spaced cusps alternating with
concavities between said cusps.
16. Chain-tensioning means according to claim 15, in which said cam
is rotatable and said cam contour is generally spiral, said spring
means acting on said cam to rotate it.
Description
The present invention relates to chain-tensioning means and in
particular to means for maintaining the cutting chain of a chain
saw under proper tension.
In a chain saw, the cutting chain customarily runs around the
periphery of a guide bar which is secured at one end to the frame
or chassis of a handheld engine and may be provided at the other
end with a nose sprocket or roller to reduce friction. The chain is
driven by a sprocket which is located in line with the inboard end
of the guide bar and is driven by the engine. The chain is provided
with inwardly projecting lugs or tangs which engage teeth of the
driving sprocket and are received in a peripheral groove of the
guide bar.
In order for the chain saw to operate efficiently and safely, the
cutting chain must be under proper tension. If it is too loose it
may jump off of the guide bar with the possibility of causing
serious injury to the operator. If the chain is too tight there may
be excessive friction between the chain and guide bar particularly
at the nose of the guide bar with the result of overheating and
loss of efficiency in operation. If a critical value of tension is
exceeded, the chain will break.
The tension of a saw chain does not remain constant. By reason of
wear in the chain elements, a chain progressively lengthens during
use thereby resulting in a decrease of tension if no compensation
is made for the increased length. The change of length and hence of
tension is more rapid during the initial period of use of a new
chain. Changes in tension also occur by reason of changes in
temperature. In the operation of a chain saw the temperature of the
chain increases more rapidly than that of the guide bar so that the
tension of the chain is decreased. When the chain and guide bar
cool overnight or during periods of nonuse, the chain becomes
tighter. Hence, in order to maintain proper operating tension of
the chain, it is necessary to provide some kind of adjustment.
Heretofore, adjustment has been effected manually when the chain
saw is not operating. The guide bar is customarily secured to the
frame or chassis of the engine by two bolts which must be loosened
in order to move the guide bar in a lengthwise direction relative
to the sprocket in order to change the tension of the chain. The
guide bar is customarily moved by means of a runner on the guide
bar engaging a screw rotatably mounted on the chassis adjacent the
inboard end of the guide bar. The bolts holding the guide bar are
then again tightened to secure the guide bar in the position to
which it has been adjusted. In order to make any adjustment the
chain saw must, of course, be stopped. The tensioning of a chain to
the proper value is dependent on the skill of the operator who
customarily determines the tightness of the chain by "feel."
It is an object of the present invention to provide improved means
for adjusting the tension of the cutting chain of a chain saw. In
accordance with the invention the guide bar, instead of being
bolted tightly on the frame or chassis of the engine, is mounted so
as to be readily slidable in a lengthwise direction while being
held against lateral movement either vertically or horizontally.
Cam means is provided for moving the guide bar longitudinally away
from the sprocket so as to tension the chain to the correct value.
In a preferred embodiment of the invention the tensioning cam is
spring operated so as to increase the chain tension automatically
in the event the chain becomes too slack. The cam is preferably
rotary with a generally spiral cam contour. Moreover, the contour
of the cam is preferably provided with alternating high and low
portions, for example spaced cusps and intervening concavities.
This configuration of the cam has been found to avoid excessive
tension on the chain and also prevents rearward movement of the
guide bar when abnormal pressure is applied to the end of the guide
bar, for example in boring.
The objects and advantages of the invention will be more fully
understood from the following description of a preferred embodiment
shown by way of example in the accompanying drawings in which:
FIG. 1 is a somewhat schematic exploded view showing
chain-tensioning means in accordance with the invention together
with associated portions of a chain saw;
FIG. 2 is an enlarged side view of a cam and associated abutment
for adjusting chain tension;
FIG. 3 is a horizontal section of chain-tensioning means in
accordance with the invention, and
FIG. 4 is a partial inside view of a cover shown in FIG. 1.
In the drawings there are shown portions of a chain saw comprising
an engine with a frame or chassis 5. The engine is ordinarily a
small two-cycle internal combustion engine although an electric
motor may be used if desired. The chain saw is provided in known
manner with suitable handles and controls for starting, regulating
and manipulating the chain saw. At one side of the engine chassis
there is provided a chain-driving sprocket 6 which is driven by the
engine preferably through a centrifugal clutch 7 (FIG. 3) so that
the sprocket is driven only when the engine speed exceeds a
predetermined value and is not driven at idling speed. Forwardly of
the sprocket 6 and in proper alignment therewith the engine chassis
is provided with a fixed block or bracket 8 for mounting a guide
bar. Two stud bolts 9 project outwardly from the mounting bracket
8. The bolts 9 are spaced apart from one another on a line
extending forwardly from the sprocket. A guide bar 10 is slidably
mounted on the chassis in alignment with the sprocket 6 so as to be
movable in a longitudinal direction toward and away from the
sprocket while being held firmly against lateral movement or
deflection either horizontally or vertically. The guide bar is
provided with a peripheral groove or slot 11 to receive the
inwardly projecting tangs of the cutting chain and may be provided
at its outer end with a roller nose or sprocket (not shown) in
known manner. Near the inner end of the guide bar there is a
longitudinally extending slot 12.
Means for mounting the guide bar on the chassis are shown by way of
example in the drawings as comprising the mounting base or bracket
8 with projecting stud bolts 9, an inner guide plate or shim 14, a
guide block 15 and an outer guide plate or shim 16. The guide block
15 and each of the shims 14 and 16 have spaced bolt holes to
receive the stud bolts 9. The guide block 15 has a thickness
slightly greater than the thickness of the guide bar 10, a width
slightly less than the width of the slot 12 in the guide bar and a
length sufficiently shorter than the length of the slot 12 to
permit the desired range of longitudinal movement of the guide bar.
The inner and outer shims 14 and 16 have a length somewhat greater
than the length of the guide block 15 and a shape corresponding
approximately to the shape of the adjacent portion of the guide
bar. When the parts are assembled as shown in FIG. 3 and nuts 9a on
the stud bolts 9 are tightened, the inner shim 14 is clamped
tightly against the mounting base or bracket 8, the mounting block
15 is received in the slot 12 of the guide bar 10 and the inner and
outer shims 14 and 16 are clamped tightly against opposite faces of
the guide block 15. By reason of the guide block being slightly
thicker than the guide bar 10, the inner and outer shims 14 and 16
are not clamped tightly against opposite faces of the guide bar but
provide sufficient clearance to permit longitudinal movement of the
guide bar (without loosing the nuts 9a) while confining it in a
lateral direction so as to prevent any substantial lateral movement
or deflection of the guide bar. In like manner, the width of the
guide block 15 is sufficiently less than the width of the slot 12
in the guide bar to provide sufficient clearance to permit free
longitudinal movement of the guide bar while preventing any
substantial up or down movement. The mounting base 8 and inner shim
14 are shown as being provided with mating slots 8a and 14 a
through which lubricant is supplied to the inner end portion of the
guide bar 10 from a suitable lubricant supply, for example an oil
tank and pump connected through a suitable duct to the slot 8a.
This provides lubrication for the chain running on the guide bar
and also lubricates the inner end portion of the guide bar to
facilitate longitudinal movement of the guide bar. The slots 8a and
14a are inclined at a small angle, for example about 6.degree. to
the centerline connecting the bolts 9.
An endless cutting chain 20 runs around the guide bar 10 and the
sprocket 6. The chain is provided in usual manner with links having
cutting teeth and depth gages and has inwardly projecting lugs or
tangs which engage the teeth of the sprocket 6 and are received in
the peripheral slot 11 of the guide bar. If, as is preferable, the
guide bar is provided with a nose sprocket, the inwardly projecting
tangs of the saw chain likewise engage the nose sprocket teeth in
known manner. It will be seen that since the cutting chain extends
around both the driving sprocket 6 and the guide bar 10, movement
of the guide bar in a longitudinal direction away from the driving
sprocket 6 applies tension to the chain.
In accordance with the present invention, cam means is provided for
moving the guide bar relative to the sprocket to tension the
cutting chain. The tension-regulating cam means is shown by way of
example in the drawings as a generally spiral cam 21 fixed on a
rotatable shaft 22 and engaging a stud or pin 23 projecting
laterally from the guide bar 10 adjacent one side of the slot 12
and projecting through a longitudinally extending slot 16a in the
outer shim 16. The cam shaft 22 is rotatably supported in a hole
25a of a cover 25 which is shaped to enclose the inner end portion
of the guide bar including the chain-tensioning means and also the
chain-driving sprocket 6 and associated centrifugal clutch 7. A
hollow knob 26 fixed on a projecting outer end portion of the cam
shaft 22 encloses a spiral spring 27, one end 27a of which is
turned inwardly so as to engage in a longitudinally extending
radial slot 22a in the shaft 22 while the opposite end 27b is
anchored by engagement with a pin 28 projecting outwardly of the
cover 25 near the shaft hole 25a. The spring 27 acts on the shaft
22 to bias it in a clockwise direction as seen in FIG. 1. The cam
shaft 22 and hence the cam 21 are rotatable in a counterclockwise
direction against the tension of the spring 27 by the knob 26,
rotation being limited by engagement of a pin 29a on the cam with a
pin 29b projecting inwardly on the inner face of the cover 25
adjacent the shaft hole 25a.
A cam cover 30 is preferably interposed between the outer shim 16
and the cam and is provided with an elongated slot 30a to
accommodate the pin 23 on the guide bar 10 and with a contoured rim
portion 30b which projects outwardly over the cam. The cam cover 30
and outer cover 25 are provided with bolt holes to receive the stud
bolts 9, the entire assembly comprising the inner shim 14, the
guide block 15, the outer shim 16, the cam cover 30 and the outer
cover 25 being secured by the nuts 9a on the stud bolts 9 with the
guide bar 10 received between the inner and outer shims, the cam 21
being accommodated between the cam cover 30 and the outer cover 25
and the cam shaft 22 extending outwardly through the hole 25a
provided in the cover 25. To facilitate assembly of the parts, the
outer shim 16 and cam cover 30 are removably secured to the outer
cover 25 by two screws 31 which extend through holes in the outer
shim and cam cover and screw into blind tapped holes in the outer
cover.
As seen more clearly in the enlarged view of FIG. 2, the cam 21 has
a cam contour 21a which is generally spiral so that an inner end
portion 21b is closer to the axis of the shaft 22 than is the outer
end portion 21c. As seen in FIG. 2 the resulting step 21d is
somewhat undercut so that the inner end portion 21b partially
underlies the outer end portion 21c. The contoured periphery 21a of
the cam 21 is preferably not smooth but comprises a plurality of
alternate high and low increments which are shown by way of example
in the drawings as spaced cusps 21e and intervening concavities
21f. The concavities 21f are preferably at least approximately
arcuate with a curvature that is slightly less than the curvature
of the pin 23 (the radius of curvature of the concavities 21f being
slightly greater than the radius of the pin 23).
In assembling the parts shown in FIG. 1, the pin 23 is fixed on the
guide bar 10, for example by being screwed into a tapped hole by
means of a hexagonal or other noncircular section provided on the
pin. The cam shaft 22 with the cam 21 fixed on the inner end
thereof is inserted through the hole 25a from the inside of the
cover and the spring 27 and knob 26 are assembled on the outer end
of the cam shaft and the knob 26 is secured on the shaft, for
example by a set screw 26a. The inner shim 14, guide block 15,
guide bar 10, outer shim 16, cam cover 30 and outer cover 25 are
then assembled on the stud bolts 9 in that order, the chain 20
being positioned around the guide bar 10 and over the drive
sprocket 20 before the cover 25 is positioned on the bolts 9. After
the cover is placed on the bolts 9 but before it is slipped in far
enough for the cam 21 to engage the pin 23 of the guide bar, the
knob 26 is turned in a counterclockwise direction against the
tension of the spring 27 until the pin 29a on the cam engages the
pin 29b on the inner face of the cover. These pins are so
positioned as to present the inner end 21b of the cam to the pin 23
on the guide bar 10. The cover 25 is then slid all the way in and
the nuts 9a are screwed onto the bolts 9 and tightened. Upon
release of the knob 26--after the cover 25 has been pushed
home--the spring 27 tends to turn the cam 21 in a clockwise
direction. The cam will turn until the cam periphery firmly engages
the pin 23. With the chain saw in a static condition the cam 21
acted on by the spring 27 may not exert sufficient force on the pin
23 to overcome static friction and move the guide bar 10
longitudinally away from the drive sprocket to tension the chain.
However, as soon as the chain saw is started, friction is reduced
by the natural vibration of the chain saw whereupon--if the chain
is not already at proper tension--the cam 21 will rotate further in
a clockwise direction until proper tension on the chain is
achieved. The contour of the cam 21 and the force of the spring 27
are selected so that the chain 20 is under proper tension when
equilibrium is reached between the force of the spring 27 and the
tension of the chain.
During operation of the saw, the chain 20 tends to become longer by
reason of the wear and setting of the chain parts and also because
of thermal expansion of the chain upon increase of temperature
occurring during cutting operations. If no correction were made,
the increase in chain length would result in a decrease of chain
tension. However, with the tensioning device herein illustrated and
described, the cam 21 turns farther in a clockwise direction
whenever slack occurs in the chain so as to maintain the chain
under proper tension.
The alternate high and low increments of the cam contour
represented by the cusps 21e and intervening concavities 21f serve
the function of avoiding excess tension on the chain. The pin 23
ordinarily sits in one of the concavities as seen in FIG. 2 and the
cam will not move until the chain has lengthened sufficiently to
permit the pin 23 to ride over the intervening cusp 21e. The pin 23
thereupon drops down into the next concavity so as to decrease the
tension slightly thereby providing the correct tension value and
preventing overtensioning.
When the saw is used in a "boring" operation, the guide bar with
the chain running thereon is pushed longitudinally into the timber
being cut so that the nose of the guide bar penetrates the timber.
This may give rise to a substantial longitudinal force on the guide
bar tending to push it back toward the drive sprocket and thereby
creating slack in the chain. The alternate cusps and concavities of
the cam contour prevent retrogressive rotation of the cam by
pressure applied by the pin 23 and thereby prevent slackening of
the chain during boring operations.
In the preferred cam contour shown by way of example in FIG. 2,
successive cusps 21e are spaced apart from one another a uniform
distance. Moreover, the radial distances between the bottoms of
successive concavities 21f from the axis of the shaft 22 increase
by equal increments from the inner end 21b of the cam contour to
the outer end 21c. This means that the guide bar is moved equal
amounts each time the pin 23 moves from one concavity to the next.
By reason of this arrangement the included angle between successive
cusps with the apex of the angle at the axis of the cam shaft 22
progressively decreases from the inner end to the outer end of the
cam contour.
In order to increase the range of movement of the guide bar 10
provided by the tensioning mechanism, the guide bar is provided
with an additional tapped hole 10a into which the pin 23 can be
screwed. The hole 10a is on the opposite side of the slot 12 and is
closer to the inner end of the guide bar by a distance
approximately equal to the throw of the cam 21. Hence, when the
chain is lengthened to an extent where the outer end 21c of the cam
contour has reached the pin 23, the guide bar 10 can be reversed
side for side and the pin 23 moved to the other hole 10a in
position to start again at the inner end 21d of the cam. Still
additional holes can, if desired, be provided on the same or
opposite side of the slot 12.
While biasing of the cam 21 by the spring 27 is desirable in order
to provide automatic regulation of the chain tension while the saw
is in operation, it will be understood that even without the spring
27, the mechanism shown and described would be useful in that it
would permit convenient manual adjustment of tension by means of
the knob 26. In this event it may be desirable to provide friction
means, for example a friction washer, to assist in holding the cam
in the position to which it is adjusted. If desired, the guide
block 15 can be omitted in which event the slot 12 is proportioned
to slide on the bolts 9 and adjustment of tension is effected by
loosening the nuts 9a on bolts 9, turning the cam-- by spring
tension or manually-- to provide the proper tension and then
tightening the nuts. Moreover, it will be understood that while a
rotary cam as described is considered preferable, a linearly
movable cam may be used in like manner for movement of the guide
bar in a longitudinal direction outwardly from the drive sprocket
to adjust the tension of the chain without loosening the nuts on
the bolts 9 by which the guide bar and associated parts are mounted
on the chain saw chassis. While the chain-tensioning means of the
present invention is particularly useful for maintaining saw chains
at proper tension, it is also applicable to tensioning other
endless chains, for example drive chains, running over a drive
sprocket and a chain guide which is movable relative to the
sprocket to tighten the chain.
* * * * *