U.S. patent application number 10/704503 was filed with the patent office on 2005-05-12 for trowel gearbox brake.
This patent application is currently assigned to Wacker Corporation. Invention is credited to Jenkins, Michael, Kruepke, Gregory.
Application Number | 20050100406 10/704503 |
Document ID | / |
Family ID | 34435594 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050100406 |
Kind Code |
A1 |
Jenkins, Michael ; et
al. |
May 12, 2005 |
TROWEL GEARBOX BRAKE
Abstract
A brake of a concrete finishing trowel responds automatically to
the cessation of drive torque delivery to the rotor to actively
brake the trowel's drive train, hence inhibiting or preventing
frame rotation. The brake preferably takes advantages of inherent
characteristics of an inclined gear, such as a worm of a gearbox,
to permit an internal component of the gearbox to shift
automatically upon the cessation of drive torque therethrough from
a brake released position to a brake engaged position. In the case
of a worm gear-based gearbox, the worm is configured such that
reaction forces that are normally generated by the delivery of
drive torque therethrough shift the worm to release the brake
during normal trowel operation. These forces are eliminated in the
absence of drive torque transfer, permitting the worm to shift to a
brake engaged position.
Inventors: |
Jenkins, Michael; (Slinger,
WI) ; Kruepke, Gregory; (Waukesha, WI) |
Correspondence
Address: |
Timothy E. Newholm
BOYLE, FREDRICKSON, NEWHOLM, STEIN & GRATZ, S.C.
250 Plaza, Suite 1030
250 East Wisconsin Avenue
Milwaukee
WI
53202
US
|
Assignee: |
Wacker Corporation
|
Family ID: |
34435594 |
Appl. No.: |
10/704503 |
Filed: |
November 7, 2003 |
Current U.S.
Class: |
404/112 |
Current CPC
Class: |
E04F 21/248 20130101;
F16H 1/16 20130101; F16D 2065/024 20130101; F16D 2127/005
20130101 |
Class at
Publication: |
404/112 |
International
Class: |
E01C 019/22 |
Claims
1. A gearbox for a rotary concrete finishing trowel, comprising:
(A) a housing; (B) a rotary input element that is supported by said
housing and that is configured to be operatively connected to an
output element of a prime mover of the trowel; (C) a rotary output
element that is supported by said housing and that is configured to
be operatively connected to a rotor of the trowel; (D) a torque
transfer system that is supported by said housing and that is
configured to transfer torque from said input element to said
output element, said torque transfer system being constructed such
that delivery of torque thereto by said input element imposes an
axial thrust on at least one component thereof that drives said
component to move axially from a first position thereof to a second
position thereof; and (E) a brake that cooperates with said torque
transfer system so as to automatically 1) release when said
component is in said second position thereof, thereby permitting
torque transfer from said input element, through said torque
transfer system, and to said output element, and 2) engage when
said component is in said first position thereof, thereby resisting
torque transfer from said output element, through said torque
transfer system, and to said input element.
2. The gearbox as recited in claim 1, wherein said component
comprises an angled drive gear that is subject to axial reaction
forces upon torque transfer between itself and a mating driven gear
of said torque transfer system.
3. The gearbox as recited in claim 2, wherein said drive gear
comprises a worm that meshes with a worm gear of said torque
transfer system, said worm gear imposing an axial thrust on said
worm upon the delivery of drive torque to said torque transfer
system by said input element.
4. The gearbox as recited in claim 3, wherein said worm is mounted
on a shaft that moves axially with said worm, and wherein said
brake is mounted on said shaft so as to be movable between the
released and engaged positions thereof upon movement of said worm
between the second and first positions thereof.
5. The gearbox as recited in claim 4, wherein said brake comprises
a cone brake including (A) a stationary cup, and (B) a cone that is
mounted on said shaft so as to contact said cup when said brake is
engaged and to be spaced from said cup when said brake is
released.
6. The gearbox as recited in claim 4, further comprising a return
spring that is configured to bias said brake toward said engaged
position.
7. The gearbox as recited in claim 6, wherein said torque transfer
system further comprises a bearing that rotatably supports said
shaft in said housing and that moves axially during movement of
said shaft between the second and first positions thereof, and
wherein said spring acts on said bearing.
8. The gearbox as recited in claim 7, further comprising a second,
axially stationary bearing that rotatably supports said shaft in
said housing and that accommodates axial movement of said shaft
relative thereto.
9. The gearbox as recited in claim 6, wherein said spring comprises
a wave spring that surrounds said shaft and that is disposed
between said bearing and a stationary internal component of said
gearbox.
10. The gearbox as recited in claim 2, wherein said torque transfer
system comprises a shaft that moves axially with said angled drive,
and wherein said brake comprises a cone brake including (A) a
stationary cup having a tapered inner peripheral surface, and (B) a
cone having a tapered outer peripheral surface, said cone being
mounted on said shaft so as to contact said cup when said brake is
engaged and to be spaced from said cup when said brake is
released.
11. The gearbox as recited in claim 10, wherein said shaft
comprises an input shaft serving as said input element.
12. A gearbox for a rotary concrete finishing trowel, comprising:
(A) a housing; (B) a rotary input shaft that is supported by said
housing and that is configured to be operatively coupled to an
output element of a prime mover of the trowel; (C) a rotary output
shaft that is supported by said housing and that is configured to
be operatively coupled to a rotor of the trowel; (D) a torque
transfer system that is supported by said housing and that is
configured to transfer torque from said input shaft to said output
shaft, said torque transfer system including a worm and a worm gear
that meshes with said worm gear, said worm being movable axially,
upon delivery of drive torque thereto by said input shaft, from a
first position thereof to a second position thereof; and (E) a
brake that cooperates with said torque transfer system so as to
automatically 1. release when said worm is in said second position
thereof, thereby permitting torque transfer from said input shaft,
through said torque transfer system, and to said output shaft, and
2. engage when said worm is in said first position thereof, thereby
resisting torque transfer from output shaft, through said torque
transfer system, and to said input shaft.
13. The gearbox as recited in claim 12, wherein said worm is fixed
to said input shaft so that said input shaft and said worm gear
move axially together, and wherein said brake is mounted on said
input shaft so as to be movable between the released and engaged
positions thereof upon movement of said worm between the second and
first positions thereof.
14. The gearbox as recited in claim 13, wherein said brake
comprises a cone brake including (A) a stationary cup mounted in
said housing, and (B) a cone that is mounted on said input shaft so
as to contact said cup when said brake is engaged and to be spaced
from said cup when said brake is released.
15. The gearbox as recited in claim 14, further comprising a return
spring that is configured to bias said cone toward the brake
engaged position.
16. The gearbox as recited in claim 15, wherein said torque
transfer system further comprises a bearing that rotatably supports
said input shaft in said housing and that moves axially with said
input shaft, and wherein said spring acts on said bearing.
17. The gearbox as recited in claim 16, further comprising a
second, axially stationary bearing that rotatably supports said
shaft in said housing and that accommodates axial movement of said
shaft relative thereto.
18. The gearbox as recited in claim 16, wherein said spring is a
wave spring that surrounds said input shaft and that is disposed
between said bearing and a stationary internal component of said
gearbox.
19. A concrete finishing trowel comprising: (A) a frame; (B) a
motor that is mounted on said frame and that has a rotatable
output; (C) a rotor that includes a plurality of blades and a hub;
(D) a torque transfer system that transfers torque from said motor
output to said rotor hub, said torque transfer system being
constructed such that delivery of drive torque thereto by said
motor output shaft imposes an axial thrust on at least one
component thereof that drives said component to move axially from a
first position thereof to a second position thereof; and (E) a
brake that cooperates with said torque transfer system so as to
automatically 1) release when said component is in said second
position thereof, thereby permitting torque transfer from said
motor output shaft, through said torque transfer system, and to
said rotor, and 2) engage when said component is in said first
position thereof, thereby resisting torque transfer from said
rotor, through said torque transfer system, and to said motor
output shaft.
20. The trowel as recited in claim 19, further comprising a gearbox
containing said torque transfer system, said gearbox comprising a
housing, an input shaft that is supported by said housing and that
couples said motor output shaft to said torque transfer system, and
a output shaft that is supported by said housing and that couples
said torque transfer system to said rotor hub.
21. The trowel as recited in claim 20, wherein said component
comprises an angled drive gear that is subject to axial reaction
forces upon torque transfer between itself and a mating driven gear
of said torque transfer system.
22. The trowel as recited in claim 21, wherein said drive gear
comprises a worm that meshes with a worm gear of said torque
transfer system, said worm gear imposing the axial thrust on said
worm upon the delivery of drive torque to said torque transfer
system by said motor output shaft.
23. The trowel as recited in claim 22, wherein said worm is fixed
to said input shaft so that said input shaft and said worm move
axially together, and wherein said brake is mounted on said input
shaft so as to be movable between the released and engaged
positions thereof upon movement of said worm between the second and
first positions thereof.
24. The trowel as recited in claim 19, wherein said brake comprises
a cone brake including (A) a stationary cup, and (B) a cone that
moves with said component so as to contact said cup when said brake
is engaged and to be spaced from said cup when said brake is
released.
25. The trowel as recited in claim 24, further comprising a return
spring that is configured to bias said cone toward the brake
engaged position.
26. The trowel as recited in claim 25, wherein said torque transfer
system further comprises a bearing that moves axially with said
component, and wherein said spring acts on said bearing.
27. The trowel as recited in claim 19, wherein said trowel is walk
behind trowel comprising a single rotor and an operator's control
handle accessible by an operator walking behind said trowel.
28. The trowel as recited in claim 24, wherein said trowel has a
magneto-based electrical system incapable of generating sufficient
electrical power to operate an electric brake.
29. A method comprising: (A) transferring drive torque from a prime
mover of a concrete finishing trowel to a torque transfer system of
said trowel; (B) transferring drive torque from said torque
transfer system to a rotor of said trowel; (C) automatically
disengaging a brake upon the delivery of drive torque to said
torque transfer system from said prime mover, thereby permitting
the delivery of drive torque to said rotor; and (D) automatically
engaging said brake in the absence of the delivery of drive torque
to said torque transfer system from said prime mover, thereby
inhibiting finishing trowel rotation.
30. The method as recited in claim 29, wherein the magnitude of
braking forces generated upon brake engagement increases with the
magnitude of a backdrive torque applied to said torque transfer
system by said rotor.
31. The method as recited in claim 29, wherein the disengaging and
engaging steps are responsive to movement of an axially moveable
component of said torque transfer system between first and said
second positions thereof upon the application and cessation of
drive torque delivery thereto, respectively.
32. The method as recited in claim 31, wherein said component
comprises an angled drive gear that is subjected to axial reaction
forces upon torque transfer between itself and a mating driven gear
of said torque transfer system.
33. The method as recited in claim 32, wherein said drive gear
comprises a worm and said mating driven gear comprises a worm
gear.
34. The method as recited in claim 31, wherein, (A) said brake
comprises a cone brake, (B) the disengaging step comprises driving
a cone of said cone brake away from a mating cup of said cone
brake, and (C) the engaging step comprises driving said cone into
engagement with said cup.
35. The method as recited in claim 34, wherein the engaging step
comprises imposing spring-generated biasing forces on said
cone.
36. The method as recited in claim 35, wherein the engaging step
additionally comprises imposing backdrive forces on said component
from said rotor, said backdrive forces driving said component and
said cone to move axially to apply braking forces that increase
with an increase in the backdriving force.
37. An apparatus comprising (A) means for transferring drive torque
from a prime mover of a concrete finishing trowel to a torque
transfer system of said trowel; (B) means for transferring drive
torque from said torque transfer system to a rotor of said trowel;
(C) means for automatically releasing a brake of said torque
transfer system, upon the delivery of drive torque to said torque
transfer system from said prime mover, thereby permitting the
delivery of drive torque to said rotor from said prime mover; and
(D) means for automatically engaging said brake in the absence of
the delivery of drive torque to said torque transfer system from
said prime mover, thereby inhibiting trowel rotation.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to concrete finishing trowels and,
more particularly, relates to a concrete finishing trowel having a
drive train that is braked automatically upon the cessation of
drive torque delivery therethrough. The invention additionally
relates to an automatically braked gearbox usable with such a
trowel and to a method of braking such a gearbox.
[0003] 2. Discussion of the Related Art
[0004] Walk behind trowels are generally known for the finishing of
concrete surfaces. A walk behind trowel generally includes a rotor
formed from a plurality of trowel blades that rest on the ground.
The rotor is driven by a motor mounted on a frame or "cage" that
overlies the rotor. The trowel is controlled by an operator via a
handle extending several feet from the cage. The rotating trowel
blades provide a very effective machine for finishing mid-size and
large concrete slabs. However, walk behind trowels have several
drawbacks.
[0005] For instance, the rotating blades impose substantial torque
on the cage that normally is counteracted by the operator through
the handle. If the operator releases the handle while the rotor is
being driven, the torque may cause the trowel to spin undesirably.
In order to avoid this potential problem, some trowels employ an
automatic disconnect or kill switch that shuts down the engine or
otherwise stops delivery of drive torque to the rotor if the
operator releases the handle and/or if a sensor determines that the
trowel is spinning at a rate indicative of loss of operator
control. A trowel employing an automatic engine shut-down mechanism
is disclosed, for example, in U.S. Pat. No. 2,734,932 to Barnes. A
trowel that relies on release of a dead man lever or similar
structure to shut down a trowel's engine or disengage a clutch upon
operator release of a steering handle is disclosed in U.S. Pat. No.
4,629,359 to Sengupta.
[0006] However, merely shutting down the engine or otherwise
ceasing the delivery of drive torque to the trowel's rotor does not
necessarily prevent the trowel from spinning because the handle and
cage have considerable momentum at the time of shut down. The
trowel may spin through a substantial arcuate range of a complete
revolution or more while that momentum is being spent. This slow
response to an attempted shut down is undesirable.
[0007] Attempts have been made to mitigate this problem through the
incorporation of active brakes in the trowel's drive train that are
designed to prevent or at least inhibit rotation of the trowel's
frame.
[0008] For instance, U.S. Pat. No. 4,280,980, assigned to Stone,
discloses a combined clutch/brake coupling a drive belt of the
trowel's drive system to the input shaft of the trowel's gearbox.
The clutch must be manually-engaged by displacing a lever mounted
on the handle. The clutch may be manually disengaged by manual
operation of the handle. It may also be disengaged automatically
through the operation of a centrifugal weight that is responsive to
undesirable rapid spinning of the trowel. In either event, movement
of the handle to the clutch disengaged position activates a
spring-applied pad-type brake that acts on the drive belt. Although
this device works reasonably well, it exhibits several drawbacks.
For instance, it is usable only with a manually actuated clutch
that is actuated by the operator only at the expense of release or
partial release of the handle. The clutch activation lever also is
open to the outside, exposing the internal components of the clutch
to contamination by dirt, debris, water, etc. In addition, the
clutch brake engages automatically only if a mechanical sensor
indicates that the trowel is undergoing unacceptable centrifugal
forces. It does not react to an "ordinary" engine shut down
situation in which torque transfer to the rotor ceases in the
absence of such centrifugal forces. The brake also imposes
significant drag on the drive belt, accelerating wear on the drive
belt. Finally, braking forces imposed on the clutch are generated
solely by the strength of the spring and, therefore, are
independent of backdrive forces imposed on the system's drive train
by the rotor.
[0009] Other brakes are available for other applications that brake
a clutch directly rather than braking a drive belt leading from the
clutch to the gearbox. Known clutch brakes lack some of the
drawbacks of the Stone clutch brake but have drawbacks of their
own, rendering them poorly suited for use with a trowel. One such
clutch brake is manufactured by North American Clutch Manufacturing
(NORAM). The NORAM brake is a manually engaged brake taking the
form of a deadman's switch operated by a control lever much like
that commonly found on a walk behind lawnmower. The brake is
engaged automatically when the operator releases the control lever
to brake the clutch. This brake also works reasonably well, but
requires that the operator squeeze the control lever at all times
while operating the trowel. This continuous squeezing requirement
leads to considerable operator fatigue and also requires that the
operator divert a substantial portion of his or her attention to
operation of the control lever, hindering his or her ability to
adequately steer the trowel. In addition, the NORAM clutch brake,
like that employed by the brake of the Stone machine, imposes
braking forces that are independent of the magnitude of backdrive
forces generated by the machine's output. In addition, as with the
Stone system, the range of clutches useful with this type of system
is considerably limiting.
[0010] Still another type of clutch brake, offered by Ogura, is
electrically powered. An electrically powered clutch brake requires
the presence of a reliable power supply to prevent the brake from
wearing or seizing. The magneto employed as an electrical power
source for a typical walk behind trowel is ineffective for the
purpose. Adding additional components to the trowel to upgrade the
power supply would add additional cost and weight to the
trowel.
[0011] The need therefore has arisen to automatically brake a drive
train of a concrete finishing trowel upon the cessation of drive
torque delivery to the trowel's rotor without interfering with the
operator's ability to steer and control the trowel and without
significantly adding to the cost or complexity of the trowel.
[0012] The need has also arisen to provide a drive train brake that
satisfies the precedingly described need and that does not
significantly add to the cost or weight of the trowel.
[0013] The need has additionally arisen to provide a brake that
satisfies the first-mentioned need while still being compatible
with a variety of different drive systems.
SUMMARY OF THE INVENTION
[0014] Pursuant to the invention, a brake is incorporated into a
concrete finishing trowel's drive train that responds automatically
to the cessation of drive torque delivery to the rotor to actively
brake the drive train, hence inhibiting or preventing rotation of
the trowel's frame. In a preferred embodiment, the brake takes
advantages of inherent characteristics of an inclined gear of a
gearbox to permit an internal component of the gearbox to shift
axially upon the cessation of drive torque delivery to the gearbox
to engage the brake. For instance, in the case of a worm gear-based
gearbox, the brake may be mounted on a worm shaft or related
component of the gearbox that is responsive to reaction forces
imposed thereon upon the delivery of drive torque thereto. The
component shifts axially in one direction to release the brake in
the presence of the reaction forces and shifts axially in an
opposite direction in the absence of the reaction forces to apply
the brake. The resulting system has several advantages over
clutch-type brakes used on other systems. It is also compatible
with any clutch and any torque delivery system. In addition,
because it is engaged and released fully automatically, it does not
in anyway interfere with or hinder the operator's ability to steer
or otherwise operate the machine.
[0015] These and other advantages and features of the invention
will become apparent to those skilled in the art from the detailed
description and the accompanying drawings. It should be understood,
however, that the detailed description and accompanying drawings,
while indicating preferred embodiments of the present invention,
are given by way of illustration and not of limitation. Many
changes and modifications may be made within the scope of the
present invention without departing from the spirit thereof, and
the invention includes all such modifications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] A preferred exemplary embodiment of the invention is
illustrated in the accompanying drawings in which like reference
numerals represent like parts throughout, and in which:
[0017] FIG. 1 is a perspective view of a walk-behind trowel
incorporating an automatic gearbox brake constructed in accordance
with a preferred embodiment of the present invention;
[0018] FIG. 2 is a sectional side elevation view of a gearbox of
the trowel of FIG. 1;
[0019] FIG. 3 is a sectional top plan view of the gearbox of FIG.
2, showing a gearbox brake in a disengaged position thereof;
[0020] FIG. 4 corresponds to FIG. 3 and shows the brake in an
engaged position thereof; and
[0021] FIG. 5 is an exploded perspective view of the brake of FIGS.
3 and 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] 1. Resume
[0023] As indicated above, the invention resides in the automatic
braking of an active component of a rotary trowel's drive train in
response to the cessation of torque transfer to the rotor from the
trowel's prime mover in order to inhibit or prevent undesired
rotation of the trowel's frame and handle. Preferably, the brake
moves with a component of the drive train that is subject to axial
reaction forces upon the delivery of drive torque therethrough. A
preferred embodiment of the invention will now be described in
conjunction with a walk behind trowel having an internal combustion
engine as a prime mover and a brake built into the trowel's
gearbox. However, the invention is not so limited. It also
potentially applies to ride on trowels and/or to a brake that acts
on components of the trowel's drive system other than the
gearbox.
[0024] 2. System Overview
[0025] Referring to FIG. 1, a walk behind trowel 10 is illustrated
that incorporates a gearbox brake assembly 100 (FIGS. 3-5)
constructed in accordance with a preferred embodiment of the
invention. In general, the walk behind trowel includes a rotor 12,
a frame or "cage" 14 that overlies and is supported on the rotor
12, an engine 16 that is supported on the frame 14, a drive train
18 operatively coupling the engine 16 to the rotor 12, and a handle
20 for controlling and steering the trowel 10. Referring to FIGS. 1
and 2, the rotor 12 includes a plurality of trowel blades 22
extending radially from a hub 24 which, in turn, is driven by a
vertical shaft 26. The shaft 26 of this embodiment comprises the
gearbox output shaft (detailed below). Alternatively, the shaft 26
could be coupled to the gearbox output shaft either directly or via
an interfering torque transfer arrangement.
[0026] The motor 16 comprises an internal combustion engine mounted
on the cage 14 immediately above the rotor 12. Referring again to
FIG. 1, the engine 16 is of the type commonly used on walk behind
trowels. It therefore includes a crankcase 30, a fuel tank 32, an
air supply system 34, an output shaft (not shown), etc. The motor
16 further includes an ignition system that includes a magneto
ignition rotor (not shown). The magneto ignition rotor rotates with
the engine output shaft and generates an electrical pulse with
every cycle that is sufficiently strong to power a spark plug but
insufficiently strong to power electronically actuated peripheral
devices such as valves and electronic clutch brakes.
[0027] The drive train 18 may be any structure configured to
transfer drive torque from the engine output shaft to the rotor
input shaft 26. In the illustrated embodiment, it comprises a
centrifugal clutch (not shown) coupled to the motor output shaft
and a gearbox 40 that transfers torque from the clutch to the rotor
input shaft 26. Referring to FIGS. 1-5, the illustrated gearbox 40
is a worm gearbox of the type commonly used on walk behind trowels.
It includes a housing 42, an input shaft 44 extending horizontally
from the housing 42, the above-described shaft 26 extending
vertically from the bottom of the housing 42, and an internal
torque transfer system 46 delivering torque from the input shaft 44
to the output shaft 26 and effecting any desired speed change
ratio. The input shaft 44 is coupled to an output shaft 48 for the
clutch, either directly or, more preferably, via an intervening
drive such as a belt drive that includes a drive pulley 50, a belt
52, and a driven pulley 54 (all best seen in FIG. 1). A gear drive
or chain drive would also suffice for this purpose.
[0028] Referring to FIGS. 2-5, the housing 42 comprises a metal
casting that is bolted to the cage 14 and that is connected to the
handle 20 by a bracket 55. The housing 42 includes inner and outer
(left and right) walls 56 and 58, side walls 59 and 60, and upper
and lower walls 61 and 62. Side and top covers 64 and 66 shield
openings in the outer and upper walls 58 and 61, respectively, and
are selectively removable to expose interior components of the
gearbox 40 for inspection or replacement. The input shaft 44
extends horizontally into the housing 42 from an outer end thereof
located outside of the housing 42 to an inner end located within
the inner wall 56. The input shaft 44 is borne by the housing 42 by
a first bearing 68 located in the side cover 64 and a second
bearing 70 located in a recess bored into the inner wall 56. As
described in more detail below, the bearings 68 and 70 are
configured to permit limited sliding movement of the input shaft 44
relative to the housing 42. The output shaft 26 extends vertically
into the housing 42 from a first end located below the housing to a
second end located above the input shaft 44. The output shaft 26 is
rotatably borne in the housing 42 by an upper bearing 72 borne by
the upper cover 66 and a lower bearing 74 borne by a boss 76 formed
in the bottom wall 62 of the housing 42.
[0029] Referring now to FIGS. 2-5, the torque transfer system 46 of
this embodiment includes a worm drive that is configured to convert
the horizontal rotation of the input shaft 44 to the vertical
rotation of the output shaft 26 while also effecting any desired
speed change. The worm drive 46 includes a worm 80 and a worm gear
82. The worm gear 82 is keyed or otherwise affixed to the output
shaft 26 between the upper and lower bearings 72 and 74 and meshes
with the worm 80 such that rotation of the worm 80 about a
horizontal axis drives the worm gear 82 and the output shaft 26 to
rotate about a vertical axis. The worm 80 is disposed on and
preferably formed integrally with the outer periphery of the input
shaft 44. As a result, the shaft 44 can be conceptually divided
into an input section and a worm section. Alternatively, a worm
could be formed from a gear mounted on the input shaft 44 or on
another shaft that mates directly or indirectly with the input
shaft.
[0030] 3. Construction and Operation of Brake
[0031] As mentioned above, the gearbox 40 is provided with a brake
100 that releases automatically in response to reaction forces on
the worm 80 and that is otherwise engaged. More specifically, like
all worms, the threads of the worm 80 are "inclined" to the extent
that they extend at an angle relative to radial bisectors of the
threads. The transfer of torque to the worm gear 82 from these
inclined teeth imposes reaction forces that tend to drive the worm
80 and the entrained input shaft 44 axially away from the worm gear
82 or to the left in FIGS. 3 and 4. Most gearboxes are designed to
prevent such axial motion. However, pursuant to an embodiment the
invention, measures are taken to permit these reaction forces to
drive the worm 80 and input shaft 44 axially relative to the
housing 42 to release the brake 100. This effect is achieved by
configuring the input shaft 44 and/or bearings 68, 70 to be movable
axially relative to the housing 42. In the illustrated embodiment,
the input shaft 44 is configured to be moveable axially relative to
the inner bearing 70, and the outer bearing 68 and input shaft 44
are configured to be movable axially relative to the housing 42.
This combination is employed because the inner bearing 70 is
relatively lightly loaded, permitting a relatively small needle
bearing to be used. Needle bearings can accommodate limited axial
movement between themselves and the supported shafts. Conversely,
the outer bearing 68 takes up most of the load and, therefore,
preferably comprises a tapered roller bearing. Tapered roller
bearings cannot accommodate movement between themselves and the
borne shaft but can be configured to move axially with the shaft,
as is the case in the present embodiment. As a result of this
configuration, the worm 80, shaft 44, and bearing 68 are driven
axially or to the left in FIGS. 3 and 4 upon the transmission of
drive torque to the worm gear 82. The stroke of this movement is
relatively small--on the order of 1 to 2 mm, but more than
sufficient to release the brake 100.
[0032] The brake 100 is configured to be automatically responsive
to input shaft motion to disengage when the input shaft 44 shifts
to the left upon the transmission of drive torque therethrough, and
to automatically engage when the input shaft 44 moves in the
opposite direction, either under backdrive forces imposed thereto
by the worm gear 82 or by a return spring (detailed below). A
variety of different brakes cooperating with the input shaft
directly or indirectly in a variety of different manners could
operate in this manner. In the currently preferred embodiment, the
brake 100 comprises a cone brake located in the vicinity of the
outer end of the input shaft 44. A cone brake is preferred because
it is simple in design, non-intrusive to the gearbox 40, and easily
incorporated into an existing gearbox design. It also does not
expose the internal components of the gearbox 40 to contamination
by dust or water.
[0033] Referring to FIGS. 2-5, the cone brake 100 includes a cup
102, a cone 104, and return spring 106 that biases the cone 104 to
an engaged position. The cup 102 surrounds the input shaft 44 and
is press-fit or otherwise mounted in a counterbore 107 in the outer
gearbox cover 64. The cone 104 is keyed to or otherwise mounted on
the outer end of the input shaft 44. It preferably includes a
powdered metal cone. The currently preferred metal is
FN-0208-105HC, which has an apparent hardness of RC31. The outer
periphery of the cone 104 has a cylindrical outer axially surface
portion 108 and an axially inner frusto-conical portion 110 that is
tapered inwardly from an outer end thereof to an inner end thereof.
The surface of the tapered inner frusto-conical portion 10 is
configured to rub against a correspondingly tapered surface 112 on
the inner periphery of the brake cup 102 to apply the brake 100.
The return spring 106 is configured to bias the shaft 44 and,
hence, the cone 104 toward the engaged position of FIG. 4, thereby
assuring automatic brake engagement upon the cessation of torque
transfer to the input shaft 44. A variety of springs could be used
and act on a variety of different components so as to directly or
indirectly perform the desired biasing effect. In the illustrated
embodiment, the spring 106 comprises a spiral wave spring that acts
on the bearing 68. It is located in a chamber formed outward of the
bearing 68 so as to rest against a radial step 114 of the outer
cover 64 at its outer end and against a spacer 116 at its inner
end. Finally, a seal 118 is disposed outwardly of the spring
chamber for sealing the interior of the housing 42 surface from the
environment while still permitting the shaft 44 to move axially
relative to the housing 42.
[0034] 4. Operation of Trowel
[0035] During normal operation of the trowel 10, torque is
transferred to the gearbox input shaft 44 from the engine's output
shaft, the clutch, and the drive train. The worm 80 then transfers
torque to the worm gear 82 which, in turn, drives the output shaft
26 to rotate counterclockwise, thereby driving the rotor 12 to
rotate. The reaction forces imposed on the teeth of the worm 80 by
the teeth of the worm gear 82 drives the input shaft 44 and bearing
68 to the position illustrated in FIG. 3 to drive the cone 104 away
from the cup 102, thereby releasing the brake 100 and permitting
unobstructed rotation of the input shaft 44 relative to the gearbox
housing 42. At some time, cessation of torque delivery to the input
shaft occurs, either because the operator shuts down the engine or
throttles it back to release the clutch or, in the case of more
sophisticated systems, because a sensor such as an accelerometer or
a gyroscope detects actual or imminent loss of operator control and
generates a signal to disable the drive system. The reaction forces
driving the shaft 44 axially to the left in the drawings therefore
are removed, permitting the shaft 44 to move to the right or to the
brake engaged position of FIG. 4, thereby inhibiting rotation of
the cage 14 and handle 20. The cone 104 meshes with the cup 102 to
engage the brake 100 as a result of this movement. This movement is
driven at least in part by the return spring 106. However, and as a
significant aspect of the invention, it may also be driven in part
by backdrive force imposed when the rotating output shaft 26
applies torque to the worm 80 through the worm gear 82. The
magnitude of this backdrive force is dependent upon the backdrive
torque. Therefore, the magnitude of braking forces generated by the
brake 100 are dependent on the magnitude of the backdrive torque,
hence resulting in more effective braking when it is most
critical.
[0036] Both brake engagement and release occur fully automatically.
As a result, operator input to both actions is completely
unnecessary. The operator therefore is free to perform the more
desired and less fatiguing steering and control functions.
[0037] Many changes and modifications could be made to the
invention without departing from the spirit thereof. For instance,
while the brake component of the drive train is preferably located
within the gearbox 40, that need not be the case. Moreover, if the
brake is incorporated into the gearbox 40, it could be used with
drives other than worm drives, so long as the drive has a torque
transfer system having a component that experiences thrust upon the
transfer or drive torque thereto. Virtually any drive component
having angled teeth, i.e., ones that extend at an angle that is
offset from the radial, could suffice.
[0038] Still other changes that could be made to the invention
without departing from the spirit thereof will become apparent from
the appended claims.
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