HST type axle driving apparatus with brake and transmission interlock

An HST type axle driving apparatus for driving axles, wherein, when a braking device provided in the apparatus is operated for braking in the state of forwardly or backwardly speed changing, a hydraulic pump or a hydraulic motor at the HST speed change apparatus is subjected to an over-load, so that in order to eliminate the overload, an association mechanism is so constructed that the HST type speed change apparatus changed of the speed forwardly or backwardly is forcibly and reliable kept in the neutral when the braking device is operated. Also, the association mechanism is disposed within a transmission case so as to make the HST type axle driving apparatus compact.

FIELD OF THE INVENTION 
The present invention relates to an HST (Hydro-Static Transmission) type 
axle driving apparatus which houses an HST type speed change apparatus in 
a transmission case for driving the axles, and more particularly to a 
mechanism for returning a speed change operation unit at the HST type 
speed change apparatus to the neutral position when a brake pedal is 
operated. 
DESCRIPTION OF THE PRIOR ART 
Conventionally, the assembly of a variable displacement hydraulic pump and 
a fixed displacement hydraulic pump for an HST type speed change apparatus 
integrally in the transmission case has been well known as disclosed in 
the U.S. Pat. No. 3,196,696. 
In a case where a braking device at the HST type axle driving apparatus 
exerts the braking action, the return of the HST type speed change 
apparatus to the neutral has been well known as disclosed in the Japanese 
Patent Laid-Open Gazette No. Sho. 59-216733. 
The providing of a cam groove at an arm to operate a variable swash plate 
so as to slowly change rotation thereof in the vicinity of the speed 
change neutral position has been well known as disclosed in the Japanese 
Utility Model Publication No. Sho. 48-25682 and No. Sho 51-7623. 
SUMMARY OF THE INVENTION 
In the axle driving apparatus equipped with the HST type speed change 
apparatus, when a separately attached braking device is operated in the 
state where the HST type speed change apparatus changes the speed 
forwardly or backwardly, the HST type speed change apparatus while 
delivering pressure oil from the variable displacement hydraulic pump to 
the fixed displacement hydraulic pump is forcibly applied with the braking 
action, which causes an overload state, thereby creating inconvenience 
such as the engine stopping, or the variable displacement hydraulic pump 
or the fixed displacement hydraulic pump, becoming broken. 
The present invention is characterized by including an association 
mechanism which, when the braking device attached to the HST speed change 
apparatus is operated to apply braking to the axles, forcibly returns to 
the neutral position the variable swash plate at the variable displacement 
hydraulic pump of the HST type speed change apparatus before breaking is 
applied to the axles. 
In the present invention, the HST type speed change apparatus is housed in 
the transmission case, whereby the association mechanism for the braking 
device and variable swash plate is constructed to be compact when housed 
in the transmission case. 
Other objects and aspects of the invention will become apparent from the 
following description of embodiments with reference to the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
Referring to FIG. 1, an HST type axle driving apparatus of the invention 
attached to a tractor will be described. 
An engine E disposed in a bonnet of the tractor is of a vertical crank 
shaft type, an output pulley 19 is fixed to the lower end of the vertical 
crank shaft, and a V-belt wound on the output pulley 19 is wound on an 
input pulley 2 at a pump shaft 6 projecting from a transmission case 1a 
and 1b. 
Axles 13 and other shafts are supported through bearings to the junction 
surface of the transmission case constituted of an upper half case 1a and 
a lower half case 16, the axles 13 laterally projecting drive driving 
wheels 14 respectively, and steering wheels 18 are disposed below the 
engine E. 
A speed change operation unit 4 for changing the speed of the HST type axle 
driving apparatus housed in the transmission case and for driving the 
axles 13 is formed as a longitudinal footpedal; and a brake pedal 25 for 
operating the braking device is juxtaposed with the speed change operation 
unit 4. 
The brake pedal 25 is connected to a brake link 43 through a shock absorber 
23. 
Referring to FIGS. 2 and 3, explanation will be given generally on 
construction of the speed change apparatus in the transmission case and 
the braking device. 
The axle driving apparatus that journals axles 13 at the rear portion of 
the transmission case comprising the upper and lower half cases 1a and 1b, 
houses the HST type speed change apparatus in the front portion of the 
same, disposes a braking device at the end of the same, and disposes at 
the outside or the inside of the upper surface of upper half case 1a an 
association mechanism for the braking device and speed change apparatus as 
discussed below. 
The HST type speed changing apparatus for changing the number of rotations 
of the axles 13 is so constructed that an oil passage plate 3 of 
substantially L-like shape when viewed from front is fixed to the lower 
surface of uppper half case 1a by bolts, a pump mounting surface 3b is 
provided at the upper surface of a horizontal plate 3a of the oil passage 
plate 3, and a variable displacement hydraulic pump is mounted on the pump 
mounting surface 3b. 
Crescent oil passages 3c and 3d are bored at the pump mounting surface 3b, 
and a pump shaft 6 is erected at the center between the oil passages 3c 
and 3d and projects upwardly from the upper half case 1a so as to fix an 
input pulley 2. 
A cylinder block 7 (FIG. 3) is fitted onto the pump shaft 6, pistons 8 each 
biased by a biasing spring 9 are fitted into the cylinder block 7 and 
disposed around the pump shaft 6, the upper end of each piston 8 abuts 
against a thrust bearing 10, and the thrust bearing 10 is fixedly fitted 
onto a variable swash plate 12. 
The upper surface in a circular arc of the variable swash plate 12 is 
slidably guided by the rear surface of the upper half case 1a. 
The variable swash plate 12 engages at the outer wall thereof with an arm 
fixed to a swash plate angle control shaft 11 so that the swash plate 
angle control shaft 11 is rotated to horizontally rotate the arm, whereby 
the upper surface in circular arc slidably moves to change inclination of 
variable swash plate 12 so as to change a discharge amount of operating 
oil. Hence, a fixed displacement hydraulic motor M is made variable of the 
rotation speed. 
Also, a motor mounting surface 3i is formed at the outer surface of a 
vertical plate 3j at the oil passage plate 3 and the fixed displacement 
hydraulic motor M is attached to the motor mounting surface 3i. 
In other words, crescent oil passages 3m and 3n are bored at the motor 
mounting surface 3i, which communicate with the crescent oil passages 3c 
and 3d through horizontal oil passages 3e and 3f provided in parallel to 
the horizontal plate 3a respectively. 
A motor shaft 5 projects from the center of the motor mounting surface 3i 
in parallel to the axles 13, a cylinder block 20 is fitted onto the motor 
shaft 5, each piston 22 biased by a biased spring 21 is fitted into the 
piston 22, and a fixed thrust bearing 24 abuts against the utmost end of 
each piston 22. 
Accordingly, when the pump shaft 6 together with the cylinder block 7 is 
rotated, for example, pressure oil generated at the crescent oil passage 
3c passes the horizontal oil passage 3e and is guided to the crescent oil 
passage 3m and then drives the motor shaft 5 together with the cylinder 
block 20. Thereafter, the pressure oil is discharged from the crescent oil 
passage 3n, passes the horizontal oil passage 3f, and is taken in the 
cylinder block 7 from the crescent oil passage 3d. 
The other ends of the horizontal oil passages 3e and 3f are closed by check 
valves 15 and 16 respectively. The check valves 15 and 16 are operated 
from the exterior to allow the horizontal oil passages 3e and 3f to 
communicate with the tank so as to make rotatable the fixed displacement 
hydraulic motor M. An oil filter 17 is disposed below the check valves 15 
and 16 so that lubricating oil in the transmission case 1 is supplied as 
operating oil to the HST type speed change apparatus by opening the check 
valves 15 and 16. 
An output gear 5a is provided on the motor shaft 5 and engages with a large 
diameter gear 27 on a counter gear 26 and a toothed gear 26a on the 
counter shaft 26 engages with a differential ring gear 28, so that a 
driving force from the motor shaft 5 is transmitted to the axles 13 
through a differential gear unit. 
A brake drum 29 is fixed to the utmost end of the motor shaft 5 and a brake 
lever 30 pivoted to a lid 31 fixed to the side surface of the transmission 
case 1 is turned, whereby brake shoes in the brake drum 29 are expanded to 
exert the braking action. 
Next, explanation will be given on a first embodiment of the invention 
shown in FIGS. 4 through 8. 
The brake lever 30 is pivotally connected with a brake link 33, the other 
end thereof is pivotally connected to one end of a bell crank 34, which is 
pivotally supported at an intermediate portion to a pivot shaft 36 
projecting from the transmission case 1, and the other end of bell crank 
34 is pivotally connected with a shock absorber 35. 
The shock absorber 35 is provided in order that, when a treading force for 
the brake pedal 25 exceeds a specified value, a spring at the shock 
absorber 35 operates not to transmit such excessive force to the brake 
lever 30. 
A slide rod 35a (FIG. 6) in the shock absorber 35, one end of which is 
connected to the bell crank 34, is inserted at the other end into a 
cylinder 35c, in which a spring 35b is fitted onto the slide rod 35a and 
biased in the compressing direction. 
A cylinder rod 35d, which is fixed at one end to the cylinder 35c, is 
pivoted at the other end to the utmost end of a brake arm 37, and a rotary 
base thereof is pivoted to a rotary shaft 40 fixed to a mounting plate 39 
fixed onto the transmission case. 
The brake arm 37 is biased toward the shock absorber 35 by a torsion spring 
41 fitted onto the rotary shaft 40 and usually retained by a stopper 38 
projecting from the transmission case. A distance between the brake arm 37 
abutting against the stopper 38 and a regulator 42a biased by the torsion 
spring 41 and positioned at the depth of a rotary groove 44d during the 
speed change as shown in FIG. 6 is an allowance gap of the invention, 
through which the regulator 42a returns a speed change cam plate 44 to the 
neutral position in a time period when the regulator 42a moves from the 
position in FIG. 6 to abut against the brake arm 37. 
Also, a regulator arm 42 which is V-like-shaped, is pivoted at the central 
portion to the rotary shaft 40, at one utmost end of the regulator arm 42 
is pivotally connected a brake link 43 connected with the brake pedal 25, 
and the regulator 42a projects from the other end of the regulator arm 42 
and is inserted into a regulation bore 44a open at one side of the speed 
change cam plate 44. 
The speed change cam plate 44 is fixed at an intermediate portion to the 
swash plate angle control shaft 11 and provided at one side with the 
regulation bore 44d, 44c of a substantially T-like shape. 
The regulation bore comprises the rotary groove 44d rotatable for the speed 
change and formed in a circular arc around the swash plate control shaft 
11 and a rotary groove 44c for the braking action, into which the 
regulator 42a can enter when the variable swash plate 12 is in neutral, 
the grooves 44d and 44c being continuously integral with each other. 
At the other end of the speed change cam plate 44 is pivoted a speed change 
link 45 connected to a speed change operation unit 4 and includes guide 
bore 44b in a circular arc around the swash plate angle control shaft 11, 
a neutral recess 44e (FIG. 6) is provided at the center of guide bore 44b, 
and a neutral position biasing member 46a projecting from one end of an 
arm 46 is fitted into the guide bore 44b and biased thereby. The arm 46 is 
pivoted at the other end to a pin 47 projecting from the mounting plate 39 
and biased toward the swash plate angle control shaft 11 by a spring 48 
fitted onto the pin 47. 
Accordingly, in the state shown in FIG. 4, the speed change cam plate 44 
and variable swash plate 12 are in the neutral positions respectively. 
In the aforesaid construction, when the speed change control unit 4 is 
operated in the state where the brake is not operated, that is, the brake 
pedal 25 is not trod, as shown in FIG. 6, the speed change link 45 rotates 
the speed change cam plate 44 in the direction of the arrow, the regulator 
42a at the regulation arm 42 is positioned in the rotary groove 44d for 
the speed changing so as not to regulate the rotation of speed change cam 
plate 44, and the speed change cam plate 44 is rotated to rotate the swash 
plate angle control shaft 11, so that an angle of the variable swash plate 
12 is changed and a discharge amount from the piston 8 is changed, thereby 
changing the number of rotations of the hydraulic motor M. 
On the other hand, in the neutral position biasing member 46a at the arm 46 
riding on the inclined surface of guide bore 44b is biased toward the 
swash plate angle control shaft 11, thereby being subjected to a force by 
which the biasing member 46a rotates toward the neutral recess 44e at the 
guide bore 44b with respect to speed change operation so as to be neutral. 
Accordingly, when the speed change control unit 4 is released from 
operation, the neutral position biasing member 46a reversely rotates the 
speed change cam plate 44, thereby returning it to the neutral position. 
When the brake pedal 25 is trod in the starting condition, the brake link 
43 is pulled and the regulation arm 42 is rotated gradually from the 
position in FIG. 6, whereby the regulator 42a, as shown in FIG. 7, rotates 
from the rotary groove 44d for the speed changing to the rotary groove 44c 
for the braking action, and the speed change cam plate 44 rotates toward 
the neutral position following the rotation of regulator 42a. Hence, the 
neutral position biasing member 46a holds the speed change cam plate 44 in 
position. 
Therefore, the variable swash plate 12 is put in the neutral position and 
the hydraulic pump P generates no oil pressure, thereby cutting off a 
driving force of the hydraulic motor M. 
Then, when the brake pedal 25 is trod, the allowance gap is consumed so 
that the regulator 42a abuts against the brake arm 37 and rotates to the 
depth of the rotary groove 44c, and the bell crank 34 rotates to pull the 
brake link 33 while compressing the spring 35b at the shock absorber 35, 
thereby turning the brake lever 30 for the braking action. 
When an operator removes his foot from the brake pedal 25, a return spring 
(not shown) reversely rotates the regulation arm 42 through the brake link 
43, thereby restoring the regulator 42a to the position shown in FIG. 4. 
Next, explanation will be given on construction of a second embodiment of 
the invention shown in FIGS. 9 through 13. 
In this embodiment, the association mechanism for the barke pedal 25 with 
the speed change unit 4 is housed in the transmission case. 
As shown in FIG. 9, however, the brake pedal 25 is connected to the upper 
portion of transmission case 1 through the brake link 43, and the speed 
change operation unit 4 transmits the operating force to the upper surface 
of transmission case 1, which is similar to the first embodiment. 
The brake link 43 is connected at the utmost end thereof to an arm 56 which 
is fixed to a regulation arm 51 in the transmission case 1 through the 
rotary shaft 40. 
The utmost end of speed change link 45 is connected to an arm 52 projecting 
outwardly from the transmission case 1, the arm 52 transmitting the 
operating force to the variable swash plate within the transmission case 1 
through the swash plate angle control shaft 11. 
Next, explanation will be given on the state where the upper half case 1a 
is removed from the transmission case 1 in accordance with the FIGS. 10 
and 11 of plan view. 
In FIG. 11, the arm 52 is fixed to the swash plate angle control shaft 11, 
and an arm 58 attached thereto rotates and is fitted at the utmost end 
into a recess at a variable swash plate engaging member 60 attached to the 
side surface of the variable swash plate 12. 
The swash plate angle control shaft 11 rotates so as to turn the arm 58, 
whereby the variable swash plate 12 is slid along the circular-arc guide 
to change an swash plate angle. 
A speed change cam plate 50, other than the arm 58, is fixed to the swash 
plate angle control shaft 11 so as to be simultaneously rotatable. 
At the speed change cam plate 50 are bored a rotary groove 50a for the 
braking action and a rotary groove 50b for the speed changing, and at the 
outer periphery of the cam plate 50 are provided an insertion recess 50c 
for a neutral position holding ball 59 and inclined surfaces 50d at both 
sides of the recess 50c. The neutral position holding ball 59 can adjust 
by a neutral position holding mechanism 57 from the exterior of 
transmission case 1 the biasing force given to the speed change cam plate 
50. 
The rotary groove 50a for the braking action and rotary groove 50b for the 
speed changing communicate with each other to constitute a T-like-shaped 
cam groove, in which a regulator 51a at the utmost end of the regulation 
arm 51 is fitted. 
The brake link 43 connected to the brake pedal 25 through the shock 
absorber 23 is connected at the utmost end to the arm 56 which transmits 
the operating force into the transmission case 1 through the rotary shaft 
40, the regulation arm 51 being fixed to the lower end of the rotary shaft 
40. 
The regulator 51a is fixed to the utmost end of regulation arm 51 and, as 
the above-mentioned, is inserted into the cam groove communicating with 
the rotary groove 50a for the braking action and rotary groove 50b for the 
speed changing at the speed change cam plate 50. 
It is required to provide at the regulation arm 51 an allowance gap for 
obtaining the allowance through which the speed change cam plate 50 
returns to the neutral position without starting the braking action of the 
braking device at first even when the brake pedal 25 is trod. Hence, in 
the second and third embodiments, the allowance gap is formed of an 
allowance elongate slot 51b provided at the regulation arm 51. 
Into the allowance elongate slot 51b is inserted one end of a connecting 
rod 53 and the other end thereof is connected to an arm 54. The arm 54 is 
fixed to a pivot shaft 36 and an arm 55 is fixed to part of the pivot 
shaft 36 projecting outwardly from the transmission case 1. A brake link 
33 is connected to the utmost end of arm 55 through the shock absorber 35. 
In this embodiment, an association mechanism for the braking device and 
speed change apparatus is disposed within the transmission case 1 and, 
especially, the association mechanism is disposed at the reverse side to 
the braking device beyond a power transmission mechanism constituted of 
the larger diameter gear 27 and differential ring gear 28, whereby it is 
required to provide a space through which the connecting rod 53 from the 
association mechanism to the braking device passes. 
The connecting rod 53 is pushed or pulled together with operation of brake 
pedal 25 so as to turn somewhat longitudinally, thereby requiring a 
sufficient gap. 
In the present invention, the connecting rod 53 passes through a space 
between a counter shaft bearing 32 for supporting a counter shaft 26a at 
the power transmission mechanism and the axle bearing 49 for supporting 
the axles 13. 
Also, as shown in FIG. 13 and FIG. 15 showing a third embodiment of the 
invention, when the association mechanism for the braking device and speed 
change apparatus is disposed in the transmission case 1, the mechanism is 
hung to be supported to shafts projected from the upper half case 1a. 
Accordingly, the state shown in FIG. 10 cannot actually exist, but for the 
convenience of explanation the association mechanism only is shown 
upwardly of the lower half case 1b. 
In addition, operation of the speed change cam plate 50 by the speed change 
operation unit 4 and that of the speed change cam plate 5 during the 
operation of brake pedal 25 are omitted as they are the same in the first 
embodiment. 
The second embodiment is different from the first embodiment in that the 
neutral holding mechanism and shock absorber 35 are different in position 
and the association mechanism is disposed in the transmission case 1, but 
similar to the first embodiment in that, when the brake pedal 25 is 
operated, the variable swash plate 12 is forcibly returned to the neutral 
position. 
Next, explanation will be given on construction of the third embodiment in 
accordance with FIGS. 14 and 15. 
In the third embodiment, as in the second embodiment, the association 
mechanism for the braking device and speed change apparatus is disposed 
within the transmission case 1, but an operation mechanism for the 
variable swash plate 12 is improved. 
In other words, the second and third embodiments are similar in that a 
rotary groove 62a for the braking action and a rotary groove 62c for the 
speed changing are open at a speed change cam plate 62 in condition of 
communicating with each other, a regulator 51a at a regulation arm 51 is 
fitted into the communicating grooves so that, when the brake pedal 25 is 
trod and the regulator 51a enters into the rotary groove 62a, the variable 
swash plate 12 is returned to the neutral position through the speed 
change cam plate 62. However, a swash plate operating cam groove 62b is 
bored in addition to the speed change cam plate 62. 
The speed change cam plate 62 is connected through a rotary shaft 68 with 
the arm 52 linked with the speed change operating unit 4 and a swash plate 
operating member 63a for an arm 63 fixed to the swash plate angle control 
shaft 11 is inserted into the swash plate operating groove 62b. In this 
case, the third embodiment is different from the first second embodiments 
in that the rotation of speed change cam plate 62 is not transmitted 
directly to the swash plate angle control shaft 11. 
The swash plate operating cam groove 62b is so formed that the variable 
swash plate 12, when the speed change operation unit 4 is operated in 
vicinity of the neutral, is rotated merely slowly, and largely at the 
position apart from neutral. 
The swash plate angle control shaft 11, as shown in FIG. 15, is supported 
rotatably to a pivot portion 11b at a neutral adjusting shaft 11a and the 
neutral adjusting shaft 11a and pivot portion 11b are formed to be 
somewhat eccentric so that the neutral adjusting shaft 11a is properly 
rotated and fixed, and the swash plate angle control shaft 11 shifts in 
position, thereby enabling the neutral position of the variable swash 
plate 12 to be set with accuracy. 
Also, in the second embodiment, in order to hold the speed change cam plate 
62 in the neutral position, the neutral holding ball 59 projects outwardly 
from the transmission case 1 so as to be biased and fitted into the 
neutral position ball insertion groove, but in the third embodiment, the 
neutral holding mechanism is constituted within the transmission case 1. 
In other words, a neutral holding arm 64 is integrally pivoted by a pivot 
shaft 64a and a neutral position biasing member 64b provided at the 
neutral holding arm 64 is adapted to be permanently biased to abut against 
the neutral position holding recess 62d at the speed change cam plate 62 
by permanently biasing springs 66 and 65 (FIG. 16). Reference numeral 67 
designates a support for blocking movement of each permanently biasing 
spring 66 or 65. 
A permanently biasing spring 66 is attached to one end of a biasing rod 65 
through a spring holder and the other end of the same is attached to a 
wall of the upper half case 1a through a spring holder. Accordingly, when 
the speed change cam plate 62 rotates to the speed change position, the 
neutral position biasing member 64b rides on an inclined surface 62e, the 
neutral holding arm 64 rotates together with the pivot shaft 64a to pull 
the biasing rod 65, thereby compressing the permanently biasing spring 66. 
Hence, the permanently biasing spring 66 gives the inverted biasing 
rotation force to the neutral holding arm 64 so that the speed change cam 
plate 62 is subjected to a force for fitting the neutral position biasing 
member 64b into the neutral position holding recess 62d. 
As shown in FIG. 15, an arm 69 is provided at part of pivot shaft 64a 
projecting from the transmission case 1 so that, when the pivot shaft 64a 
is rotated as above-mentioned, a neutral position detecting switch 70 is 
adapted to be off. 
The third embodiment is different from the first embodiment in the rotation 
mechanism for the neutral holding mechanism or the variable swash plate 
12, but similar to the same in operation of returning the variable swash 
plate 12 to the neutral position by operating the brake pedal 25. 
Next, the effect of the present invention will be described with reference 
to construction to be claimed as follows: 
The variable swash plate 12 at the variable displacement hydraulic pump P 
is first returned to the neutral position while the allowance gap provided 
between the link and the arm is being operated during the operation of 
brake operating unit, and thereafter the braking device exerts the braking 
action, whereby the hydraulic pump P and hydraulic motor M are not 
subjected to an overload and also the engine E is not subjected thereto, 
resulting in that the engine does not stop occurs. 
Also, the regulator 42a rotates and shifts while holding the speed change 
cam plate 44 in the neutral position and the brake arm 37 and link are 
operated to actuate the braking device and the speed change cam plate 44 
is kept stable in the neutral position by the regulator 42a for the 
braking action, thereby enabling the hydraulic pump P or hydraulic motor M 
to be prevented from a breakdown. 
The speed change cam plate 44 is formed in a flat plate and the rotary 
groove 44d for the speed changing and the rotary groove 44c for the 
braking action are constituted so that the association mechanism can be 
smaller in thickness when viewed from the lateral side, thereby enabling 
the apparatus to be compact. 
When the regulator 42a returns the speed change cam plate 44 to the neutral 
position, the neutral mechanism biases the speed change cam plate 44 
toward the neutral position, thereby enabling the return time to be 
quicker. Also, since the speed change cam plate 44 can reliably be fixed 
in the neutral position, even when the brake operating unit is abruptly 
operated in the emergency braking action, the swash plate 12 can reliably 
be held in the neutral position. 
An excessive braking force is cut by the shock absorber 35 interposed 
between the regulation arm and the break lever, thereby protecting of the 
braking device. 
Since the association mechanism for the braking device and variable swash 
plate is disposed in the transmission case 1, part of the association 
mechanism can be lubricated by lubricating oil within the transmission 
case and be prevented from rusting or mudding, thereby enabling a stable 
operation. 
Since the association mechanism for the braking device and variable swash 
plate is disposed in a dead space at the HST type speed change apparatus, 
the transmission case 1 can be compact. 
The association mechanism for the braking device and swash plate is 
supported at the upper half casing 1a, thereby being simple to assemble. 
The association mechanism is disposed in a space at the HST type speed 
change apparatus and the connection rod 53, which transmits the operating 
force to the braking device disposed at the reverse side to the 
association mechanism beyond the power transmission mechanism, can be 
pushed or pulled smoothly without any hindrance. 
Although several embodiments have been described, they are merely exemplary 
of the invention and not to be constructed as limiting, the invention 
being defined solely by the appended claims.