Transmission

Details Transmission Transmission

1998-11-06

2001-07-10

Marmor, Charles A (Department: 3681)

Machine element or mechanism

Gearing

Interchangeably locked

C074S473100, C074S473250

Reexamination Certificate

active

06257084

ABSTRACT:

DETAILED DESCRIPTION OF THE INVENTION
The present invention concerns a transmission for use on a vehicle such as a tractor.
PRIOR ART
Prior Art 1
In the transmission technique, it is conventionally known to change the rotation of a power source to a desired rotational speed by using a gear type change speed mechanism provided with a synchromesh device and then transmit the modified rotation to the axles.
The gear type change speed mechanism is, as described in
FIG. 2
, a device for changing a speed with a gear train located between a drive shaft
110
and a driven shaft
120
which shafts are disposed substantially in parallel to each other, the gear type change speed mechanism comprising a rotatable gear
101
relatively rotatably supported on one of the above mentioned shafts, a synchromesh device
100
supported on the said one of the above-mentioned shafts so as to be axially slidable but non-rotatable relative thereto, and a fixed gear
102
supported on the other of the shafts in a non-rotatable manner relative thereto and operatively connected to the rotatable gear.
FIG. 2
shows a change speed gear mechanism of the type in which a pair of rotatable gears share one synchromesh device.
The synchromesh device
100
has an inner tapered engaging face
100
a
and internal teeth
100
b.
The rotatable gear
101
is provided with an outer tapered engaging face
101
a
and external teeth
101
b
corresponding to the inner tapered face and the internal teeth of the synchromesh device, respectively. The synchromesh device
100
is moved axially so that the tapered engaging face
100
a
is pressed on the tapered face
101
a
of the rotatable gear. Then, both the engaging faces are synchronized through frictional engagement so as to mesh the internal teeth
100
b
with the external teeth
101
b,
whereby the rotatable gear is synchronized with the one shaft to obtain a change speed ratio between the drive shaft and the driven shaft based on the gear ratio between the rotatable gear and the fixed gear operatively coupled therewith. In the figure, the reference numeral
121
denotes an output gear integrally formed with a driven shaft
120
.
As described above, the synchromesh device is designed so that the tapered engaging face
100
a
is frictionally engaged with the engaging face
101
a
of the rotatable gear and consequently the internal teeth
100
b
is meshed with the external teeth
101
b
of the rotatable gear. Accordingly, both the synchromesh device and the rotatable gear are subjected to load with some torque in meshing with each other. By the way, the engine output to be transmitted to the transmission is proportional to the product of a torque and a rotational speed of the engine. Accordingly, in order to reduce the load applied on the rotatable gear and the synchromesh device, it is desirable that both the members are connected on the drive shaft that rotates at a higher rotational speed than a driven shaft. Thus, for this reason, according to the conventional transmission, the synchromesh device and the rotatable gear in all the speed change gear mechanism are provided on the drive shaft. Such a construction has encountered the following disadvantages.
Specifically, the change gear ratio is denoted by the gear ratio between the meshing gears, whereas the pitches of the meshing gears are equal. Accordingly, the gear ratio is indicated by the ratio between the pitch circle diameters thereof. Therefore, in order to reduce the distance between the drive shaft and the driven shaft while maintaining a given gear ratio, it is necessary to reduce the pitch circle diameters of the meshing gears as much as possible while maintaining the ratio between the pitch circle diameters thereof. In the case where the transmission is of the multi speed type, since the gear ratio for the 1st speed (or the reversing) is the highest, the 1st speed gear (or the reversing gear) supported on the drive shaft has the smallest pitch circle diameter. Accordingly, it is possible to make the whole transmission compact by reducing the pitch circle diameter of the 1st speed gear supported on the drive shaft.
However, if the pitch circle diameter is reduced, the thickness of the portion (hereinafter referred as “the body portion of the gear”) sandwiched between the position defined by the pitch circle radius and the position defined by the radius of the center hole is reduced. Accordingly, if the pitch circle diameter is reduced excessively, the gear is deficient in strength. Further, it is necessary to attach a rotatable member such as a bearing between the gear and the drive shaft in order to support the gear rotatably relative to the drive shaft. Therefore, it is necessary to take into consideration the minimum required thickness of the body portion and also the thickness of the rotatable member in addition to the gear ratio so as to determine the pitch circle diameter of the gear in a gear type change speed mechanism.
For example, provided that the minimum required thickness of the body portion of the 1st speed gear on the drive shaft, the thickness of the rotatable member and the radius of the drive shaft are denoted at “h”, “t” and “r”, respectively, the pitch circle diameter D
1
of the 1st speed gear supported on the drive shaft is expressed by D
1
=2(r+t+h). Provided that the change gear ratio is “5”, the pitch circle diameter D
2
of the 1st speed gear mounted on the driven shaft is expressed by D
2
=5D
1
=10(r+t+h). Accordingly, the distance L
1
between the drive shaft and the driven shaft is expressed by L
1
=(D
1
+
5
D
1
)/2=6(r+t+h). Thus, according to the prior art construction in which the rotatable gears of all the change speed gear mechanisms are mounted on the drive shaft, the actual distance between the drive and driven shafts is larger than the distance
6
(r+h) determined only based on the change gear ratio and the gear strength by 6 times as the thickness “t” of the rotatable member, thereby increasing the size of the whole transmission.
Further, since both the engaging faces of the rotatable gear and the synchromesh device are frictionally engaged with each other, before meshing the external teeth of the rotatable gear with the internal teeth of the synchromesh device, both the engaging faces are required to have an adequate radial thickness from the view of strength. Therefore, from the point of this, in order to rotatably mount the gear of the minimum pitch circle diameter on the drive shaft, the diameter of the gear has to be greater than the diameter that can be determined based on only the gear ratio. This results in further increased size of the whole transmission itself.
Prior Art 2
In order to obtain the drive power for driving the front wheels, it has been known, as disclosed in Japanese Unexamined Patent Publication No. 193679/97, to mount a front wheel drive unit on the transmission of the type that transmits drive power from a drive source to a pair of rear wheels. However, the four wheel drive types and the two wheel drive types of the conventional transmission have been produced in separate production lines.
Specifically, since it Is necessary to determine whether to install the front drive unit within the transmission casing depending on the four wheel type or the two wheel type before assembling the transmission, the four wheel types and the two wheel types have to be produced in separate lines. Accordingly, the conventional transmission has encountered difficulties in enhancing the assembling efficiency of the transmission or in enhancing the stock management efficiency.
Further, with this construction, when a customer wishes to change the specifications of the transmission mounted, i.e., he wishes to change the two wheel drive type to the four wheel drive type or vice verse after the transmission is assembled, it is necessary to attach or detach the front wheel drive unit after disassembling of the transmission itself. Accordingly, it is difficult to change the specifications of the t

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