Patent Description:
A power transmission device for a work vehicle may include a transmission case, a gear transmission contained in the transmission case and configured to receive motive power from a power source, and a differential mechanism contained in the transmission case and configured to receive motive power from the gear transmission and transmit the motive power to a left wheel and a right wheel.

<CIT> discloses a power transmission structure as an example power transmission device of the above type. The power transmission structure includes a gear transmission ("speed-changing power transmission device", "forward/backward travel switching device") and a differential mechanism ("rear-wheel differential mechanism").

<CIT> shows a power transmission device according to the preamble of claim <NUM>.

<CIT>, <CIT> and <CIT> each show a differential mechanism contained in a case together with a gear cover covering a portion of a ring gear of the differential mechanism.

There has been a demand for a work vehicle capable of high-speed travel for efficient work. Power transmission devices of the above type are configured such that the transmission case stores lubricating oil inside and that the ring gear of the differential mechanism has a lower portion under the surface of the lubricating oil. Designing a work vehicle capable of traveling at a higher speed leads to the ring gear, whose lower portion is below the surface of lubricating oil, rotating and stirring the lubricating oil at a higher speed. This in turn poses a larger drive load on the ring gear, with the result of an increased loss of power during transmission through the differential mechanism.

The present invention has an object of providing a power transmission device for a work vehicle that is capable of driving wheels at high speeds without increasing the loss of power during transmission through a differential mechanism caused by a ring gear being partially under the surface of lubricating oil.

The power transmission device for a work vehicle according to the present invention is defined in claim <NUM>.

The above power transmission device is configured as follows: While the ring gear rotates, its lower portion stirs the lubricating oil. The gear cover serves to prevent such stirring from affecting the entire lubricating oil in the lubricating oil storage space in the transmission case. This allows the rotation speed of the ring gear to be increased without increasing the drive load on the ring gear, thereby making it possible to drive wheels at high speeds without increasing the loss of power during transmission through the differential mechanism.

The gear cover includes: a front cover portion facing and covering a front side face of the lower portion, the front side face being a first side face of opposite side faces of the lower portion which first side face is provided with teeth; and a lower cover portion extending from a lower end of the front cover portion under the lower portion to cover the lower portion from below.

The above power transmission device is configured as follows: The front cover portion prevents stirring of lubricating oil by the ring gear from affecting that portion of the lubricating oil storage space which faces the front side face of the ring gear. The lower cover portion prevents stirring of lubricating oil by the ring gear from affecting that portion of the lubricating oil storage space which is below the ring gear. This makes it possible to effectively prevent stirring of lubricating oil by the ring gear from affecting the entire lubricating oil in the lubricating oil storage space, thereby making it possible to drive wheels at higher speeds without increasing the loss of power during transmission through the differential mechanism.

The power transmission device according to the present invention may preferably be arranged such that the gear cover has a through hole that is in a lower end portion of the front cover portion which lower end portion is downstream of the lower portion in a direction in which the ring gear rotates and that allows lubricating oil to flow out.

The above power transmission device is configured as follows: The lower portion of the ring gear forces lubricating oil between the lower portion and the gear cover downstream of the lower portion in the rotation direction. The through hole allows the lubricating oil thus forced to flow out of the gear cover. The through hole thereby prevents lubricating oil heated as a result of stirring by the ring gear from stagnating between the gear cover and the lower portion. This prevents a rise in the temperature of lubricating oil between the gear cover and the lower portion.

The power transmission device according to the present invention may preferably be arranged such that the gear cover includes: an upstream fixing portion fixing to the transmission case a first end of the gear cover which first end is upstream of the lower portion in a direction in which the ring gear rotates; and a downstream fixing portion fixing to the transmission case a second end of the gear cover which second end is downstream of the lower portion in the direction in which the ring gear rotates.

The above power transmission device is configured as follows: The upstream fixing portion fixes to the transmission case that end of the gear cover which is upstream of the lower portion in the rotation direction. The downstream fixing portion fixes to the transmission case that end of the gear cover which is downstream of the lower portion in the rotation direction. This allows the gear cover to be fixed to the transmission case firmly against the oil pressure applied to the gear cover by stirring of lubricating oil by the ring gear.

The power transmission device according to the present invention may preferably be arranged such that the gear cover includes: a front cover portion facing and covering a front side face of the lower portion, the front side face being a first side face of opposite side faces of the lower portion which first side face is provided with teeth; a lower cover portion extending from a lower end of the front cover portion under the lower portion to cover the lower portion from below; and a reverse cover portion standing on an end of the lower cover portion which end is opposite to the front cover portion and facing and covering a reverse side face of the lower portion, the reverse side face being a second side face of the opposite side faces of the lower portion which second side face is not provided with teeth.

The above power transmission device is configured as follows: The front cover portion prevents stirring of lubricating oil by the ring gear from affecting that portion of the lubricating oil storage space which faces the front side face of the ring gear. The lower cover portion prevents stirring of lubricating oil by the ring gear from affecting that portion of the lubricating oil storage space which is below the ring gear. The reverse cover portion prevents stirring of lubricating oil by the ring gear from affecting that portion of the lubricating oil storage space which faces the reverse side face of the ring gear. This makes it possible to effectively prevent stirring of lubricating oil by the ring gear from affecting the entire lubricating oil in the lubricating oil storage space, thereby making it possible to drive wheels at higher speeds without increasing the loss of power during transmission through the differential mechanism.

The description below deals with an example embodiment of the present invention with reference to drawings.

The embodiment described below is for a tractor (which is an example of the "work vehicle") including a body. <FIG> shows arrow F to indicate the forward direction of the body, arrow B to indicate the backward direction of the body, arrow U to indicate the upward direction of the body, and arrow D to indicate the downward direction of the body. The front side of the drawing corresponds to the leftward direction of the body, whereas the back side of the drawing corresponds to the rightward direction of the body.

The body of the tractor includes (i) a body frame <NUM> including components such as an engine <NUM>, (ii) a pair of left and right drivable front wheels <NUM> disposed at a front portion of the body frame <NUM> and serving as travel devices, and (iii) a pair of left and right drivable rear wheels <NUM> disposed at a back portion of the body frame <NUM> and serving as travel devices. The body frame <NUM> includes an engine <NUM>, a clutch housing <NUM> behind the engine <NUM>, a transmission case <NUM> coupled to the clutch housing <NUM>, and a front frame portion <NUM>. The front wheels <NUM> are swingable about an axis extending in the up-down direction of the body (not shown in the drawings) to be turned. The body includes at a front portion a motor section <NUM> containing the engine <NUM>, and also includes a driver section <NUM> at a back portion. The driver section <NUM> includes a driver's seat <NUM>, a steering wheel <NUM> for use to turn the front wheels <NUM>, and a cabin <NUM> defining a driver space. The body frame <NUM> has a back portion provided with a link mechanism <NUM> and a power take-off shaft <NUM>. The link mechanism <NUM> allows any of various work devices such as a rotary tiller device (not shown in the drawings) to be coupled to the body frame <NUM> in such a manner as to be capable of being lifted and lowered. The power take-off shaft <NUM> takes off motive power from the engine <NUM> and transmits the motive power to a work device coupled to the body frame <NUM>.

The tractor includes a power transmission device <NUM> illustrated in <FIG>. The power transmission device <NUM> transmits motive power from the engine <NUM> to the front wheels <NUM> and the rear wheels <NUM>. The transmission case <NUM> is included in the power transmission device <NUM> in such an orientation as to have a front-back direction identical to that of the body.

As illustrated in <FIG>, the power transmission device <NUM> includes a transmission case <NUM> that contains a gear transmission <NUM>, a rear-wheel differential mechanism <NUM>, and a front-wheel power transmission device <NUM>. The gear transmission <NUM> includes (i) a main transmission section <NUM> coupled to the input shaft <NUM> of the transmission case <NUM>, (ii) a multi-stage power transmission section <NUM> configured to receive the output of the main transmission section <NUM>, (iii) a forward/rearward travel switching device <NUM> configured to receive the output of the multi-stage power transmission section <NUM>, (iv) a rear-wheel gear interlocking mechanism <NUM> configured to transmit the output of the forward/rearward travel switching device <NUM> to the rear-wheel differential mechanism <NUM>, and (v) a front-wheel gear interlocking mechanism <NUM> configured to transmit the output of the forward/rearward travel switching device <NUM> to the front-wheel power transmission device <NUM>.

The transmission case <NUM> includes a front case portion 3a, a middle case portion 3b, and a back case portion 3c as segmented in the front-back direction of the transmission case <NUM>. The front case portion 3a contains the main transmission section <NUM>, the multi-stage power transmission section <NUM>, and the front-wheel power transmission device <NUM>. The middle case portion 3b contains the rear-wheel gear interlocking mechanism <NUM> and the front-wheel gear interlocking mechanism <NUM>. The back case portion 3c contains the forward/rearward travel switching device <NUM> and the rear-wheel differential mechanism <NUM>.

The main transmission section <NUM> includes a planetary gear device 18A and a continuously variable transmission device 18B. The planetary gear device 18A has a front portion coupled to a first gear interlocking mechanism <NUM> configured to allow motive power from the input shaft <NUM> to be received by the planetary gear device 18A. The continuously variable transmission device 18B has a back portion coupled to a second gear interlocking mechanism <NUM> configured to allow motive power from the input shaft <NUM> to be received by the continuously variable transmission device 18B. The continuously variable transmission device 18B has a front portion coupled to a third gear interlocking mechanism <NUM> configured to allow the output of the continuously variable transmission device 18B to be received by the planetary gear device 18A. The forward/rearward travel switching device <NUM> has a front portion coupled to the rear-wheel gear interlocking mechanism <NUM>, which is configured to transmit the output of the forward/rearward travel switching device <NUM> to the rear-wheel differential mechanism <NUM>. The front-wheel power transmission device <NUM> has a back portion coupled to the front-wheel gear interlocking mechanism <NUM>, which is configured to transmit the output of the forward/rearward travel switching device <NUM> to the front-wheel power transmission device <NUM>.

As illustrated in <FIG>, the power transmission device <NUM> is configured as follows: Motive power outputted by the engine <NUM> from its output shaft 1a is transmitted through a main clutch <NUM> to the input shaft <NUM> of the transmission case <NUM>; the input shaft <NUM> transmits its motive power to the main transmission section <NUM>; the main transmission section <NUM> outputs its motive power to the multi-stage power transmission section <NUM>; the multi-stage power transmission section <NUM> outputs its motive power to the forward/rearward travel switching device <NUM>; and the forward/rearward travel switching device <NUM> outputs its motive power through the rear-wheel gear interlocking mechanism <NUM> to the input shaft 22a of the rear-wheel differential mechanism <NUM>. The power transmission device <NUM> includes a pair of steering brakes <NUM> and a pair of deceleration mechanisms <NUM> as a power transmission system for transmitting the output of the rear-wheel differential mechanism <NUM> to the respective rear wheels <NUM>. The deceleration mechanisms <NUM> are each in the form of a planetary gear mechanism. The power transmission device <NUM> includes a rotary shaft <NUM> and a front-wheel differential mechanism <NUM>. The front-wheel gear interlocking mechanism <NUM> transmits motive power from the input shaft 22a of the rear-wheel differential mechanism <NUM> to the front-wheel power transmission device <NUM>, which then transmits the motive power through the rotary shaft <NUM> to the front-wheel differential mechanism <NUM>. The front-wheel gear interlocking mechanism <NUM> is provided with a parking brake <NUM>.

As illustrated in <FIG>, the power transmission device <NUM> includes a work transmission device <NUM>. The work transmission device <NUM> receives motive power from the input shaft <NUM> through the rotary shaft <NUM> and a rear rotary shaft <NUM>, varies the motive power, and transmits the resulting motive power to the power take-off shaft <NUM>.

The main transmission section <NUM>, as illustrated in <FIG>, includes a planetary gear device 18A and a continuously variable transmission device 18B. The planetary gear device 18A includes two planetary gear sections <NUM> and <NUM> arranged in the front-back direction of the transmission case <NUM>. The forward one of the two planetary gear sections <NUM> and <NUM>, that is, the planetary gear section <NUM>, is coupled to the input shaft <NUM> with the first gear interlocking mechanism <NUM> in-between. The continuously variable transmission device 18B is of a hydrostatic type, and includes a hydraulic pump P with a variable capacity and a hydraulic motor M. The continuously variable transmission device 18B includes a pump shaft coupled to the input shaft <NUM> with the second gear interlocking mechanism <NUM> and the rotary shaft <NUM> in-between. The continuously variable transmission device 18B includes a motor shaft coupled to the planetary gear section <NUM> with the third gear interlocking mechanism <NUM> in-between.

The main transmission section <NUM> is configured as follows: The continuously variable transmission device 18B varies motive power from the engine <NUM>. The planetary gear device 18A receives the varied motive power and motive power transmitted from the engine <NUM> through the third gear interlocking mechanism <NUM>. The planetary gear sections <NUM> and <NUM> composite the two motive powers. The planetary gear device 18A allows the composite motive power to be outputted from a first output shaft 37a, a second output shaft 37b, and a third output shaft 37c, which coaxially overlap with one another.

The multi-stage power transmission section <NUM>, as illustrated in <FIG>, includes an output shaft <NUM> and first to fourth clutches CL1 to CL4 disposed on the output shaft <NUM> for respective stages and each configured to receive the output of the planetary gear device 18A.

The multi-stage power transmission section <NUM> is configured such that appropriately operating the continuously variable transmission device 18B and the four clutches CL1 to CL4 allows the composite motive power from the planetary gear device 18A to be outputted from the output shaft <NUM> in one of four speed ranges.

<FIG> is a graph that shows the relationship among the speed change state of the continuously variable transmission device 18B, the speed range, and the rotation speed V of the output shaft <NUM> of the multi-stage power transmission section <NUM>. The graph has a vertical axis indicative of the rotation speed V of the output shaft <NUM> and a horizontal axis indicative of the speed change state of the continuously variable transmission device 18B. The symbol "N" indicates the neutral state, "+MAX" indicates the state of changing the speed to its maximum in the normal rotation direction, and "-MAX" indicates the state of changing the speed to its maximum in the reverse rotation direction.

Engaging the first clutch CL1 and operating the continuously variable transmission device 18B causes motive power from the first output shaft 37a to be varied by a first-gear interlocking mechanism 39a and the first clutch CL1 and then outputted from the output shaft <NUM>. As illustrated in <FIG>, the output shaft <NUM> has a rotational speed within a first-gear range. Varying the continuously variable transmission device 18B from "-MAX" to "+MAX" increases the rotation speed V of the output shaft <NUM> continuously from zero speed ("<NUM>") to the maximum speed within the first-gear range ("V1").

Engaging the second clutch CL2 and operating the continuously variable transmission device 18B causes motive power from the third output shaft 37c to be varied by a second-gear interlocking mechanism 39b and the second clutch CL2 and then outputted from the output shaft <NUM>. As illustrated in <FIG>, the output shaft <NUM> has a rotational speed within a second-gear range, which covers higher speeds than the first-gear range. Varying the continuously variable transmission device 18B from "+MAX" to "-MAX" increases the rotation speed V of the output shaft <NUM> continuously from the minimum speed within the second-gear range ("V1") to the maximum speed within the second-gear range ("V2").

Engaging the third clutch CL3 and operating the continuously variable transmission device 18B causes motive power from the second output shaft 37b to be varied by a third-gear interlocking mechanism 39c and the third clutch CL3 and then outputted from the output shaft <NUM>. As illustrated in <FIG>, the output shaft <NUM> has a rotational speed within a third-gear range, which covers higher speeds than the second-gear range. Varying the continuously variable transmission device 18B from "-MAX" to "+MAX" increases the rotation speed V of the output shaft <NUM> continuously from the minimum speed within the third-gear range ("V2") to the maximum speed within the third-gear range ("V3").

Engaging the fourth clutch CL4 and operating the continuously variable transmission device 18B causes motive power from the third output shaft 37c to be varied by a fourth-gear interlocking mechanism 39d and the fourth clutch CL4 and then outputted from the output shaft <NUM>. As illustrated in <FIG>, the output shaft <NUM> has a rotational speed within a fourth-gear range, which covers higher speeds than the third-gear range. Varying the continuously variable transmission device 18B from "+MAX" to "-MAX" increases the rotation speed V of the output shaft <NUM> continuously from the minimum speed within the fourth-gear range ("V3") to the maximum speed within the fourth-gear range ("V4").

The forward/rearward travel switching device <NUM>, as illustrated in <FIG>, includes (i) an input shaft <NUM> coupled to the output shaft <NUM> of the multi-stage power transmission section <NUM>, (ii) a forward-travel clutch CLF and a rearward-travel clutch CLR both disposed on the input shaft <NUM>, and (iii) an output shaft <NUM> coupled to the forward-travel clutch CLF with a forward-travel gear mechanism 41f in-between and to the rearward-travel clutch CLR with a rearward-travel gear mechanism 41r in-between. As illustrated in <FIG>, the rearward-travel gear mechanism 41r includes a reverse rotation gear <NUM> meshing with teeth on an output rotary member of the rearward-travel clutch CLR. The reverse rotation gear <NUM> is supported by the input shaft 22a of the rear-wheel differential mechanism <NUM> in such a manner as to be rotatable relative to the input shaft 22a.

The forward/rearward travel switching device <NUM> is configured as follows: Engaging the forward-travel clutch CLF causes motive power transmitted from the multi-stage power transmission section <NUM> to the input shaft <NUM> to be converted by the forward-travel clutch CLF and the forward-travel gear mechanism 41f into forward-travel motive power and then outputted from the output shaft <NUM>. Engaging the rearward-travel clutch CLR causes motive power transmitted from the multi-stage power transmission section <NUM> to the input shaft <NUM> to be converted by the rearward-travel clutch CLR and the rearward-travel gear mechanism 41r into rearward-travel motive power and then outputted from the output shaft <NUM>. The output shaft <NUM> transmits the forward-travel motive power and the rearward-travel motive power to the rear-wheel gear interlocking mechanism <NUM>, which then transmits them to the input shaft 22a of the rear-wheel differential mechanism <NUM>.

The front-wheel power transmission device <NUM>, as illustrated in <FIG>, includes (i) an input shaft <NUM> coupled to the input shaft 22a of the rear-wheel differential mechanism <NUM> with the front-wheel gear interlocking mechanism <NUM> in-between, (ii) a constant-rate clutch CLT and a rate-increasing clutch CLH both disposed on the input shaft <NUM>, and (iii) an output shaft <NUM> coupled to the constant-rate clutch CLT with a constant-rate gear mechanism 44a in-between and to the rate-increasing clutch CLH with a rate-increasing gear mechanism 44b in-between.

The front-wheel power transmission device <NUM> is configured as follows: Engaging the constant-rate clutch CLT causes motive power transmitted from the input shaft 22a of the rear-wheel differential mechanism <NUM> to the input shaft <NUM> to be further transmitted through the constant-rate clutch CLT and the constant-rate gear mechanism 44a to the output shaft <NUM> and then to the front-wheel differential mechanism <NUM>. This achieves four-wheel drive with a constant rate for the front and rear wheels, that is, the tractor drives the front wheels <NUM> and the rear wheels <NUM> at respective average circumferential speeds substantially equal to each other. Engaging the rate-increasing clutch CLH causes motive power transmitted from the input shaft 22a of the rear-wheel differential mechanism <NUM> to the input shaft <NUM> to be further transmitted through the rate-increasing clutch CLH and the rate-increasing gear mechanism 44b to the output shaft <NUM> and then to the front-wheel differential mechanism <NUM>. This achieves four-wheel drive with an increased rate for the front wheels, that is, the tractor drives the front wheels <NUM> at a first average circumferential speed and the rear wheels <NUM> at a second average circumferential speed, the first average circumferential speed being higher than the second average circumferential speed.

The planetary gear device 18A, as illustrated in <FIG>, includes two planetary gear sections <NUM> and <NUM> arranged in the front-back direction of the transmission case <NUM> (composite planetary gear device). The description below refers to the front one of the two planetary gear sections <NUM> and <NUM> as "first planetary gear section <NUM>" and the back one thereof as "second planetary gear section <NUM>".

The first planetary gear section <NUM>, as illustrated in <FIG>, includes (i) a first sun gear <NUM>, (ii) a first planetary gear <NUM> meshing with the first sun gear <NUM>, (iii) a first internally toothed gear <NUM> meshing with the first planetary gear <NUM>, and (iv) a first carrier <NUM> that supports the first planetary gear <NUM> in such a manner that the first planetary gear <NUM> is rotatable and that is configured to rotate about the axis of the first sun gear <NUM> together with the first planetary gear <NUM> revolving about the first sun gear <NUM>.

The second planetary gear section <NUM>, as illustrated in <FIG>, includes (i) a second sun gear <NUM>, (ii) a second planetary gear <NUM> meshing with the second sun gear <NUM>, (iv) a second internally toothed gear <NUM> meshing with the second planetary gear <NUM>, and (iv) a second carrier <NUM> that supports the second planetary gear <NUM> in such a manner that the second planetary gear <NUM> is rotatable and that is configured to rotate about the axis of the second sun gear <NUM> together with the second planetary gear <NUM> revolving about the second sun gear <NUM>.

The first planetary gear section <NUM> includes a power transmission gear (not shown in the drawings) meshing with the first planetary gear <NUM>. The planetary gear device 18A includes a coupling member (not shown in the drawings) extending from the first planetary gear section <NUM> to the second planetary gear section <NUM> and coupling the power transmission gear to the second planetary gear <NUM> in an interlocking manner. The first carrier <NUM> of the first planetary gear section <NUM> is coupled to the second carrier <NUM> of the second planetary gear section <NUM> in such a manner that the first carrier <NUM> and the second carrier <NUM> are rotatable integrally with each other.

As illustrated in <FIG>, the second-gear interlocking mechanism 39b includes (i) an output power transmission gear <NUM> disposed on the third output shaft 37c and (ii) an input gear 82a disposed on an input member of the second clutch CL2 and meshing with the output power transmission gear <NUM>.

As illustrated in <FIG>, the second gear interlocking mechanism <NUM> includes a pump power transmission gear <NUM> disposed backward of the output power transmission gear <NUM> and coupled to the rotary shaft <NUM>. The pump power transmission gear <NUM> is configured to take off motive power, which has been transmitted from the input shaft <NUM> to the rotary shaft <NUM>, from the rotary shaft <NUM> and transmit the motive power to the hydraulic pump P.

The first gear interlocking mechanism <NUM>, as illustrated in <FIG>, includes (i) a first power transmission gear <NUM> disposed on the input shaft <NUM>, (ii) a second power transmission gear <NUM> coupled to the first internally toothed gear <NUM>, (iii) a first relay gear <NUM> meshing with the first power transmission gear <NUM>, and (iv) a second relay gear <NUM> meshing with the second power transmission gear <NUM>. The first gear interlocking mechanism <NUM> further includes a counter shaft <NUM> coupling the first relay gear <NUM> to the second relay gear <NUM> and supporting the first relay gear <NUM> and the second relay gear <NUM>.

As illustrated in <FIG>, the rear-wheel gear interlocking mechanism <NUM> includes a power transmission gear <NUM> meshing with the output gear 20a of the forward/rearward travel switching device <NUM> and configured to transmit motive power from the output gear 20a to the input shaft 22a of the rear-wheel differential mechanism <NUM>. The power transmission gear <NUM> is supported by the input shaft 22a.

As illustrated in <FIG>, the front-wheel gear interlocking mechanism <NUM> includes a first power transmission gear <NUM> disposed on the input shaft 22a of the rear-wheel differential mechanism <NUM> and a second power transmission gear <NUM> meshing with the first power transmission gear <NUM>. The second power transmission gear <NUM> includes a shaft 78a below the input shaft 22a as the shaft of the first power transmission gear <NUM>.

The front-wheel gear interlocking mechanism <NUM> allows the output of the forward/rearward travel switching device <NUM> to be transmitted to the front-wheel power transmission device <NUM> as follows: The output gear 20a of the forward/rearward travel switching device <NUM> transmits motive power through the power transmission gear <NUM> and the input shaft 22a to the first power transmission gear <NUM>, which then transmits the motive power through the second power transmission gear <NUM> and the shaft 78a to the input shaft <NUM> of the front-wheel power transmission device <NUM>.

As illustrated in <FIG>, the rear-wheel differential mechanism <NUM> is contained in the back case portion 3c of the transmission case <NUM>. As illustrated in <FIG>, the rear-wheel differential mechanism <NUM> includes an input shaft 22a at a front portion and a differential case <NUM> at a back portion. The input shaft 22a is provided with a drive pinion <NUM>. The differential case <NUM> has a lateral side portion provided with a ring gear <NUM> meshing with the drive pinion <NUM>. The differential case <NUM> contains (i) a pair of differential pinions <NUM>, (ii) a left side gear <NUM> meshing with the differential pinions <NUM> and configured to transmit motive power to the left rear wheel <NUM>, and (iii) a right side gear <NUM> meshing with the differential pinions <NUM> and configured to transmit motive power to the right rear wheel <NUM>. The differential case <NUM> has a lateral side portion provided with a differential lock device <NUM>.

The rear-wheel differential mechanism <NUM> is configured such that when it transmits forward-travel motive power to the rear wheels <NUM>, the ring gear <NUM> rotates so that a lower portion 95a thereof rotates in the rotation direction R indicated with an arrow in <FIG>.

The transmission case <NUM> is configured to store, in its internal space, lubricating oil for components such as the gear transmission <NUM> and the rear-wheel differential mechanism <NUM>. The lubricating oil is stored as illustrated in <FIG> to have a surface S at a level between the rotation axis X and lower end of the ring gear <NUM>. The rear-wheel differential mechanism <NUM> is positioned such that the ring gear <NUM> has a lower portion 95a below the surface S of the lubricating oil.

As illustrated in <FIG>, the rear-wheel differential mechanism <NUM> includes a gear cover <NUM> covering the lower portion 95a of the ring gear <NUM>. While the ring gear <NUM> rotates, its lower portion 95a stirs the lubricating oil. The gear cover <NUM> serves to prevent such stirring from affecting the entire lubricating oil in the lubricating oil storage space in the transmission case.

The gear cover <NUM>, as illustrated in <FIG>, includes a front cover portion <NUM> and a lower cover portion <NUM>. The front cover portion <NUM> faces and covers a front side face 95f of the lower portion 95a, the front side face 95f being that one of the opposite side faces of the ring gear <NUM> which is provided with teeth. The lower cover portion <NUM> extends from the lower end of the front cover portion <NUM> under the lower portion 95a to cover the lower portion 95a from below. The front cover portion <NUM> serves to prevent stirring of lubricating oil by the ring gear <NUM> from affecting that portion of the lubricating oil storage space which faces the front side face 95f. The lower cover portion <NUM> serves to prevent stirring of lubricating oil by the ring gear <NUM> from affecting that portion of the lubricating oil storage space which is under the ring gear <NUM>.

As illustrated in <FIG>, <FIG>, and <FIG>, the gear cover <NUM> includes an upstream vertical cover portion <NUM> standing on that end of the lower cover portion <NUM> which is upstream of the lower portion 95a in the rotation direction R. The upstream vertical cover portion <NUM> is connected with that end of the front cover portion <NUM> which is upstream of the lower portion 95a in the rotation direction R. The gear cover <NUM> includes a downstream vertical cover portion <NUM> standing on that end of the lower cover portion <NUM> which is downstream of the lower portion 95a in the rotation direction R. The downstream vertical cover portion <NUM> is connected with that end of the front cover portion <NUM> which is downstream of the lower portion 95a in the rotation direction R.

As illustrated in <FIG>, <FIG>, and <FIG>, the gear cover <NUM> has a through hole <NUM> in that lower end portion of the front cover portion <NUM> which is downstream of the lower portion 95a in the rotation direction R. The through hole <NUM> allows lubricating oil to flow out. The lower portion 95a of the ring gear <NUM> forces lubricating oil between the lower portion 95a and the gear cover <NUM> downstream of the lower portion 95a in the rotation direction R. The through hole <NUM> allows the lubricating oil thus forced to flow out of the gear cover <NUM>. The through hole <NUM> thereby prevents lubricating oil heated as a result of stirring by the ring gear <NUM> from stagnating between the gear cover <NUM> and the lower portion 95a.

The gear cover <NUM>, as illustrated in <FIG>, includes an upstream fixing portion <NUM> at that end which is upstream of the lower portion 95a in the rotation direction R and a downstream fixing portion <NUM> at that end which is downstream of the lower portion 95a in the rotation direction R. As illustrated in <FIG>, the upstream fixing portion <NUM> is coupled to a first support <NUM> of the transmission case <NUM> with use of a coupling bolt <NUM>, and the downstream fixing portion <NUM> is coupled to a second support <NUM> of the transmission case <NUM> with use of a coupling bolt <NUM>. This results in the gear cover <NUM> being detachably fixed to the transmission case <NUM>.

Claim 1:
A power transmission device (<NUM>) for a work vehicle,
the power transmission device (<NUM>) comprising:
a transmission case (<NUM>) configured to store lubricating oil in its internal space;
a gear transmission (<NUM>) contained in the transmission case (<NUM>) and configured to receive motive power from a power source (<NUM>);
a differential mechanism (<NUM>) contained in the transmission case (<NUM>) and configured to receive motive power from the gear transmission (<NUM>) and transmit the motive power to a left wheel and a right wheel (<NUM>); and
a gear cover (<NUM>) covering a lower portion (95a) of a ring gear (<NUM>) of the differential mechanism (<NUM>) below a surface (S) of the stored lubricating oil and including a front cover portion (<NUM>) facing and covering a front side face (95f) of the lower portion(95a), the front side face(95f) being a first side face of opposite side faces of the lower portion (95a) which first side face is provided with teeth;
characterized in that
the gear cover (<NUM>) includes a lower cover portion (<NUM>) extending from a lower end of the front cover portion (<NUM>) under the lower portion (95a) to cover the lower portion (95a) from below.