CA1055277A - Transmission for tractors - Google Patents

Abstract

TITLE OF THE INVENTION
Transmission for Tractors ABSTRACT OF THE DISCLOSURE
A speed change mechanism for a transmission com-prises a main shaft provided with a plurality of power trans-mitting gears, and a propelling speed change gear system and a young takeoff speed change gear system supported on the opposite sides of the main shaft in parallel thereto.
Each of the propelling and power takeoff speed change systems includes a speed change shaft freely rotatably carrying a suitable number of speed change gears meshing with the power transmitting gears on the main shaft at all times. The speed change shaft accommodates an axially slidable speed change rod which, when moved, forces pins outward or inward radially of the shaft to couple the speed change shaft to the corresponding speed change gear or disconnect the shaft from the gear. The freely rotatable speed change gears are held against axial movement, while axially slidable engaging members which are otherwise provided between the adjacent speed change gears are eliminated. The transmission therefore has a shortened axial length and does not require a clutch which must be usually provided between the engine and the main shaft. The propelling speed change system further incorporates a creeping assembly of the planetary roller type for super-speed reduction and is provided with a secon-dary speed change assembly. The transmission also includes a power takeoff assembly for driving the front wheels of the tractor utilizing the secondary speed change assembly.

Description

BACKCROUND OF THE INVE~TIO~
The present invention relates to a tran~miasion for transmitting the power of an engine to propelling and power takeoff systems at the desired number of ~peed change steps, and more particularly to improvement~ in speed change mechanism.
Transmissions of agricultural and civil engineering tractors have ~uch higher speed reduction ratios at low - speed~ than motor vehicle~ and require many steps of speed chaDges. Thus transmi~aions comprising a gear speed change mechanism include very complex speed change gear systems, ~re very cumbersome to design ~nd make and involve diffi-culties in accommodating shafts in a limited space.
Tractor transmissions comprising a gear speed change mechanism are generally of the selective mesh type End con~nt ~esh type. In either type, gears or engaging members or clutch pawls are slidingly moved in the axial direction to give desired speed changes 80 that the mecha-nism requires an elongated axial length, is subject to damage to gears and gives off a nolse. ~oreoverj each ~peed change procedure requires engagement and diseneagement of the clutch.
~ ore specifically, ~ith the selective mesh type in which a speed change gear on the speed change shaft is Qlidingly moved to give the desired speed change, the ~hs~t must have a length ~hich is greater by at l~ast the aistance of the ~liding movement of the gear. ~he use of geurs*hich must be in the form of a spur gear further gives off a loud noise snd invol~eo difficulties in meshing gears 1055Z7~7 due to the difference in peripheral speed between the gears, possibly leading to damage to the gears.
With the constant mesh type, gears on the main shaft are always in mesh with speed change gears on the speed change shaft which is provided with clutch pawls slidable thereon.
Accordingly the shaft must be elongated by at least the sliding distance of the clutch pawls. In addition, since the clutch pawls are adapted to engage with or disengage from rotating wheels, the mechanism is subject to damage due to impact.
The gear speed change mechanism invariably requires a clutch which is provided between the engine and the main shaft and which must be disengaged for a speed change. Because the main shaft continues to idly rotate even after the clutch has been disengaged, the speed change action involves a delay especially in the case of the selective mesh type. Tractor transmissions comprising two systems, i.e. a propelling and a power take off system further necessitate a double clutch which is expensive.
SUMMARY OF THE INVENTION
The primary object of the present invention is to overcome one or more of the aforementioned deficiencies of conventional tractor transmissions.
The present invention is broadly defined as a trans-mission of the three-system type for a tractor comprising an input system having a main input shaft carrying a plurality of gears for transmitting the torque of an engine, and a propelling speed change system and a power takeoff speed change system supported on the opposite sides of the input system respectively rw/~

~055Z77 in parallel thereto, each of the propelling and power takeoff speed change systems including a speed change shaft having a power output end and having a plurality of speed change gears freely rotatably mounted thereon side by side and meshing with the gears of the input system respectively at all times, the speed change shaft being a hollow shaft internally formed with an axial bore, each of the speed change gears being formed in its inner peripheral surface with grooves, the speed change shaft incorporating pins movable radially of the speed change shaft and engageable in and disengageable from the grooves in the corresponding speed change shaft and engageable and disen-gageable from the grooves in the corresponding speed change gear, the speed change shaft accomodating in its axial bore an axailly movable speed change rod being axially movable to engage the pins in the grooves of the corresponding speed change gear and the speed change rod partially extending outwardly from the speed change shaft and operating force transmitting means pro-vided at an end opposite to the power output end coupled to the partially extending portion of the speed change rod for axially moving the speed change rod, whereby the speed change gear is coupled to the speed change shaft to transmit the torque to rear wheels and a power takeoff shaft with desired speed changes.
Other features of this invention will become apparent from the following description of the preferred embodiments with reference to the accompanying drawings. Various modifi-cations and alterations in the details of the construction are included within the scope of the appended claims.

~ _ 4 _ rw/~

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a side elevation showing the appearance of a tractor incorporating the transmission of this invention;
Fig. 2 is a sectional view showing the details of the interior construction of the transmission of this invention;
Fig. 3 is an enlarged side elevation in section showing the principal part of a speed change assembly of the pin engageable type;

~ - 4a -rw/~ ~s Fig. 4 is an enlarged view in section showing the assembly of Fig. 3 88 it is seen from the right side of FiB. 3;
` Fig. 5 is an enlarged side elevation showing another embodiment of the speed change assembly;
Fig. 6 is an enlarged side elevation showing the assembly of Fig. 5 as it is seen from the rieht side of Fig. 5; and Fig. 7 is a sectional view showing a creeping assembly of the planetary roller type as it is ~een from the front.

DETAILED DES~RIPT~ON 0~ ~HE INVE~TION
Fig. 1 shows a tractor including a frame 1, front ~heels 2 and rear wheels 3. The rear wheels 3 are drive wheels, while the front wheels 2 are also serviceable a~ drive wheels by gear:change. An engine 4 mounted on a front portion of the fr&me 1 is connected to a transmission case 5.
- With reference to Pig. 2, a flywheel 7 i8 fixodly mounted on one end of the crank shaft 6 of the engine 4.
A main input shaft 8 provided with a group of three power tr~nsmission gears ~ iB supported at it~ opposite ends by the transmis6ion case 5 by bearing~ 10. A fle~ible ~hock-absorbing coupling 11 interconnects the flywheel 7 and the front end of the main shaft 8 to transmit the power o~ the engine to the main shaft 8. lhe coupling 11 com-prises ~n annular member 12 made of an oil-resistant elastic ~atsrial ~uch as rubbsr and front and rear connecting arm~

~' i~' ''6-1~ and t4. ~he a~m 13 i8 bolted to the flywheel 7, while the other arm 14 has its cylindrical portion splined tb the main shaft 8.
A propelling speed change ~haft 15 and a power 5 takeoff speed change shaft 16 extend in parallel to each other above and below the main sha$t 8 and ~re supported at their opposite ends by bearings 17, 18 respectively.
~oth the shafts 15, 16 are hollo~ and have axial bores 15a, 16a. ~he shafts 15, 16 are sheathed with sleeves 19, 20 10 ~hich are keyed or splined to the shafts. As shown in Figs. 5 and 6, the sleeves can be omitted.
~hen the shafts 15, 16 are sheathed with the sleeves 19, 20, propelling speed change gears 21, 22, 23 r ,' snd power takeoff speed change gears 24, 25, 26 are t5 ~reely rotatably mounted on the sleeve~ 19, 20 respectively.
(See ~iga. 2 to 4.) ~hen the sleeves sre omitted, the speed c~ange gears are freely rotata'oly mounted directly on the shafts, as seen in Figs. 5 and 6 representatively showing the propelling speed change ~haft 15. In either case, the propelling speed change gears 21, 22, 23 and the power takeoff speed change gears 24, 25, 26 mesh ~ith the gear group 9 on the main shaft 8 at all times.
lhe gearo in corresponding relation to each other, n3mely the propelling first speed gear 21 snd the power takeoff first speed gear 24, the prop~lllr~ second ~oea gesr 22 and the po~er takeoff second opeed gear 25, and the propelling third speea gear 23 snd the power takeoff th~rd speed gear 26 are respeotively iaenticsl in ~hape ~nt d ~e.

:., .

The hollo~ speea change ehafts 15, 16 accommoaate ln their axial boree 15a, 16a ~xislly ~lidable speed change rod~ 27, 28 reepectively. The speed change rods 27, 28 are formed with a~nular pro~ections 29, 30 and have front S ends e~tending outward from the bores 15a, 16a, ~ propelling lever bar 31 i8 supported on ths case tur~ably about its lateral axis and is integral with le~er 33 engaged ~ith or pivoted to the propelling speed ohange rod 27. A power takeoff lever bar 32 is supported on the case turnably about it~ lateral axis and i8 integr~1 ~ith a lever 34 engaged with or pivoted to the power ta~eoff speed change rod 28. As shown in Fig~. 3 ana 4 i~ greater detsil, the shafts 15, 16 are formed ~ith ~in bores 35 C ~ 6 R 6 r~
jl ~hich are equiaistantly spaced apart o ~ of the shafts. ~lthough Figs. 3 and 4 show the propelling system, the po~er takeoff system is identic~l thereto.
In the illustrated embodi~ent, three radial pin bor~ 35 are formea ~ich accommodate radially movable pins 36. ~ach of the pin~ 36 has a flange 36a in contact with a rollable ball 37 and a rounded top end. When the speea change shaft 15 iB provided with the ~leeve t9, ooiled spri~gs 38 are di~poeed between the n an~es 36a of the pins 36 and the inner surface of the sleeve 19. The spri~gs 38 urge the pins 36 radi~1ly inwardly of the shaft 15 Bt all ti~es. ~hen the sleeve is not used as illustrated in ~igs. 5 sna 6, each of tho bores 35 ia provided w$th a - ~ri~g receiving portion 35a, in ~hich case tho ~pring 38 beare ag ~nst tho portion 35a and the flEnge 36a of the pin 36. As H~ll be apparent from Fig~. 5 and 6, when the lQSSZ77 epring receiving portion 35a is *or.med, a gulde bore 35b . must be provided for inserting the pin 36. Since the guide boros 35b reduce the ~trength of the shaft 15, 16, it 1~
preferable to sheath them ~ith ths sleeves 19, 20. When the eleeves 19, 20 ~re used, guide holes 35c ~re formed in the sleevea in register with the pin bores 35 to render the pins 36 radially movable in~vard or outward through the holes 35c.
Each of the propelling speed change gears 21, 22, 23 and the power takeoff apeed change gear~ 24, 25, 26 is fo~med in the inner peripher~l ~urface with grooves 39 in correspondine relation to the pin bores 35. As shown ~n Figs., 3 and 4, for example 8iX groove~ 39 are for;ned as equidistantly spaced :.apart in the circumfere~tial di-rection. 5uppose the gear is idly rotatable in the direction of the arro~ in Fig. 4, the groove 39 is 80 shaped that its depth prcgressively reducee in the direction of.rotation o~ the gesr. q~he groove 39 ma;y be a semicircular groove for recelving the pin 36 as shovn ia~ Figs. 5 and 6, but ~hen it i8 an elongated groove having a reducing depth as etatea above, the impact at the time o* engagement can be mitigated as will be aescribed lster~
~ith reference to Figs. 2 ~nd 7, a creeping a~sembly 40 Or the planetary rollar type is provided at .. a resr portion of the propelling ~peed change shaft 15.
25 ~ne creepirg assembly 40 comprisea a sun roller 41, an annular hous1ng 42 ~ncentric with the roller 41 and ~llmotary rollers 43 diuposea betlreen the sun roller 41 ana the housing 42. 7he planeta2y rollers 43 are revolvable ~hlle rotatiDg about their axes. T~e sun roller 41 i8 mude of tnotal material and i8 ~pllned or otherlise secured to the shaft 15. When the sun roller 41 is secured in position, a spline portion 15b is left exposed at the shaft end.
~he housing 42 surrounding the sun roller 41 hss a circular inner surface and is secured to the transnission case 5, Each of the-planetary rollers 43 comprises inner and outer metal rings 44, 45 and a rubber or like elastic member 46 provided between and ~oined to the rings. As seen in Fig.
7, three planetary rollers are rollably supported on shafts 10 47. The roller shafts 47 are supported by a housing 49 hsving an internal gear 48.
1~ secondary speed change shaft 50 i8 supported st t~o rear end portions thereof by bearings 51 and has a ~ront end inserted in the axial bore 15a of tho propelling 15 ~peed change shaft 15 with a needle bearing 52 or the like pro~ided therebet~oen. Thus the ~haft 50 extends rearward ~o~ially with the shaft 15. The secondary speed change -shaft 50 fixedly carries largs and small gears 53, 54.
The ~m~ll gear 54 has an increased thickness. A clutch 20 gesr 55 splined to the front end of the seconda~y speed oha~ge shaft 50 i8 a~cially slidable. When slidingly moved iert~lard from the position in Fig. 2, the gear 55 is splined to tho spline portion 15b of the ahaft 15, directly connecting the ~ t 15 to the shaft 50, fhereas ~hen moved right~ard 25 end engaged ~ith the internal gear 48, the shaft 15 is ~ '~
coupled to the shaft 50 via the creeping assembly 40.
The pol~er takeof~ ~peed change ohaft 16 i~ con-n~ct~d by a couplin~S 56 to a po~er takeoff ~haft 57, the r~ na o~ which pro~ects out~rard from the rear ~all Or . .
~-: q ~055277 the tran~misaion case 5. A~ seen in Fig. 2, the ~ropelling secondary speed change shaft 50 is in parallel to the power tskeof~ shaft 57. An intermediate eha~t may be provided bet~een the shafta 15 and 50 ~ connected thereto with couplings. Si~ilarly the Jhaf~ 16 and 57 m~y be inter-connected by an intermediate ~hsft with use of couilings.
~ drive pinion shaft 58 i8 interpo~ed between the ~econdary speed chan~e shaft 50 and the power t~keoff - shaft 57 in a rear portion of the transmis3ion case 5.
? The shaft 58 is supported at its opposite ends by bearin~
59 and is parallel to the shafts 50, 57. The dri~e pinion . ~haft 58 i3 in the form of a hollow shaft haYing a bevel .. .. pinion 61 at its front end and an axial bore 63. ~he bevel - pinion 61 meshea with a bevel gear 60 of rear wheel differ-- 15 ential urit 62. : .: .....
- . The axial bore 63 of the drive pinlon shaft 58 accommodates the ~ame speed change means of the pin engageable ..
type as one already de~cribed. A slidable speed change rod . .64 having an annular pro~ection 65 is insertea into the

2~ a~ial bore 63 from its rear end. A secondary speed change ; leve.~ bar 66 turnable about its lateral axI9 fixedly carries a lever 67, the free end of which i8 engaged ~ith or pivoted to the rear end of the speed change rod 64. A cover 69 bolted to a bearing case 68 encloses the lever 67 and the like.
- ~he ~rive pinion shaft 58 is sheathed with a .-. d eevo 70 which is aplined or keyea thereto. ~no secondary - 8~eea change gear~ 71, 72 are freely rotatably mounted on the slesve 70. The eocondary speea dange gear 71 mesheo .
, 'lOSSZ77 .

~lth the gear 53 on the secondary ~peed change shaft 50, ~nd the secondary speed change gear 72 ~ith the gear 54 at all times. The sleeve 70 supports two other gears 73, 74. She front gear 73 is fixedlyv-mounted on the sleeve 70, whereas the rear gear 74 is freely rotatably mounted on the ~leeve 70. The gear 73 serves for rearward ~peed in corporation with an idle gear (not ~hown) provided between the thick gear 54 on the shaft 50 and the gear 74.
- ~he gear 73 serve~ to-tran~mit the torque for driving the front wheele.
Each of the secondary epeed change gear~ 71, 72 and rearwardly propelling gear 74 is formed in the inner peripheral surface ~ith grooves èquidlstantly spaced apart clrcumferentially thereof. The grooves hsve the same conatruction as those shown 1n Fig~. 3 and 4.
In corresponding relation to the grooves, the plnion shaft 58 and the sleeve 70 are fonmed with pin bores accommodating pins ~hich are pro~eotable and retractable and ~hich ~re biased radially inwardly of the shaft by coiled ~prings st all times. The grooves, pins and balls and like parts for the secondary speed change assembly hsve the same construction as those ~hoNn ln Figs. 3 and 4 in detail.
As illustrated in Figo. 5 and 6, the sleeve 70 can be diminatea from the secondary speed chP~ge a~sembly.
~ith reference to Fig. 2, a po~er takeoff assembly 75 for dri~1ng front ~heels i8 mountea on the bottom of t;he transml~sion ca~c 5. ~he po~er takeofr assombly 75 60mprisss a ca~e 76 detachably mounted on the bottom of th~ tranem~sion ca~e 5, a tranamitting ahaft 77 support2a by the case 76, B change gear 78 slidsbly mounted on the transmitting shsft 77 and an idle gear 80 freely rotstably mounted on a cyllnder 79 secured to an upper portion of the caee 76. The idle ~ear ôO meshes with the gear 73 ~ecured to the sleeve 70 at all time~. By the selective engagement of the change gear 78 with the idle gear 80, the transmitting shaft 77 i8 given a power in oynchronism ~ith the rear wheel driving system. The transmitting shaft 77 and the drive pinion shaft of a front wheel differ-1~ entisl unit is interconnected by a propeller shaft 82 bymeans of coupling 81 and the like, whereby the front wheels can be driven. As will be apparent from Fig. 2, the power ta~eoff shaft 57 extends through the cylinder 79 eccentrically there~ith. The space between the power takeoff shaft 57 ana the cylinder 79 resulting from the eccentric arrangement permits changes in the number of the teeth of the idle gear 80 ana afiords freedom of design although the distances - betveen the drive pinion shaft 58, power takeoff shaft 57 snd the transm1tting shaft 77 are not variable. Preferably, the propeller shaft 82 is covered uith a tubular coveri~g 83 and thereby protected from ~ater and mud.
According to this invention, the engine po~er i8 transmitted in the folloning manner. The torque of the engine 4 i~ delivered at ~11 times from the flywheel 7 to the input main shaft 8 which i9 connected to the fly~heel 7 ~y the fle~ible ~hock-~bsorbing coupling 11.
2he group of gearo 9 on the ~ain shaft 8 are al~ays in mesh nith tha propelling speea change gearg 21, 22~ 2~ and the power takeoff epeed chsnge gears 24, 25, 26 which are freely rotatable on the speea change shafts 15, 16. Accordingly the po~er wlll not be delivered to . the propslling syatem or the power takeoff system, unless the pin~ 36 biased by the coiled springs 33 radially inwardly S Of the shaft are forced outward against the springa and thereby engaged in the grooves in one of the speed change eears. Uhen the lever 33 is turned, slidingly ~oving the speed change rod 27 within the speed change shaft 15, the annular projection 29 projects the pins 36, opposing for e~ample the speed change gear 21, against the springs 38 and engages them into the groo~es 39 formed in the inne~
peripheral surface of the gear 21. The gear 21 and the speed chsnge shaft 15 are therefore coupled by the pins - 36 for the transmission of the torque. With ~he illustrated embodiment wherein the three speed change gesrs 21, 22, 23 sre mounted side by side on the speed change shaft 15, t~e three steps of speed change~ of first speed ~eutr~l ~ second speed ----~ neutral ~ thlrd speed are available merely by sxially moving the single speed change rod 27. Such speed chsnges are effected smoothly nith a smRll shock ~ithout the necessity of slowing down the engine 4, since the grooves 39 ha~e a progressively re~ucing depth in the direction of rotstion of the gears as ~llustrated in Fig. 4. The speed change mechanism is further operable smoothly by virtue of the pro~ision of tha rollable balls 37 for the pins 36. The shock-absorbing coupling 11 effectively mitigates the slight shock produced by the speea change action, eli~insti~g the necs~sity of ~roviding a clutch between the engine 4 and the main shsft 8.
, lOSSZ77 Similarly a8 above, the power takeof~ speed ohenge rod 28, ~hen slidingly moved in the axial bore 16a of the shaft 16, cause~ the pins ~6 to selectively couple the speed change gears 24, 25, 26 to the speed change shaft 16, giving three steps of poNer takeoff speed changes to deliver the desirea torque to the po~er takeoff shaft 57.
~hen the clutch gear 55 i8 moved right~ard in Fig. 2, the torque is delivered to the ~econdary speed ohange shaft 50 at a greatly reduced speed via the creeping asgembly 40 interpoged between the speed chsnge shaft 15 and the aecondary speed change shaft 50. Conversely when the ¢lutch gear 55 is moved leftward in ~ig. 2, the speed change shaft 15 is connected directly to the secondary speed ohange shaft 50. Wit~ the lllustrated embodiment wherein the planetary roller 43 compriees the inner and outer metal rings 45, 44 ~hich are joined together by the elastic .
member 46, the desired torque can be delivered, whereas e~¢ees torque is absorbed by the defonmation of the elastic member 46 and slippage of the roller 4~, with the result the shafts, gears and other parts are rendered durable and free Or serious damage.
When the torque is tr~n~mitted to the secondary speed change shaft 50 directly or by way of the creeping assembly ~0, high and low secondary upeed changes are e ffected in the following manner, penmitting the torgue to be deliverea from the drive pinion shaft 58 to the ~ifferentisl unit 62, from ~hich the torgue is fhrther tran~mlttea to the terminal apeed reduction means, driving . ~h~ rear wheels 3. ~he ~econdary speed change shaft 50 --I

lOSSZ77 flxedly carriea the large cna small gesra 53, 54 al~ayu m~ehing ~ith high and low seconaary ~peed change gears 71, 72 ~hich are freely rotatably mounted on the drive p~nion shaft 58 directly or with the slee~e 70 interposea there-between. The drive p~nion shaft 58 incorporates, incorresponaing relation to the gears 71, 72, pins 36 ~hich are biasea by the epringQ 38 radially inwardly of the shaft 58. The pins 36 are projectsble radially outwardly of the shaft 58 by tha annular pro~ection 65 when the secondary speed change rod 64 i~ move~. Thus when the sliding movement of the rod 64 causes the pins 35 to engage in the groove 39 of the gear 71 or the pins 36 to engage in the groove ~9 of the gear 72, the gear 71 or 72 is coupled to the - drive pinion shaft 58, whereby the torque i~ delivered to the differential unit 62 with a high or low secondary epaed change.
The rearn~rdly propelling gear 74 idly rotatably mounted on the drive pinion shaft 58 i~ operatively coupled to the gear 54 by means of an idle gear (not ~hown).
~hen the gesr 74 is coupled to the drive pinion shaft 58 by the sllding movement of the secondary speed change rod 64 by virtue of the engagement of the pins 36 in the grooves 39, the rear wheels 3 are d;riven reversely.
Yurther when the change gear 78 is selectively engagea ~ith the idle gear ôO, the transmitting ~haft 77 is driven ln ~ynchronism ~ith the rear wheel system.

.
S

.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A transmission of the three-system type for a tractor comprising an input system having a main input shaft carrying a plurality of gears for transmitting the torque of an engine, and a propelling speed change system and a power takeoff speed change system supported on the opposite sides of the input system respectively in parallel thereto, each of the propelling and power takeoff speed change systems includ-ing a speed change shaft having a power output end and having a plurality of speed change gears freely rotatably mounted there-on side by side and meshing with the gears of the input system respectively at all times, the speed change shaft being a hollow shaft internally formed with an axial bore, each of the speed change gears being formed in its inner peripheral surface with grooves, the speed change shaft incorporating pins movable radially of the speed change shaft and engageable in and disengageable from the grooves in the corresponding speed change shaft and engageable and disengageable from the grooves in the corresponding speed change gear, the speed change shaft accommodating in its axial bore an axially movable speed change rod being axially movable to engage the pins in the grooves of the corresponding speed change gear and said speed change rod partially extending outwardly from said speed change shaft and operating force transmitting means provided at an end opposite to said power output end coupled to said partially extending portion of said speed change rod for axially moving said speed change rod, whereby the speed change gear is coupled to the speed change shaft to transmit the torque to rear wheels and a power takeoff shaft with desired speed changes.

2. A transmission as defined in claim 1 wherein the pins engageable in each of the speed change gears are equidistantly spaced apart circumferentially of the speed change shaft and spring-biased radially inwardly of the shaft at all times, and the grooves formed in each of the speed change gears are equidistantly spaced apart circumferentially of the gear, the speed change gears being freely rotatably mounted on a sleeve fixedly sheathing each of the speed change shafts.

3. A transmission as defined in claim 1 wherein each of the grooves formed in the inner peripheral surface of each of the speed change gears had a reducing depth in the direction of rotation of the gear.

4. A transmission as defined in claim 1 further including a flexible shock-absorbing coupling for transmitting the power of an engine to said main input shaft, a power take-off shaft connected to the power takeoff speed change shaft, a creeping assembly provided between the speed change shaft and a secondary speed change shaft, a drive pinion shaft freely rotatably carrying a rearwardly propelling gear and high and low secondary speed change gears always meshing with large and small gears on the secondary speed change shaft respectively to transmit the power to a differential unit, and secondary speed change means of the pin engageable type incorporated in the drive pinion shaft coaxially therewith.

5. A transmission as defined in claim 4 wherein the creeping assembly is of the planetary roller type com-prising planetary rollers, each of the rollers including inner and outer metal rings joined together by an elastic member at least dimetrically deformable.

6. A transmission of the three-system type for a tractor comprising an input system having a main input shaft carrying a plurality of gears for transmitting the torque of an engine, a propelling speed change system and a power takeoff speed change system supported on the opposite sides of the input system respectively in parallel thereto, each of the propelling and power takeoff speed change system includ-ing a speed change shaft having a power output end and having a plurality of speed change gears freely rotatably mounted there-on side by side and meshing with the gears of the input system respectively at all times, the speed change shaft being a hollow shaft internally formed with an axial bore, each of the speed change gears being formed in its inner peripheral surface with grooves, the speed change shaft incorporating pins movable radially of the speed change shaft and engageable in and disen-gageable from the grooves in the corresponding speed change

claim 6 continued.

shaft and engageable and disengageable from the grooves in the corresponding speed change gear, the speed change shaft accommodating in its axial bore an axially movable speed change rod being axially movable to engage the pins in the grooves of the corresponding speed change gear and said speed change rod partially extending outwardly from said speed change shaft and operating force transmitting means provided at an end opposite to said power output end coupled to said partially extending portion of said speed change rod for axially moving said speed change rod, whereby the speed change gear is coupled to the speed change shaft to transmit the torque to rear wheels and a power takeoff shaft with desired speed changes, and a flexible shock-absorbing coupling provided between the engine and the main input shaft, said coupling mediating the impact resulting from engagement of the pins in the grooves for a speed change.