Speed change operation apparatus for stepless speed changer device

A speed change operation apparatus includes a pivotal body operatively connected to a trunnion shaft to be pivotally displaced in response to an operational displacement by an operational tool, a positioning body operatively coupled with the pivotal body so as to be pivotally displaced in response to a pivotal displacement of the pivotal body, a main urging mechanism for applying an urging force to the pivotal body via the positioning body for returning the pivotal body from an operational position to the neutral position, and an auxiliary urging mechanism for applying an auxiliary urging force to the pivotal body for returning the pivotal body from the operational position to the neutral position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a speed change operation apparatus for a stepless speed changer device, configured to increase a rotational output in response to increase in rotational displacement of a trunnion shaft from a neutral position to an operational position.

2. Description of the Related Art

According to one type of speed change operation apparatus, the apparatus includes a pivotal body operably coupled to a trunnion shaft, a positioning body supported to be pivotable about a positioning axis parallel with a pivot axis of the pivotal body, and a cam mechanism for positioning the trunnion shaft at the neutral position. At the time of positioning, into a recessed portion of a cam (a constituent element of a cam mechanism) formed in one of the pivotal body and the positioning body, a cam follower (another constituent element of the cam mechanism) is engaged. And, the positioning body is pivotally urged by a spring so as to bring the cam and the cam follower into contact with other.

For instance, a speed change operation apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2009-48331 includes a first member as a pivotal body operably coupled to a trunnion shaft, a second member as a positioning body and a spring and an auxiliary spring that pivotally urge the second member. The spring is connected to the second member and pivots the second member with an urging force that progressively increases as the trunnion shaft reaches the higher speed side. The auxiliary spring is connected to a rod having one end thereof engaged in a cam hole formed in the second member. The rod is supported to be pivotable with the portion thereof opposite to the side engaged in the cam hole acting as the pivot. When the trunnion shaft is located at a neutral position or an area adjacent the neutral position, one terminal end of the rod contacts an inclined portion of the cam hole by the urging force of the auxiliary spring, thereby to press the second member toward the first member. When the trunnion shaft is located at a distant area more distant from the neutral position than the adjacent area, the one terminal end of the rod is pressed against an arcuate portion of the cam hole by the urging force of the auxiliary spring. That is, while the trunnion shaft is located in the adjacent area, the urging force of the auxiliary spring urges the trunnion shaft to the neutral position. Whereas, while the trunnion shaft is located at the distant area, the urging force of the auxiliary spring does not urge the trunnion shaft to the neutral position. With this speed change operation apparatus, even with appropriate setting of the urging force applied to the positioning member when the trunnion shaft is located at the neutral position and the urging force applied to the positioning member when the trunnion shaft is located at a highest speed position, the urging force of the spring becomes weaker as the trunnion shaft is located closer to the neutral position. Further, the urging force of the auxiliary spring is applied to the positioning member for only a portion in the operational range of the trunnion shaft adjacent the neutral. Because of these two respects, in the course of an operation of the trunnion shaft from one of the neutral position and the highest speed position to the other, there occurs change in the pressing force applied from the positioning member to the pivotal body, thus tending to invite change in the rotational resistance of the trunnion shaft. As a result, there arises a problem of variation occurring in the operational resistance.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a speed change operation apparatus for a stepless speed changer device that is capable of effecting, with high precision, urging of the trunnion shaft to the neutral position and that provides good speed change operation performance.

For accomplishing the above-noted object, a speed change operation apparatus according to the present invention comprises:

an operational tool whose operational displacement causes rotational displacement of a trunnion shaft between a neutral position and an operational position;

a pivotal body operably coupled with the trunnion shaft to be pivotally displaced in response to an operational displacement of said operational tool;

a positioning body operably coupled with said pivotal body to be pivotally displaced in response to pivotal displacement of said pivotal body;

a main urging mechanism for providing an urging force to said pivotal body via said positioning body, said urging force returning said pivotal body from the operational position to the neutral position; and

an auxiliary urging mechanism for providing an auxiliary urging force to said pivotal body, said auxiliary urging force returning said pivotal body from the operational position to the neutral position;

wherein a returning (neutral maintaining) urging force provided by the main urging mechanism at the neutral position of the trunnion shaft is stronger than a returning urging force provided by the main urging mechanism at the operational position of the trunnion shaft; and

wherein an auxiliary urging force provided by the auxiliary urging mechanism at the neutral position of the trunnion shaft is weaker than an auxiliary urging force provided by the auxiliary urging mechanism at the operational position of the trunnion shaft.

With the above construction, with optimization of application of force to the pivotal body through cooperative action of the main urging mechanism and the auxiliary urging mechanism, it becomes possible e.g.:

(1) to decrease variation in the operational force through the entire operational range of the stepless speed changer device;

(2) to progressively increase the operational force as the stepless speed changer device is operated toward the maximum speed side.

Further, with an arrangement for allowing ready adjustment of the auxiliary urging force of the auxiliary adjusting mechanism from outside the apparatus, it becomes possible to optimize the operational force of the operational tool as desired to suit each operator's preference.

According to another speed change operation apparatus of the present invention, the apparatus comprises:

a pivotal body operably coupled with the trunnion shaft;

a positioning body supported to be pivotable about a positioning axis extending parallel with a pivotal axis of the pivotal body;

a cam mechanism for positioning the trunnion shaft at the neutral position through engagement of a recessed portion of a cam and a cam follower, said cam being formed in one of the pivotal body and the positioning body, said cam follower being formed in the other of the pivotal body and the positioning body;

first and second springs that pivotally urge the positioning body in a pivotal direction for bringing the cam follower and the cam into contact with each other;

wherein said cam is configured to vary a rotational resistance of the trunnion shaft over the entire operational range of the trunnion shaft in such a manner that the rotational resistance becomes maximum when the cam follower disengages from the recessed portion and the rotational resistance progressively decreases as the cam follower moves away from the recessed portion after its disengagement therefrom; and

wherein said second spring and said positioning body are operably coupled with each other to vary a pressing force applied from the positioning body to the pivotal body through pivotal urging of the positioning body by the second spring in such a manner that the pressing force becomes minimum when the trunnion shaft is located at the neutral position and the pressing force progressively increases as the trunnion shaft approaches a highest speed position.

With the above-described construction, in response to pivotal urging of the positioning body by the first spring, the positioning body is pressed against the pivotal body to bring the cam and the cam follower into contact with each other; and also, in response to pivotal urging of the positioning body by the second spring, the positioning body is pressed against the pivotal body to bring the cam and the cam follower into contact with each other, whereby the trunnion shaft is urged toward the neutral position. And, the pivotal urging of the positioning body by the first spring varies a rotational resistance of the trunnion shaft over the entire operational range of the trunnion shaft in such a manner that the rotational resistance becomes maximum when the cam follower disengages from the recessed portion and the rotational resistance progressively decreases as the cam follower moves away from the recessed portion after its disengagement therefrom. On the other hand, the pivotal urging of the positioning body by the second spring varies a pressing force applied from the positioning body to the pivotal body in such a manner that the pressing force becomes minimum when the trunnion shaft is located at the neutral position and the pressing force progressively increases as the trunnion shaft approaches a highest speed position. Therefore, it is possible to cause the rotational resistance that occurs due to the pivotal urging of the positioning body by the first spring and the pivotal urging of the positioning body by the second spring not to change at all or not to change much. Hence, even when an arrangement is made to allow neutral urging force of an appropriate magnitude to be applied to the trunnion shaft, whether the trunnion shaft is operated to the neutral position, the highest speed position or any operational position in its operational range, it is possible to operate the trunnion shaft through the entire operational range thereof, without significant increase or decrease in the operational resistance received due to the neutral urging of the trunnion shaft between the neutral position and the highest speed position.

Therefore, the trunnion shaft can be returned to the neutral position with an urging force of appropriate strength and can be maintained at the neutral position in a stable manner. At the same time, superior operational performance for providing smooth speed change operations can be realized with no or less change in the operational resistance over the entire operational range of the trunnion shaft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a side view showing, in its entirety, a tractor equipped with an HST10to which the speed change operation apparatus relating to the present invention is applied. The tractor includes a pair of front wheels1,1as steering wheels and driving wheels, a pair of right and left rear wheels2,2as driving wheels, an engine section3provided at a front portion of the vehicle body supported by the front and rear wheels, and a riding type driving section4provided at a rear portion of the vehicle body. To a rear portion of a vehicle body frame5of this tractor, there is attached a link mechanism6having a pair of right and left lift arms6apivotally liftable up/down, and at a rear portion of the vehicle body frame5, there is mounted a power takeoff (PTO) shaft9.

To this tractor, at its rear portion of the vehicle body for instance, a rotary cultivator unit (not shown) is connected via the link mechanism6to be lifted up/down. Further, a power is transmitted via the PTO shaft9to the rotary cultivator unit. With these, the tractor is constituted as a riding type cultivator, or various kinds of utility implements can be connected thereto to be liftable and drivable, thus constituting various kinds of riding type work machines.

The vehicle body frame5includes the engine section3having an engine3a, an intermediate case7having a front end thereof connected to the rear portion of the engine3a, and a transmission case8having a front end thereof connected to the rear end portion of the intermediate case7.

As shown inFIG. 1andFIG. 2, to the front portion of the transmission case8, the HST10is mounted as being disposed inside the intermediate case7. The HST10is configured to convert drive force transmitted from the engine3avia a transmission shaft (not shown) mounted in the intermediate case7into a forward travel drive force and a reverse travel drive force and to transmit these to a transmission unit (not shown) mounted within the transmission case8.

As shown inFIG. 1, in the driving section4, a forward travel pedal20aand a reverse travel pedal20bare disposed laterally outside of the intermediate case7. As shown inFIG. 2, the forward travel pedal20aand the reverse travel pedal20bare pivoted to the intermediate case7, via a support shaft22on which a boss portion21provided at the base portions of the forward travel pedal20aand the reverse travel pedal20bare engaged and are operably coupled to a trunnion shaft11(seeFIG. 3) of the HST10via a coupling mechanism26having a pivotal arm25provided on the boss portion21to be rotatable therewith.

Therefore, when the forward travel pedal20ais stepped on about the axis of the support shaft22, the HST10is speed-changed to the forward travel state to transmit the forward travel driving force to the front wheels1and the rear wheels2, whereby the self-propelled vehicle body travels forward. When the reverse travel pedal20bis stepped on about the axis of the support shaft22, the HST10is speed-changed to the reverse travel state to transmit the reverse travel driving force to the front wheels1and the rear wheels2, whereby the self-propelled vehicle body travels in reverse.

As shown inFIG. 2, the boss portion21of the forward travel pedal20aand the reverse travel pedal20bis provided as a common boss portion shared by the forward travel pedal20aand the reverse travel pedal20b, thus operably coupling the forward travel pedal20aand the reverse travel pedal20bwith each other. As shown inFIG. 2,FIG. 3andFIG. 6, the coupling mechanism26for coupling the forward travel pedal20aand the reverse travel pedal20bwith the trunnion shaft11of the HST10includes the pivotal arm25, a pivotal body31operably coupled to the trunnion shaft11, and a coupling rod27having one end thereof pivotally connected to the free end of the pivotal body31via a coupling shaft27aand having the other end thereof pivotally connected to the free end of the pivotal arm25. The coupling rod27is disposed outside the intermediate case7. The coupling shaft27acoupling the coupling rod27with the pivotal body31extends through an arcuate through hole7aprovided in the intermediate case7and centered about the rotational axis of the trunnion shaft11. At the terminal end of the coupling rod27connected to the pivotal body31, there is provided an anti-vibration rubber28for restraining transmission of vibration from the trunnion shaft11to the forward travel pedal20aand the reverse travel pedal20b. To the pivotal arm25, there is connected a dumper29for preventing too-rapid returning of the forward travel pedal20aand the reverse travel pedal20bto the neutral positions.

As shown inFIG. 3, the core component of the speed change operation apparatus for operating the HST10is the neutral urging mechanism30incorporating the pivotal body31. The neutral urging mechanism30urges the trunnion shaft11to return it to the neutral position [N] when the HST10is speed-changed to the forward travel state, i.e. when the trunnion shaft11is operated to an operational position offset toward one side (forward travel side) in its operational range A relative to the neutral position [N]. Whereas, when the HST10is operated to the neutral state, the neutral urging mechanism30fixes the trunnion shaft11at the neutral position [N] so that the shaft11is not to be displaced from this neutral position [N].

Next, the neutral urging mechanism30will be described in greater details.

As shown inFIG. 3andFIG. 5, the neutral urging mechanism30includes, in addition to the pivotal body31, a positioning body32disposed on the opposite side to the side of the trunnion shaft11relative to the pivotal body31, a first spring33disposed upwardly of the pivotal body31and operably coupled to the positioning body32, a second spring34disposed forwardly of the pivotal body31and operably coupled to the positioning body32, and a cam mechanism35provided between and across the positioning body32and the pivotal body31.

The pivotal body31is connected to the trunnion shaft11via a connecting portion31bprovided at the base of this pivotal body31, so that the pivotal body31is pivoted in operative connection with the trunnion shaft11with a rotational axis of the trunnion shaft11acting as a pivotal axis31c. The connecting portion31bof the pivotal body31is connected and fastened to an end of the trunnion shaft11by means of a fastening bolt31d.

As shown inFIG. 3andFIG. 6, the positioning body32is supported to the supporting member37via a coupling body36(seeFIG. 7) having its support portion36arotatably engaged within a circular attaching hole32a(seeFIG. 7) provided at the base of the positioning body32, and the positioning body32pivots about a positioning axis32bextending through the center of the attaching hole32aand parallel with the pivotal axis31cof the pivotal body31. The supporting member37includes a connecting portion37a(seeFIG. 7) formed by a bent end portion provided at the rear end of the supporting member37and the supporting member37is attached via this connecting portion37ato the case of the HST10.

As shown inFIG. 3, the cam mechanism35includes a cam38formed in an end face of the pivotal body31and a roller-shaped cam follower39formed in the positioning body32by attaching a bearing via a support shaft39a. The cam38is configured such that the cam follower39is slid in association with a pivotal movement of the pivotal body31. The cam38includes a pair of inclined cam faces38b,38bhaving a gentle slope angle provided in distribution to one end and the other end of the cam follower39in the sliding direction and a recessed portion38adisposed between the pair of inclined cam faces38b,38band recessed toward the side where the pivotal axis31cis located.

As shown inFIG. 9,FIG. 10andFIG. 11, the forward travel side and reverse travel side inclined cam faces38bare formed with such inclinations that a distance D from a portion of the inclined cam face38bcontacting or tangent to the cam follower39to the pivotal axis31cof the pivotal body31progressively increases as the cam follower39moves from the neutral position [N] to the highest speed position [Fmax], [Rmax] and also the distance becomes maximum when the cam follower39reaches the highest speed position [Fmax], [Rmax]. Further, the inclined cam faces are formed as a shape downwardly inclined toward the recessed portion38arelative to the normal at the portion contacted by the cam follower39at the highest speed position [Fmax], [Rmax], or any other speed position.

As the cam follower39of the positioning body32engages into the recessed portion38aof the cam38, the cam mechanism35fixedly positions the trunnion shaft11at the neutral position [N] by fixedly positioning the pivotal body31at its pivotal position corresponding to the neutral position [N] of the trunnion shaft11.

One end of the first spring33is supported to a spring supporting portion40provided in the case of the HST10. A hook33aprovided at the other end of the first spring33is engaged with an engaging portion provided at a free end32cof the positioning body32and the other end of the first spring33is connected to the free end32cof the positioning body32, and the first spring33pivotally urges the positioning body32about the positioning axis32bso as to bring the cam follower39and the cam38into contact with each other.

As shown inFIG. 3andFIG. 6, one end of the second spring34is supported to a spring supporting portion37cwith providing a spring adjusting screw to a lower portion37bof the supporting member37. The other end of the second spring34is connected to the positioning body32with connecting means46having a driven cam45formed in the positioning body32.

The connecting means46includes, in addition to the driven cam45, an urging body48(seeFIG. 7) having, at its free end, a bent portion48a(“free end portion48a” hereinafter) engaged into an elongate-hole like supporting hole47provided in the positioning body32to form a cam face for the driven cam45.

The driven cam45is constituted from an inner wall of the supporting hole47. The urging body48is attached to the supporting hole47of the positioning body and an attaching hole49(seeFIG. 9) provided in the supporting member37and the urging body48pivots with the free end48asliding along the driven cam45, about a pivotal axis49awhich is the axis extending through the center of the attaching hole49. The urging body48has its free end48aconnected to the second spring34so that the urging body48is pivotally urged about the pivot axis49aby the second spring34. By the urging force of the second spring34, the free end48ais pressed against the driven cam45, and the positioning body32is pivotally urged about the positioning axis32bso as to bring the cam follower39and the cam38into contact with each other.

Therefore, with the neutral urging mechanism30in operation, the pivotal body31is caused to provide the pressing action with the pivotal urging of the positioning body32by the first spring32and the pivotal urging via the connecting means46by the second spring34. And, when the trunnion shaft11is rotatably operated, the cam follower39is caused to slide relative to the cam38to come into engagement into the recessed portion38aor to disengage from the recessed portion38ato be positioned at the forward travel side or reverse travel side inclined cam face38b, thereby to fixedly position the trunnion shaft11at the neutral position [N] or to return it to the neutral position [N].

That is,FIG. 3is a side view showing the neutral urging mechanism30under the condition where the trunnion shaft11is operated to the neutral position [N]. As shown in this figure, with the neutral urging mechanism30in operation, when the trunnion shaft11is operated to the neutral position [N], the pivotal body31pivots in operative association with the trunnion shaft11, so that the cam follower39of the positioning body32and the recessed portion38aof the cam38are brought into opposition to each other, and with the pivotal urging by the first spring33and the second spring34, the positioning body32urges the cam follower39into engagement in the recessed portion38a, thereby to fixedly position the trunnion shaft11at the neutral position [N].

FIG. 4is a side view showing the neutral urging mechanism30under the condition when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax]. As shown in this figure, with the neutral urging mechanism30in operation, when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax], the pivotal body31pivots in operative association with the trunnion shaft11, so that the cam follower39of the positioning body32disengages from the recessed portion38aof the cam38and comes into opposition to the forward travel side inclined cam face38band the positioning body32, as being pivotally urged by the first spring33and the second spring34, presses the cam follower39against the forward travel side inclined cam face38b, thus urging the trunnion shaft11to be returned to the neutral position [N]. Though not shown, when the trunnion shaft11is operated to an operational position of a speed before the forward travel side highest speed position [Fmax], like the case of the trunnion shaft11being operated to the highest speed position [Fmax], the neutral urging mechanism30operates such that the positioning body32presses the cam follower39against the forward travel side inclined cam face38b, thus urging the trunnion shaft11to be returned to the neutral position [N].

FIG. 5is a side view showing the neutral urging mechanism30under the condition when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax]. As shown in this figure, with the neutral urging mechanism30in operation, when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax], the pivotal body31pivots in operative association with the trunnion shaft11, so that cam follower39of the positioning body32disengages from the recessed portion38aof the cam38and comes into opposition to the reverse travel side inclined cam face38band the positioning body32, as being pivotally urged by the first spring33and the second spring34, presses the cam follower39against the reverse travel side inclined cam face38b, thus urging the trunnion shaft11to be returned to the neutral position [N]. Though not shown, when the trunnion shaft11is operated to an operational position of a speed before the reverse travel side highest speed position [Rmax], like the case of the trunnion shaft11being operated to the highest speed position [Rmax], the neutral urging mechanism30operates such that the positioning body32presses the cam follower39against the reverse travel side inclined cam face38b, thus urging the trunnion shaft11to be returned to the neutral position [N].

FIG. 8is an explanatory view showing an operational state of the neutral urging mechanism30when the cam follower39disengages from the recessed portion38aof the cam38toward the forward travel side. As shown in this figure, when the cam follower39disengages from the recessed portion38aof the cam38toward the forward travel side, the cam follower39comes into contact with the forward travel side entrance/exit of the recessed portion38aof the cam38, so that a pressing force F1of the first spring33is applied to a position P1located at the forward travel side entrance/exit of the recessed portion38aand having an arm length L1from the pivotal axis31cof the pivotal body31, whereby the trunnion shaft11is subjected to a pressing moment M1=arm length L1×pressing force F1, about the pivotal axis31c, as a rotational resistance due to the pivotal urging of the positioning body32by the first spring33.

When the cam follower39disengages from the recessed portion38aof the cam38toward the reverse travel side, the cam follower39comes into contact with the reverse travel side entrance/exit of the recessed portion38aof the cam38. In this, the portion of the cam38contacted by the cam follower39differs from that when the cam follower39disengages from the recessed portion38atoward the forward travel side. As the shape of the recessed portion38ais same on the side of the forward travel side entrance/exit and on the side of the reverse travel side entrance/exit, when the cam follower39disengages from the recessed portion38atoward the reverse travel side, the trunnion shaft11receives a pressing moment M1about the pivotal axis31c, as a rotational resistance of substantially same strength as that the shaft11receives when the cam follower39disengages from the recessed portion38atoward the forward travel side.

FIG. 9is an explanatory view showing the operational condition of the neutral urging mechanism30when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax]. As shown in this figure, when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax], the cam follower39comes into contact with an end portion of the forward travel side inclined cam face38b, so that a pressing force F2of the first spring33is applied to a position P2located at the forward travel side entrance/exit of the recessed portion38aand having an arm length L2from the pivotal axis31cof the pivotal body31, whereby the trunnion shaft11is subjected to a pressing moment M2=arm length L2×pressing force F2, about the pivotal axis31c, as a rotational resistance due to the pivotal urging of the positioning body32by the first spring33.

FIG. 10is an explanatory view showing the operational condition of the neutral urging mechanism30when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax]. As shown in this figure, when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax], the cam follower39comes into contact with an end portion of the reverse travel side inclined cam face38b, so that a pressing force F3of the first spring33is applied to a position P3located at the reverse travel side entrance/exit of the recessed portion38aand having an arm length L3from the pivotal axis31cof the pivotal body31, whereby the trunnion shaft11is subjected to a pressing moment M3=arm length L3×pressing force F3, about the pivotal axis31c, as a rotational resistance due to the pivotal urging of the positioning body32by the first spring33.

The extension lengths of the first spring33when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the reverse travel highest speed position [Rmax] are greater than the extension length of the first spring33when the cam follower39disengages from the recessed portion38a. However, the arm length L2when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the arm length L3when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax] are significantly shorter than the arm length L1when the cam follower39disengages from the recessed portion38a. And, the pressing moment M2as the rotational resistance received by the trunnion shaft11when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the pressing moment M3as the rotational resistance received by the trunnion shaft11when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax] are smaller than the pressing moment M1as the rotational resistance received by the trunnion shaft11when the cam follower39disengages from the recessed portion38a.

The extension lengths of the first spring33when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the extension lengths of the first spring33when the trunnion shaft11is operated to the reverse travel highest speed position [Rmax] are substantially same. The arm length L2when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the arm length L3when the trunnion shaft11is operated to the reverse travel highest speed position [Rmax] are substantially same.

Therefore, as the trunnion shaft11is rotatably operation, the rotational resistance of the trunnion shaft11that occurs in association with the pivotal urging of the positioning body32by the first spring33varies over the entire pivotal range A of the trunnion shaft11, due to the effect of the cam38that is rotated in operative connection with the trunnion shaft11. Hence, whether the trunnion shaft11is rotatably operated from the neutral position [N] to the forward travel side or the reverse travel side, the rotational resistance becomes maximum when the cam follower39disengages from the recessed portion38aand becomes smaller as the cam follower39moves away from the recessed portion38aafter its disengagement therefrom.

FIG. 11is an explanatory view showing the operational condition of the neutral urging mechanism30when the trunnion shaft11is operated to the neutral position [N]. As shown in this figure, when the trunnion shaft11is operated to the neutral position [N], the free end48aof the urging body48is located at a position P4having an arm length L4from the positioning axis32bof the positioning body32, so that the urging body48applies an operational force F4of the second spring34to the position P4of the positioning body32. This operational force F4of the second spring34is divided into a component force F4afor pivotally urging the positioning body32about the positioning axis32band a component force F4bthat does not affect the pivotal urging of the positioning body32. Therefore, under the condition of the trunnion shaft11being located at the neutral position [N], the positioning body32has a pressing moment M4=arm length L4×component force F4a, about the positioning axis32b, as a pressing force for pressing the pivotal body31due to the pivotal urging by the second spring34.

As shown inFIG. 9, when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax], the free end48aof the urging body48is located at a position P5having an arm length L5from the positioning axis32bof the positioning body32, so that the urging body48applies an operational force F5of the second spring34to the position P5of the positioning body32. This operational force F5of the second spring34is divided into a component force F5afor pivotally urging the positioning body32about the positioning axis32band a component force F5bthat does not affect the pivotal urging of the positioning body32. Therefore, under the condition of the trunnion shaft11being located at the neutral position [N], the positioning body32has a pressing moment M5=arm length L5×component force F5a, about the positioning axis32b, as a pressing force for pressing the pivotal body31due to the pivotal urging by the second spring34.

As shown inFIG. 10, when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax], the free end48aof the urging body48is located at a position P6having an arm length L6from the positioning axis32bof the positioning body32, so that the urging body48applies an operational force F6of the second spring34to the position P6of the positioning body32. This operational force F6of the second spring34is divided into a component force F6afor pivotally urging the positioning body32about the positioning axis32band a component force F6bthat does not affect the pivotal urging of the positioning body32. Therefore, under the condition of the trunnion shaft11being located at the neutral position [N], the positioning body32has a pressing moment M6=arm length L6×component force F6a, about the positioning axis32b, as a pressing force for pressing the pivotal body31due to the pivotal urging by the second spring34.

When the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the reverse travel side highest speed position [Rmax], the free end48aof the urging body48is located at positions having greater distances from the spring supporting portion37cthan the case of the trunnion shaft11being operated to the neutral position [N], so that the second spring34is extended greater than the case of the trunnion shaft11being operated to the neutral position [N], and the operational forces F5, F6of the second spring34in the cases of the trunnion shaft11being operated to the forward travel side highest speed position [Fmax] and the reverse travel side highest speed position [Rmax] are greater than the operational force F4in the case of the trunnion shaft11being operated to the neutral position [N]. Further, the arm lengths L5, L6when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the reverse travel side highest speed position [Rmax] are greater than the arm length L4in the case of the trunnion shaft11being operated to the neutral position [N]. And, the pressing moment M5present in the positioning body32when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the pressing moment M6present in the positioning body32when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax] are greater than the pressing moment M4present in the case of the trunnion shaft11being operated to the neutral position [N].

The free end48aof the urging body48is located at substantially same position of the positioning body32when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax]; and the pressing moment M5present in the positioning body32when the trunnion shaft11is operated to the forward travel side highest speed position [Fmax] and the pressing moment M6present in the positioning body32when the trunnion shaft11is operated to the reverse travel side highest speed position [Rmax] are pressing moments of substantially same strength.

Therefore, due to the function of the connecting means46, the pressing force applied by the positioning body32to press the pivotal body31with the pivotal urging of the positioning body32by the second spring34is minimum under the condition when the trunnion shaft11is located at the neutral position [N], and the pressing force becomes greater as the trunnion shaft11moves closer to the forward travel side highest speed position [Fmax] and the reverse travel side highest speed position [Rmax], thus, the pressing force is varied over the entire operational range A of the trunnion shaft11.

FIG. 12is an explanatory view showing relationship among the pressing forces (the pressing forces F1, F2, F3applied by the first spring33) applied from the positioning body32to the pivotal body31with the pivotal urging by the first spring33, rotational resistances M1, M2, M3of the trunnion shaft11that occur in association with the pivotal urging of the positioning body32by the first spring33, the component forces F4a, F5a, F6aof the operational forces of the second spring34for pivotally urging the positioning body32, the pressing forces present in the positioning body32to pressing the pivotal body31with the pivotal urging by the second spring34(the pressing forces M4, M5, M6of the positioning body32) and speed change operation resistances received from the first spring33and the second spring34in the course of speed change operations by the forward travel pedal20aand the reverse travel pedal20b. In this figure, the magnitudes or strengths of the respective component forces, the respective rotational resistances and the respective speed change operation resistances are indicated in the form of ratios relative to the strength of the component force F5aof the operational force of the second spring34applied to the positioning body32as a reference when the trunnion shaft11is operated to the neutral position [N]. It should be noted; however, that the strength of the component force F5aof the operational force of the second spring34applied to the positioning body32in the case of the trunnion shaft11being operated to the neutral position [N] is assumed to have a value of 0.75.

As shown in this figure, with the neutral urging mechanism30in operation, the positioning body32is pivotally urged by the first spring33and the second spring34thereby to bring the cam follower39and the cam38into contact with each other and the rotational resistance generated in the trunnion shaft11due to the pivotal urging of the positioning body32by the first spring33is varied by the function of the cam38over the entire operational range A of the trunnion shaft11such that the resistance becomes maximum when the trunnion shaft11moves away from the neutral position [N] and the resistance becomes smaller as the trunnion shaft11approaches the forward travel side highest speed position [Fmax] and the reverse travel side highest speed position [Rmax] after its departure from the neutral position [N]. And, the pressing force present in the positioning body32to press the pivotal body31by the pivotal urging by the second spring34is varied by the function of the connecting means46over the entire operational range A of the trunnion shaft11such that the pressing force becomes minimum when the trunnion shaft11is located at the neutral position [N] and the pressing force becomes greater as the trunnion shaft11approaches the forward travel side highest speed position [Fmax] and the reverse travel side highest speed position [Rmax]. Hence, the operational resistance that occurs when the HST10is speed-changed is rendered substantially same strength over the entire operational range A of the trunnion shaft11; and also the trunnion shaft11is fixedly positioned at the neutral position [N] by both the pivotal urging of the positioning body32by the first spring33and the pivotal urging of the pivotal body32by the second spring34.

OTHER EMBODIMENTS

(1) In the foregoing embodiment, there was explained an example using the pivotal body31directly connected to the trunnion shaft11. Instead, the invention can be embodied with using a pivotal body operably coupled to the trunnion shaft11via a coupling mechanism including a coupling rod, a pivotal link, etc.

(2) In the foregoing embodiment, there was explained an example wherein the speed change operation resistances are rendered substantially same over the entire operational range A of the trunnion shaft11. Instead, the invention can be embodied with using an arrangement where the speed change operation resistances are rendered substantially same over the entire operational range of the trunnion shaft11, depending on the shapes of the driven cam, the cam follower, etc.

(3) In the foregoing embodiment, there was explained an example where the cam38is formed in the pivotal body31and the cam follower39is formed in the positioning body32. Instead, the invention can be embodied with forming the cam38in the positioning body32and forming the cam follower39in the pivotal body31.

The present invention can be used as a neutral urging mechanism to be incorporated in a speed change operation apparatus for a stepless speed changer device to be mounted on various kinds of vehicle, not only a tractor, but also a combine, a rice planter, a load carrying vehicle, etc.