Abstract:
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.

Description:
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&#39;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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing a tractor in its entirety, 
         FIG. 2  is a side view showing a speed change operation apparatus for a hydrostatic stepless speed changer device, 
         FIG. 3  is a side view showing a neutral urging mechanism under a condition when a trunnion shaft is operated to the neutral position, 
         FIG. 4  is a side view showing the neutral urging mechanism under a condition when the trunnion shaft is operated to the highest speed position on the forward travel side, 
         FIG. 5  is a side view showing the neutral urging mechanism under a condition when the trunnion shaft is operated to the highest speed position on the reverse travel side, 
         FIG. 6  is a front view in vertical section showing the neutral urging mechanism, 
         FIG. 7  is a perspective view showing a positioning body, a coupling body, a supporting member and an urging body, 
         FIG. 8  is an explanatory view showing an operational state of the neutral urging mechanism when a cam follower disengages from a recessed portion, 
         FIG. 9  is an explanatory view showing an operational state of the neutral urging mechanism when the trunnion shaft is operated to the highest speed position on the forward travel side, 
         FIG. 10  is an explanatory view showing an operational state of the neutral urging mechanism when the trunnion shaft is operated to the highest speed position on the reverse travel side, 
         FIG. 11  is an explanatory view showing an operational state of the neutral urging mechanism when the trunnion shaft is operated to the neutral position, and 
         FIG. 12  is an explanatory view showing relationship among an operational position of the trunnion shaft, a pressing force of a first spring, a rotational resistance of the trunnion shaft, a component force of a second spring, a pressing force of a positioning body and a speed change operational resistance. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, preferred embodiments of the present invention will be described with reference to the accompanying drawings. 
       FIG. 1  is a side view showing, in its entirety, a tractor equipped with an HST  10  to which the speed change operation apparatus relating to the present invention is applied. The tractor includes a pair of front wheels  1 ,  1  as steering wheels and driving wheels, a pair of right and left rear wheels  2 ,  2  as driving wheels, an engine section  3  provided at a front portion of the vehicle body supported by the front and rear wheels, and a riding type driving section  4  provided at a rear portion of the vehicle body. To a rear portion of a vehicle body frame  5  of this tractor, there is attached a link mechanism  6  having a pair of right and left lift arms  6   a  pivotally liftable up/down, and at a rear portion of the vehicle body frame  5 , there is mounted a power takeoff (PTO) shaft  9 . 
     To this tractor, at its rear portion of the vehicle body for instance, a rotary cultivator unit (not shown) is connected via the link mechanism  6  to be lifted up/down. Further, a power is transmitted via the PTO shaft  9  to 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 frame  5  includes the engine section  3  having an engine  3   a , an intermediate case  7  having a front end thereof connected to the rear portion of the engine  3   a , and a transmission case  8  having a front end thereof connected to the rear end portion of the intermediate case  7 . 
     As shown in  FIG. 1  and  FIG. 2 , to the front portion of the transmission case  8 , the HST  10  is mounted as being disposed inside the intermediate case  7 . The HST  10  is configured to convert drive force transmitted from the engine  3   a  via a transmission shaft (not shown) mounted in the intermediate case  7  into 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 case  8 . 
     As shown in  FIG. 1 , in the driving section  4 , a forward travel pedal  20   a  and a reverse travel pedal  20   b  are disposed laterally outside of the intermediate case  7 . As shown in  FIG. 2 , the forward travel pedal  20   a  and the reverse travel pedal  20   b  are pivoted to the intermediate case  7 , via a support shaft  22  on which a boss portion  21  provided at the base portions of the forward travel pedal  20   a  and the reverse travel pedal  20   b  are engaged and are operably coupled to a trunnion shaft  11  (see  FIG. 3 ) of the HST  10  via a coupling mechanism  26  having a pivotal arm  25  provided on the boss portion  21  to be rotatable therewith. 
     Therefore, when the forward travel pedal  20   a  is stepped on about the axis of the support shaft  22 , the HST  10  is speed-changed to the forward travel state to transmit the forward travel driving force to the front wheels  1  and the rear wheels  2 , whereby the self-propelled vehicle body travels forward. When the reverse travel pedal  20   b  is stepped on about the axis of the support shaft  22 , the HST  10  is speed-changed to the reverse travel state to transmit the reverse travel driving force to the front wheels  1  and the rear wheels  2 , whereby the self-propelled vehicle body travels in reverse. 
     As shown in  FIG. 2 , the boss portion  21  of the forward travel pedal  20   a  and the reverse travel pedal  20   b  is provided as a common boss portion shared by the forward travel pedal  20   a  and the reverse travel pedal  20   b , thus operably coupling the forward travel pedal  20   a  and the reverse travel pedal  20   b  with each other. As shown in  FIG. 2 ,  FIG. 3  and  FIG. 6 , the coupling mechanism  26  for coupling the forward travel pedal  20   a  and the reverse travel pedal  20   b  with the trunnion shaft  11  of the HST  10  includes the pivotal arm  25 , a pivotal body  31  operably coupled to the trunnion shaft  11 , and a coupling rod  27  having one end thereof pivotally connected to the free end of the pivotal body  31  via a coupling shaft  27   a  and having the other end thereof pivotally connected to the free end of the pivotal arm  25 . The coupling rod  27  is disposed outside the intermediate case  7 . The coupling shaft  27   a  coupling the coupling rod  27  with the pivotal body  31  extends through an arcuate through hole  7   a  provided in the intermediate case  7  and centered about the rotational axis of the trunnion shaft  11 . At the terminal end of the coupling rod  27  connected to the pivotal body  31 , there is provided an anti-vibration rubber  28  for restraining transmission of vibration from the trunnion shaft  11  to the forward travel pedal  20   a  and the reverse travel pedal  20   b . To the pivotal arm  25 , there is connected a dumper  29  for preventing too-rapid returning of the forward travel pedal  20   a  and the reverse travel pedal  20   b  to the neutral positions. 
     As shown in  FIG. 3 , the core component of the speed change operation apparatus for operating the HST  10  is the neutral urging mechanism  30  incorporating the pivotal body  31 . The neutral urging mechanism  30  urges the trunnion shaft  11  to return it to the neutral position [N] when the HST  10  is speed-changed to the forward travel state, i.e. when the trunnion shaft  11  is 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 HST  10  is operated to the neutral state, the neutral urging mechanism  30  fixes the trunnion shaft  11  at the neutral position [N] so that the shaft  11  is not to be displaced from this neutral position [N]. 
     Next, the neutral urging mechanism  30  will be described in greater details. 
     As shown in  FIG. 3  and  FIG. 5 , the neutral urging mechanism  30  includes, in addition to the pivotal body  31 , a positioning body  32  disposed on the opposite side to the side of the trunnion shaft  11  relative to the pivotal body  31 , a first spring  33  disposed upwardly of the pivotal body  31  and operably coupled to the positioning body  32 , a second spring  34  disposed forwardly of the pivotal body  31  and operably coupled to the positioning body  32 , and a cam mechanism  35  provided between and across the positioning body  32  and the pivotal body  31 . 
     The pivotal body  31  is connected to the trunnion shaft  11  via a connecting portion  31   b  provided at the base of this pivotal body  31 , so that the pivotal body  31  is pivoted in operative connection with the trunnion shaft  11  with a rotational axis of the trunnion shaft  11  acting as a pivotal axis  31   c . The connecting portion  31   b  of the pivotal body  31  is connected and fastened to an end of the trunnion shaft  11  by means of a fastening bolt  31   d.    
     As shown in  FIG. 3  and  FIG. 6 , the positioning body  32  is supported to the supporting member  37  via a coupling body  36  (see  FIG. 7 ) having its support portion  36   a  rotatably engaged within a circular attaching hole  32   a  (see  FIG. 7 ) provided at the base of the positioning body  32 , and the positioning body  32  pivots about a positioning axis  32   b  extending through the center of the attaching hole  32   a  and parallel with the pivotal axis  31   c  of the pivotal body  31 . The supporting member  37  includes a connecting portion  37   a  (see  FIG. 7 ) formed by a bent end portion provided at the rear end of the supporting member  37  and the supporting member  37  is attached via this connecting portion  37   a  to the case of the HST  10 . 
     As shown in  FIG. 3 , the cam mechanism  35  includes a cam  38  formed in an end face of the pivotal body  31  and a roller-shaped cam follower  39  formed in the positioning body  32  by attaching a bearing via a support shaft  39   a . The cam  38  is configured such that the cam follower  39  is slid in association with a pivotal movement of the pivotal body  31 . The cam  38  includes a pair of inclined cam faces  38   b ,  38   b  having a gentle slope angle provided in distribution to one end and the other end of the cam follower  39  in the sliding direction and a recessed portion  38   a  disposed between the pair of inclined cam faces  38   b ,  38   b  and recessed toward the side where the pivotal axis  31   c  is located. 
     As shown in  FIG. 9 ,  FIG. 10  and  FIG. 11 , the forward travel side and reverse travel side inclined cam faces  38   b  are formed with such inclinations that a distance D from a portion of the inclined cam face  38   b  contacting or tangent to the cam follower  39  to the pivotal axis  31   c  of the pivotal body  31  progressively increases as the cam follower  39  moves from the neutral position [N] to the highest speed position [Fmax], [Rmax] and also the distance becomes maximum when the cam follower  39  reaches the highest speed position [Fmax], [Rmax]. Further, the inclined cam faces are formed as a shape downwardly inclined toward the recessed portion  38   a  relative to the normal at the portion contacted by the cam follower  39  at the highest speed position [Fmax], [Rmax], or any other speed position. 
     As the cam follower  39  of the positioning body  32  engages into the recessed portion  38   a  of the cam  38 , the cam mechanism  35  fixedly positions the trunnion shaft  11  at the neutral position [N] by fixedly positioning the pivotal body  31  at its pivotal position corresponding to the neutral position [N] of the trunnion shaft  11 . 
     One end of the first spring  33  is supported to a spring supporting portion  40  provided in the case of the HST  10 . A hook  33   a  provided at the other end of the first spring  33  is engaged with an engaging portion provided at a free end  32   c  of the positioning body  32  and the other end of the first spring  33  is connected to the free end  32   c  of the positioning body  32 , and the first spring  33  pivotally urges the positioning body  32  about the positioning axis  32   b  so as to bring the cam follower  39  and the cam  38  into contact with each other. 
     As shown in  FIG. 3  and  FIG. 6 , one end of the second spring  34  is supported to a spring supporting portion  37   c  with providing a spring adjusting screw to a lower portion  37   b  of the supporting member  37 . The other end of the second spring  34  is connected to the positioning body  32  with connecting means  46  having a driven cam  45  formed in the positioning body  32 . 
     The connecting means  46  includes, in addition to the driven cam  45 , an urging body  48  (see  FIG. 7 ) having, at its free end, a bent portion  48   a  (“free end portion  48   a ” hereinafter) engaged into an elongate-hole like supporting hole  47  provided in the positioning body  32  to form a cam face for the driven cam  45 . 
     The driven cam  45  is constituted from an inner wall of the supporting hole  47 . The urging body  48  is attached to the supporting hole  47  of the positioning body and an attaching hole  49  (see  FIG. 9 ) provided in the supporting member  37  and the urging body  48  pivots with the free end  48   a  sliding along the driven cam  45 , about a pivotal axis  49   a  which is the axis extending through the center of the attaching hole  49 . The urging body  48  has its free end  48   a  connected to the second spring  34  so that the urging body  48  is pivotally urged about the pivot axis  49   a  by the second spring  34 . By the urging force of the second spring  34 , the free end  48   a  is pressed against the driven cam  45 , and the positioning body  32  is pivotally urged about the positioning axis  32   b  so as to bring the cam follower  39  and the cam  38  into contact with each other. 
     Therefore, with the neutral urging mechanism  30  in operation, the pivotal body  31  is caused to provide the pressing action with the pivotal urging of the positioning body  32  by the first spring  32  and the pivotal urging via the connecting means  46  by the second spring  34 . And, when the trunnion shaft  11  is rotatably operated, the cam follower  39  is caused to slide relative to the cam  38  to come into engagement into the recessed portion  38   a  or to disengage from the recessed portion  38   a  to be positioned at the forward travel side or reverse travel side inclined cam face  38   b , thereby to fixedly position the trunnion shaft  11  at the neutral position [N] or to return it to the neutral position [N]. 
     That is,  FIG. 3  is a side view showing the neutral urging mechanism  30  under the condition where the trunnion shaft  11  is operated to the neutral position [N]. As shown in this figure, with the neutral urging mechanism  30  in operation, when the trunnion shaft  11  is operated to the neutral position [N], the pivotal body  31  pivots in operative association with the trunnion shaft  11 , so that the cam follower  39  of the positioning body  32  and the recessed portion  38   a  of the cam  38  are brought into opposition to each other, and with the pivotal urging by the first spring  33  and the second spring  34 , the positioning body  32  urges the cam follower  39  into engagement in the recessed portion  38   a , thereby to fixedly position the trunnion shaft  11  at the neutral position [N]. 
       FIG. 4  is a side view showing the neutral urging mechanism  30  under the condition when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax]. As shown in this figure, with the neutral urging mechanism  30  in operation, when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax], the pivotal body  31  pivots in operative association with the trunnion shaft  11 , so that the cam follower  39  of the positioning body  32  disengages from the recessed portion  38   a  of the cam  38  and comes into opposition to the forward travel side inclined cam face  38   b  and the positioning body  32 , as being pivotally urged by the first spring  33  and the second spring  34 , presses the cam follower  39  against the forward travel side inclined cam face  38   b , thus urging the trunnion shaft  11  to be returned to the neutral position [N]. Though not shown, when the trunnion shaft  11  is operated to an operational position of a speed before the forward travel side highest speed position [Fmax], like the case of the trunnion shaft  11  being operated to the highest speed position [Fmax], the neutral urging mechanism  30  operates such that the positioning body  32  presses the cam follower  39  against the forward travel side inclined cam face  38   b , thus urging the trunnion shaft  11  to be returned to the neutral position [N]. 
       FIG. 5  is a side view showing the neutral urging mechanism  30  under the condition when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax]. As shown in this figure, with the neutral urging mechanism  30  in operation, when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax], the pivotal body  31  pivots in operative association with the trunnion shaft  11 , so that cam follower  39  of the positioning body  32  disengages from the recessed portion  38   a  of the cam  38  and comes into opposition to the reverse travel side inclined cam face  38   b  and the positioning body  32 , as being pivotally urged by the first spring  33  and the second spring  34 , presses the cam follower  39  against the reverse travel side inclined cam face  38   b , thus urging the trunnion shaft  11  to be returned to the neutral position [N]. Though not shown, when the trunnion shaft  11  is operated to an operational position of a speed before the reverse travel side highest speed position [Rmax], like the case of the trunnion shaft  11  being operated to the highest speed position [Rmax], the neutral urging mechanism  30  operates such that the positioning body  32  presses the cam follower  39  against the reverse travel side inclined cam face  38   b , thus urging the trunnion shaft  11  to be returned to the neutral position [N]. 
       FIG. 8  is an explanatory view showing an operational state of the neutral urging mechanism  30  when the cam follower  39  disengages from the recessed portion  38   a  of the cam  38  toward the forward travel side. As shown in this figure, when the cam follower  39  disengages from the recessed portion  38   a  of the cam  38  toward the forward travel side, the cam follower  39  comes into contact with the forward travel side entrance/exit of the recessed portion  38   a  of the cam  38 , so that a pressing force F 1  of the first spring  33  is applied to a position P 1  located at the forward travel side entrance/exit of the recessed portion  38   a  and having an arm length L 1  from the pivotal axis  31   c  of the pivotal body  31 , whereby the trunnion shaft  11  is subjected to a pressing moment M 1 =arm length L 1 ×pressing force F 1 , about the pivotal axis  31   c , as a rotational resistance due to the pivotal urging of the positioning body  32  by the first spring  33 . 
     When the cam follower  39  disengages from the recessed portion  38   a  of the cam  38  toward the reverse travel side, the cam follower  39  comes into contact with the reverse travel side entrance/exit of the recessed portion  38   a  of the cam  38 . In this, the portion of the cam  38  contacted by the cam follower  39  differs from that when the cam follower  39  disengages from the recessed portion  38   a  toward the forward travel side. As the shape of the recessed portion  38   a  is 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 follower  39  disengages from the recessed portion  38   a  toward the reverse travel side, the trunnion shaft  11  receives a pressing moment M 1  about the pivotal axis  31   c , as a rotational resistance of substantially same strength as that the shaft  11  receives when the cam follower  39  disengages from the recessed portion  38   a  toward the forward travel side. 
       FIG. 9  is an explanatory view showing the operational condition of the neutral urging mechanism  30  when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax]. As shown in this figure, when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax], the cam follower  39  comes into contact with an end portion of the forward travel side inclined cam face  38   b , so that a pressing force F 2  of the first spring  33  is applied to a position P 2  located at the forward travel side entrance/exit of the recessed portion  38   a  and having an arm length L 2  from the pivotal axis  31   c  of the pivotal body  31 , whereby the trunnion shaft  11  is subjected to a pressing moment M 2 =arm length L 2 ×pressing force F 2 , about the pivotal axis  31   c , as a rotational resistance due to the pivotal urging of the positioning body  32  by the first spring  33 . 
       FIG. 10  is an explanatory view showing the operational condition of the neutral urging mechanism  30  when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax]. As shown in this figure, when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax], the cam follower  39  comes into contact with an end portion of the reverse travel side inclined cam face  38   b , so that a pressing force F 3  of the first spring  33  is applied to a position P 3  located at the reverse travel side entrance/exit of the recessed portion  38   a  and having an arm length L 3  from the pivotal axis  31   c  of the pivotal body  31 , whereby the trunnion shaft  11  is subjected to a pressing moment M 3 =arm length L 3 ×pressing force F 3 , about the pivotal axis  31   c , as a rotational resistance due to the pivotal urging of the positioning body  32  by the first spring  33 . 
     The extension lengths of the first spring  33  when the trunnion shaft  11  is 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 spring  33  when the cam follower  39  disengages from the recessed portion  38   a . However, the arm length L 2  when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax] and the arm length L 3  when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax] are significantly shorter than the arm length L 1  when the cam follower  39  disengages from the recessed portion  38   a . And, the pressing moment M 2  as the rotational resistance received by the trunnion shaft  11  when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax] and the pressing moment M 3  as the rotational resistance received by the trunnion shaft  11  when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax] are smaller than the pressing moment M 1  as the rotational resistance received by the trunnion shaft  11  when the cam follower  39  disengages from the recessed portion  38   a.    
     The extension lengths of the first spring  33  when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax] and the extension lengths of the first spring  33  when the trunnion shaft  11  is operated to the reverse travel highest speed position [Rmax] are substantially same. The arm length L 2  when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax] and the arm length L 3  when the trunnion shaft  11  is operated to the reverse travel highest speed position [Rmax] are substantially same. 
     Therefore, as the trunnion shaft  11  is rotatably operation, the rotational resistance of the trunnion shaft  11  that occurs in association with the pivotal urging of the positioning body  32  by the first spring  33  varies over the entire pivotal range A of the trunnion shaft  11 , due to the effect of the cam  38  that is rotated in operative connection with the trunnion shaft  11 . Hence, whether the trunnion shaft  11  is 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 follower  39  disengages from the recessed portion  38   a  and becomes smaller as the cam follower  39  moves away from the recessed portion  38   a  after its disengagement therefrom. 
       FIG. 11  is an explanatory view showing the operational condition of the neutral urging mechanism  30  when the trunnion shaft  11  is operated to the neutral position [N]. As shown in this figure, when the trunnion shaft  11  is operated to the neutral position [N], the free end  48   a  of the urging body  48  is located at a position P 4  having an arm length L 4  from the positioning axis  32   b  of the positioning body  32 , so that the urging body  48  applies an operational force F 4  of the second spring  34  to the position P 4  of the positioning body  32 . This operational force F 4  of the second spring  34  is divided into a component force F 4   a  for pivotally urging the positioning body  32  about the positioning axis  32   b  and a component force F 4   b  that does not affect the pivotal urging of the positioning body  32 . Therefore, under the condition of the trunnion shaft  11  being located at the neutral position [N], the positioning body  32  has a pressing moment M 4 =arm length L 4 ×component force F 4   a , about the positioning axis  32   b , as a pressing force for pressing the pivotal body  31  due to the pivotal urging by the second spring  34 . 
     As shown in  FIG. 9 , when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax], the free end  48   a  of the urging body  48  is located at a position P 5  having an arm length L 5  from the positioning axis  32   b  of the positioning body  32 , so that the urging body  48  applies an operational force F 5  of the second spring  34  to the position P 5  of the positioning body  32 . This operational force F 5  of the second spring  34  is divided into a component force F 5   a  for pivotally urging the positioning body  32  about the positioning axis  32   b  and a component force F 5   b  that does not affect the pivotal urging of the positioning body  32 . Therefore, under the condition of the trunnion shaft  11  being located at the neutral position [N], the positioning body  32  has a pressing moment M 5 =arm length L 5 ×component force F 5   a , about the positioning axis  32   b , as a pressing force for pressing the pivotal body  31  due to the pivotal urging by the second spring  34 . 
     As shown in  FIG. 10 , when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax], the free end  48   a  of the urging body  48  is located at a position P 6  having an arm length L 6  from the positioning axis  32   b  of the positioning body  32 , so that the urging body  48  applies an operational force F 6  of the second spring  34  to the position P 6  of the positioning body  32 . This operational force F 6  of the second spring  34  is divided into a component force F 6   a  for pivotally urging the positioning body  32  about the positioning axis  32   b  and a component force F 6   b  that does not affect the pivotal urging of the positioning body  32 . Therefore, under the condition of the trunnion shaft  11  being located at the neutral position [N], the positioning body  32  has a pressing moment M 6 =arm length L 6 ×component force F 6   a , about the positioning axis  32   b , as a pressing force for pressing the pivotal body  31  due to the pivotal urging by the second spring  34 . 
     When the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax] and the reverse travel side highest speed position [Rmax], the free end  48   a  of the urging body  48  is located at positions having greater distances from the spring supporting portion  37   c  than the case of the trunnion shaft  11  being operated to the neutral position [N], so that the second spring  34  is extended greater than the case of the trunnion shaft  11  being operated to the neutral position [N], and the operational forces F 5 , F 6  of the second spring  34  in the cases of the trunnion shaft  11  being 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 F 4  in the case of the trunnion shaft  11  being operated to the neutral position [N]. Further, the arm lengths L 5 , L 6  when the trunnion shaft  11  is 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 L 4  in the case of the trunnion shaft  11  being operated to the neutral position [N]. And, the pressing moment M 5  present in the positioning body  32  when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax] and the pressing moment M 6  present in the positioning body  32  when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax] are greater than the pressing moment M 4  present in the case of the trunnion shaft  11  being operated to the neutral position [N]. 
     The free end  48   a  of the urging body  48  is located at substantially same position of the positioning body  32  when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax] and when the trunnion shaft  11  is operated to the reverse travel side highest speed position [Rmax]; and the pressing moment M 5  present in the positioning body  32  when the trunnion shaft  11  is operated to the forward travel side highest speed position [Fmax] and the pressing moment M 6  present in the positioning body  32  when the trunnion shaft  11  is 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 means  46 , the pressing force applied by the positioning body  32  to press the pivotal body  31  with the pivotal urging of the positioning body  32  by the second spring  34  is minimum under the condition when the trunnion shaft  11  is located at the neutral position [N], and the pressing force becomes greater as the trunnion shaft  11  moves 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 shaft  11 . 
       FIG. 12  is an explanatory view showing relationship among the pressing forces (the pressing forces F 1 , F 2 , F 3  applied by the first spring  33 ) applied from the positioning body  32  to the pivotal body  31  with the pivotal urging by the first spring  33 , rotational resistances M 1 , M 2 , M 3  of the trunnion shaft  11  that occur in association with the pivotal urging of the positioning body  32  by the first spring  33 , the component forces F 4   a , F 5   a , F 6   a  of the operational forces of the second spring  34  for pivotally urging the positioning body  32 , the pressing forces present in the positioning body  32  to pressing the pivotal body  31  with the pivotal urging by the second spring  34  (the pressing forces M 4 , M 5 , M 6  of the positioning body  32 ) and speed change operation resistances received from the first spring  33  and the second spring  34  in the course of speed change operations by the forward travel pedal  20   a  and the reverse travel pedal  20   b . 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 F 5   a  of the operational force of the second spring  34  applied to the positioning body  32  as a reference when the trunnion shaft  11  is operated to the neutral position [N]. It should be noted; however, that the strength of the component force F 5   a  of the operational force of the second spring  34  applied to the positioning body  32  in the case of the trunnion shaft  11  being operated to the neutral position [N] is assumed to have a value of 0.75. 
     As shown in this figure, with the neutral urging mechanism  30  in operation, the positioning body  32  is pivotally urged by the first spring  33  and the second spring  34  thereby to bring the cam follower  39  and the cam  38  into contact with each other and the rotational resistance generated in the trunnion shaft  11  due to the pivotal urging of the positioning body  32  by the first spring  33  is varied by the function of the cam  38  over the entire operational range A of the trunnion shaft  11  such that the resistance becomes maximum when the trunnion shaft  11  moves away from the neutral position [N] and the resistance becomes smaller as the trunnion shaft  11  approaches 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 body  32  to press the pivotal body  31  by the pivotal urging by the second spring  34  is varied by the function of the connecting means  46  over the entire operational range A of the trunnion shaft  11  such that the pressing force becomes minimum when the trunnion shaft  11  is located at the neutral position [N] and the pressing force becomes greater as the trunnion shaft  11  approaches 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 HST  10  is speed-changed is rendered substantially same strength over the entire operational range A of the trunnion shaft  11 ; and also the trunnion shaft  11  is fixedly positioned at the neutral position [N] by both the pivotal urging of the positioning body  32  by the first spring  33  and the pivotal urging of the pivotal body  32  by the second spring  34 . 
     OTHER EMBODIMENTS 
     (1) In the foregoing embodiment, there was explained an example using the pivotal body  31  directly connected to the trunnion shaft  11 . Instead, the invention can be embodied with using a pivotal body operably coupled to the trunnion shaft  11  via 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 shaft  11 . 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 shaft  11 , depending on the shapes of the driven cam, the cam follower, etc. 
     (3) In the foregoing embodiment, there was explained an example where the cam  38  is formed in the pivotal body  31  and the cam follower  39  is formed in the positioning body  32 . Instead, the invention can be embodied with forming the cam  38  in the positioning body  32  and forming the cam follower  39  in the pivotal body  31 . 
     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.