Patent Publication Number: US-6341488-B1

Title: Mechanism of returning to neutral for axle driving apparatus

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of application Ser. No. 09/192,315; filed Nov. 16, 1998, which is a Continuation-in-Part of application Ser. No. 08/872,672; filed Jun. 11, 1997. The disclosures of both identified applications are incorporated in their entirety by reference hereto. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mechanism for automatically returning a movable swash plate to the neutral position in an axle driving apparatus which drives the axles thereof by a hydrostatic transmission (hereinafter referred to as an “HST”) for steplessly changing the rotational speed of an engine, wherein resistance is given to the movable swash plate and pressure generated in a closed fluid circuit of the HST escapes therefrom when the movable swash plate returns to the neutral position, so as to prevent the vehicle from a sudden stop. 
     2. Related Art 
     Conventionally, an HST comprising a hydraulic pump and a hydraulic motor, which are fluidly connected through a closed fluid circuit, has a control arm which engages with a movable swash plate of the hydraulic pump. The quantity of oil discharged from the hydraulic pump can be changed in a stepless fashion by rotation of the control arm. The control arm is regularly biased by a neutral return spring, whereby the movable swash plate is automatically returned to its neutral position by the neutral return spring when an operator stops speed changing operation and releases the operational power. Accordingly, the movable swash plate suddenly returns to the neutral position when the operating force is released at a high speed position of the movable swash plate and operating oil which has smoothly circulated in the closed fluid circuit theretofore is blocked in the hydraulic pump so that the rotation of the hydraulic motor is suddenly stopped. In other words, a dynamic brake is applied. Therefore, a control arm is often provided with a shock absorber to prevent sudden returning to its neutral position, as disclosed in U.S. Pat. No. 5,094,077, for example. 
     A shock absorber which is provided to prevent the control arm from suddenly returning to its neutral position is disposed on the outside of the HST housing so as to enable mounting and exchange thereof. However, a shock absorber which uses gas or fluid for shock absorption is heated by radiation of the housing which is heated by the rising temperature of the oil therein and is affected by the temperature of the outside air. The gas or fluid expands or contracts because of such a change of temperature so that operation of the shock absorber differs according to temperature or, in some cases, the neutral position can not be fixed at a high or a low temperature. 
     Further, a shock absorber mounted on the outside of the housing requires space for it to be mounted which can restrict the shape of the axle driving apparatus. As a result, the entire axle driving apparatus becomes larger. Also, because the shock absorber is mounted outside of the housing, the movable portion of the shock absorber is conventionally covered with a protector such as a rubber boot to avoid penetration of water, dust or other foreign matter, and therefore, requires an increase in the number of parts and in the cost. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide a mechanism of returning to neutral for an axle driving apparatus, which can moderate a shock of deceleration by returning to neutral in advancing or reversing of a vehicle. 
     The second object of the present invention is to provide a mechanism of returning to neutral for an axle driving apparatus, which can moderate a starting shock of vehicle for advancing and reversing. 
     The third object of the present invention is to provided a mechanism of returning to neutral for an axle driving apparatus, which can prevent the vehicle left on a slope in neutral from suddenly self-descending. 
     The axle driving apparatus regarding to the present invention is constructed as follows: 
     Axles and a hydrostatic transmission as a combination of a variable capacity hydraulic pump and a hydraulic motor for driving axles are contained in fluid sump formed within a housing for the axle driving apparatus. The hydrostatic transmission is constructed such that suction and discharge ports of the hydraulic pump are respectively connected with discharge and suction ports of the hydraulic motor through fluid passages. The hydrostatic transmission is provided with a capacity changing member, which can be shifted between a neutral position making the hydraulic pump discharge substantially no fluid and an acting range making the hydraulic pump discharge fluid. A biasing member biases the capacity changing member in the acting range so as to return it to the neutral position. Orifices are provided which are connected to the fluid passages. A shutting member faces the orifices so as to be operable to open and shut the orifices toward the fluid sump. The shutting member is interlocked with the capacity changing member so that at least one of the orifices is open to the fluid sump during returning of the capacity changing member from the acting range to the neutral position. Pistons with the orifices provided therethrough are slidably fit to the oil passages and the outer surfaces thereof are pressed against the shutting member by hydraulic pressure in the oil passages. The shutting member is provided with grooves so as to communicate with the orifices of the piston pressed as such. Other biasing members for pushing the pistons are smaller than the biasing member for returning the capacity changing member to neutral. 
     In such a construction, to achieve the main object, a predetermined position is provided in the acting range, so that when returning the capacity changing member in the acting range to the neutral position, the shutting member shuts both the orifices of the pistons until it reaches the predetermined position, and after the capacity changing member passes the predetermined position until the neutral position, it allows one of the orifices to open toward the fluid sump while still shutting the other orifice. 
     Alternatively, the shutting member is constructed such that during returning of the capacity changing member in the acting range to the neutral position, it shuts one of the orifices and allows the other orifice to open toward the fluid sump at any position in the acting range. 
     To achieve the second object, the shutting member is constructed such that at the moment that the capacity changing member leaves the neutral position, it allows both the orifices to open toward the fluid sump, and when the capacity changing member is further apart from the neutral position, the shutting member shuts one of the orifices. 
     To achieve the third object, the shutting member is constructed such that when the capacity changing member is in the neutral position, it allows both the orifices to open partly toward the fluid sump, or it allows both the orifices to open fully in case that the orifices are diametrically small enough, or the shutting member shuts both the orifices in case that another member for giving a dead zone of the neutral position is provided, or it shuts one of the orifices. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a plan view, partly in cross section, of an axle driving apparatus of the present invention, from which an upper half housing has been removed; 
     FIG. 2 is a cross-sectional view looking in the direction of arrows  2 — 2  in FIG. 1; 
     FIG. 3 is a cross-sectional view looking in the direction of arrows  3 — 3  in FIG. 1; 
     FIG. 4 is a cross-sectional view looking in the direction of arrow s  4 — 4  in FIG. 1; 
     FIG. 5 is a plan view of center section  5  of the present invention; 
     FIG. 6 is a view looking in the direction of arrow  6  in FIG. 5; 
     FIG. 7 is a cross-sectional view looking in the direction of arrows  7 — 7  in FIG. 6; 
     FIG. 8 is a cross-sectional view looking in the direction of arrows  8 — 8  in FIG. 5; 
     FIG. 9 is a cross-sectional view looking in the direction of arrows  9 — 9  in FIG. 5; 
     FIG. 10 is a partial cross-sectional side view of the axle driving apparatus of the present invention showing a side view of operational members of movable swash plate  11 ; 
     FIG. 11 is a perspective view of control arm  39 ; 
     FIG. 12 is a cross-sectional view looking in the direction of arrows  12 — 12  in FIG. 10; 
     FIG. 13 is a cross-sectional view of a principal part of another embodiment of the present invention showing the disposition of pistons  64  in center section  5 ; 
     FIG. 14 is a cross-sectional view of a principal part of a further embodiment of the present invention showing the shape of groove  39   d  on the surface of control arm  39  in contact with piston  64 ; 
     FIG. 15 is a plan view of center section  5 ′; 
     FIG. 16 is a cross-sectional plan view of center section  5 ′; 
     FIG. 17 is a cross-sectional view looking in the direction of arrows  17 — 17  in FIG. 15; 
     FIG. 18 is a cross-sectional view looking in the direction of arrows  18 — 18  in FIG. 15; 
     FIG. 19 is a cross-sectional view looking in the direction of arrows  19 — 19  in FIG. 15; 
     FIG. 20 is a side view looking in the direction of arrow  20  in FIG. 15; 
     FIG. 21 is a cross-sectional plan view of center section  5 ″; 
     FIG. 22 is a cross-sectional view looking in the direction of arrows  22 — 22  in FIG. 21; 
     FIG. 23 is a vertical rear elevation view of another embodiment of an axle driving apparatus containing center section  5 ″ of FIG. 21; 
     FIG. 24 is a cross-sectional view looking in the direction of the arrows  24 — 24  in FIG. 23; and 
     FIG. 25 is a perspective view of a movable swash plate  11 ′ for the axle driving apparatus shown in FIG.  23 . 
     FIG. 26 is a plan view, partly in cross section, of another axle driving apparatus of the present invention suiting center section  5 ′ shown in FIGS. 15 through 20 in condition that upper half housing I has been removed therefrom; 
     FIG. 27 is a cross-sectional view looking in the direction of arrows  27 — 27  in FIG. 26, being provided with upper half housing  1 ; 
     FIG. 28 is a cross-sectional view looking in the direction of arrows  28 — 28  in FIG. 26, being provided with upper half housing  1 ; 
     FIG. 29 is a cross-sectional view of a principal part of the axle driving apparatus shown in FIGS. 26 through 28 showing the disposition of upper and lower pistons  64 U and  64 L in center section  5 ′; 
     FIG. 30 is a perspective view of a preferred first embodiment of a control arm  39 ′ which is suitable to center section  5 ′ with pistons  64 U and  64 L; 
     FIG. 31 is a side view of the same in a neutral position N; 
     FIGS.  32 ( a ) through  32 ( d ) are views showing the same control arm  39 ′ at various positions in series of its advancing range A; 
     FIGS.  33 ( a ) and  33 ( e ) through  33 ( g ) are views showing the same control arm  39 ′ at various positions in series of its reversing range R; 
     FIG. 34 is a hydraulic circuit diagram of an HST provided with center section  5 ′ and control arm  39 ′ according to the first embodiment shown in FIGS. 31,  32 ,  36 ,  37 ,  39  and  40 ; 
     FIG. 35 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the preferred first embodiment of control arm  39 ′ shown in FIGS. 31 and 32; 
     FIG. 36 is a side view of a first modification of the first embodiment of control arm  39 ′ in neutral position N; 
     FIG. 37 is a side view of a second modification of the first embodiment of control arm  39 ′ in neutral position N; 
     FIG. 38 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the second modification of the first embodiment control arm  39 ′ shown in FIG. 37; 
     FIG. 39 is a side view of a third modification of the first embodiment of control arm  39 ′ in neutral position N; 
     FIG. 40 is a side view of a fourth modification of the first embodiment of control arm  39 ′ in neutral position N; 
     FIG. 41 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the third and fourth modifications of the first embodiment of control arm  39 ′ shown in FIGS. 39 and 40; 
     FIG. 42 is a perspective view of a preferred second embodiment of control arm  39 ′ which is suitable to center section  5 ′ with pistons  64 U and  64 L; 
     FIG. 43 is a hydraulic circuit diagram of an HST provided with center section  5 ′ and control arm  39 ′ according to the second embodiment shown in FIGS. 42,  45  and  47 ; 
     FIG. 44 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the preferred second embodiment of control arm  39 ′ shown in FIG. 42; 
     FIG. 45 is a side view of a first modification of the second embodiment of control arm  39 ′ in neutral position N; 
     FIG. 46 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the first modification of the second embodiment of control arm  39 ′ shown in FIG. 45; 
     FIG. 47 is a side view of a second modification of the second embodiment of control arm  39 ′ in neutral position N; 
     FIG. 48 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the second modification of the second embodiment of control arm  39 ′ shown in FIG. 47; 
     FIG. 49 is a side view of a preferred third embodiment of control arm  39 ′ which is suitable to center section  5 ′ with pistons  64 U and  64 L; 
     FIG. 50 is a hydraulic circuit diagram of an HST provided with center section  5 ′ and control arm  391  according to the third embodiment shown in FIGS. 49,  52  and  54 ; 
     FIG. 51 is a schematic diagram showing opening conditions of orifices  60 U and  80 L in relation to the rotational positions of the preferred third embodiment of control arm  39 ′ shown in FIG. 49; 
     FIG. 52 is a side view of a first modification of the third embodiment of control arm  39 ′ in neutral position N; 
     FIG. 53 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the first modification of the third embodiment of control arm  39 ′ shown in FIG. 52; 
     FIG. 54 is a side view of a second modification of the third embodiment of control arm  39 ′ in neutral position N; 
     FIG. 55 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the second modification of the third embodiment of control arm  39 ′ shown in FIG. 54; 
     FIG. 56 is a side view of a preferred fourth embodiment of control arm  39 ′ which is suitable to center section  5 ′ with pistons  64 U and  64 L; 
     FIG. 57 is a hydraulic circuit diagram of an HST provided with center section  5 ′ and control arm  39 ′ according to the fourth embodiment shown in FIG. 56; 
     FIG. 58 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the preferred fourth embodiment of control arm  39 ′ shown in FIG. 56; 
     FIG. 59 is a side view of a fifth embodiment of control arm  39 ′; 
     FIG. 60 is a perspective view of control arm  39 ′ of FIG. 59; 
     FIG. 61 is a sectional front view of a principal portion of the axle driving apparatus which employs the fifth embodiment of control arm  39 ′; and 
     FIG. 62 is a schematic diagram showing opening conditions of orifices  80 U and  80 L in relation to the rotational positions of the fifth embodiment of the control arm  39 ′. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Explanation will first be given on the entire construction of an axle driving apparatus having the neutral return mechanism of the present invention. FIGS. 1,  2 ,  3  and  4  show a housing constructed by joining an upper half housing  1  and a lower half housing  2  along horizontal joint surfaces. At the joint surfaces is provided a bearing for a motor shaft  4 . Bearings for axles  7  are shifted upwardly from the joint surfaces of the housing and are disposed in upper half housing  1  to rotatably support axles  7 . Axles  7  are differentially coupled by a differential gear unit  23 . One end of each axle  7  projects laterally from the housing. 
     The interior of the housing is divided by an inner wall  8  into a first chamber R 1  for housing an HST and a second chamber R 2  for housing differential gear unit  23 , a transmission gear unit for transmitting power from motor shaft  4  to differential gear unit  23  and axles  7 . First and second chambers R 1  and R 2  are filled with lubricating oil in common so as to form an oil sump. An air reservoir (not shown) is formed above differential gear unit  23  in upper half housing  1 . An oil supply hole is bored through the portion of the housing which is above the air reservoir, enabling oil to be supplied thereto. 
     Within first chamber R 1  is mounted a center section  5  which is L-like shaped when viewed from the side and has a horizontal portion  500  and a vertical portion  501 . At the peripheral portions of horizontal portion  500  are vertically open through bores Sf at three positions, as shown in FIG. 5. A mounting bolt  30  is inserted into each through bore  5   f  from below to fix center section  5  to the inside of upper half housing  1 . On the upper surface of horizontal portion  500  of center section  5  is formed a pump mounting surface  40 . A cylinder block  16  is rotatably and slidably disposed thereon. Pistons  12  are fitted. through biasing springs, into a plurality of cylinder bores in cylinder block  16  and are reciprocally movable. A thrust bearing  11   a  of a movable swash plate  11  abuts against the heads of pistons  12 . An opening  11   b  is provided at the center of movable swash plate  11  so as to enable a pump shaft  3  to perforate therethrough. Pump shaft  3  is also used as an input shaft and is vertically disposed and is not relatively rotatably retained onto the axis of rotation of cylinder block  16 , thereby constituting an axial piston type hydraulic pump. Pump shaft  3  projects outwardly at the upper end thereof from upper half housing  1 . An input pulley  43 , with a cooling fan  44 , is fixed onto pump shaft  3 . Input pulley  43  receives power from a prime mover (not shown) through a belt transmitting mechanism (also not shown). 
     As shown in FIG. 6, at the outside surface of vertical portion  501  of center section  5  is formed a motor mounting surface  41  on which a cylinder block  17  is rotatably supported. A plurality of pistons  13  are fitted into a plurality of cylinder bores in cylinder block  17 . Pistons  13  are reciprocally movable whereby the heads thereof abut against a fixed swash plate  37  which is fixedly sandwiched between upper half housing  1  and lower half housing  2 . Motor shaft  4  is horizontally disposed on the axis of rotation of cylinder block  17  and is not relatively rotatably retained thereto so as to constitute an axial piston type hydraulic motor. Motor shaft  4  is also rotatably supported by a bearing bore provided on vertical portion  501  of center section  5  and by a bearing  4   a  with a seal held at the joint surfaces of upper half housing  1  and lower half housing  2 . 
     Transmission gear unit for transmitting power from motor shaft  4  to differential gear unit  23  is shown in FIGS. 1 and 2. A gear  25  engageable with a larger diameter gear  24  on counter shaft  26  is provided on motor shaft  4  where it enters into second chamber R 2 . A smaller diameter gear  21  on counter shaft  26  engages with a ring gear  22  of differential gear unit  23 . Ring gear  22  drives differential gear unit  23  so as to transmit power to left and right axles  7 . 
     As shown in FIG. 2, a brake disk  19  is fixed onto an axial end of motor shaft  4  positioned in second chamber R 2 . A brake operating shaft  14  is supported by upper half housing  1  through a bush  15 . A brake arm  27  is fixed to the outer end of brake operating shaft  14  projecting from the housing. When brake arm  27  is rotated, balls  20  ride on cam grooves provided on a flange  14   a . Brake operating shaft  14  slides toward brake disk  19 , so that the brake disk  19  is put between the inner end surface of brake operating shaft  14  and a brake pad  29 , thereby exerting a braking action to motor shaft  4 . 
     As shown in FIG. 5, a pair of arcuate ports  40   a  and  40   b  are open along pump mounting surface  40  on horizontal portion  500  of center section  5  so that the feed oil discharged from cylinder block  16  is introduced into center section  5 . As shown in FIG. 6, a pair of arcuate ports  41   a  and  41   b  are open on the motor mounting surface  41  of vertical portion  501  thereby introducing feed discharge oil into center section  5  from cylinder block  17 . 
     As shown in FIG. 7, a linear first oil passage  5   a  and a linear second oil passage  5   b  are drilled in parallel with each other within the thick horizontal portion  500  of center section  5  forming a closed fluid circuit for circulating operating oil between the hydraulic pump and the hydraulic motor. As shown in FIGS. 8 and 9, oiling pipes  52  and  53  communicating with first and second oil passages  5   a  and  5   b  are disposed on the lower surface of horizontal portion  500  of center section  5  and are exposed at the lower ends thereof at the outer bottom surface of lower half housing  2 . After the axle driving apparatus has been assembled, the closed fluid circuit is filled with operating oil from the open end of each oiling pipe  52  and  53 . Thereafter, the open end of each oiling pipe  52  and  53  is closed by a plug. 
     As shown in FIGS. 3,  7 ,  8  and  9 , oil holes  5   c  and  5   d  are branched downwardly from the intermediate portion of first and second oil passages  5   a  and  5   b . Oil holes  5   c  and  5   d  are open on the lower surface of horizontal portion  500  of center section  5 . Check valves  54  and  55 , which automatically open merely when oil is supplied, are disposed in the open end of oil holes  5   c  and  5   d , respectively. As shown in FIG. 3, valve casings  54   a  and  55   a  are supported on projections  2   a  which project upwardly from the inner bottom surface of lower half housing  2 . An annular oil filter  56  is disposed in the oil sump between the lower surface of center section  5  and the inner bottom surface of lower half housing  2  and surrounds projections  2   a.    
     As shown in FIGS. 2 and 4, a by-pass operating arm  60  is disposed on upper half housing  1  so as to open first and second oil passages  5   a  and  5   b  into the oil sump for enabling axles  7  to be idle when the vehicle is hauled. In particular, by-pass operating arm  60  is fixed at the base thereof to an upper end of a by-pass shaft  61  which is vertically and pivotally supported to the upper wall of upper half housing  1 . By-pass shaft  61  extends at the lower end thereof into vertical portion  501  of center section  5 , so that a flat surface  61   a  formed at the side surface of the lower end thereof abuts one end of a push pin  62  which can abut at the other end thereof against the rotatable slidable surface of cylinder block  17  supported on vertical portion  501 . When an operator operates by-pass operating arm  60  outside of the housing for hauling the vehicle, by-pass shaft  61  is rotated and flat surface  61  a at the lower end thereof diagonally presses push pin  62  toward cylinder block  17 . First and second oil passages  5   a  and  5   b  communicate with the oil sump in the housing through arcuate ports  41   a  and  41   b , thereby enabling motor shaft  4  to be idle. 
     As shown in FIGS. 7,  8 ,  9  and  12 , pistons  64  constituting the neutral return means of the present invention are horizontally and slidably disposed in the open end portions of first and second oil passages  5   a  and  5   b  which are open at the side surface of horizontal portion  500  of center section  5 , opposite to vertical portion  501 . Each piston  64  is cylindrical and has a large diametric flange  64   a  formed at the outside end thereof. Each piston  64  is slidably inserted into an axial opening through a bush  65  which is screwed into the open end of each of first and second oil passages  5   a  and  5   b . As best seen in FIG. 12, a seal  66  is disposed between each bush  65  and the external surface of piston  64 . A seal  67  is disposed between each bush  65  and center section  5 . Seals  66  and  67  prevent oil from leaking. An oil passage  64   b  is bored along the axis of piston  64 . A discharging oil hole  64   c  having a diameter smaller than oil passage  64   b , is formed in the closed end of oil passage  64   b . Each oil passage  5   a  and  5   b  can be opened to the outside of the closed fluid circuit through oil passage  64   b  and discharging oil hole  64   c.    
     Movable swash plate  11  is constructed for slanting movement. As shown in FIGS. 1,  2  and  10 , a bush  51  is disposed on a side wall of upper half housing  1  positioned on a phantom extension line of the center of curvature X of convex portion  11   c  at a back surface of movable swash plate  11 . Bush  51  rotatably supports a control shaft  35 . Onto the outer end of control shaft  35 , outside of the housing is fixed a control lever  38  to enable movable swash plate  11  to be slantingly operated from the exterior of the housing. Control lever  38  is connected through a control rod (not shown) and may be pushed or pulled longitudinally to control a speed changing member (not shown) of the vehicle, such a lever or a pedal (not shown) provided on the vehicle. 
     As shown in FIG. 10, a control arm  39  is fixed to the inner end of control shaft  35  within the housing, and comprises a first arm  39   a , a second arm  39   b  and a radially extending, fan-shaped contact plate  39   c . Contact plate  39   c  may be divided from control arm  39  so long as contact plate  39   c  rotates following control arm  39 . 
     As best seen in FIG. 11, first arm  39   a  extends horizontally and forms, at one end thereof, an engaging portion  39   a ′ for paralleling control shaft  35 . Second arm  39   b  extends upwardly and forms, at one end thereof, an engaging portion  39   b ′ for paralleling control shaft  35 . Engaging portions  39   a ′ and  39   b ′ project in opposite directions from each other. 
     Engaging portion  39   b ′ is connected to a groove  11   d  provided in a side surface of movable swash plate  11 . Groove  11   d  is formed between a pair of engaging projections le disposed on the side surface of movable swash plate  11  and are longitudinally spaced apart at a predetermined interval. In such a construction, when control arm  38  is rotated around an axis lateral to the vehicle body, resulting in being rotated longitudinally of the vehicle body, control arm  39  rotates longitudinally around control shaft  35  to enable movable swash plate  11  to be slantingly operated and the hydraulic pump to be operated to change the quantity and direction of its discharging oil. 
     As shown in FIGS. 2 and 10, a coiled neutral return spring  31  is fitted onto bush  51 . Both ends of neutral return spring  31  are crossed to extend in the direction of first arm  39   a  and sandwich between them an eccentric shaft  33 . As shown in FIG. 1, eccentric shaft  33  is mounted to an inside wall of upper half housing  1  near control shaft  35  and engaging portion  39   a ′ formed at one end of control arm  39 . 
     Accordingly, when control lever  38  is turned to change the vehicle speed, control arm  39  is turned and one end of neutral return spring  31  is moved away from the other end, which is received by eccentric shaft  33 , thereby applying a biasing force to control lever  38  to return it to the neutral position. When operating force to the speed changing member is released, a restoring force generated at one end of neutral return spring  31  returns engaging portion  39   a ′ toward eccentric shaft  33  so as to hold control arm  38  in the neutral position. The extension of eccentric shaft  33  outside of the housing creates an adjusting screw. When the adjusting screw is loosened and eccentric shaft  33  is rotatably shifted, control arm  39  is shifted around control shaft  35  through neutral return spring  31  so that movable swash plate  11  can be adjusted to be in an accurate neutral position. 
     Contact plate or shutting member  39   c  is fan-shaped around a center of curvature X so as to abut against pistons  64  along its entire rotational range between the furthest forward and furthest rearward positions, including a neutral position. As shown in FIGS. 10,  11  and  12 , grooves  39   d  are formed in contact plate  39   c  at positions which abut against discharging oil holes  64   c  of pistons  64  when control arm  39  is in the neutral position, and extend therefrom to the fringe of fan-shaped contact plate  39 . 
     The interior of the closed fluid circuit is connected with the oil sump in the housing through grooves  39   d  and discharging oil holes  64   c . When control arm  39  is rotated from the neutral position so that movable swash plate  11  is slantingly rotated beyond a predetermined angle, discharging oil holes  64   c  are cut off from contact with grooves  39   d . The surface of contact plate  39   c  abutting oil holes  64   c  at this time is smooth and plain. As shown in FIG. 2, a retaining plate  68  is disposed at a side of contact plate  39   c  opposite to oil holes  64   c  and is fixed to the inner portion of lower half housing  2 . When pistons  64  are advanced by oil pressure, contact plate  39   c  is sandwiched between pistons  64  and retaining plate  68  so as to be given a rotational resistance against the biasing force of neutral return spring  31 . 
     Alternatively, contact plate  39   c  may be disposed between pistons  64  and the inner wall of lower half housing  2 , without a retaining plate  68  so as to provide rotational resistance directly by the inner wall of lower half housing  2 . Further, rather than being screwed into bushes  65  in the open ends of first oil passage  5   a  and second oil passage  5   b , pistons  64  can be directly, slidably inserted into the open ends of first oil passage  5   a  and second oil passage  5   b , as shown in FIG.  13 . 
     When control lever  38  is rotated by operating a speed changing member, control arm  39  is rotated by control shaft  35  so as to slantingly rotate movable swash plate  11  which is connected by engaging portion  39   b ′ to engaging projections  11   e  thereof, thereby changing the quantity of fluid discharged from the hydraulic pump. Accordingly, the rotational direction and speed of motor shaft  4  of the hydraulic motor is shifted to correspond with the rotational direction and degree of the speed changing member, so as to transmit driving force to axles  7 . 
     In this case, whichever of first oil passage  5   a  and second oil passage  5   b  has higher pressure oil receives pressure in proportion to the load on axles  7  so that one of pistons  64  slides outwardly and pushes against contact plate  39   c  of control arm  39 . The friction force generated by such pushing is set to be smaller than the biasing force of neutral return spring  31 . Therefore, an operator must operate the speed changing member with an operating force exceeding the friction force and the biasing force. After movable swash plate  11  is slantingly rotated beyond the predetermined position, discharging oil holes  64   c  of pistons  64  are sealed by the smooth and plain surface of contact plate  39   c , whereby operating oil circulating in the closed fluid circuit does not leak therefrom so as to maintain the volume efficiency of the HST. 
     In such a condition, when the operator releases operating force applied to the speed changing member, control arm  39  is rotated toward the neutral position by the biasing force of neutral return spring  31 . Pressure of piston  64  generates a friction force against contact plate  39   c  of contact plate  39 , as above mentioned, causing a resistant against the rotation toward the neutral position. Thereby, control arm  39  is gradually rotated toward the neutral position. As a result, a dynamic brake is not applied so that a vehicle does not stop suddenly. 
     When control arm  39  reaches the proximity of the neutral position, discharging oil holes  64   c  of pistons  64  communicate with grooves  39   d  so that the pushing force of pistons  64  against contact plate  39   c  and any remaining pressure in the closed fluid circuit escapes, thereby moderating the braking shock and enlarging the range of the neutral position of the HST. 
     One of pistons  64  is disposed in the high-pressure oil passage for running the vehicle in a forward direction, the other is disposed in the high-pressure oil passage running the vehicle in a reverse direction. Both of the high-pressure oil passages, first oil passage  5   a  and second oil passage  5   b , are separated from each other so that an operational condition of one piston  64  does not interfere with that of the other piston  64 . Hence, the operational condition of each piston  64  can be adjusted individually so as to enable each of the friction forces against contact plate  39   c  to meet the individual requirements to braking the vehicle when advancing and when backing the vehicle up. As a result, a vehicle can avoid stop shock when braking both in the case of an advancing vehicle and one that is backing up. 
     The operational condition of each piston  64  can be adjusted by modifying the diameter of discharging oil hole  64   c  and/or the width or shape of groove  39 d. In FIG. 14 is disclosed an alternative embodiment of the present invention in which the shape of groove  39   d  has been modified. In this embodiment, the depth of the grove varies in that groove  39   d  is shallower the closer it is to communicating with discharging oil hole  64   c . This modification of groove  39   d  can also be applied to the embodiment of the invention in which bush  65  is interposed between piston  64  and center section  5 , as shown in FIG.  12 . 
     As shown in FIGS. 8 and 9, the above mentioned center section  5  includes first oil passage  5   a  and second oil passage  5   b  in parallel to each other on a common horizontal plane. In FIGS. from  15 - 20 , an alternative embodiment of the present invention will be described which comprises a center section  5 ′ instead of center section  5 . In this embodiment, center section  5 ′ includes a horizontal first oil passage  5 ′ a  and a horizontal second oil passage  5 ′ b  disposed in parallel to each other along a common vertical plane, so that first and second oil passages  5 ′ a  and  5 ′ b  overlap with each other as seen in the cross-sectional plan view of FIG.  16 . As best seen in FIG. 17, each of kidney ports  41 ′ a  and  41 ′ b  which are open at a motor mounting surface  41 ′ formed on the vertical portion  501 ′ of center section  5 ′, communicates with one end of each of first and second oil passages  5 ′ a  and  5 ′ b , respectively. 
     Kidney ports  40 ′ a  and  40 ′ b  are open at a pump mounting surface  40 ′ formed on a horizontal portion  500 ′ of center section  5 ′. Kidney port  40 ′ a  is open above first oil passage  5 ′ a  and extends downwardly to directly communicate with oil passage  5 ′ a . As best seen in FIG. 18, kidney port  40 ′ b  is above, but to one side of second oil passage  5 ′ b  and communicates with second oil passage  5 ′ b  through a connecting oil passage  40 ′ c  which is slantingly and downwardly disposed from the outside of center section  5 ′ to second oil passage  5 ′ b . The outer open end of connecting oil passage  40 ′ c  is closed by a plug  69 . 
     As seen in FIG. 17, the other ends of first and second oil passages  5 ′ a  and  5 ′ b  are open along an outside surface of horizontal portion  500 ′. A check valve  54  is disposed in each of the open ends of oil passages  5 ′ a  and  5 ′ b  for supplying oil to the closed fluid circuit. Each check valve  54  is closed by a plug  70 . Each outer end of plug  70  abuts against a projection  71  formed along an inner surface of the housing to prevent the plugs  70  from slipping out. 
     A supply port  5 ′ g  is open at the lower surface of horizontal portion  500 ′ and extends upwardly within center section  5 ′ communicating with the entrance ports of check valves  54  in both first and second oil passages  5 ′ a  and  5 ′ b . Supply port  5 ′ g  opens within an oil filter  56  disposed between the bottom of center section  5 ′ and the bottom surface of lower half housing  2 , as the previously described embodiments. Each of first and second oil passages  5 ′ a  and  5 ′ b  is supplied with oil from the housing which is filtered by oil filter  56  through supply port  5 ′ g  and check valves  54 . 
     Oil holes  5 ′ e  and  5 ′ f  are horizontally branched from first and second oil passages  5   a ′ and  5 ′ b , respectively toward one side surface of center section  5 ′. The outer opening end of each of oil holes  5 ′ e  and  5 ′ f  is provided with a piston  64 . In this embodiment, a flange and bush, as in the previous embodiments are not used. 
     Both oil holes  5 ′ e  and  5 ′ f  are disposed in parallel to each other with their axes along a common vertical plane so that pistons  64  are disposed in a vertical row, as shown in FIGS. 18 and 20. Accordingly, only one groove  39 ′ d  is formed on contact plate  39 ′ along a surface which abuts against pistons  64  and communicates with both discharging oil holes  64   c  of pistons  64  simultaneously when contact plate  39 ′ c  is positioned in the neutral position. Because contact plate  39 ′ c  has only one groove  39 ′ d , it can be more narrow as compared with contact plate  39   c  of the previous embodiments so as to make the space around it more compact. 
     When it is desired to moderate the difference in braking shock caused by advancing and backing of the vehicle, the braking shocks may be individually moderated by modifying the diameter of discharging oil holes  64   c  of pistons  64 . 
     Center sections  5  and  5 ′ mentioned above, which form pump mounting surfaces  40  and  40 ′, respectively and motor mounting surfaces  41  and  41 ′, respectively which are perpendicular to each other, may be adapted to an HST having a pump shaft and a motor shaft which are disposed in parallel to each other. Such a center section is shown in FIGS. 21 and 22 in which center section  5 ″ forms a plate for an HST having a pump shaft  3  and a motor shaft  4  which are disposed in parallel to each other. 
     Center section  5 ″ is formed as a thin plate. Both a pump mounting surface  40 ′ and a motor mounting surface  41 ″ are formed along a the top surface thereof. A first oil passage  5 ″ a  and a second oil passage  5 ″ b  are bored in center section  5 ″ below pump mounting surface  40 ″ and motor mounting surface  41 ″. First and second oil passages  5 ″ a  and  5 ″ b  are disposed in parallel to each other on a common horizontal plane. Kidney ports  40 ″ a  and  40 ″ b  are open at pump mounting surface  40 ″. Kidney ports  41  ″ a  and  41 ″ b  are open at motor mounting surface  41 ″. Kidney ports  40 ″ a  and  41 ″ a  extend downwardly and communicate with first oil passage  5 ″ a . Kidney ports  40 ″ b  and  41 ″ b  extend downwardly and communicate with second oil passage  5 ″ b.    
     An oil hole  5 ″ c  and an oil hole  5 ″ d  extend downwardly from first and second oil passages  5 ″ a  and  5 ″ b , respectively. In the opening of each of oil holes  5 ″ c  and  5 ″ d  which are open at the bottom surface of center section  5 ″ is disposed a check valve  54  so as to enable operating oil to be supplied from the oil sump in the housing to each of first and second oil passages  5 ″ a  and  5 ″ b.    
     One end of each of first and second oil passages  5 ″ a  and  5 ″ b  is open at one side surface of center section  5 ″ and is closed by plug  70 . Each plug  70  abuts against a projection  71  formed at the interior of the housing. First and second oil passages  5 ″ a  and  5 ″ b  differ in length so that the other end of first passage  5 ″ a  is offset from the other end of second oil passage  5 ″ b . Horizontal oil holes  5 ″ e  and  5 ″ f  are branched perpendicularly from approximate the inner ends of first and second oil passages  5 ″ a  and  5 ″ b , respectively. Both oil holes  5 ″ e  and  5 ″ f  are open at another side surface of center section  5 ″. A piston  64 , as discussed above, is disposed in each opening of oil holes  5 ″ e  and  5 ″ f . At the exterior side of pistons  64  is disposed contact plate  39  having a pair of grooves  39   d , as discussed above. 
     Another embodiment of an axle driving apparatus will be described with reference to FIGS. 23,  24  and  25 . The HST of the axle driving apparatus of previously described embodiments includes a pump shaft  3  and motor shaft  4  which are disposed perpendicular to each other and have a movable swash plate  11  of a cradle-type which is separated from a control arm  39  or  39 ′ for the hydraulic pump. The axle driving apparatus of this alternative embodiment has a center section  5 ′, as shown in FIGS. 21 and 22 for supporting pump shaft  3  and motor shaft  4  in parallel to each other and has a trunnion-type movable swash plate  11 ′ which forms a control shaft as single body. Center section  5 ″ in FIGS. 21 and 22 differs from that in FIG. 23 and 24 in, among other things, appearance and the shape of the oil holes, however, the technical idea of the latter center section  5 ″ is the same as that of the former. 
     With specific reference to FIGS. 23-25, center section  5 ″ is disposed within a lower half housing  2 ′. On pump mounting surface  40 ″ and motor mounting surface  41 ″ formed on the upper surface thereof, is mounted a cylinder block for the hydraulic pump and a cylinder block for the hydraulic motor, respectively, thereby constituting an HST. Pump shaft  3  is connected with cylinder block  16  of the hydraulic pump and is vertically disposed and rotatably supported through bearings  75  at the upper portion of upper half housing  1 ′. The lower end thereof is rotatably inserted into center section  5 ″. Movable swash plate  11 ′, which is of a trunnion-type, is disposed above cylinder block  16  in upper half housing  1 ′. 
     The entire movable swash plate  11 ′ is shown in FIG. 25. A pair of trunnion shafts  11 ′ a  and  11 ′ b  are formed on movable swash plate  11 ′ and project in opposite directions from both sides thereof. A fan-shaped contact plate  11 ′ c , equivalent to contact plate  39   c , is formed below the base end of trunnion shaft  11 ′ b  , equivalent to control shaft  35 . A pair of grooves  11 ′ d , equivalent to grooves  39   d , are formed at an inner surface of contact plate  11 ′ c.    
     In upper half housing  1 ′, trunnion shaft  11 ′ a  is supported by a side wall thereof through a bush  73 . Trunnion shaft  11 ′ b  is supported by a lid  72  attached to upper half housing  1 ′ through an other bush  74 . Control lever  38  is fixed to the exterior portion of trunnion shaft  11 ′ b  projecting from lid  72 . The surface of contact plate  11 ′ c  forming the pair of grooves  11 ′ d  abuts against the utmost ends of pistons  64  inserted into center section  5 ″. Restraining plate  68  is interposed between the inner wall of lower half housing  2 ′ and contact plate  11 ′ c . Such a construction constitutes a neutral return position member for returning the movable swash plate  11 ′ to the neutral position effecting the same as that comprising pistons  64  and contact plate  39   c , described above. 
     Next, an axle driving apparatus having center section  5 ′ as described in FIGS. 15 through 20 will be explained in accordance with FIGS. 26 through 58. The members whose construction and function are identical and similar with those shown in FIGS. 1 through 20 are marked with the same reference numerals and the detailed descriptions of them are hereinafter omitted. 
     As shown in FIGS. 26 through 28, center section  5 ′, which is substantially similar to that shown in FIGS. 15 through 20, is compactly disposed in first chamber R 1  within the housing of the axle driving apparatus. Hydraulic pump  45  having vertical pump shaft  3  is disposed between axles  7  and hydraulic motor  46  having horizontal motor shaft  4  is disposed parallel to axles  7 . 
     As shown in FIGS. 27 and 28 according to this embodiment, first chamber R 1  and second chamber R 2  are filled with oil so as to constitute an oil sump. Chambers R 1  and R 2  are connected with each other through a passage  9  so as to allow the inner oil of housing to interflow between chambers R 1  and R 2 . Passage  9  is covered at the open top thereof with a lid  78  and provided on the bottom thereof with a seat  63 . A discoid oil filter  77  is vertically disposed between lid  78  and seat  63  for removal of such impurities as metal fragments, which are generated by the rubbing of gears against each other, from oil in chambers R 1  and R 2 . 
     Center section  5 ′ is provided therein with first and second oil passages  5 ′ a  and  5 ′ b  horizontally disposed in a vertical row as the above. Oil holes  5 ′ e  and  5 ′ f  horizontally branch in a vertical row from oil passages  5 ′ a  and  5 ′ b , respectively, and are plugged at the outward open ends thereof with an upper piston  64 U and a lower piston  64 L, respectively. Pistons  64 U and  64 L constitute an internal damping system (IDS) which is provided mainly for moderating the dynamic braking shock during returning of the speed changing member to neutral and secondly for moderating the shock on starting of advancing and reversing. 
     For this embodiment, as shown in FIG. 29, pistons  64 U and  64 L, which are bored therein by respective outward open upper and lower orifices  80 U and  80 L serving as the above-mentioned discharging oil holes  64   c , are slidably fitted in outward open cylindrical portions  5 ′ h  formed at the ends of oil holes  5 ′ e  and  5 ′ f , respectively. Seal  66  is interposed between the wall of cylindrical portion  5 ′ h  and the periphery of each of pistons  64 U and  64 L for avoiding an oil leak. Coiled springs  79  are interposed between the inner ends of pistons  64 U and  64 L and the inner ends of cylindrical portions  5 ′ h  so as to bias pistons  64 U and  64 L outwardly. The biasing force of coiled spring  79  is set to be smaller than that of coiled and twisted neutral return spring  31  winding around control shaft  35 . 
     Next, referring to FIGS. 30 through 62, explanation will be given on various embodiments and modifications regarding control arm  39 ′ with contact plate  39 ′ c  suiting pistons  64 U and  64 L fitted in center section  5 ′ applied in the embodiment shown in FIGS. 26 through 29. 
     A preferred first embodiment of control arm  39 ′, shown in FIGS. 30 and 31, has contact plate  39 ′ c , which is provided on the surface thereof facing pistons  64 U and  64 L with vertical groove  39 ′ d  similar to that shown in FIG.  20 . Vertical groove  39 ′ d has a width which is larger than the diameters of orifices  80 U and  80 L. When control arm  39 ′ is positioned in neutral, orifices  80 U and  80 L are in communication with vertical groove  39 ′ d , so that oil within the closed fluid circuit is released from orifices  80 U and  80 L through vertical groove  39 ′ d  into the oil sump formed in the housing of the axle driving apparatus. Thus, even if an error with respect to the neutral positioning of movable swash plate  11  occurs so that the closed fluid circuit is wrongly supplied with oil by hydraulic pump  45 , hydraulic motor  46  is prevented from wrong slight rotation. 
     When the vehicle is left on a slope or slanted ground when the HST is in neutral and axles  7  are not locked for parking, the weight of vehicle is applied on axles  7  so as to rotate them in the descending direction. Then, the load on axles  7  generates a back-pressure in the closed circuit through hydraulic motor  46 . In this case, if the inner oil of the closed circuit is drained through wide-open orifices  80 U and  80 L and vertical groove  39 ′ d  into the oil sump, such reduced hydraulic pressure cannot hold hydraulic motor  46 , whereby hydraulic motor  46  freely follows the rotation of axles  7  and the vehicle descends the slope. However, each of orifices  80 U and  80 L of this embodiment is made smaller with its diameter than conventional one, whereby the oil cannot be drained therefrom perfectly. Hence, hydraulic motor  46  is held by the increased hydraulic pressure against the rotational force of axles  7  generated by the weight of vehicle, thereby enabling the vehicle to stay. 
     As shown in FIGS. 30 and 31, contact plate  39 ′ c  is provided with upper and lower first transverse grooves  39 ′ g  and  39 ′ h  and upper and lower second transverse grooves  39 ′ i  and  39 ′ j  branching transversely from vertical groove  39 ′ d.    
     The relation of arrangement between first and second oil passages  5 ′ a  and  5 ′ b  may be reversed. However, this embodiment and others hereinafter will be described on the premise that first oil passage  5 ′ a  is disposed above second oil passage  5 ′ b . It will be understood that in the rotational range of control arm  39 ′ for advancing, first oil passage  5 ′ a  is lower-pressured and second oil passage  5 ′ b  is higher-pressured during acceleration, however, first oil passage  5 ′ a  becomes pressured higher than second oil passage  5 ′ b  because of back-pressure occurred by the rotation of hydraulic motor  46  following the rotation of axles  7  during deceleration by the returning of control arm  39 ′ to neutral. Similarly, in the rotational range of control arm  39 ′ for reversing, second oil passage  5 ′ b , which is lower-pressured during acceleration, becomes pressured higher than first oil passage  5 ′ a  during the returning of control arm  39 ′ to neutral. 
     For this embodiment, upper first transverse groove  39 ′ g  having a slight range is disposed so as to communicate with upper orifice  80 U of upper piston  64 U connected to first oil passage  5 ′ a  when control arm  39 ′ is positioned in its rotational range for reversing (hereinafter, reversing range R). Lower first transverse groove  39 ′ h  having a slight range is disposed so as to communicate with lower orifice  80 L of lower piston  64 L connected to second oil passage  5 ′ b  when control arm  39 ′ is positioned in its rotational range for advancing (hereinafter, advancing range A). An angle α1 which the center line of the width of vertical groove  39 ′ d  passing center  0  of control shaft  35  (when control arm  39 ′ is in neutral position N, the center line is common with a neutral line NL passing center  0  and centers of orifices  80 U and  80 L) forms with the line passing the utmost end of upper first transverse groove  39 ′ g  and center  0  is as large as an angle α  2  which the same center line forms with the line passing the utmost end of lower first transverse groove  39 ′ h  and center  0 . 
     Angles α1 and α2 may be different. They can be optionally determined so as to enable the advancing and reversing vehicle to start without a shock. 
     On the start of advancing, the increase of hydraulic pressure closed fluid circuit within center section  5 ′ is slightly delayed because higher-pressured second oil passage  5 ′ b  is in communication with lower first transverse groove  39 ′ h  through lower orifice  80 L, and after control arm  39 ′ is rotated so that lower orifice  80 L passes the end of lower first transverse groove  39 ′ h , the amount of oil supplied to hydraulic motor  46  by hydraulic pump  45  is increased so much as to enable the vehicle to start. On the start of reversing, similarly, the hydraulic pressure within the closed fluid circuit reaches the degree capable of driving hydraulic motor  46  after control arm  39 ′ is rotated so as to let upper orifice  80 U pass the end of upper first transverse groove  39 ′ g . Thus, the starts of advancing and reversing are comfortably moderated. 
     Upper second transverse groove  39 ′ i  having the predetermined range is disposed so as to communicate with upper orifice  80 U of upper piston  64 U connected to first oil passage  5 ′ a  when control arm  39 ′ is positioned in its advancing range A. Lower second transverse groove  39 ′ j  having the predetermined range is disposed so as to communicate with lower orifice  80 L of lower piston  64 L connected to second oil passage  5 ′ b  when control arm  39 ′ is positioned in its reversing range R. An angle of β1 which the center line of vertical groove  39 ′ d  passing center  0  of control shaft  35  forms with the line passing the utmost end of upper second transverse groove  39 ′ i  and center  0  is as large as an angle β2 which the same center line forms with the line passing the utmost end of lower second transverse groove  39 ′ j  and center  0 . 
     Angles β1 and β2 may be different. They can be optionally determined so as to enable the advancing and reversing vehicle to stop without a braking shock. 
     When a speed changing member on the vehicle is released by an operator during advancing or reversing of the vehicle, control arm  39 ′ returns to neutral by biasing of neutral return spring  31 . In case of the absence of second transverse grooves  39 ′ i  and  39 ′ j  so far, until control arm  39 ′ reaches its neutral position, a dynamic brake regarded as an engine brake is applied so as to decelerate the vehicle rapidly. When reaching the neutral position, the pressure oil in the closed fluid circuit is drained from both of orifices  80 U and  80 L to the oil sump through vertical groove  39 ′ d , whereby the hydraulic pressure is reduced suddenly so that hydraulic motor  46  is made freely rotatable and axles  7  run idle. Accordingly, in case of deceleration in advancing, an operator leans forward until reaching neutral, and when reaching neutral, the decelerating force is suddenly lost, thereby making the operator bend backward. Thus, the operator is forced to change his/her posture greatly, and it will make him/her very uncomfortable. 
     In case of employing the above constructed contact plate  39 ′ c  having second transverse grooves  39 ′ i  and  39 ′ j , when the speed changing member is released during advancing (or reversing), the increased pressure oil in one of oil passages  5 ′ a  ( 5 ′ b ), which is pressured higher by back-pressure generated from self-rotating of hydraulic motor  46  while it is set to be lower-pressured by movable swash plate  11 , is drained from corresponding orifice  80 U ( 90 L) to the oil sump through corresponding second transverse grooves  39 ′ i  ( 39 ′ j ) for a short time before returning control arm  39 ′ reaches its neutral position N. Hence, the back pressure in the closed circuit is reduced until it reaches the peak thereof, thereby enabling the vehicle to stop smoothly. 
     The hydraulic action and its effect on the movement of vehicle in relation to the whole rotational positions of control arm  39 ′ will be described according to FIGS. 32,  33  and  35 . 
     In FIG. 35, “FO” designates the condition that orifice  80 U or  80 L is fully open toward the oil sump when it is located in one of grooves  39 ′ d ,  39 ′ g ,  39 ′ h ,  39 ′ i  and  39 ′ j . “PO” designates that orifice  80 U or  80 L is partly open toward the oil sump when it is put on the end of one of first and second transverse grooves  39 ′ g ,  39 ′ h ,  39 ′ i  and  39 ′ j  so as to be partly shut by contact plate  39 ′ c . And “S” designates that orifice  80 U or  80 L is completely shut by contact plate  39 ′ c . The same designations are used in FIGS. 38,  41 ,  44 ,  46 ,  48 ,  51 ,  53 ,  55  and  58 . 
     The attitudes of control arm  39 ′ in the rotational movement thereof marked as (a)-(g) shown in FIG. 32,  33  and  35  will be employed in similarly in all embodiments of control arm  39 ′ hereinafter. 
     Referring to FIGS. 32 and 35, the hydraulic action and its effect on the movement of vehicle in the advancing range A of control arm  39 ′ will be described. 
     When an operator operates the speed changing member provided on the vehicle for acceleration in advancing so as to rotate control lever  38  of the axle driving apparatus from its neutral position into its advancing range, control arm  39 ′ is rotated together with control lever  38  and control shaft  35  so as to change its position from (a) to (d) through (b) and (c) in FIG.  32 . 
     During the rotation of control arm  39 ′, movable swash plate  11  of hydraulic pump  45  is shifted so as to accelerate the advancing rotation of hydraulic motor  46  with motor shaft  4 , thereby accelerating the advancing rotation of axles  7 . Higher-pressured second oil passage  5 ′ b  is back-pressured further in proportion to the load applied on axles  7 , thereby making lower piston  64 L slide outwardly and press against contact plate  39 ′ c . The friction force between friction plate  68  and contact plate  39 ′ c  pressed there against through lower piston  64 L by the maximum hydraulic pressure and the biasing force of coiled spring  79  is set to be smaller than the biasing force of neutral return spring  31 , so that control arm  39 ′ c  automatically returns to neutral when releasing the speed changing member. For acceleration of the advancing speed, an operator must apply an operating force which exceeds the amount of such friction force and the biasing force of neutral return spring  31 , onto the speed changing member. 
     At the position (a), both of orifices  80 U and  80 L are in communication with vertical groove  39 ′ d , so that even if hydraulic pump  45  wrongly discharges oil into the closed circuit by an error of connecting means or linkage so as to apply hydraulic pressure to either oil passage  5 ′ a  or  5 ′ b  higher than the predetermined, the increased pressure oil therein. is drained to the oil sump through either of orifices  80 U and  80 L. Also, when the vehicle is left in neutral on a slope so that hydraulic motor  46  is wrongly driven so as to apply back-pressure to either oil passage  5 ′ a  or  5 ′ b , the increased pressure oil therein cannot be drained perfectly therefrom because orifices  80 U and  80 L have such small diameters as described above, thereby preventing the vehicle from suddenly descending the slope. 
     During the rotation of control arm  39 ′ from the position (a) to the position (b), second oil passage  5 ′ b  is essentially higher pressured slightly by hydraulic pump  45 , however, it is the fact that the hydraulic pressure therein tends to become higher than the predetermined base on the position of movable swash plate because of the load of staying axles  7  thereby causing the vehicle to start suddenly. According to this embodiment, lower orifice  80 L enters lower first transverse groove  39 ′ h , so that part of pressure oil in second oil passage  5 ′ b  by the back-pressure is drained, thereby moderating the starting shock. In addition, upper orifice  80 U in communication with lower-pressured first oil passage  5 ′ a  enters upper second transverse groove  39 ′ i.    
     When passing the position (c), lower orifice  80 L passes the end of lower first transverse groove  39 ′ h , so that lower piston  64 L pushes contact plate  39 ′ c  to shut lower orifice  80 L, whereby higher-pressured second oil passage  5 ′ b  is increased in its hydraulic pressure. However, upper orifice  80 U is still open to second transverse groove  39 ′ i.    
     When reaching the position (d), upper orifice  80 U also passes the end of upper second transverse groove  39 ′ i  so as to be shut by contact plate  39 ′ c , thereby enabling the entire capacity of the HST to perform advancing acceleration. In this state, contact plate  39 ′ c  is slidably rotated against friction plate  68 , thereby preventing control arm  39 ′ from rapid rotation. Hence, the vehicle is prevented from a rapid acceleration. 
     In case that an operator releases the operating force applied on the speed changing member during advancing so as to automatically rotate control arm  39 ′ from the position (d) to the position (a) through the positions (c) and (b), in other words, in case of returning to neutral, the rotation of axles  7 , whose force exceeds that of motor shaft  4 , drives hydraulic motor  46  so as to function as a hydraulic pump, whereby lower-pressured first oil passage  5 ′ a  is changed to be pressured higher than second oil passage  5 ′ b . In this condition, after control arm  39 ′ passes the position (c), the excessive pressure oil in first oil passage  5 ′ a  is drained through upper orifice  80 U and upper second transverse groove  39 ′ i , thereby preventing the vehicle from sudden deceleration. Thus, the vehicle is moderated in its deceleration and stopping. 
     Referring to FIGS. 33 and 35, the hydraulic action and its effect on the movement of vehicle in the reversing range R of control arm  39 ′ will be described. In case of reversing acceleration from neutral, an operator shifts the speed changing member so as to rotate control arm  39 ′ from the position (a) to the position (g) through the positions (e) and (f). On starting, control arm  39 ′ is positioned at the position (e), part of oil in higher pressured first oil passage  5 ′ a , which is increased by hydraulic pump  45  and the load of axles  7 , is drained through upper orifice  80 U and first transverse groove  39 ′ g , thereby preventing the vehicle from sudden start. In case of reversing deceleration by releasing the speed changing member so as to rotate control arm  39 ′ by biasing of neutral return spring  31  from the position (g) to the position (a) through the positions (f) and (e), lower orifice  80 L enters lower second transverse groove  39 ′ j  after control arm  39 ′ passes the position (f), so that the excessive pressure oil in second oil passage  5 ′ b , which is pressured higher by the rotational force of axles  7 , is drained, thereby preventing the vehicle from sudden deceleration and rapid stopping. 
     The first embodiment of control arm  39 ′ (including the following modifications thereof) comprising contact plate  39 ′ c , vertical grooves  39 ′ d  and second transverse grooves  39 ′ i  and  39 ′ j  between the position (c) and the position (f) through the position (a) as neutral position N constitutes the valves shown in FIG. 34 which control the oil releasing of orifices  80 U and  80 L. In FIG. 34, first transverse grooves  39 ′ g and  39 ′ h are out of consideration. The orifice opening and closing action of control arm  39 ′ is controlled together with movable swash plate  11  of hydraulic pump  45  by operation of speed changing member and biasing of neutral return spring  31 . According to the opening and closing action, both upper and lower orifices  80 U and  80 L are fully open to the oil sump at neutral position N. In advancing range A, upper orifice  80 U is opened to drain the pressure oil from first oil passage  5 ′ a  and lower orifice  80 L is closed. In reversing range B, lower orifice  80 L is opened to drain the pressure oil from second oil passage  5 ′ b  and upper orifice  80 U is closed. 
     Next, various modifications of the first embodiment of control arm  39 ′ as shown in FIGS. 30 and 31 will be described according to FIGS. 36 through 41. 
     A first modification thereof shown in FIG. 36, in which first transverse grooves  39 ′ g  and  39 ′ h  are eliminated, is adapted to such case that the moderation of starting shock does not have to be considered. In FIG. 35, “PO” of upper orifice  80 U in the position (e) and lower orifice  80 L in the position (b) are changed into “S”. 
     With regard to a second modification shown in FIG. 37, vertical groove  39 ′ d  and first transverse grooves  39 ′ g  and  39 ′ h  are similar to the preferred first embodiment shown in FIGS. 30 and 31. One of second transverse grooves  39 ′ i  and  39 ′ j  is extended so that either orifice  80 U or  80 L is in communication with the extended second transverse groove at any position of one of advancing and reversing ranges A and R of control arm  39 ′. In the case shown in FIG. 38, an extending groove  39 ′ n  is formed so as to extend from the end of lower second transverse groove  39 ′ j , whereby lower orifice  80 L is in communication with either  39 ′ j  or  39 ′ n  at all the positions between the neutral position N and the maximum position of reversing range R of control arm  39 ′. Lower second transverse groove  39 ′ j  and extending groove  39 ′ n  are formed in a bending shape corresponding to the locus of lower orifice  80 L. They may be formed in an arcuate shape. Accordingly, when the speed changing member in the reversing range is released, the excessive pressure oil in second oil passage  5 ′ b  is drained through lower orifice  80 L and the series of extending groove  39 ′ n  and lower second transverse groove  39 ′ j  in all the reversing range R of control arm  39 ′, so that dynamic braking is rarely applied during deceleration of reversing between the maximum position and the neutral position. Such variation of opening condition of orifices  80 U and  80 L in relation to the whole rotational positions of control arm  39 ′ according to the second modification of the first embodiment is shown in FIG.  38 . 
     With regard to a third modification shown in FIG. 39 serving as a modification of the second modification shown in FIG. 37, extending grooves  39 ′ m  and  39 ′ n  are extended from the ends of both second transverse grooves  39 ′ i  and  39 ′ j , respectively, so that the two series of grooves  39 ′ i  and  39 ′ m , and  39 ′ j  and  39 ′ n  form a bending shape corresponding to the locus of orifices  80 U and  80 L in all of the advancing and reversing rotational ranges A and R of control arm  39 ′. Thus, even in the event of returning to neutral from the maximum position of advancing and reversing of the speed changing member, the excessive pressure oil in the higher-pressured one of oil passages  5 ′ a  and  5 ′ b  is drained until the neutral position, thereby preventing the vehicle from dynamic braking during deceleration of advancing and reversing. 
     With regard to a fourth modification shown in FIG. 40, which is a modification of that shown in FIG. 39, the cross-sectional areas of second transverse grooves  39 ′ i  and  39 ′ j  and extending grooves  39 ′ m  and  39 ′ n  are large enough to drain the excessive pressure oil in the higher pressured one of oil passages  5 ′ a  and  5 ′ b  sufficiently, thereby allowing vertical groove  39 ′ d  to be removed. Hence, control arm  39 ′ becomes so simple as to be produced at low cost. 
     Such variation of conditions of orifices  80 U and  80 L in relation to the whole rotational positions of control arm  39 ′ according to the third and fourth modifications of the first embodiment is shown in FIG.  41 . 
     Next, explanation will be given on a second embodiment of control arm  39 ′ in accordance with FIGS. 42 through 48. As shown in FIGS. 43,  44 , 46  and  48 , the second embodiment of control arm  39 ′ is characterized in that both orifices  80 U and  80 L are partly open toward the oil sump in neutral position N. 
     A preferred second embodiment of control arm  39 ′ shown in FIG. 42 is provided on the surface of contact plate  39 ′ c  thereof with upper and lower second transverse grooves  39 ′ i  and  39 ′ j  which have the construction and function similar to the first embodiment. Vertical groove  39 ′ d  of the first embodiment is replaced with upper vertical groove  39 ′ d ′ and lower vertical groove  39 ′ d ″, which are connected with upper second transverse groove  39 ′ i  and lower second transverse groove  39 ′ j , respectively, and are open at the upper and lower ends thereof toward the oil sump. Orifices  80 U and  80 L are diametrically larger than those of the first embodiment, so that when the vehicle in neutral is left on a slope, the increased pressure oil by back pressure is drained so much as to become impossible to hold hydraulic motor  46 . It will be advantageous in manufacturing and will prevent orifices  80 U and  80 L, which are diametrically larger, from stop up. 
     Furthermore, the center line of the width of upper vertical groove  39 ′ d ′ is offset from the vertical line serving as the horizontal center of contact plate  39 ′ c , which is common with neutral line NL in neutral position N, toward upper second transverse groove  39 ′ i , and that of lower vertical groove  39 ′ d ″ is offset therefrom toward lower second transverse groove  39 ′ j , so that when control arm  39 ′ is at neutral position N, upper orifice  80 U is put on a vertical edge of upper vertical groove  39 ′ d ′ in opposite to upper second transverse groove  39 ′ i , and lower orifice  80 L is put on that of lower vertical groove  39 ′ d ″ in opposite to lower second transverse groove  39 ′ j . Thus, at the position (a) as neutral position N shown in FIGS. 43 and 44, the openings of orifices  80 U and  80 L are partly shut by contact plate  39 ′ c , so as to be tightened. According to such construction, when the vehicle in neutral is left on a slope, the increased pressure oil by back pressure in the closed fluid circuit is hard to be drained, thereby preventing the vehicle from easily descending the slope. 
     At the positions (b) and (e) of control arm  39 ′ shown in FIG. 44 as the starting of advancing and reversing, one of orifices  80 U and  80 L is fully open to corresponding one of grooves  39 ′ d ′,  39 ′ d ′,  39 ′ i  and  39 ′ j , and the other is shut by contact plate  39 ′ c , thereby enabling the vehicle to start for advancing and reversing swiftly. 
     At the positions (c) and (f) shown in FIG. 44, one of orifices  80 U and  80 L is put on the end of corresponding second transverse groove  39 ′ i  or  39 ′ j  so that the opening is tightened. During deceleration of advancing and reversing by returning to neutral, when control arm  39 ′ passes the positions (c) or (f), the excessive pressure oil in the higher-pressured oil passage  5 ′ a  or  5 ′ b  is drained through corresponding orifice  80 U or  80 L and second transverse groove  39 ′ i  or  39 ′ j , thereby moderating the decelerating shook. At the positions (d) and (g), both orifices  80 U and  80 L are shut so that the HST can be operated with the whole of its capacity. 
     Referring to FIGS. 45 through 48, modifications of the second embodiment of control arm  39 ′ will be described. 
     With regard to a first modification shown in FIG. 45, lower vertical groove  39 ′ d ″ is removed. Extending groove  39 ′ n  is extended from the end of lower second transverse groove  39 ′ j , whereby lower orifices  80 L is fully open to the series of grooves  39 ′ j  and  39 ′ n  during the whole reversing range R as shown in FIG. 46, so that the excessive pressure oil in higher-pressured second oil passage  5 ′ b  is drained so as to moderate the dynamic brake during deceleration of advancing. The opening patterns of orifices  80 U and  80 L in relation to the whole rotational positions of control arm  39 ′ of the first modification is shown in FIG.  46 . 
     With regard to a second modification shown in FIG. 47, upper vertical groove  39 ′ d ′ is also removed and extending groove  39 ′ m  is also extended from the end of upper second transverse groove  39 ′ i  in addition to the construction of the first modification. Due to this construction, upper and lower orifices  80 U and  80 L are fully open to the respective series of grooves  39 ′ i  and  39 ′ m , and  39 ′ j  and  39 ′ n  during the whole advancing and reversing ranges A and R as shown in FIG. 48, so that the excessive pressure oil in higher-pressured one of oil passages  5 ′ a  and  5 ′ b  is drained to moderate the dynamic brake during deceleration of advancing and reversing. 
     Next, explanation will be given on a third embodiment of control arm  39 ′in accordance with FIGS. 49 through 55. The third embodiment is characterized in that both orifices  80 U and  80 L are shut by contact plate  39 ′ c when control arm  39 ′ is located in neutral position N or the position (a) as shown in FIGS. 50,  51 ,  53  and  55 . 
     A preferred third embodiment of control arm  39 ′ shown in FIG. 49 is provided with second transverse grooves  39 ′ i  and  39 ′ j  formed similar to those of the first and second embodiments of control arm  39 ′. Upper vertical groove  39 ′ d ′, whose top is open to the oil sump, is offset from the vertical line constituting a horizontal center of contact plate  39 ′ c , which is common with neutral line NL passing center  0  and both the centers of orifices  80 U and  80 L in neutral position N, so as to be connected at the lower end thereof with the intermediate portion of upper second transverse groove  39 ′ i . Lower vertical groove  39 ′ d ″, whose lower end is open to the oil sump, is offset from the same vertical line so as to be connected at the upper end thereof with the intermediate portion of lower second transverse groove  39 ′ j . At the position (a) of control arm  39 ′ as neutral position N shown in FIGS. 50 and 51, both orifices  80 U and  80 L are shut by contact plate  39 ′ c , however, an orifice  81  is interposed between first and second oil passages  5 ′ a  and  5 ′ b  in the closed fluid circuit so as to be disposed in parallel to check valve  54  as shown in FIG. 50, thereby giving a dead zone of the HST in the neutral position thereof. The pressure oil, which is drained through orifice  81  at the neutral position, is so limited as to prevent the vehicle in neutral left on a slope from suddenly descending. 
     As shown in FIG. 51, in case of acceleration of advancing, at the position (b) of control arm  39 ′, lower orifice  80 L connected to higher-pressured second oil passage  5 ′ b  is shut by contact plate  39 ′ c , thereby enabling the vehicle to start swiftly. 
     In case of deceleration of advancing by returning to neutral, at the position (d) of control arm  39 ′, the closed fluid circuit is back pressured so that a dynamic brake is applied on the vehicle. At the position (c), upper orifice  80 U connected to higher-pressured first oil passage  5 ′ a  is put on the end of upper second transverse groove  39 ′ i  to be tightened with its opening and afterward enters it, thereby moderating the dynamic brake. Just before the neutral position as the position (b), upper orifice  80 U is tightened and is finally shut by contact plate  39 ′ c  at the neutral position (a), thereby applying the dynamic brake on the vehicle again so as to stop it. 
     In case of acceleration of reversing, at the position (e) of control arm  39 ′, upper orifice  80 U connected to higher-pressured first oil passage  5 ′ a  is shut by contact plate  39 ′ c , thereby enabling the vehicle to start swiftly. 
     In case of deceleration of reversing by returning to neutral, at the position (g) of control arm  39 ′ c , the closed fluid circuit is back pressured so that a dynamic brake is applied on the vehicle. At the position (f), lower orifice  80 L connected to higher-pressured second oil passage  5 ′ b  is put on the end of lower second transverse groove  39 ′ i  to be tightened with its opening and afterward enters it, thereby moderating the dynamic brake. Just before the neutral position as the position (e), lower orifice  80 L is tightened and is finally shut by contact plate  39 ′ c  at the neutral position (a), thereby applying the dynamic brake on the vehicle again so as to stop it. 
     Referring to FIGS. 52 through 55, modification of the third embodiment of control arm  39 ′ will be described. 
     For a first modification shown in FIG. 52, lower vertical groove  39 ′d″ is removed and extending groove  39 ′ n  is extended from the end of lower second transverse groove  39 ′ j . For a second modification shown in FIG. 54, lower vertical groove  39 ′ d ″ is similarly removed and extending groove  39 ′ m  is additionally extended from the end of upper second transverse groove  38 ′ i.    
     According to the first modification, the dynamic brake is moderated during deceleration of advancing by returning to neutral, and at the neutral position, dynamic brake is applied so as to stop the vehicle. According to the second modification, the same phenomenon also occurs during deceleration of advancing by returning to neutral. 
     Next, explanation will be given on a fourth embodiment of control arm  39 ′ in accordance with FIGS. 56 through 58. This is a mixture of first and third embodiments. In this regard, as shown in FIG. 56, vertical groove  39 ′ d  is divided into upper vertical groove  39 ′ d ′ and lower vertical groove  39 ′ d ″, which are open to the oil sump. Similar to the first embodiment, lower vertical groove  39 ′ d ″, which is located along neutral line NL in neutral position N, is connected with lower first and second transverse grooves  39 ′ h  and  39 ′ j . Similar to the third embodiment, upper vertical groove  39 ′ d ′ is offset so as to be connected with the intermediate portion of upper second transverse groove  39 ′ i , so that it does not communicate with upper orifice  80 U at neutral position N. 
     Due to such construction, as shown in FIGS. 57 and 58, at the position (a) as neutral position N, only lower orifice  80 L is fully open to the oil sump and upper orifice  80 U is shut by contact plate  39 ′ c , so as to limit the drained excessive pressure oil in the closed circuit in neutral position N. Hence, the vehicle in neutral left on a slope is prevented from descending. 
     At the position (b) as starting of advancing, the excessive pressure oil in higher-pressured second oil passage  5 ′ b  is drained through lower orifice  80 L which is fully open to first transverse groove  39 ′ h  and lower vertical groove  39 ′ d ″, thereby moderating the advancing starting shock. In this embodiment, moderation of the shock of reversing start is out of consideration so that upper first transverse groove  39 ′ g  is eliminated. 
     For a short time until reaching the neutral position during returning to neutral in advancing and reversing, one of orifices  80 U and  80 L enters corresponding second transverse groove  39 ′ i  or  39 ′ j , thereby moderating the shock of deceleration and braking. Second transverse grooves  39 ′ i  or  39 ′ j  may be extended so as to form extending grooves  39 ′ m  or  39 ′ n  as the above. In this case, for all the time of returning to neutral in advancing or reversing, dynamic braking can be moderated. 
     As the fourth embodiment, the shape of groove on control arm  39 ′ can be constructed by mixing any two of the above three embodiments. Also, it can be constructed by employing any two of the above various modifications. Accordingly, the orifices  80 U and  80 L can be made in communication with the oil sump in various timing patterns. 
     Next, description will be given on a fifth embodiment of control arm  39 ′ to in accordance with FIGS. 59 through 62. This embodiment does not provide an oil draining to the oil sump in the housing, which causes the problem that either orifice  80 U or  80 L which is negatively pressurized badly absorbs oil with air bubbles from the oil sump in the housing. 
     For details of the problem referring to the first embodiment, when control arm  39 ′ (movable swash plate  11 ) reaches the vicinity of its neutral position so as to make orifices  80 U and  80 L open to the oil sump in the housing through grooves, pressure oil is drained from either orifice  80 U or  80 L of higher pressurized oil passage  5 ′ a  or  5 ′ b . However, as oil drains from the orifice, the other orifice of negatively pressurized oil passage  5 ′ b  or  5 ′ a  absorbs oil with air bubbles thereinto from the oil sump through the corresponding groove. Then, the mixing of air bubbles into the oil circulated between both oil passages  5 ′ a  and  5 ′ b  brings the vehicle into a freewheeling condition, where the vehicle unexpectedly descends a slope by inertia. 
     Referring to the fifth embodiment for solving the problem, as shown in FIGS. 59 and 61, on the surface of contact plate  39 ′ c  abutting pistons  64 U and  64 L are opened a hole  39 ′ x  and a groove  39 ′ i ′ separated from each other in correspondence to upper orifice  80 U, and a hole  39 ′ y  and a groove  39 ′ j ′ separated from each other in correspondence to lower orifice  80 L, The distances between hole  39 ′ x  and groove  39 ′ i ′ and between hole  39 ′ y  and groove  39 ′ j ′ may be reduced as required. Holes  39 ′ x  and  39 ′ y , which have a larger diamter than orifices  80 U and  80 L, are bored perpendicular to the surface of contact plate  39 ′ c . The openings of holes  39 ′ x  and  39 ′ y  are positioned so as to communicate with orifices  80 U and  80 L when control arm  39 ′ (movable swash plate  11 ) is at the neutral position or its vicinity. A connection hole  39 ′ z  is bored in control arm  39 ′ in parallel to the surface of contact plate  39 ′ c  so as to join with holes  39 ′ x  and  39 ′ y . Connection hole  39 ′ z  is drilled from the tail end of contact plate  39 ′ c  and its opening is plugged with a cap  90 , thereby preventing oil leak from connection hole  39 ′ z  to the oil sump in the housing. 
     Grooves  39 ′i′ and  39 ′j′ are provided for moderation of dynamic brake. similar to grooves  39 ′ i  and  39 ′ j . However, groove  39 ′ i ′ is spaced from hole  39 ′ x  at such a distance as to make orifice  80 U partly open during the relative motion of orifice  80 U between hole  39 ′ x  and groove  39 ′i′. The distance may be determined so as to close orifice  80 U entirely for an extremely short time while moving between hole  39 ′ x  and groove  39 ′ i ′. Similarly, groove  39 ′ j ′ and hole  39 ′ y  are spaced from each other. 
     When control arm  39 ′ (movable swash plate  11 ) is located at the neutral position and its vicinity so as to make orifices  80 U and  80 L communicate with respective holes  39 ′ x  and  39 ′ y , oil is drained from either orifice  80 U or  80 L of either oil passage  5 ′ a  or  5 ′ b , which has been higher pressurized, into connection hole  39 ′ z . and absorbed into the other negative pressurized oil passage  5 ′ b  or  5 ′ a  through the other orifice  80 L or  80 U. The oil circuit consisting of holes  39 ′ x ,  39 ′ y  and  39 ′ z  is closed so as to be hydraulically tightened, whereby oil passage  5 ′ b  or  5 ′ a is fed with only the oil circulated through within center section  5 ′, control arm  39 ′ and corresponding piston  64 L or  64 U without the oil mixed with air bubbles in the housing. As a result, both oil passages  5 ′ a  and  5 ′ b , when movable swash plate  11  and control arm  39 ′ are located at their neutral position, are prevented from the entry of air bubbles, so as to bring the vehicle safe from the freewheeling condition. 
     Referring to FIG. 62, both orifices  80 U and  80 L are fully open to respective holes  39 ′ x  and  39 ′ y  when control arm  39 ′ (movable swash plate  11 ) is at the neutral position and within a certain advancing and reversing ranges from the neutral position. Since the rotational speed of lower orifice  80 L which is further from control shaft  35  as a pivot than upper orifice  80 U, is greater than that of orifice  80 U while holes  39 ′ x  and  39 ′ y  are of the same diameter, the full open range of control arm  39 ′ including the neutral position for orifice  80 L is narrower than that for orifice  80 U. Alternatively, the diameter of hole  39 ′ y  may be greater than that of hole  39 ′ x , thereby extending the width of full open range for orifice  80 L to coincide with that for orifice  80 U. 
     While orifice  80 U relatively moves from hole  39 ′ x  to groove  39 ′ i ′ by rotation of control arm  39 ′ for advancing acceleration, orifice  80 U is partly open to hole  39 ′ x  or groove  39 ′ i ′, and then, fully open to groove  39 ′ i ′. At last, orifice  80 U passes groove  39 ′ i ′ and is shut by contact plate  39 ′ c . Simultaneously, orifice  80 L leaves hole  39 ′ y , and then, becomes shut by contact plate  39 ′ c . When control arm  39 ′ is rotated for advancing deceleration to the neutral position, oil passage  5 ′ a  becomes higher pressurized so that excessive pressure oil is brake, drained from orifice SOU to groove  39 ′ i ′, thereby moderating a dynamic brake. When both holes  39 ′ x  and  39 ′ y  reach respective orifices  80 U and  80 L, both oil passages  5   a  and  5   b  are evenly pressurized, thereby stopping the vehicle. 
     Orifices  80 U and  80 L are similarly opened and closed in relation to holes  39 ′ x  and  39 ′ y  and groove  39 ′ j ′ so as to give the similar effect during the rotation of control arm  39 ′ for acceleration and deceleration of reversing.