Abstract:
A housing cannot be made compact by only securing a space corresponding to a reservoir tank at the upper part of the housing, and the space at the upper part of the housing cannot be utilized if the reservoir tank is integrally provided to the upper part of the housing. An axle driving apparatus comprising a tubular reservoir tank erected on the upper surface of the upper wall surface of the housing for accommodating an HST, wherein, since the reservoir tank is thinly elongated and is freely bendable, it can be disposed in a limited space. The housing is divided into a first room and a second room with which the reservoir tank communicates. Further, a first magnet is disposed at a position where the first and second rooms are connected, and a second magnet is disposed in the proximity of an oil filter connected to a closed circuit of the HST so that impurities can be removed.

Description:
This application is a continuation of U.S. application No. 09/555,764, filed Jun. 5, 2000, now U.S Pat. No. 6,401,869 which is a 371 of PCT/JP98/04598, filed Oct. 12, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a reservoir tank attached to an axle driving system in which a hydrostatic transmission (hereafter referred to as HST), an axle, and a drive train for connecting the HST and the axle for driving are housed in a common housing. 
     2. Related Art 
     Conventionally, a housing that houses an HST is filled with hydraulic oil that is also used as lubricating oil. A temperature of the hydraulic oil is increased by driving the HST. The increase in temperature of the hydraulic oil results in an increase in its volume. In order to receive the increase in the volume, a technique of providing a reservoir tank on an outside of the housing or forming space having a volume corresponding to the increase within the housing to form a reservoir room is known. 
     For example, there is a technique disclosed in U.S. Pat. No. 4,987,796 or U.S. Pat. No. 5,440,951. 
     However, an axle driving apparatus that utilizes a reservoir tank structure requires additional space, a mounting member for mounting the reservoir tank, and piping for connecting the reservoir tank and the housing. In addition, the number of man-hours required for assembly increases, the reservoir tank is difficult to handle, and cost increases. 
     By positioning the reservoir tank at an upper portion of the housing, a vertical length of the housing increases to inhibit miniaturization of the axle driving system and an air layer is created in the housing to cause air to be swallowed in oil when the oil within the housing is stirred. If this oil is charged into a closed circuit of the HST as the hydraulic oil, volume efficiency of the HST is reduced, which may generate noise or reduce durability. 
     Further, it is difficult to integrally form the reservoir tank at the upper portion of the housing for a lawn tractor having a rear discharge method, because a chute passes above the axle driving system. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a tubular reservoir tank is mounted to stand on an upper face of an upper wall-face of a housing that houses an HST including a hydraulic pump and a hydraulic motor. The reservoir tank communicates with an oil reservoir within the housing. Therefore, it is possible to easily mount the reservoir tank by a simple structure, to form the reservoir tank from low-priced members, and to reduce cost. 
     The reservoir tank is formed of a tubular member. Therefore, if there is a member such as a chute that is related to the vehicle main machine above the housing, it is possible to mount the reservoir tank by bending the reservoir tank. Therefore, it is possible to easily mount the reservoir tank by causing the reservoir tank to make way for the member that interferes with the reservoir tank, thereby increasing the number of applications in which the axle driving system may be used. 
     The housing is partitioned into a first room, housing the HST and a second room, housing an axle and a gear train for transmitting power from the hydraulic motor to the axle. The first and second rooms communicate with each other such that the oil with which both the rooms are filled can circulate between both the rooms. The reservoir tank is disposed on the second room. Therefore, it is possible to position a mounting portion of the reservoir tank in a high position, thereby easily purging air generated within the reservoir tank. Because the reservoir tank may be disposed in a position isolated from the pump shaft, it is possible to easily prevent the reservoir tank from interfering with a rotary member such as an input pulley or a cooling fan. 
     A communicating position connecting the first room and the second room allows oil to pass there between. A filter member for removing impurities included in oil is disposed in the communicating position. Therefore, it is possible to remove impurities included in the oil when the oil circulates between the first room and the second room due to the variation in volume of the respective rooms which occurs with a change in oil temperature or stirring by actuation action of the HST or rotation of a gear. The impurities can be removed by means of the filter member disposed in an oil hole for hydraulic oil of the HST, the oil within the first room can be cleaned, and durability of the HST can be further improved. 
     Because the filter member is formed of a magnet, the filter member can be formed at low cost and can be miniaturized. Therefore, limitations to a place where the filter member is disposed can be reduced, durability of the filter member is high, and the filter member can be cleaned easily. 
     A closed circuit is formed by providing an oil path to a center section to which a hydraulic pump and a hydraulic motor in the HST are mounted, an oil hole opening at the center section for supplying hydraulic oil to the closed circuit is connected to an oil filter disposed in an oil reservoir, and a magnet is disposed in the oil reservoir in a vicinity of the oil filter. Therefore, it is possible to cause the magnet to collect iron powder before it is drawn into the oil hole and to easily remove the harmful iron powder before it enters the closed circuit of the HST. 
     The magnet is disposed such that the magnet acts on the hydraulic oil after passing through the oil filter and before entering the oil hole. Therefore, if there is extremely fine iron powder that has passed through the oil filter, the iron powder can be collected by the magnet and prevented from entering the closed circuit of the HST, thereby increasing life of the HST. Because only small iron powder is collected by the magnet, an amount of iron powder collected by the magnet is small even after use of the magnet for a long term and frequency of maintenance can be reduced. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a general side view of a lawn tractor to which an axle driving system of the present invention is mounted. 
     FIG. 2 is a sectional view taken in a direction of an arrow A—A in FIG.  1 . 
     FIG. 3 is a back perspective view of an axle driving system. 
     FIG. 4 is a sectional view taken in a direction of an arrow B—B in FIG.  2 . 
     FIG. 5 is a sectional view taken in a direction of an arrow C—C in FIG.  4 . 
     FIG. 6 is a sectional view taken in a direction of an arrow D—D in FIG.  4 . 
     FIG. 7 is a sectional view taken in a direction of an arrow E—E in FIG.  4 . 
     FIG. 8 is a sectional view taken in a direction of an arrow F—F in FIG.  4 . 
     FIG. 9 is a sectional view showing another embodiment in which a magnet is disposed in the vicinity of an oil filter. 
     FIG. 10 is a perspective view of a mounting seat. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1,  2 , and  3 , a lawn tractor of a rear discharge method has a vertical crankshaft-type engine  100  disposed on a front portion of a body work frame  101 . Steerable left and right (front) follower wheels  102 ,  102  are suspended from the front portion of the bodywork frame  101 , an axle driving system  6  having left and right (rear) driving wheels  103 ,  103  is suspended from a rear portion, a grass box  104  is mounted to a rear end by a working machine mounting device (not shown) such that the grass box  104  can be hoisted and lowered, and a mower  105  is mounted to a lower portion of the bodywork between the follower wheels  102 ,  102  and the driving wheels  103 ,  103  through a hoisting and lowering mechanism (not shown). A discharge opening of the mower  105  and an entrance of the grass box  104  are connected through a chute  106 . The chute  106  extends diagonally upward and rearward from an upper portion of the mower  105 , passes above the axle driving system and between the left and right driving wheels  103 ,  103 , and connects to the entrance of the grass box  104 . Therefore, grass mowed by the mower  105  is blown rearward, passes through the chute  106  and between the left and right driving wheels  103 ,  103 , and is housed in the grass box  104 . 
     As shown in FIG. 1, two pulleys  108  and  109  are fixed onto a vertical output shaft  107  of the engine  100 . The pulley  108  has an electromagnetic clutch and transmits power through a belt  110  to an input pulley  112  fixed onto an input shaft  111  of the mower  105  to drive the mower  105 , thereby rotating a cutting blade  115  of the mower  105 . As shown in FIG. 3, five tension pulleys  114 , pivoted on the bodywork frame  101 , allow a belt  113  to navigate around the chute  106 . The belt  113  is attached to the other pulley  109  and an input pulley  43 , which is fixed onto a pump shaft  3 . Pump shaft  3  is an input shaft projecting upward from the housing of the axle driving system  6 . A reference numeral  44  designates a cooling fan for cooling the axle driving system and fixed to a lower face of the input pulley  43  on the pump shaft  3 . 
     As shown in FIG. 3, the axle driving system  6  is hung on left and right mount members  101   a ,  101   a  which are parts of the bodywork frame  101 . Left and right axles  7 L and  7 R project from side faces of the housing of the axle driving system  6  and the driving wheels  103 ,  103  are mounted to end portions of the axles  7 L and  7 R. 
     Next, in FIGS. 4 to  8 , a general structure of the axle driving system  6  will be described. The housing of the axle driving system  6  is formed of two housing members, i .e., an upper housing  1  and a lower housing  2 , which are joined to each other at their flat and peripheral joint faces in a horizontal plane. At the joint face of the housing, bearing portions of a motor shaft  4  and a counter shaft  26  are provided. The axles  7 L and  7 R are disposed in parallel with the joint face of the housing. The bearing portions of the axles  7 L and  7 R are displaced upward from the joint face and disposed within the upper housing  1  to rotatably support the axles  7 L and  7 R. The respective axles  7 L and  7 R are connected in a differential manner by a differential gear  23  and opposite ends of the axles  7 L and  7 R project outward from left and right outer walls of the housing. 
     An inside of the housing is partitioned by an inner wall  8  into a first room R 1  for housing the HST and a second room R 2  for housing a drive train. The drive train is formed of a plurality of gears for transmitting power from the motor shaft  4  to the differential gear  23  and the axles  7 L and  7 R. The inner wall  8  is formed of a portion extending along and in parallel with a longitudinal direction of the axles  7 L and  7 R and a portion extending in a direction perpendicular to the longitudinal direction. These portions are formed seamlessly and the first room R 1  is adjacent to the second room R 2 . The inner wall  8  is formed of a wall portion hanging from an inside of the upper housing  1  toward the joint face and a wall portion standing from an inner wall of the lower housing  2  toward the joint face. End faces of the upper and lower wall portions are placed on each other at the joint face, thereby partitioning the inside of the housing into two independent rooms. 
     The first room R 1  is disposed at the rear of the axles  7 L and  7 R and on the left of a power transmission device for transmitting power from the motor shaft  4  to the differential gear  23  in the housing as shown in FIG.  4 . In the first room R 1 , a center section  5  constituting the HST is detachably mounted to the inside of the upper housing  1 . The center section  5  is disposed such that a longitudinal direction of the center section  5  is in a longitudinal direction of the bodywork and is substantially perpendicular to the axles  7 L and  7 R. A vertical face is formed at a rear portion of the center section  5 , a motor attachment face  41  is formed on the vertical face, and a hydraulic motor  51  is disposed on the motor attachment face  41 . A horizontal face is formed at a front portion of the center section  5 , a pump attachment face  40  is formed on the horizontal face, and a hydraulic pump  50  is disposed on the pump attachment face  40 . Therefore, the hydraulic pump  50  is disposed between the hydraulic motor  51  and the axles  7 L and  7 R when viewed in the longitudinal direction of the bodywork. The pump shaft  3  is vertically supported at a center of the pump attachment face  40 . The pump shaft  3  extends between the hydraulic motor  51  and the axles  7 L and  7 R, passes through an upper wall of the upper housing  1 , and projects wherein the input pulley  43  is mounted thereon. 
     A cylinder block  16  is disposed for rotation and sliding on the pump attachment face  40 . Pistons  12 ,  12 , are fitted for reciprocation in a plurality of cylinder bores of the cylinder block  16  through biasing springs. A thrust bearing  11   a  of a movable swash plate  11  is in contact with bead portions of the pistons  12 ,  12 . An opening portion  11   b  is formed at a center of the movable swash plate  1  such that the pump shaft  3  can pass through the opening portion  11   b . The pump shaft  3  also functions as an input shaft, is disposed on a center rotational axis of the cylinder block  16 , and is locked to the cylinder block  16  such that the pump shaft  3  and the cylinder block  16  cannot rotate with respect to each other, thereby forming the hydraulic pump  50  of an axial piston type. 
     By inclining a piston contact face of the movable swash plate  11  through an arbitrary angle from a horizontal position with respect to the center rotational axis of the cylinder block  16 , it is possible to change a discharge amount or a discharge direction of oil from the hydraulic pump  50 . A back face of the movable swash plate  11  is formed into a projecting arc portion. A recessed arc portion corresponding to a shape of the projecting arc portion is formed on an inner face of an upper portion of the upper housing  1 . The projecting arc portion of movable swash plate  11  is formed as a cradle-type movable swash plate that slides in close contact with the recessed arc portion of the upper housing  1 . 
     In order to incline the movable swash plate  11 , as shown in FIGS. 4 and 7, a control shaft  35  parallel with the axle  7  is rotatably supported on a right wall of the upper housing  1  on an opposite side to the drive train for transmitting power to the differential gear  23 . A control arm  38  is mounted to an end portion of the control shaft  35  extending to an outside of the housing and a swinging arm  39  is mounted to a housing inner end portion of the control shaft  35 . The swinging arm  39  is formed of a first arm  39   a  and a second arm  39   b  extending radially from the control shaft  35  and a contact plate  39   e  formed into a shape of a sector. 
     The contact plate  39   e  extends to a vicinity of an end face of the horizontal portion of the center section  5  in the lower housing  2  and forms a sector-shaped contact face around an axial center of the control shaft  35  such that the contact plate  39   e  can be in contact with piston bodies  64 ,  64  which will be described below in an entire range covered when the control lever  38  is rotated from a neutral position to a forward-side maximum position and a rearward-side maximum position as shown in FIG.  7 . The contact plate  39   e  may be formed as a separate member independent of the control arm  39  if the contact plate  39   e  rotates with the control arm  39 . 
     A projection  39   c  is-formed at a tip end portion of the second arm  39   b . Because the axial center of the control shaft  35  is aligned with a center of inclining of the movable swash plate  11 , it is possible to directly engage the projection  39   c  with a groove portion formed in a side face of the movable swash plate  11 . The control arm  39  is connected to a speed change operating tool (not shown) such as a lever and a pedal provided to the vehicle through a linkage (not shown). 
     With the above structure, when the control arm  38  is rotated along the longitudinal direction of the bodywork, the swinging arm  39  rotates in the longitudinal direction of the bodywork about the control shaft  35  to incline the movable swash plate  11 , thereby changing output of the hydraulic pump  50 . 
     An engaging pin  39   d  is formed to project from a tip end of the first arm  39   a  (FIG.  4 ). A neutral position recovering spring  31  of a helical torsion spring type is fitted to an outside of the control shaft  35  in the housing. Opposite ends of the neutral position recovering spring  31  cross and extend toward the first arm  39   a . The opposite end portions of the neutral position recovering spring  31  pinch an eccentric shaft  33  mounted to an inner wall of the upper housing l in the vicinity of the control shaft  35  and the engaging pin  39   d  between the opposite end portions. 
     Therefore, when the control arm  38  is rotated to rotate the swinging arm  39  in order to change speed, one end side of the neutral position recovering spring  31  is moved away from the other end side of the neutral position recovering spring  31  by the engaging pin  39   d  while the other end side is stopped by the eccentric shaft  33 , thereby applying biasing force for recovering the neutral position to the control lever  38 . If operating force applied to the control arm  38  is cancelled, the engaging pin  39   d  is retained in a neutral position determined by the eccentric shaft  33  by the recovering force generated on the one end side of the neutral position recovering spring  31 . A portion of the eccentric shaft  33  extending to the outside of the housing is formed into an adjusting screw. By arbitrarily rotating and displacing the eccentric shaft  33  through the screw portion, the swinging arm  39  is displaced about the control shaft  35  through the neutral position recovering spring  31  and it is possible to adjust the movable swash plate  11  such that the movable swash plate  11  is positioned in the accurate neutral position. 
     Pressure oil discharged from the hydraulic pump  50  is sent to the hydraulic motor  51  through an oil path in the center section  5 . A structure of the hydraulic motor  51  is shown in FIGS. 4 and 8. A cylinder block  17  is disposed for rotation and sliding on the motor attachment face  41  formed on the vertical face of the center section  5 . A plurality of pistons  13 ,  13 , are fitted for reciprocating in a plurality of cylinder bores of the cylinder block  17  through biasing springs. Head portions of the pistons  13 ,  13  are in contact with a fixed swash plate  37 . The fixed swash plate  37  is sandwiched and fixed between the upper housing l and the lower housing  2 . 
     A pair of arcuate ports (not shown) open at the pump attachment face  40  of the horizontal portion of the center section  5  such that supplied or discharged oil from the cylinder block  16  can be introduced through the ports. As shown in FIG. 5, a pair of arcuate ports  41   a  and  41   b  also open on the motor attachment face  41  of the vertical portion such that supplied or discharged oil from the cylinder block  17  can be introduced through the ports. 
     In a thick-walled portion of the center section  5 , a straight first oil path  5   a  and a straight second oil path  5   b  are formed in parallel with each other in upper and lower positions for respectively connecting the arcuate ports on the pump attachment face  40  and the arcuate ports  41   a  and  41   b  on the motor attachment face  41 . One of the arcuate ports on the pump attachment face  40  communicates with the oil path  5   d  formed in a diagonal direction shown in FIG.  7  and the oil path  5   d  communicates with the second oil path  5   b . These oil paths constitute a closed circuit for circulating the hydraulic oil between the hydraulic pump  50  and the hydraulic motor  51 . As shown in FIGS. 5 and 7, check valves  54  and  55  are disposed at front end portions of the first oil path  5   a  and second oil path  5   b  in their extending direction. Also, an oil hole  5   c  that communicates with both the check valves  54  and  55  and opens downward at a lower face of the center section is provided to a primary side (front portion) of the check valves  54  and  55  such that the check valves  54  and  55  automatically open only in supplying the hydraulic oil. 
     As shown in FIG. 7, piston bodies  64 ,  64  constituting means for lessening a shock in recovering of the neutral position are arranged in a vertical direction at oil path ends formed perpendicularly to the first oil path  5   a  and second oil path  5   b . Each piston body  64  has a cylindrical shape and is formed at an axial center portion with an orifice  64   b  to open the oil paths  5   a  and  5   b  to an outside of the closed circuit through the orifice  64   b . In other words, when the control arm  39  is in the neutral position, straight and vertical groove portions  39   f  each having a width slightly larger than a diameter of each of ends of the orifices  64   b ,  64   b  are formed on the contact plate  39   e  that faces the orifices  64   b ,  64   b  of the piston bodies  64 ,  64  and open at the lower face of the contact plate  39   e.    
     By the groove  39   f , the closed circuit is allowed to communicate with an oil reservoir within the housing through the orifices  64   b ,  64   b . By rotating the control arm  39  such that the movable swash plate is inclined through an angle greater than a predetermined angle from the neutral position, the orifices  64   b ,  64   b  and the groove portions  39   f ,  39   f  are separated from each other. A surface of the contact plate  39   e  excluding the groove portions  39   f ,  39   f  and facing the orifices  64   b ,  64   b  is formed to be smooth and a friction plate  68  is disposed at a portion facing the orifices  64   b ,  64   b  through the contact plate  39   e  and is sandwiched and fixed between the upper housing  1  and the lower housing  2 . As described below, if the piston bodies  64 ,  64  advance while receiving hydraulic pressure, the contact plate  39   e  is sandwiched between the piston bodies  64 ,  64  and the friction plate  68 , thereby applying rotational resistance to the contact plate  39   e.    
     In such a structure, if the speed change operating tool of the vehicle is operated to rotate the control lever  38  of the axle driving system, the control arm  39  is rotated through the control shaft  35 , the engaging projection  39   c  is engaged with the engaging groove of the movable swash plate  11  to incline the movable swash plate  11 , the discharge amount of the hydraulic oil of the hydraulic pump is changed, and a rotation number of the motor shaft  4  of the hydraulic motor is changed according to a rotation direction and a rotation amount of the speed change operating tool to transmit power to the axle  7 . 
     At this time, pressure proportional to load of the axle  7  is applied to the higher-pressured oil, i.e., the first oil path  5   a  or the second oil path  5   b  and one of the piston bodies  64  is slid outward by this pressure to push the contact plate  39   e  of the control arm  39 . Friction force generated by the pushing force is set at a smaller value than the spring force of the neutral position recovering spring  31 . 
     An operator operates the speed change operating tool with operating force that exceeds the friction force and the spring force. Because the orifice  64   b  of the piston body  64  is closed by the smooth face of the contact plate  39   e  after inclining the movable swash plate through-the predetermined angle, the hydraulic oil circulating through the closed circuit does not leak from the orifice  64   b  and the volumetric efficiency of the HST is maintained satisfactorily. 
     In such a state, if the operator releases the operating force applied to the speed change operating tool, the control arm  39  is biased by the spring force of the neutral position recovering spring  31  such that the control arm  39  rotates toward the neutral position. However, as described above, because the friction force is generated between the piston body  64  and the contact plate  39   e  of the control arm  39 , resistance is applied to the rotation toward the neutral position and the control arm  39  gradually rotates toward the neutral position. Thus, sudden dynamic braking is not applied and a quick stop is not generated. When the control arm  39  rotates to the vicinity of the neutral position, the orifice  64  of the piston body  64  communicates with the groove portion  39   f  of the control arm  39 , pushing pressure applied to the contact plate  39   e  and residual pressure in the closed circuit are allowed to escape, a shock of stop is lessened, and a neutral range of the HST is widened. 
     A bypass operating lever  60  for causing the first oil path  5   a  and second oil path  5   b  open into the oil reservoir is disposed at an upper portion of the upper housing  1  such that the axle can be freewheeling in tow. In other words, as shown in FIGS. 5 and 8, a base portion of the bypass operating lever  60  is fixed to an upper end of a bypass shaft  61  axially supported for rotation in a vertical direction by an upper wall of the upper housing  1  and a lower end of the bypass shaft  61  extends along one side face of a vertical portion of the center section  5 . A pushing pin  62  that can come in contact with a rotating sliding face of the cylinder block  17  supported on the other side face of the vertical portion is slidably supported in the vertical portion. An end face of the pushing pin  62  is brought into contact with a flat face  61   a  formed on a side face of the lower end of the bypass lever shaft  61 . 
     Therefore, if the operator operates the bypass operating lever  60  on the outside of the housing in tow of the vehicle, the bypass lever shaft  61  is rotated, the flat face  61   a  at the lower end of the bypass lever shaft  61  inclines to push the pushing pin  62  toward the cylinder block  17 , the pushing pin  62  separates the motor attachment face  41  and the cylinder block  17  which have been in close contact with each other, and the first oil path  5   a  and second oil path  5   b  open into the oil reservoir of the housing through the arcuate ports  41   a  and  41   b , thereby allowing the motor shaft  4  to rotate freely. 
     An annular oil filter  56  is disposed between a lower face of the center section  5  and an inner bottom face of the lower housing  2  such that a periphery of an opening portion at a lower end of the oil hole  5   c  is covered with the oil filter  56 . The oil filter  56  is formed by sandwiching a ring-shaped porous member that is a filter main body between upper and lower cover plates  57  and  58  and disposing a cylindrical support cylinder  59  such as net or punching metal at an inner peripheral face portion of the porous member as shown in FIGS. 5 and 7. Because an opening  57   a  is formed at a center portion of the upper cover plate  57 , filtered oil that has flowed from an outer peripheral portion of the oil filter  56  into an inside of the oil filter  56  can flow into the oil hole  5   c  of the center section  5  through the opening  57   a.    
     The cover plates  57  and  58  are formed of a steel sheet such as an iron sheet and a nickel sheet which can be magnetized and a magnet  63  is placed on an.upper face of an inner face of the lower cover plate  58  to magnetize the lower cover plate  58 . In this manner, iron powder included in the reservoir of the first room R 1  is collected by the lower cover plate  58  and even extremely fine iron powder that has passed through the oil filter  56  is collected by the magnet  63  or the lower cover plate  58  and is not introduced into the closed circuit, thereby preventing damaging of the sliding face and the like. 
     A position where the magnet  63  is disposed is not limited to the upper face of the lower cover plate  58  and the magnet  63  may be also disposed on a lower face of the lower cover plate  58  or at the upper cover plate  57 . If the magnet  63  is disposed at the upper cover plate  57 , it is necessary to provide a bridge, a mount, or the like such that the magnet  63  can be hung within the oil filter  56 . 
     If the magnet  63  is provided to the lower face of the lower cover plate  58 , as shown in FIG. 9, a depression or a recessed portion  2   a  is formed at the lower housing  2  at a lower portion where the oil filter  56  is positioned to form a space where dust and the like accumulate. Three or more projections  2   b ,  2   b , are formed on an inner face of the lower housing  2  in peripheral positions of the space where the oil filter  56  is positioned and the lower face of the lower cover plate  58  is placed on the projections  2   b ,  2   b , thereby supporting the oil filter  56 . Thus, a passage is formed by the projections  2   b ,  2   b , between the lower cover plate  58  and the bottom face of the lower housing  2  and between the projections  2   b ,  2   b , dust and the like pass through the passage and accumulate in the recessed portion  2   a  that is less subject to flow of oil when the oil is stirred, and the iron powder is collected by the magnet  63 . By the projections  2   a ,  2   a , the lower cover plate  58  is disposed in a higher position, thereby forming a space where the magnet  63  is disposed. 
     Furthermore, if there is extremely fine iron powder that has passed through the oil filter  56 , the iron powder is collected by an upper face of the lower cover plate  58  because the lower cover plate  58  is magnetized by the magnet  63 . 
     The motor shaft  4  is locked onto a rotation axial center of the cylinder block  17  such that the motor shaft  4  cannot rotate with respect to the cylinder block  17 . The hydraulic motor shaft  4  is oriented in a substantially horizontal direction. One end of the motor shaft  4  is supported in a bearing hole in the motor attachment face  41  of the center section  5 , the other side of the motor shaft  4  is supported through a bearing  76  on the inner wall  8  formed at the joint face between the upper housing  1  and the lower housing  2 , and a tip end projects into the second room R 2 . A bearing with a seat is used as the bearing  76  so as to separate the first room R 1  and the second room R 2 . 
     The drive train for transmitting power from the motor shaft  4  to the differential gear system  23  is formed of a gear  25  secured onto the motor shaft  4  projecting into the second room R 2 , a large-diameter gear  24  that constantly meshes with the gear  25  and is supported on the counter shaft  26 , a small-diameter gear  21  supported on the counter shaft  26  rotating with the large diameter gear  24 , and a ring gear  22  of the differential gear  23  with which the small-diameter gear  21  constantly meshes as shown in FIGS. 4 and 6. The counter shaft  26  is disposed within the second room P 2  to be adjacent to and orthogonal to the pump shaft  3 . 
     One end of the counter shaft  26  is supported on aside wall of the housing, i.e., at the joint face between the upper housing  1  and the lower housing  2  and the other end of the counter shaft  26  is supported on the inner wall  8  of the housing, i.e., at the joint face between the upper housing  1  and the lower housing  2 . Speed of rotation output from the motor shaft  4  is reduced by the large-diameter gear  24 , the small diameter gear  21 , and the ring gear  22  to drive the axles  7 L and  7 R through the differential gear  23 . 
     A brake disc  19  formed integrally with the gear  25  is secured onto a tip end portion of the motor shaft  4  positioned within the second room R 2 . As shown in FlG.  4 , a brake pad  29  is disposed between a rear side of an upper portion of the brake disc  19  and an inner face of the upper housing  1  and a pressing body  72  and a brake operating shaft  14  are disposed successively with respect to the brake disc  19  in a portion surrounded by the brake disc  19  and the inner wall  8  of the housing on a side (right side of the brake disc  19 ) of the brake disc  19  opposite to the brake pad  29 . 
     The brake operating shaft  14  is disposed in a vertical direction and supported rotatably by the upper housing  1  and the lower housing  2 . An upper end of the brake operating shaft  14  projects upward from the housing and a brake arm  27  (FIG. 3) is fixed to the upper end. A flat notch  14   a  is formed on an outer face of a midway portion of the brake operating shaft  14  within the housing such that the operating shaft  14  has a D shape in a plan sectional view, the pressing body  72  is brought into contact with the notch  14   a , movement of the pressing body  72  in rightward and leftward directions is limited by the notch  14   a , and front and rear opposite sides of the pressing body  72  are guided by the inner face of the upper housing  1  such that the pressing body  72  can slide only in leftward and rightward directions. Therefore, if the brake arm  27  is rotated, the brake operating shaft  14  is rotated, an end portion of the notch  14   a  pushes a back face of the pressing body  72 , and the brake disc  19  is sandwiched between the brake pad  29  and the pressing body  72  to brake the motor shaft  4 . 
     The first room R 1  and the second room R 2  are filled with common lubricating oil to form the oil reservoir. As shown in FIGS. 4 to  8 , a communicating room  9  is formed at an upper portion of the inner wall  8  that partitions the inside of the housing into the first room R 1  and the second room R 2 . An oil inlet  9   a  communicating with the first room R 1  and an oil inlet  9   b  communicating with the second room R 2  open into a side face and a bottom of the communicating room  9 . An opening portion  1   c , to which a filter member that will be described below is mounted, is formed at the upper housing above the oil inlets, and the opening portion  1   c  is closed by a lid  65 . 
     A mounting seat  66  formed by press forming an iron sheet into a cup shape is placed on a bottom face of the communicating room  9  and a magnet  71  as a filter member for removing impurities is placed and fixed onto the mounting seat  66 . The magnet  71  is formed into a disc shape, and as shown in FIG. 10, an elongated hole  66   a  is formed on an upper face of the mounting seat  66 . By inserting the magnet  71  into the elongated hole  66   a  and bringing tongue chips  66   b  extending from opposite sides of the elongated hole  66   a  to face each other into resilient contact with opposite faces of the magnet  71 , the magnet  71  and the mounting seat  66  are integrated with each other. The integrated magnet  71  and mounting seat  66  are inserted into the communicating room  9  that opens at an upper wall of the upper housing  1  and the opening is closed by the lid  65 , thereby achieving completion. 
     A reference numeral  66   c  in FIG. 10 designates notches, each formed by partially notching a lower end face of the mounting seat  66 . A width of the notch  66   c  is equal to a thickness of a portion of the inner wall  8  positioned at the bottom face of the communicating room  9 . By inserting the portions of the inner wall  8  into the notches  66   c  when the mounting seat  66  is placed on the bottom face of the communicating room  9 , rotation of the magnet  71  within the communicating room can be prevented. 
     The magnet  71  is fixed such that the magnet  71  is oriented to be diagonal with respect to the oil inlets  9   a  and  9   b  so as to increase a contact area in flowing of the oil. However, the magnet  71  may have a rectangular shape and a shape of the magnet  71  is not limited. 
     With the above structure, iron powder generated in the first room R 1  and the second room R 2  when the oil within the housing circulates between the first room R 1  and the second room R 2  is attracted by the magnet  71  and adheres to the magnet  71  and the mounting seat  66  to maintain the oil that has accumulated in the first room R 1  in a clean state, thereby improving durability of the HST. 
     As shown in FIGS. 2 and 6, an oil circulating hole  1   a  opens at the upper wall face of the upper housing  1  forming the second room R 2  in a position where the hole does not interfere with the input pulley  43  and the cooling fan  44 . A joint pipe portion  1   b  is formed on a periphery of the oil circulating hole l a . A tubular reservoir tank  10  formed of a rubber hose or a resilient member is fitted with the joint pipe portion  1   b  such that the reservoir tank  10  communicates with the second room R 2  in a fluidic manner. A lower portion of the reservoir tank  10  is fastened and fixed by the joint pipe portion  1   b  and bands  69   a . An opening at an upper end of the reservoir tank  10  is closed by a breather cap  70  and fastened and fixed by bands  69   b.    
     If the first and second rooms R 1  and R 2  are filled with oil after assembling the axle driving system, an oil level OL is determined within both the rooms R 1  and R 2  as shown in FIGS. 5 to  8 . The oil level OL is uniform because both the rooms R 1  and R 2  communicate with each other through the communicating room  9 . If a volume of the oil within the first room R 1  varies depending on whether the HST is driven, the oil level OL shifts upward or downward from the position shown in the drawings. An air layer with a variable volume is formed above the oil level OL within the reservoir tank  10  that opens into the atmosphere through the breather cap  70 . The air layer allows variation in the volume of the oil. 
     Thus, the hydraulic oil expands and the volume of the hydraulic oil increases when a temperature of the hydraulic oil within the oil reservoir increases due to driving of the HST. The volume can be adjusted by causing the increase in the volume to flow into the reservoir tank  10  and a vertical length of the tube portion of the reservoir tank  10  is set at such a value as to keep at least a volume corresponding to an oil amount by which the volume of the housing increases. 
     The reservoir tank  10  can be easily mounted by only inserting the reservoir tank  10  in to the joint pipe portion l b  on the outer wall of the housing and fastening the reservoir tank  10  by the band  69   a . As shown in FIG. 2, if the chute  106  and the like exist above the housing, the reservoir tank  10  can be bent naturally to make way for the chute  106  and the like because of resiliency of the reservoir tank  10  itself. 
     Therefore, it is possible to reduce limitations on disposition of the reservoir tank  10 . 
     As described above, the axle driving system according to the invention is a system suitable for driving an axle of a field-work vehicle, a vehicle of construction equipment, a mowing vehicle, a snow removal vehicle, or the like and especially suitable to a lawn tractor of a rear discharge method having a chute above the housing.