Patent Publication Number: US-6705028-B2

Title: Self-propelled snowplow vehicle

Description:
FIELD OF THE INVENTION 
     The present invention relates to a self-propelled snowplow vehicle having a driving wheel for driving the snowplow vehicle and an auger for removing snow. 
     DESCRIPTION OF THE RELATED ART 
     In a snowplow vehicle equipped with a snow-removing anger, a system is employed to ensure that the vertical level or height of the auger can be changed in view of snow-removing conditions. When the snowplow vehicle moves from one place to another, the auger is preferably kept in a raised position to facilitate smooth movement of the snowplow vehicle. On the other hand, when a snow-removing operation is to be achieved, the auger is preferably moved to a lower position to achieve the snow-removing operation with improved efficiency. During the snow-removing operation, the auger is frequently raised and lowered in harmony with angulations on the ground surface. Frequent rising and lowering operation of the auger, when achieved manually, is laborious. To lighten the load on the human operator, an improved self-propelled snowplow vehicle has been proposed, which is equipped with a power-operated vertically swingable auger, as disclosed in Japanese Patent Laid-open Publication No. HEI 4-194109. 
     The disclosed snowplow vehicle includes a propelling frame equipped with left and right crawler belts, a vehicle frame equipped with an auger and pivotally connected to the propelling frame, and a lift control device operable to lift a front end portion of the vehicle frame up and down relative to the propelling frame. The lift control device is comprised of a cylinder actuator operable, under the control of a control unit, to extend or contract its piston rod to thereby lift the vehicle frame front end portion and the auger in an upward or a downward direction in response to pivotal movement of a manual operating lever provided on an operating part of the snowplow vehicle. 
     The cylinder actuator constituting the lift control device needs a power source for operation thereof. In the case where the cylinder actuator is an oil hydraulic cylinder actuator, a separate hydraulic power unit must be provided. Accordingly, the overall size of the lift control device is relatively large. Thus, the use of the oil hydraulic cylinder actuator is quite disadvantageous when the snowplow vehicle is relatively small in size. 
     In order to achieve downsizing of the lift control device, use of an electro-hydraulic cylinder actuator may be considered. The electro-hydraulic cylinder actuator has an electric motor drivable to produce a hydraulic pressure used for reciprocating a piston rod of the cylinder actuator. The electric motor and a hydraulic power unit such as a pump are assembled with a cylinder of the cylinder actuator, so that the electro-hydraulic cylinder actuator is relatively small in size. The electric motor is controlled to extend or contract the piston rod of the cylinder actuator to thereby raise or lower the auger in response to on-off operation of an operation switch. 
     Since the height of the auger is changed in view of snow-removing conditions, it may occur that the operation switch is kept in the activated state even after the piston rod arrives at its fully extended or fully contracted position. On this occasion, the electric motor is subjected to a heavy load for a long time. Additionally, during snow-removing operation, since the height of the auger is frequently changed in harmony with angulations of the ground surface, the electro-hydraulic cylinder actuator is forced to operate repeatedly with high frequencies. Under such condition, the duty cycle of the electric motor is very high and generation of heat from the electric motor is promoted. 
     To deal with this problem, use of a continuously operable electric motor may be considered. The continuously operable electric motor is, however, expensive and hence increases the cost of the snowplow vehicle. As an alternative measure, use of a thermo-breaker may be considered for the purpose of protecting the motor from overheating. The thermo-breaker is generally built in the electric motor and operates to cut off or open a power supply circuit to the electric motor when the electric motor heats up above a given temperature. 
     The thermo-breaker is designed to continue the “open” state of the power supply circuit until the electric motor cools to a satisfactory operating temperature. Accordingly, a downtime occurs each time the thermo-breaker operates. In case where the operating temperature of the thermo-breaker is set to a relatively low value, the power supply circuit to the electric motor may be frequently cut off by the thermo-breaker. Alternatively, when the operating temperature of the thermo-breaker is set to a relatively high value, the power supply circuit to the electric motor may be cut off infrequently. In the latter case, however, the thermo-breaker requires a relatively long time to recover its original inoperating state. To enable smooth snow-removing operation, the frequency of operation of the thermo-breaker should preferably be reduced 
     To this end, an arrangement may be considered, in which a detection switch is associated with the electro-hydraulic cylinder actuator such that when arrival of the piston rod of the cylinder actuator at its fully extended or fully contracted position is detected by the detection switch, the detection switch generates a signal to stop operation of the electric motor. This arrangement may reduce the occurrence of overloaded condition of the electric motor. However, use of the detection switch necessarily increases the number of parts of the cylinder actuator and requires an electric wiring system, leading to an increased cost of the snowplow vehicle. 
     FIGS. 16A to  16 C are diagrammatical views illustrative of the operation of a conventional self-propelled snowplow vehicle  500 . In FIG. 16A, the snowplow vehicle  500  is shown with a snow-removing auger  503  disposed in a lowermost horizontal position. The snowplow vehicle  500  is moving forward by the action of crawlers  501  (one being shown) while removing snow by means of the auger  503  and a blower  504  rotatably driven by an engine  502 . The auger  503  collects snow and the blower  504  blows the collected snow away from the snowplow vehicle  500  through a shooter  505 . In this instance, a travel control lever  511  provided on a control board  510  is disposed in an “F” (forward) position, and an auger lift control lever  512  also provided on the control board  510  is disposed in a “DN” (down) position. 
     Due to a large amount of snow to be removed or in order to change the advancing direction of the snowplow vehicle  500 , the snowplow vehicle  500  is occasionally moved backward. In this instance, as shown in FIG. 16B, the travel control lever  511  on the control board  510  is shifted from the “F” (forward) position to an “N” (neutral) position as indicated by the arrow {circle around ( 1 )} whereupon the snowplow vehicle  500  stops moving in the forward direction. Then, the auger lift control lever  512  is shifted from the “DN” (down) position to an “UP” (up) position as indicated by the arrow {circle around ( 2 )} whereupon lift cylinder actuators  506  (one being shown) operate to extend their piston rods to thereby lift a front end portion of a vehicle frame  508  upward relative to a propelling frame  507  on which the crawlers  501  (FIG. 16A) are mounted. The auger  503  is thus raised to an uppermost elevated inclined position. 
     Then as shown in FIG. 16C, the travel control lever  511  on the control board  510  is shifted from the “N” (neutral) position to an “R” (reverse) position as indicated by the arrow {circle around ( 3 )} whereupon the snowplow vehicle  500  moves backward. As described above, in order to reverse the snowplow vehicle while moving in the forward direction, the conventional snowplow vehicle requires three consecutive steps of manual operation as indicated by the arrows {circle around ( 1 )}-{circle around ( 3 )}. Conversely, when the snowplow vehicle while moving backward is to be moved in the forward direction, the snowplow vehicle is first stopped from moving backward. Then, the auger is lowered from the uppermost inclined position to the lowermost horizontal position. Finally, the snowplow vehicle is moved in the forward direction. Thus, three consecutive steps of manual operation are also required. Due to complicated manual operations of the two levers  511 ,  512  to be done in a correct order, the maneuverability of the conventional snowplow vehicle is relatively low. 
     To deal with this problem, an improved snowplow vehicle has been proposed, wherein a snow-removing unit such as an auger is automatically raised when a reversing operation of the snowplow vehicle is selected, as disclosed in Japanese Utility Model Laid-open Publication No. SHO 64-28416. As shown in FIG. 17A, when a travel control lever  611  on a control board  610  is shifted to an “F” (forward) position, the snowplow vehicle  600  moves forward as indicated by the arrow while, at the same time, an auger  603  rotates to thereby achieve snow-removing operation. When the travel control lever  611  on the control board  610  is shifted to an “R” (reverse) position, as shown in FIG. 17B, the auger  603  is moved upward from the lowermost horizontal position of FIG. 17A through a neutral position (not shown) to an uppermost inclined position of FIG.  17 B. Upon arrival of the auger  603  at the uppermost inclined position, rotation of the auger  603  is stopped by disengaging an auger clutch (not shown) disposed between the auger  603  and an engine (not designated). At the same time, the snowplow vehicle  600  is driven to move in the reverse direction as indicated by the arrow shown in FIG.  17 B. 
     Since the auger  603  is lifted up to the uppermost inclined position each time the reverse position is selected by the travel control lever  611 , this means that when the snowplow vehicle  600  is then to be moved forward to achieve a snow-removing operation, the auger  603  needs to be lifted down from the uppermost inclined position to the lowermost horizontal position. Due to a long downward stroke of the auger  603 , an interruption occurs in the snow-removing operation each time the “F” (forward) position is selected immediately after the reversing mode of the snowplow vehicle. In other words, lifting of the auger  603  to the uppermost inclined position in preparation for the backward movement of the snowplow vehicle will lower the efficiency of the snow-removing operation. Due to this difficulty, the snowplow vehicle  500  shown in FIGS. 16A-16C is normally used notwithstanding the fact that the snowplow vehicle  500  is not satisfactory in terms of the maneuverability and lightening of load on the operator. 
     SUMMARY OF THE INVENTION 
     It is, accordingly, an object of the present invention to provide a self-propelled snowplow vehicle, which can be manufactured at a relatively low cost, is able to lighten the load on an electric motor of a electro-hydraulic cylinder actuator used to raise or lower a snow-removing member such as an auger, and is capable of achieving a snow-removing operation smoothly and efficiently. 
     According to the present invention, there is provided a self-propelled snowplow vehicle comprising: a propelling frame equipped with driving wheels for driving the snowplow vehicle; a vehicle frame equipped with an auger at a front end portion thereof for removing snow, the vehicle frame being pivotally connected to the propelling frame; a frame lift mechanism for lifting the front end portion of the vehicle frame up and down relative to the propelling frame, the frame lift mechanism including an electro-hydraulic cylinder actuator having a piston rod and an electric motor rotatably driven to produce a fluid pressure for reciprocating the piston rod between a fully contracted position and a fully extended position; an operation switch adapted to be manually activated to drive the electric motor in either direction; and a control unit for controlling operation of the electric motor thereby to control operation of the frame lift mechanism. 
     In one preferred form of the present invention, the control unit is arranged to forcibly stop the electric motor when a predetermined time has elapsed after the operation switch is activated, the predetermined time being equal to an operating time of the cylinder actuator which is required to extend or contract the piston rod over a maximum stroke defined between the fully extended position and fully contracted position. 
     By thus forcibly stopping the electric motor, it is possible to cut down the operating time of the electric motor. Since the electric motor is released from a heavily loaded condition soon after the arrival of the piston rod at its fully extended or contracted position, the load on the frame lift mechanism including the electric motor is lessened and the durability of the frame lift mechanism is increased. 
     Additionally, since the electric motor is stopped when the piston rod moves over the maximum stroke, generation of heat from the electric motor can be suppressed. The thermo-breaker built in the electric motor does not operate, so that the operator is allowed to continue snow-removing operation of the snowplow vehicle without considering a downtime of the snowplow vehicle which may occur when the thermo-breaker operates. The snow-removing operation can, therefore, be achieved smoothly and efficiently. Furthermore, the electro-hydraulic cylinder actuator (frame lift mechanism) can operate smoothly and reliably without requiring detection switches provided for detecting the piston rod arrived at the fully extended position and the fully contracted position. The snowplow vehicle is, therefore, formed by a reduced number of parts used and has a relatively simple electric wiring system. This achieves cost cutting of the snowplow vehicle. 
     It is preferable that the control unit continues to stop the electric motor when the operation switch is still in the activated state even after the lapse of the predetermined time. 
     When the operation switch is still in the activated state even after the electric motor is forcibly stopped upon the lapse of the preset reference time (which is equal to an operating time required for the electro-hydraulic cylinder actuator to move the piston rod over the maximum stroke), the control unit continues to stop the electric motor. Thus, a heavily loaded condition of the electric motor does not recur with the result that the total load exerted on the frame lift mechanism including the electric motor is reduced and the durability of the frame lift mechanism is increased. Additionally, since the thermo-breaker is kept in the off or inactivated state, a downtime does not occur. Thus, the snow-removing operation can be continued smoothly and efficiently. 
     In another preferred form of the present invention, the control unit is arranged to add up running times of the electric motor during which the electric motor is rotating and forcibly stop the electric motor when a total sum of the running times reaches a predetermined reference value. The predetermined reference value corresponds to a time which is required for the electric motor to heat up above a predetermined temperature. By forcibly stopping the electric motor, it is possible to protect the electric motor from overheating and eventually improve the durability of the electric motor. Additionally, the electric motor is stopped rapidly without operating the thermo-breaker built in the electric motor The control of the electric motor depends on time and does not rely on the thermo-breaker which requires a relatively long time for recover its original inoperating state. It is, therefore, possible to resume rotation of the electric motor in a relatively short period of time. Since snow-removing operation of the snowplow vehicle can be continued without considering a downtime which may occur when the thermo-breaker operates, the efficiency of the snow-removing operation is very high. 
     It is preferable that the total sum (Tm) of the running times is obtained by the expression 
     
       
         
           Tm=Tr−Ts 
         
       
     
     where Tr represents an accumulated total of the running times during which the electric motor is rotating, and Ts represents an accumulated total of the rest times during which the electric motor is at a standstill. 
     It may be considered that the cumulative running time Tr is a total sum of the running times of the motor during which the electric motor heats up while it is rotating, and the cumulative rest time Ts is a total sum of the rest times of the motor during which the electric motor cools down while it is at a standstill. By using the integrated value or total sum Tm of rotating times which is represented by the expression Tm=Tr−Ts, control of the electric motor is achieved in close match with actual heat-developing and -releasing conditions of the electric motor. Since the cumulative rest time (heat-releasing time) Ts of the electric motor is subtracted from the cumulative running time (heat-developing time) Tr, it is possible to elongate the time during which the integrated value or total sum Tm of running times reaches the preset reference value. This means that the time period during which the motor continues to rotate before it is forcibly stopped can be extended. The snow-removing operation of the snowplow vehicle can be achieved with improved efficiency. 
     It is further preferable that the control unit continues to stop the electric motor until a preset fixed time has passed after forcible stop of the electric motor. Since the heat developed in the electric motor is further released, the electric motor is protected from overheating with higher safeness and hence has a higher degree of durability. 
     Preferably, the running times of the electric motor have a fixed value and are added up at the lapse of a unit time, and the rest times of the electric motor have a fixed value and are added up at the lapse of the unit time, and wherein the fixed value of the running times is larger than the fixed value of the rest times. 
     In still another preferred form of the present invention, the snowplow vehicle has three modes of operation including a manual-up mode in which the auger is raised manually, a manual-down mode in which the auger is lowered manually, and an auto-up mode in which the auger is automatically raised, wherein the control unit is arranged such that when the manual-down mode is selected, the control unit determines and stores an amount of contraction of the piston rod achieved in the selected manual-down mode, and when the manual-down mode is followed by the auto-up mode and information representing reversing of the direction of rotation of the driving wheels is received, the control unit performs an auto-up control of the piston rod in which the piston rod is extended by an amount equal to the amount of contraction of the piston rod determined with respect to the preceding manual-down mode. 
     The travel condition of the snowplow vehicle, which may occur immediately before the manual-down mode is selected, is considered to be a road traveling condition in which the snowplow vehicle travels on a road surface with the auger held in an uppermost position, or a reversing condition in which the snowplow vehicle travels backwards on a snow-covered road surface with the auger held in an elevated position intermediate between the uppermost inclined position and a lowermost horizontal position. The auger, as it is in the elevated intermediate position, does not interfere with snow while the snowplow vehicle is moving backward. From this, it is preferable that when the auto-up mode is selected, the auger is raised to the elevated intermediate position. The auger is thus automatically returned to the previous position, so that there is no possibility of interference occurring between the auger and snow when the snowplow vehicle is moving backward. Furthermore, at the time of forward movement of the snowplow vehicle, the auger is lifted down from the elevated intermediate position to the lowermost horizontal position. Thus, the time required for lowering the auger is reduced to one-half of the conventional snowplow vehicle discussed above with reference to FIGS. 17A and 17B, so that the efficiency of the snow-removing operation is increased correspondingly. In addition, since the auger is automatically lifted to the elevated intermediate position, the operator is not subjected to undue load or pressure. 
     It is preferable that the piston rod of the electro-hydraulic cylinder actuator is extended and contracted at the same speed, and the amount of contraction of the piston rod is determined depending on time. This arrangement obviates the need for a stroke sensor provided for measuring the amount of extension or contraction of the piston rod, which sensor is expensive, is susceptible to malfunction due to adhesion of snow or dirt, and requires wire harnesses. 
     Preferably, the self-propelled snowplow vehicle further includes an auger clutch disposed between a power source and the auger for transmitting rotational power from the power source to the auger, wherein when the auger clutch is in an disengaged state, the control unit disables the auto-up control of the piston rod of the cylinder actuator. 
     The above and other objects, features and advantages of the present invention will become manifest to those versed in the art upon making reference to the following description and accompanying sheets of drawings in which certain preferred structural embodiments incorporating the principle of the invention are shown by way of illustrative examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a walk behind self-propelled crawler snowplow vehicle according to an embodiment of the present invention; 
     FIG. 2 is a diagrammatical plan view of the snowplow vehicle, showing a propelling power transmission line extending from electric motors to crawler belts and a snow-removing power transmission line extending from an engine to a snowplow mechanism; 
     FIG. 3 is a view in the direction of the arrow  3  shown in FIG. 1; 
     FIG. 4 is an exploded perspective view of a frame lift mechanism; 
     FIGS. 5A and 5B are diagrammatical side views illustrative of the operation of the frame lift mechanism; 
     FIG. 6 is a circuit diagram including a control unit and related parts thereof; 
     FIG. 7 is a flowchart showing a control procedure achieved by a control unit of the snowplow vehicle; 
     FIG. 8 is a time chart explanatory of the operation of the control unit; 
     FIG. 9 is a flowchart showing a modified control procedure achieved by the control unit; 
     FIG. 10 is a circuit diagram showing the control unit and related parts thereof according to a modification of the present invention; 
     FIG. 11 is a diagrammatic plan view of a walk behind self-propelled crawler snowplow according to another embodiment of the present invention; 
     FIGS. 12A and 12B are diagrammatical views illustrative of the operation of an auger clutch equipped in the snowplow vehicle shown in FIG. 11; 
     FIG. 13 is an enlarged plan view of a control board; 
     FIG. 14 is a flowchart showing a control procedure achieved by a control unit of the snowplow vehicle shown in FIG. 11; 
     FIG. 15 is a flowchart similar to FIG. 14, but showing a modified control procedure of the control unit; 
     FIGS. 16A to  16 C are diagrammatical views illustrative of the operation of a conventional snowplow vehicle; and 
     FIGS. 17A and 17B are diagrammatical views illustrative of the operation of another conventional snowplow vehicle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is merely exemplary in nature and is in no way intended to limit the invention or its application or use. 
     Referring to the drawings and FIG. 1 in particular, there is shown a walk behind self-propelled crawler snowplow vehicle  10  according to an embodiment of the present invention. The snowplow vehicle  10  generally comprises a propelling frame  12  carrying thereon left and right crawler belts (only the left crawler belt  11 L being shown), a vehicle frame  15  carrying thereon a snowplow mechanism  13  and an engine (prime motor)  14  for driving the snowplow mechanism  13 , a frame lift mechanism  16  operable to lift a front end portion of the vehicle frame  15  up and down relative to the propelling frame  12 , and a pair of left and right operation handlebars  17 L and  17 R extending from a rear portion of the propelling frame  12  obliquely upward in a rearward direction of the snowplow vehicle  10 . The operation handlebars  17 L,  17 R each have a grip  18 L,  18 R at the distal end (free end) thereof The propelling frame  12  and the vehicle frame  15  jointly form a vehicle body  19 . The propelling frame  12  also carries thereon left and right drive wheels  23 L,  23 R and left and right driven wheels  24 L,  24 R. 
     The operation handlebars  17 L,  17 R are adapted to be gripped by a human operator (not shown) walking behind the snowplow vehicle  10  in order to maneuver the snowplow vehicle  10 . In the illustrated embodiment, a control board  41 , a control unit  28  and batteries  28  are arranged in a vertical space defined between the left and right handlebars  17 L,  17 R and they are mounted to the handlebars  17 L,  17 R in the order named when viewed from the top to the bottom of FIG.  1 . 
     The engine  14  serves as a power source for the snowplow mechanism  13  and generates motive power which is transmitted via a snowplow power transmission mechanism  34  to the snowplow mechanism  13 . The snowplow power transmission mechanism  34  is arranged such that power from an output shaft (crankshaft)  35  of the engine  14  can be transmitted via a driving pulley  36  and a power transmission belt  37  to the snowplow mechanism  13 . To this end, an electromagnetic clutch  45  is mounted on the output shaft  35  of the engine  14 . The driving pulley  36  is freely rotatably mounted on the output shaft  35  of the engine  14  and is connected in driven relation to the output shaft  35  when the electromagnetic clutch  45  is actuated or placed in the engaged state. 
     The snowplow mechanism  13  has an auger  31 , a blower  32  and a discharge duct or shooter  33  that are mounted to a front portion of the vehicle frame  15 . The auger  31  and the blower  32  are rotatably mounted on a rotating shaft  39 . The rotating shaft  39  has a driven pulley  38  connected in driven relation to the driving pulley  36  by means of the power transmission belt  37 . 
     In operation, the power from the engine output shaft  35  is transmitted via the electromagnetic clutch  45  to the driving pulley  36 , and rotation of the driving pulley  36  is transmitted via the power transmission belt  37  to the driven pulley  38 . The driven pulley  38  is thus rotated. Rotation of the driven pulley  38  causes the rotating shaft  39  to rotate the auger  31  and the blower  32  concurrently. The auger  31  cuts snow away from a road surface, for example, and feeds the snow into the blower  32 . The blower  32  blows out the snow through the discharge duct or shooter  33  to a distant place. 
     In FIG. 1, reference numeral  26   a  denotes an auger case, numeral  26   b  denotes a blower case, numeral  26   c  denotes a scraper formed integrally with a lower edge of the auger case  26   a,  numeral  26   d  denotes a charging generator for charging the batteries  29 , numeral  26   e  denotes a lamp, numeral  26   f  denotes a cover for protecting the generator  26   d  and the electromagnetic clutch  50 , and numeral  26   g  denotes a stabilizer for urging each crawler belt  11 L,  11 R downward against the ground surface. 
     As diagrammatically shown in FIG. 2, the left and right crawler belts  11 L,  11 R are driven by left and right electric motors  21 L,  21 R, respectively. The crawler belts  11 L,  11 R are each entrained around the driving wheel  23 L,  23 R and the driven wheel  24 L,  24 R provided in pair. The driving wheel  23 L,  23 R is disposed on a rear side of the crawler belt  11 L,  11 R, and the driven wheel  24 L,  24 R is disposed on a front side of the crawler belt  11 L,  11 R. 
     Power from each electric motor  21 L,  21 R is transmitted through a propelling power transmission mechanism  22 L,  22 R to the corresponding driving wheel  23 L,  23 R to thereby drive the associated crawler belt  11 L,  11 R. The propelling power transmission mechanism  21 L,  22 R comprises a speed reducer  22 L,  22 R assembled integrally with the electric motor  21 L,  21 R. The speed reducer  22 L,  22 R has an output shaft that forms a rear axle on which each driving wheel  23 L,  23 R is fixed. Thus, the left and right crawler belts  11 L,  11 R are separately drivable with power from the corresponding electric motors  21 L,  21 R. Reference numeral  25  denotes a front axle on which the left and right driven wheels  24 L,  24 R are rotatably mounted. 
     In order to drive the charging generator  26   d , a generator driving pulley  27   a  is mounted to the engine output shaft (crankshaft)  35 , and a generator driven pulley  27   b  is mounted to a shaft of the charging generator  26   d . The driving and driven pulleys  27   a ,  27   b  are connected by a V-belt  27   c , so that rotation of the engine output shaft  35  is transmitted to the charging generator  26   d.    
     FIG. 3 shows the general arrangement of an operating part  40  of the snowplow vehicle. The operating part  40  generally comprises the control board  41  disposed between the left and right handlebars  17 L,  17 R, a travel-ready lever  43  and a left turn control lever  44 L that are mounted to the left handlebar  17 L in the proximity of the grip  18 L, and a right turn control lever  44 R mounted to the right handlebar  17 R in the proximity of the grip  18 R. The travel-ready lever  43  is operated to place the snowplow vehicle  10  in a ready-to-travel condition. 
     The control board  41  is composed of a box-shaped body  45  extending between the left and right handlebars  17 L,  17 R, and a control panel  46  covering an upper opening of the box-shaped control board body  45 . The body  45  is provided with an auger switch (clutch switch)  45 A for switching on-off operation of the electromagnetic clutch  50  (FIG.  1 ), a main switch (key switch)  45 B, a choke knob  45 C that may be used when the engine  14  (FIG. 1) is started, a light button  45 D for turning on and off the lamp  26   e  (FIG.  1 ), and a failure lamp  45 D designed to be turned on when a failure occurs. 
     The control panel  46  is provided with a lift control lever  46 A for controlling operation of the frame lift mechanism  16  (FIG.  1 ), a shooter control lever  46 B for changing the direction of the shooter  33  (FIG.  1 ), a throttle lever  46 C for controlling speed (revolutions per minute) of the engine  14 , and a forward/reverse speed control lever  76  for controlling the direction and speed of the electric motors  21 L,  21 R (FIG.  1 ). The control panel  46  has a generally flat body portion  47   a  forming a major part of the control panel  46 , a cover portion  47   b  of an inverted U-shaped cross section for covering the travel-ready lever  43 , and a guide groove  48  formed in the body portion  47   a  for guiding movement of the forward/reverse speed control lever  76 . 
     The lift control lever  46 A has an auto-return mechanism so that when the lift control lever  46 A is released from the human operator, it automatically returns to the original neutral position shown in FIG.  3 . When the lift control lever  46 A is pulled or tilted rearward of the snowplow vehicle, the frame lift mechanism  16  (FIG. 1) operates to raise the snowplow mechanism  13  (FIG.  1 ). Conversely, when the lift control lever  46 A is pushed or tilted forward of the snowplow vehicle, the frame lift mechanism  16  operates to lower the snowplow mechanism  13 . 
     As shown in FIG. 4, the propelling frame  12  is composed of a pair of parallel spaced left and right side members  61 ,  61  extending in the longitudinal direction of the vehicle body  19 , a front cross member  62  interconnecting respective front portions of the side members  61 ,  61 , and a rear cross member  63  interconnecting respective rear portions of the side members  61 ,  61 . The propelling frame  12  further has a pair of side brackets  64 ,  64  connected to left and right end portions of the rear cross member  63  adjacent to the side members  61 , and a central bracket  65  connected to a central portion of the rear cross member  63  which corresponds in position to a widthwise central portion of the propelling frame  12 . 
     The electric motors  21 L,  21 R assembled integrally with the speed reducers (not designated) are mounted to respective rear end portions of the side members  61 ,  61 . The rear axles (not designated) that are formed by output shafts of the speed reducers are rotatably supported by the rear end portions of the side members  61 ,  61 . Respective front end portions of the side members  61 ,  61  have a longitudinal slot (not designated) for receiving therein a longitudinal portion of the front axle  25  so that the front axle  25  is rotatably supported on the front end portions of the side members  61 ,  61 . 
     The left and right side brackets  64  are each comprised of a vertically extending channel member having a U-shaped cross section. The left and right handlebars  17 L,  17 R have respective lower end portions bolted to the opposite outer sides of the left and right side brackets  64 . The side brackets  64  each have a horizontal through-hole  64   a  formed in an upper end portion thereof 
     The vehicle frame  15  is comprised of a pair of parallel spaced left and right side members  71 ,  71  extending in the longitudinal direction of the vehicle body  19 , and a horizontal mount base  72  extending between the side members  71 ,  71  astride a rear half of the side members  71  for mounting the engine  14 . The vehicle frame  15  also has a support arm  73  connected to a central portion of the front edge of the mount base  72 . The side members  71  each have a horizontal through-hole  71   a  formed in a rear end portion thereof 
     The vehicle frame  15  is pivotally connected to the propelling frame  12  by means of pivot pins  74  (one being shown) inserted successively through the horizontal through-holes  64   a  in the side brackets  64  and the horizontal through-holes  71   a  in the side members  71 . With this pivotal connection, a front end portion of the vehicle frame  15  is movable up and down in a vertical plane relative to the propelling frame  12 . 
     The frame lift mechanism  16  is comprised of a cylinder actuator having a cylinder tube  81  and a piston rod  82  reciprocally movable to project from or retract into the cylinder tube  81 . The cylinder actuator is of the electro-hydraulic type, in which the piston rod  82  is reciprocated by a fluid pressure generated from a pump (not shown) driven by an electric motor  85  (FIG.  2 ). The electric motor  85  is mounted on one side of the cylinder tube  81 . 
     The front end of the rod  82  is pivotally connected by a pin  84  to the support arm  73  of the vehicle frame  15 , and the rear end of the cylinder tube  81  is pivotally connected by a pin  83  to the central bracket  65  of the propelling frame  12 . With this arrangement, the vehicle frame  15  is movable to swing up and down in the vertical plane about the pivoted rear end portion thereof in response to extending and contracting movement of the cylinder actuator (frame lift mechanism)  16 . 
     The frame lift mechanism  16  of the foregoing construction operates as follows. As shown in FIG. 5A, when the cylinder actuator (frame lift mechanism)  16  of the snowplow vehicle  10  is in the fully contracted state (in which the piston rod  82  shown in FIG. 4 is disposed in a fully contracted position), the auger  31  of the snowplow mechanism  13  and the front portion of the vehicle frame  15  are disposed in a lowest horizontal position. 
     Conversely, as shown in FIG. 5B, when the cylinder actuator (frame lift mechanism)  16  is in the fully extended state (in which the piston rod  82  shown in FIG. 4 is disposed in a fully extended position), the auger  31  of the snowplow mechanism  13  and the front portion of the vehicle frame  15  are disposed in a highest inclined position. 
     Since the crawler belts  11 L,  11 R carried on the propelling frame  12  are in contact with the ground surface Gr, the height of the propelling frame  12  is always constant. On the other hand, the vehicle frame  15 , which is pivotally connected by the pivot pins  74  to the propelling frame  12 , is pivotally movable to swing up and down about the pivot pins  74  relative to the propelling frame  12 . 
     It will be appreciated that by properly manipulating the lift control lever  46 A (FIG. 3) so as to extend or contract the cylinder actuator (frame lift mechanism)  16 , the vehicle body  15  swings up and down relative to the propelling frame  12  to thereby raise or lower the auger  31  of the snowplow mechanism  13  mounted to the front portion of the vehicle frame  15 . When the snowplow vehicle  10  is to be moved from one place to another, the auger  31  is preferably disposed in the highest inclined position of FIG. 5B so as to enable the snowplow vehicle  10  to travel smoothly. During snow-removing operation of the snowplow vehicle  10 , the auger  31  is preferably disposed in the lowest horizontal position of  5 A so as to insure highly efficient snow-removing operation by the snowplow  31 . It is preferable that during the snow-removing operation, the vertical position of the auger  31  is adjusted to accommodate angulations of the ground surface Gr. 
     The frame lift mechanism  16  and lift control lever  46 A (FIG. 3) are operatively connected together so that when the lift control lever  46 A (FIG. 3) returns to its original neutral position, the cylinder actuator (frame lift mechanism)  16  retains its length given at that time, thereby keeping a swing angle of the auger  31  and the vehicle frame  15  relative to the propelling frame  12 . 
     FIG. 6 is a circuit diagram showing an electric circuit  90  including the control unit  28  and related parts thereof. The electric circuit  90  also includes an operation switch  100  connected directly to the control unit  28 . In the electric circuit  90 , the control unit  28 , a control relay  110 , an auger-up relay  120 , an auger-down relay  130  and a control lamp  140  are connected via a main switch  45   b  to the batteries  29 . 
     The operation switch  100  comprises a lift control switch composed of the lift control lever  46 A and a switch mechanism  101  that are assemble together so as to control operation of the electric motor  85  of the frame lift mechanism  16 . 
     The switch mechanism  101  of the lift control switch (operation switch)  100  has the function of a three-position toggle switch having a movable contact  102  and two fixed contacts  103 ,  104 . The switch mechanism  101  and the lift control lever  46 A are operatively connected together such that when the lift control lever  46 A is in the neutral position Ne, the movable contact  102  of the operation switch  100  is also disposed in the neutral position where the movable contact  102  does not engage either of the two fixed contacts  103 ,  104 . In this instance, the operation switch  100  is in the off state and no signal is generated from the operation switch  100 . When the lift control lever  46 A is pulled or tilted rearward (rightward in FIG. 6) to an up position Up, the movable contact  102  comes in contact with the first fixed contact  103 . This makes the operation switch  100  turn on and an “on” signal is generated from the operation control switch  100 . Similarly, when the lift control lever  46 A is pushed or tilted forward (leftward in FIG. 6) to a down position Dw, the movable contact  102  comes in contact with the second fixed contact  104 . This makes the operation switch  100  turn on and an “on” signal is generated from the operation switch  100 . 
     The control unit  28  has a first function of forcibly stopping the electric motor  85  when a preset reference time T1 has passed after the operation switch  100  is turned on or activated (or when activation of the operation switch  100  continues till a lapse of the reference tim t1). The control unit  28  also has a second function of continuing stopping the electric motor  85  when the operation switch  100  is still in the activated state after a lapse of the reference time t1. The reference tim t1 is equal to an operating time of the cylinder actuator (frame lift mechanism)  16  which is required to move the piston rod  82  over a maximum stroke between the fully extended position and the fully contacted position. 
     The control unit  28  performs various control operations, as enumerated below. 
     (1) When the operation switch  100  is in the off state (i.e., in the absence of an “on” signal from the operation switch  100 ), an excitation coil  111  of the control relay  110  is kept de-energized to maintain the original “off” position of a normally open contact  112 . The control relay  110  is thus kept in the off state. 
     (2) When the operation switch  100  is turned on or activated (that is, when an “on” signal is produced from the operation switch  100 ), the excitation coil  111  of the control relay  110  is energized to move the normally open contact  112  to an “on” or dosed position. The control relay  110  is thus turned on or activated. 
     (3) When the “on” signal from the operation switch  100  continues to present until the reference time T1 has elapsed after the operation switch  100  is turned on or activated, the excitation coil  111  of the control relay  110  is forcibly de-energized to return the normally open contact  112  to the original “off” or open position. The control relay  110  is thus forcibly turned off or deactivated 
     (4) When the “on” signal from the operation switch  100  is still present even after the lapse of the reference time T1, the de-energized state of the excitation coil  111  is continued to thereby keep the “off” or open position of the contact  112 . The control relay  110  is continuously held in the de-activated state. 
     (5) When the control relay  110  is in the on or activated state, this means that the electric motor  85  is operating or rotating. Under such condition, the control lamp  141  is in the on or activated state. 
     The auger-up relay  120  and the auger-down relay  130  are disposed between the control relay  110  and the operation switch  100  so that they operate under the control of the control relay  110  and the operation switch  100 . Furthermore, the electric motor  85  and a thermo-breaker  86  for protection of the electric motor  85  are also disposed between the auger-up relay  120  and the auger-down relay  130  so that they operate also under the control of the control relay  110  and the operation switch  100 . 
     The thermo-breaker  86  is a protection member incorporated in the electric motor  85  for protecting the electric motor from overheating. The thermo-breaker  86  is designed to cut off supply of electric current to the electric motor  85  when the electric motor  85  heats up to a given (overheat) temperature due to continued activation or frequent on-off operations of the operation switch  100 . 
     When the left control lever  46 A is in the neutral position Ne, or when the control relay  110  is in the off state (with the normally open contact  112  disposed in the original “off” or open position), the auger-up relay  120  and the auger-down relay  130  are both placed in the off condition. Under such condition, the electric motor  85  connected between the auger-up relay  120  and the auger-down relay  130  is in the off or de-energized state. 
     When the lift control lever  46 A is pulled or tilted down toward the “Up” side to bring the movable contact  12  into contact with the first fixed contact  103  and, at the same time, the control switch  110  is in the on state (with the normally open contact  112  disposed in the “on” or activated position), the auger-up relay  120  is turned on or activated whereupon the electric motor  85  starts rotating in a forward direction. 
     Conversely, when the lift control lever  46 A is pushed or tilted down toward the “Dw” side to bring the movable contact  12  into contact with the second fixed contact  104  and, at the same time, the control switch  110  is in the on state (with the normally open contact  112  disposed in the “on” or activated position), the auger-down relay  120  is turned on or activated whereupon the electric motor  85  starts rotating in a reverse direction. 
     The control unit  28  shown in FIG. 6 is comprised of a microcomputer and can operate to achieve a control procedure as illustrated in the flowchart shown in FIG.  7 . The control procedure achieved in the microcomputer (control unit)  28  will be described below in conjunction with the circuit diagram shown in FIG.  6 . 
     The control procedure shown in FIG. 1 begins when the main switch  45 B (FIG. 6) is turned on. At a first step ST 01 , a timer in a central processing unit of the microcomputer (control unit)  28  is reset to zero (Tc=0). Then ST 02  reads a signal from the operation switch  100 . Subsequently, ST 03  judges whether or not the count in the timer has not exceeded the preset reference time T1. If the result of judgment is “YES” (Tc≦T1), this means that the preset reference time has not elapsed after activation of the operation switch  100 . In this condition, the control procedure goes on to ST 04 . Alternatively, if the judgment result at ST 03  is “NO” (Tc&gt;T1), this means that the preset reference time T1 has passed after activation of the operation switch  100 . Under such condition, the control procedure branches to ST 13 . 
     ST 04 , which follows ST 03 , makes a judgment to determine whether or not the “on” signal from the operation switch  100  is present. If, the result of judgment is “YES”, this means that the lift control lever  46 A has been tilted down toward the “Up” side or the “Dw” side. Under such condition, the control procedure advances to ST 05 , which turns on the control relay  110  to thereby rotate the electric motor  85 . Alternatively, if the judgment result at ST 04  is “NO”, this means that the lift control lever  46 A is in the neutral position “Ne”. The control procedure then branches to ST 14 , which turns off the control relay  110  to thereby stop the electric motor  85 . 
     ST 05  is followed by ST 06 . At ST 06 , a judgment is made to determine whether or not the timer is still operating. If the result of judgment is “YES”, the control procedure advances to ST 09 . Conversely, if the judgment result is “NO”, the control procedure branches to ST 07 . At ST 07 , the timer is reset to zero (Tc=0). The timer is subsequently started at ST 08 . After ST 08 , the control procedure advances to ST 09 . 
     ST 09  judges whether or not the count in the timer Tc has exceed the preset reference time T1 (Tc&gt;T1). If the result of judgment is “YES”, this means that the preset reference time T1 has elapsed after activation of the operation switch  100 . Under such condition, the control goes on to ST 10  and turns off the control relay  100  to thereby forcibly stop the electric motor  85 . Alternatively, if the judgment result at ST 09  is “NO”, this means that the preset reference time T1 has not elapsed after activation of the operation switch  100 . The control procedure then returns to ST 02 . 
     ST 10  is followed by ST 11  where the timer in the control unit  28  is stopped. Subsequently, the control procedure advances to ST 12 , which makes a judgment to determine whether or not the control procedure is to be terminated. If the result of judgment is “YES”, the control procedure is stopped. Alternatively, if the judgment result is “NO”, the control procedure returns to ST 02 . 
     At ST 13 , which is branched off from ST 03 , a judgment is made to determine whether or not the “on” signal from the operation switch  100  is present. If the result of judgment is “YES”, this means that the lift control lever  46 A is still tilted down toward the “Up” side or the “Dw” side even after the lapse of the preset reference time T1. Under such condition, the control procedure goes on to ST 14  where the control relay  110  is turned off or deactivated to thereby stop the electric motor  85 . ST 14  is followed by ST 12  described previously. Alternatively, if the judgment result at ST 13  is “NO”, this means that the lift control lever  46 A is in the neutral position Ne after the lapse of the preset reference time T1. The control procedure then jumps to ST 12  where, as previously described, judgment is made to determine whether or not the control procedure is to be terminated. 
     It will be appreciated from the foregoing description that when the operation switch  100  (FIG. 6) is turned on or activated to rotate the electric motor  85  in the forward or the reverse direction, the electro-hydraulic cylinder actuator frame lift mechanism)  16  generates a fluid pressure to extend or contract the piston rod  82 . By virtue of the extending or contracting movement of the piston rod  82 , the front end portion of the vehicle frame  15  and the auger  31  mounted thereto are lifted up and down, as illustrated in FIGS. 5A and 5B. 
     When the preset reference time T1 has elapsed after activation of the operation switch  100  (T1 being equal to an operating time of the electro-hydraulic cylinder actuator  16  which is required to move the piston rod  82  over a maximum stroke defined between the fully extended position and the fully contracted position of the piston rod  82 ), the control unit  28  forcibly stops the electric motor  85  even if the operation switch  100  in the “on” or activated state. By thus forcibly stopping the electric motor  85 , it is possible to cut down the operating time of the electric motor. Since the electric motor  85  is released from a heavily loaded condition soon after the arrival of the piston rod  82  at its fully extended or contracted position, the load on the frame lift mechanism  16  including the electric motor  85  is lessened and the durability of the frame lift mechanism  16  is increased. 
     Additionally, since the electric motor  85  is stopped when the piston rod  82  moves over the maximum stroke, generation of heat from the electric motor  85  can be suppressed The thermo-breaker  85  built in the electric motor  86  does not operate, so that the operator is allowed to continue snow-removing operation of the snowplow vehicle  10  without considering a downtime of the snowplow vehicle  10  which may occur when the thermo-breaker  86  operates. The snow-removing operation can, therefore, be achieved smoothly and efficiently. 
     Furthermore, the electro-hydraulic cylinder actuator (frame lift mechanism)  16  can operate smoothly and reliably without requiring detection switches provided for detecting the piston rod  82  arrived at the fully extended position and the fully contracted position. The snowplow vehicle  10  is, therefore, formed by a reduced number of parts used and has a relatively simple electric wiring system. This achieves cost cutting of the snowplow vehicle  10 . 
     Additionally, when the operation switch  100  is still in the activated state even after the electric motor  85  is forcibly stopped upon the lapse of the preset reference time T1 (which is equal to an operating time required for the electro-hydraulic cylinder actuator  16  to move the piston rod  82  over the maximum stroke), the control unit  28  continues to stop the electric motor  85 . Thus, a heavily loaded condition of the electric motor  85  does not recur with the result that the total load exerted on the frame lift mechanism  16  including the electric motor  85  is reduced and the durability of the frame lift mechanism  16  is increased. Additionally, since the thermo-breaker  86  is kept in the off or inactivated state, a downtime does not occur. Thus, the snow-removing operation can be continued smoothly and efficiently. 
     When the stroke of the piston rod  82  is changed due to the influence of snow, dirt, mud and other foreign matter, the control unit  28  forcibly stops the electric motor  85  upon the lapse of the predetermined reference time T1 regardless of the operation switch  100  being in the on or activated state. As a result, a heavily loaded condition of the electric motor  85  is immediately removed. This ensures that the total load applied to the frame lift mechanism  16  including the electric motor  85  is reduced and the durability of the frame lift mechanism  16  is increased. Additionally, by virtue of the forcible stop of the electric motor  85 , generation of heat from the electric motor  85  can be suppressed. The thermo-breaker  85  built in the electric motor  65  does not operate. 
     The control unit  28  shown in FIG. 6 may be modified to have a function of integrating or adding up the running time Trα of the electric motor  85  during which the electric motor  85  is rotating and forcibly stopping the electric motor  85  when the integrated value (total sum of the running times) Tm reaches a predetermined reference value (reference time) T2. The predetermined reference value T2 corresponds to a time which is required for the electric motor  85  to heat up above a predetermined temperature. For instance, if the cumulative running time and cumulative rest time of the electric motor are represented by Tr and Ts, respectively, the integrated value (total sum) Tm of the running times is obtained by Tm=Tr−Ts. 
     The modified control unit, designated by  28   a  in FIG. 6 for purposes of explanation, further has a function of continuing stopping of the electric motor  85  until a predetermined fixed time (reference time) T3 has passed. 
     More specifically, the modified control unit  28   a  performs various control operations, as enumerated below 
     (1) When the main switch  45 B (FIG. 6) is turned on or activated, the control relay  110  is turned on or activated. 
     (2) Time periods during which the “on” state signal from the operation switch  100  is present (i.e., running times Trα of the electric motor  85  during which the electric motor  85  is rotating) are integrated or added up, and when an integrated value (total sum) Tm of the running times Trα reaches the reference value T2, the control relay  100  is forcibly turned off or deactivated. 
     (3) After forcible de-activation of the control relay  110 , the “off” or deactivated state of the control relay  110  is continuously maintained until the reference time T3 has passed. 
     (4) When the control relay  110  is in the “on” state, this means that the electric motor  85  is running or rotating. Under such condition, the control lamp  141  is kept in the on or activated state. 
     Stated in more concretely, the cumulative running time Tr is updated each time a predetermined time has passed. That is, each time the predetermined time has passed, a running time Trα is added to the accumulated total Tr of the running times during the preceding interval (Tm=Tr+Trα). The running time Trα has a predetermined value such as 11 milliseconds (ms), which is added up, at an interval of 100 milliseconds (ms). 
     On the other hand, the cumulative rest time Ts is updated each time a predetermined time has passed. That is, each time the predetermined time has passed, a rest time Trβ during which the electric motor  85  is stopping or not rotating is added to the accumulated total Ts of the rest times during the preceding interval (Ts=Ts+Trβ). The rest time Trβ has a predetermined value such as 10 ms, which is added up, at an interval of 100 ms. 
     The thus obtained cumulative rest time Ts is subtracted from the cumulative running time Tr to thereby obtain an integrated value or total sum Tm of the rotating times (Tm=Tr−Ts). 
     The running time Trα (i.e., 11 ms) which is added up at intervals of 100 ms is set to be larger than the rest time (i.e., 10 ms) which is also added up at intervals of 100 ms, the reason for which is as follows. 
     In general, a heat-developing time, which is required for the electric motor  85  to heat up from the room temperature to a predetermined elevated temperature while it is rotating, is shorter than a heat-releasing time which is required for the electric motor  85  to cool down from the elevated temperature to the room temperature while it is at a standstill. If the running time Trα is set to be equal to the rest time Trβ, it may occur that the integrated value or total sum Tm of the running times becomes zero even though the electric motor  85  has not cooled down to the room temperature. To preclude the occurrence of this problem, the running time Trα added up at intervals of 100 ms is set to be longer than the rest time Trβ added up at intervals of 100 ms. 
     FIG. 8 is a timing chart illustrative of operation of the modified control unit  28   a  (FIG.  6 ). In FIG.  8 ( a ), the horizontal axis represents time (ms), and the vertical axis represents the state of the operation switch  100  (FIG.  6 ). In FIG.  8 ( b ), the horizontal axis represents time (ms), and the vertical axis represents an integrated value or total sum Tm (ms) of running times of the electric motor  85 . Similarly, in FIG.  8 ( c ), horizontal axis represents time (ms), and the vertical axis represents the state of the control relay  110  (FIG.  6 ). 
     As shown in FIG.  8 ( b ), the integrated value or total sum Tm of running times of the electric motor  85  increases gradually as long as a signal indicative of the “on” or activated state of the operation switch  100  is present (namely, when the electric motor  85  is rotating). Alternatively, when the “off” or deactivated state of the operation switch  100  is present (namely, when the electric motor  85  is at rest), the integrated value or total sum Tm of running times of the electric motor  85  decreases gradually. When the total sum Tm of running times reaches the reference value T2, the control relay  110  is forcibly changed or shifted from the “on” or activated state to the “off” or de-activated state. 
     As shown in FIG.  8 ( c ), after forcible stopping of the control relay  110 , the “off” or deactivated state of the control relay  110  is maintained until the reference time T3 has passed. During that time, the electric motor  85  continues to stop even when the “on” state signal is received from the operation switch  100 . Thus, the total sum Tm of running times gradually decreases until the reference time T3 has passed. 
     FIG. 9 is a flowchart showing a control procedure achieved by the CPU incorporated in the modified control unit  28   a  shown in FIG.  6 . 
     At a first step ST 101 , all the values are initialized. Namely, the cumulative running time Tr, cumulative rest time Ts and the integrated value or total sum Tm of running times are all reset to zero. Then, a signal from the operation switch  100  is read in at ST 102  and, subsequently, ST 103  judges whether or not the signal from the operating switch  100  is in the “on” or activated state. If the result of judgment is “YES”, this means that the lift control lever  46 A (FIG. 6) has been tilted down toward the “Up” side or the “Dw” side. Under such condition, the control procedure advances to ST 104  where the control relay  110  is turned on or activated to thereby rotate the electric motor  85 . Alternatively, if the judgment result at ST 103  is “NO”, this means that the lift control lever  46 A is in the neutral position “Ne”. The control procedure then branches to ST 106  where the control relay  110  is turned off or deactivated to thereby stop the electric motor  85 . 
     ST 104  is followed by ST 105  where a cumulative running time Tr is determined by adding a running time Trα of the electric motor  85  to the accumulated total Tr of running times during the preceding interval (Tr=Tr+Trα). ST 106  is followed by ST 107  where a cumulative rest time Ts is determined by adding a rest time Tsβ of the electric motor  85  is added to the accumulated total Ts of rest times during the preceding interval (Ts=Ts+Tsβ). The thus determined cumulative rest time Ts is subtracted from the cumulative running time Tr so that an integrated value or total sum Tm of running times (Tm=Tr−Ts) is obtained at ST 108 . 
     Subsequently, ST 109  judges whether or not the total sum Tm of running times has reached the predetermined value T2 (Tm≧T2). If the result of judgment is “YES”, the control procedure goes on to ST 110  where the control relay  110  is forcibly turned off to thereby stop rotation of the electric motor  85 . Alternatively, if the judgment result at ST 109  is “NO”, the control procedure branches to ST 115 . 
     ST 110  is followed by ST 111  where the total sum Tm of running times is reset to zero (Tm=0). The control procedure goes on to ST 112  where the internal timer of the control unit  28  is reset to zero (Tc=0). The internal timer is started again at ST 113 , and at the next following step ST 114  a judgment is made to determine whether or not a count TC of the timer has exceeded the reference time T3 (Tc&gt;T3). If the result of judgment is “YES”, the control procedure goes on to ST 115  Alternatively, if the judgment result at ST 114  is “NO”, ST 114  will repeat the same judgment process until Tc exceeds T3. 
     At ST 115 , a judgment is made to determine whether or not the control procedure is to be stopped. If the result of judgment is “ES” (for instance, when the main switch  45 B has been turned off), the control procedure is terminated. Alternatively, if the result of judgment at ST 115  is “NO”, the control procedure returns to ST 102 . 
     It will be appreciated from the foregoing description that in the modified arrangement shown in FIGS. 5,  6 ,  8  and  9 , when the operation switch  100  (FIG. 6) is turned on or activated to rotate the electric motor  85  in the forward or the reverse direction, a hydraulic pressure is produced, and by the hydraulic pressure, the piston rod  82  of the electro-hydraulic cylinder actuator (frame lift mechanism)  16  is extended or contracted. By thus extending or contracting the piston rod  82 , the front end portion of the vehicle frame  15  and the auger  31  mounted thereto are lifted up and down, as illustrated in FIGS. 5A and 5B. 
     While the electric motor  85  is rotating, the running time Trα of the electric motor  85  is added up at uniform intervals of time, and when an integral value or total sum Tm of the running times reaches the predetermined reference time T2, the electric motor  85  is forcibly stopped by the control unit  28   a  regardless of the operation switch  100  being in the “on” or activated state. By thus forcibly stopping the electric motor  85 , the motor  85  is protected from overloading and thus has a higher degree of durability. 
     Additionally, the electric motor  85  is stopped rapidly without operating the thermo-breaker  86  built in the electric motor  85 . The control of the electric motor  85  depends on time and does not rely on the thermo-breaker  86  which requires a relatively long time to recover its original in operating state. It is, therefore, possible to resume rotation of the electric motor  85  in a relatively short period of time. Since snow-removing operation of the snowplow vehicle  10  can be continued without considering a downtime, which may occur when the thermo-breaker  86  operates, the efficiency of the snow-removing operation is very high. 
     In the arrangement using the control unit  28   a  shown in FIG. 6, the cumulative running time Tr and the cumulative rest time Ts of the electric motor  85  are represented by Tr and Ts, respectively, so that we can obtain an integrated value or total sum Tm of the running times of the motor  85  from the expression Tm=Tr−Ts. 
     It may be considered that the cumulative running time Tr is a total sum of the running times of the motor during which the electric motor  85  heats up while it is rotating, and the cumulative rest time Ts is a total sum of the rest times of the motor  85  during which the electric motor  85  cools down while it is at a standstill. By using the integrated value or total sum Tm of rotating times which is represented by the expression Tm=Tr−Ts, control of the electric motor  85  is achieved in dose match with actual heat-developing and -releasing conditions of the electric motor  85 . Since the cumulative rest time (heat-releasing time) Ts of the electric motor  85  is subtracted from the cumulative running time (heat-developing time) Tr, it is possible to elongate the time during which the integrated value or total sum Tm of running times reaches the preset reference value T2. This means that the time period during which the motor  85  continues to rotate before it is forcibly stopped can be extended The snow-removing operation of the snowplow vehicle can be achieved with improved efficiency. 
     Furthermore, after forcible stop of the electric motor  85 , the control unit  28   a  continues to stop the electric motor  85  until the preset reference time T3 has passed. During that time, heat developed in the electric motor  85  is released. The electric motor  85  is thus prevented from overheating and hence has an improved degree of durability. 
     FIG. 10 is a circuit diagram showing a control unit  28   b  and related parts thereof according to a further modification of the present invention; 
     The electric circuit  90 A shown in FIG. 10 differs from the electric circuit  90  shown in FIG. 6 only in that the control relay  110  is omitted, and the control unit  28   b  performs on-off control of the auger-up relay  120  and auger-down relay  130  by directly energizing or de-energizing the excitation coils  121 ,  131  of the relays  120 ,  130 . These parts which are identical to those shown in FIG. 6 are designated by the same reference characters, and a further description thereof can be omitted. 
     The control unit  28   b  is designed to perform various control operations as enumerated below. 
     (1) When the “on” state signal from operation switch  100  is not present, the auger-up relay  120  and the auger-down relay  130  are kept in the off or deactivated state. 
     (2) When the lift control lever  46 A is tilted down toward the “Up” side, an “on” state signal from the operation switch  100  is received whereupon the auger-up relay  120  is turned on or activated. 
     (3) When the lift control lever  46 A is tilted down toward the “Dw” side, an “on” state signal from the operation switch  100  is received whereupon the auger-down relay  130  is turned on or activated. 
     (4) As to the function of controlling the auger-up relay  120  and the auger-down relay  130 , which is achieved through the control relay  110  in the case of the control unit  28 ,  28   a  shown in FIG.  6  and described above with reference to FIGS. 7 and 9, the control unit  28   b  has substantially the same function even though the relays  120 ,  130  are directly controlled by the control unit  28   b.    
     (5) when the control relay  110  is in the “on” state, this means that the electric motor  85  is running or rotating. Under such condition, the control lamp  141  is kept in the on or activated state. 
     The control procedure shown in the flowchart of FIG.  7  and the control procedure shown in the flowchart of FIG. 9 may be combined to attain the advantageous effects achieved by the two control procedures. The control procedures thus combined can be achieved by appropriately modifying the control unit  28 ,  28   a  or  28   b.    
     FIG. 11 schematically shows in plan view a walk behind self-propelled crawler snowplow vehicle according to another embodiment of the present invention. 
     The snowplow vehicle  210  includes a propelling body  220  having a propelling frame  221 , and a vehicle frame  230  pivotally connected at  228 ,  228  to the propelling frame  221 . A snow removing unit or mechanism including an auger  231  and a blower  232  is mounted on a front end portion of the vehicle frame  230 . 
     The propelling body  220  further has a pair of left and right driving wheels  222 L,  222 R and a pair of left and right driven wheels  223 L,  223 R mounted to the propelling frame  221 . A pair of left and right crawler belts  224 L,  224 R is entrained around a pair of driving and driven wheels  222 L and  223 L or  222 R and  223 R on either side of the propelling frame  221 . Each of the driving wheels  222 L,  222 R is connected to an electric motor  226 L,  226 R via a speed reducer  225 L,  225 R. The vehicle frame  230  carries thereon an engine  235 , an auger clutch  236  and a rotating shaft  237  connected in driven relation to the engine  235  via the auger clutch  236 . The rotating shaft  237  is connected in driving relation to an auger shaft  238  of the auger  231 . The auger  231  and the blower  232  are housed in an auger housing  239  mounted on the front end portion of the vehicle frame  230 . 
     Left and right lift cylinder actuator  233 L,  233 R are disposed on opposite outer sides of the vehicle frame  230  and connected between the vehicle frame  230  and the propelling frame  221  such that in response to extending and contracting movements of respective piston rods  234 L,  234 R of the cylinder actuators  233 L,  233 R, the front end portion of the vehicle frame  230  and the auger  231  are lifted up and down relative to the propelling frame  221   
     Preferably, the lift cylinder actuators  233 L,  233 R comprise an electric linear actuator or an electro-hydraulic cylinder actuator that can perform extending and contracting motions at the same speed. The electric linear actuator comprises an electric motor as a power source, and a ball-screw mechanism composed of a screw rotatably driven by the electric motor within a cylinder and a nut threaded with the screw and connected at one end of an actuator rod slidably received in the cylinder. When the electric motor is driven to rotate the screw in one direction, rotary motion of the screw is converted by the nut into an extending or contracting movement of the actuator rod relative to the cylinder. The motor is designed to rotate in the forward and reverse directions at the same speed, so that the actuator rod can perform extending and contracting motions at the same speed. The electro-hydraulic cylinder actuator is formed by a combination of a hydraulic cylinder actuator and a motor-driven hydraulic pump. The pump is driven by an electric motor to produce a fluid pressure used for reciprocating a piston rod of the cylinder actuator. The electro-hydraulic cylinder actuator is designed such that an extending motion and a contacting motion occur at the same speed. In the illustrated embodiment, the lift cylinder actuators  233 L,  233 R are of the electro-hydraulic type including an electric motor for driving a hydraulic pump to produce a fluid pressure for reciprocating the piston rod  234 L,  234 R of the cylinder actuator. The electric motor and the hydraulic pump are not shown in FIG. 11 but they are assembled with a cylinder of each cylinder actuator  233 L,  233 R in the same manner as described above with respect to the embodiment shown in FIGS. 1-10. 
     In FIG. 11, reference character  41  denotes a battery for supplying electric power to the electric motors  226 L,  226 R. Reference characters  42 L,  42 R denote left and right handlebars extending from a rear portion of the vehicle frame  230  obliquely upward in a rearward direction of the snowplow vehicle  210 . Reference numeral  244  denotes a control board, and reference numeral  245  denotes a control unit disposed in the control board  244 . The snowplow vehicle may be a wheeled vehicle having front and rear wheels wearing tires, or a half-crawler vehicle having front wheels wearing tires and intermediate and rear wheels connected by a crawler belt. The snow removing mechanism may include a dozer blade. 
     FIGS. 12A and 12B are diagrammatical views illustrative of the arrangement and operation of the auger clutch  236 . The auger clutch  236  comprises a first or driving pulley  246  firmly connected to an output shaft (not designated) of the engine  235 , a second or driven pulley  247  firmly connected to the rotating shaft  237 , an endless belt  248  entrained around the driving and driven pulleys  246 ,  247 , and a clutch actuator  249  disposed on one side of the belt  248  for applying a tension to the belt  248 . The clutch actuator  249  is preferably comprised of a solenoid-operated plunger. 
     As shown in FIG. 12A, when the clutch actuator  249  operates to tension the belt  248 , rotational motion of the driving pulley  246  is transmitted via the belt  248  to the driven pulley  247 , thereby rotating the rotating shaft  237  The auger  231  and the blower  232  that are coupled to the rotating shaft  237  are thus rotated The auger clutch  236  shown in FIG. 12A is in the ON or engaged state. 
     When the clutch actuator  249  is disposed in its original inoperating position shown in FIG. 12B, the belt  248  is in a free or loose state and hence has no function of transmitting rotational motion of the driving pulley  246  to the driven pulley  247 . Since the driven pulley  247  is thus isolated from rotation of the driving pulley  246 , the rotating shaft  237  does not rotate. The auger  231  and the blower  232  that are coupled to the rotating shaft  237  do not rotate. The auger clutch  236  shown in FIG. 12B is in the OFF or disengaged state. 
     FIG. 13 is a top plan view of the control board  244  of the snowplow vehicle  210  shown in FIG.  11 . As shown in FIG. 13, the control board  244  is equipped with an auger lift control lever (hereinafter referred to, for brevity, as “lift control lever”)  251  for raising or lowering the auger  231  (FIG. 11) by extending or contracting the lift cylinder actuators  233 L,  233 R (FIG.  11 ), an auger clutch lever  252  for engaging or disengaging the auger clutch  236  by activating or deactivating the clutch actuator  249  (FIGS.  12 A and  12 B), a travel control lever  253  for making or breaking a power line from the batteries  241  to the electric motors  226 L,  226 R to allow or prevent rotation of the electric motors  226 L,  226 R, and a direction/speed control lever  255  for controlling the direction and speed of rotation of the electric motors  226 L,  226 R. 
     The lift control lever  251  is movable between a first position (auto-up position) in which an auto-up mode is selected, a second position (manual-up position) in which a manual-up mode is selected, and a third position (manual-down position) in which a manual-down mode is selected. The direction/speed control lever  254  is operatively connected with a potentiometer (variable resistor)  255  that produces a voltage signal continuously variable within a range corresponding to a range of movement of the direction/speed control lever  254  defined between a forward high speed position and a forward low speed position, and a voltage signal continuously variable within a range corresponding to a range of movement of the direction/speed control lever  254  defined between a reverse high speed position and a reverse low speed position. Based on the variable voltage signals from the potentiometer  255 , the direction and speed of travel of the snowplow vehicle  210  (FIG.  11 ). 
     A control procedure achieved by the control unit  245  will be described below with reference to the flowchart shown in FIG.  14 . 
     The control procedure begins at ST 201  where a judgment is made to determine the current position of the lift control lever  251 . When the lift control lever  251  is disposed in the manual-up position and, hence, the manual-up mode of operation is selected, the control procedure advances to ST 202  where the lift cylinder actuators  233 L,  233 R are extended with the result that the auger  231  is raised to an elevated position. 
     When the result of judgment at ST 201  indicates that the lift control lever  251  is disposed in the manual-down position and, hence, the manual-down mode of operation is selected, the control procedure branches to ST 204  where the lift cylinder actuators  233 L,  233 R are contracted. ST 204  is followed by ST 205  where the measurement of operating time of the lift cylinder actuators  233 L,  233 R is started by using a clock friction of the control unit  245 . Stated more specifically, a motor current flowing through the electric motor of one lift cylinder actuator  233 L or  233 R is monitored, and when the motor current exceeds a predetermined value, the internal clock of the control unit  245  starts to measure time (operating time of the lift cylinder actuators  233 L,  233 R). As a result of contracting movement of the lift cylinder actuators  233 L,  233 R, the auger  231  is moved downward at ST 206 . When the lift control lever  251  is shifted from the manual-down position to the auto-up position or the manual-up position, downward movement of the auger  231  is stopped. At this time, ST 207  determines an operating time Td of the lift cylinder actuators  233 L,  233 R, which starts when the motor current exceeds the predetermined value and is ended when the auto-up position or the manual-up position is selected by the lift control lever  251 . The operating time Td thus determined is stored in the control unit  245  at ST 208 . The stored operating time Td is updated each time a shift from the manual-down mode to another operation mode occurs. 
     When the result of judgment at ST 201  indicates that the lift control lever  251  is disposed in the auto-up position and, hence, the auto-up operation mode is selected, the control procedure branches to ST 209  where a judgment is made to determine whether or not the direction/speed control lever  254  is disposed in the reverse position. If the result of judgment is “NO”, the control procedure goes to an end. Alternatively, if the judgment result is “YES”, the control procedure advances to ST 210  where the lift cylinder actuators  233 L,  233 R are extended for a time which is equal the operating time Td stored in the control unit  245 . By thus extending the lift cylinder actuators  233 L,  233 R, the auger  231  is raised to an elevated position at ST 211 . It is important to note that the amount of upward movement of the auger  231  (corresponding to the amount of extension of the lift cylinder actuators  233 L,  233 R) achieved by ST 210  to ST 211  in the auto-up operation mode is set to be equal to the amount of downward movement of the auger  231  (corresponding to the amount of contraction of the lift cylinder actuators  233 L,  233 R) achieved by ST 204  to ST 207  in the manual-down operation mode. 
     The travel condition of the snowplow vehicle  210 , which may occur immediately before the manual-down mode is selected, is considered to be a road traveling condition in which the snowplow vehicle travels on a road surface with the auger  231  held in an uppermost position, or a reversing condition in which the snowplow vehicle travels backwards on a snow-covered road surface with the auger  231  held in an elevated position intermediate between the uppermost inclined position and a lowermost horizontal position. The auger  231 , as it is in the elevated intermediate position, does not interfere with snow while the snowplow vehicle  210  is reversing. From this, according to the present invention, when the auto-up mode is selected, the auger  231  is raised to the elevated intermediate position. The auger  231  is thus automatically returned to the previous position, so that there is no possibility of interference occurring between the auger  231  and snow when the snowplow vehicle is moving backwards. 
     FIG. 15 is a flowchart showing a modified form of the control procedure shown in FIG.  14 . The modified control procedure makes a judgment at ST 301  so as to determine the position of the lift control lever  251  (FIG.  13 ), which may take one position among the auto-up position, the manual-up position and the manual-down position. When the lift control lever  251  is disposed in the manual-up position and, hence, the manual-up mode of operation is selected, the control procedure advances to ST 302  where the lift cylinder actuators  233 L,  233 R are extended with the result that the auger  231  is raised to an elevated position. 
     When the result of judgment at ST 301  indicates that the lift control lever  251  is disposed in the manual-down position and, hence, the manual-down mode of operation is selected, the control procedure branches to ST 304  where the lift cylinder actuators  233 L,  233 R are contracted. ST 304  is followed by ST 305  where the measurement of operating time of the lift cylinder actuators  233 L,  233 R is started by using a dock function of the control unit  245 . Stated more specifically, a motor current flowing through the electric motor of one lift cylinder actuator  233 L or  233 R is monitored, and when the motor current exceeds a predetermined value, the internal dock of the control unit  245  starts to measure time (operating time of the lift cylinder actuators  233 L,  233 R). As a result of contracting movement of the lift cylinder actuators  233 L,  233 R, the auger  231  is moved downward at ST 306 . When the lift control lever  251  is shifted from the manual-down position to the auto-up position or the manual-up position, downward movement of the auger  231  is stopped. At this time, ST 307  determines an operating time Td of the lift cylinder actuators  233 L,  233 R, which starts when the motor current exceeds the predetermined value and is ended when the auto-up position or the manual-up position is selected by the lift control lever  251 . The operating time Td thus determined is stored in the control unit  245  at ST 308 . The stored operating time Td is updated each time a shift from the manual-down mode to another operation mode occurs. 
     When the result of judgment at ST 301  indicates that the lift control lever  251  is disposed in the auto-up position and, hence, the auto-up operation mode is selected, the control procedure branches to ST 309  where a judgment is made to determine whether or not the auger clutch  236  is in the “on” or engaged state. When the result of judgment is “NO”, this means that the auger clutch  236  is in the “off” or disengaged state. In this condition, the auger  231  and the blower  232  are not rotating and, hence, they do not exert any load on the engine  235 . Accordingly, from the viewpoint of engine load, there is no difficulty caused from the forward or reverse movement of the snowplow vehicle with the auger kept in the lowermost horizontal position. Thus, the control procedure is terminated. 
     When the judgment result at ST 309  is “YES”, this means that the auger clutch  236  is in the “on” or engaged state. In this condition, since the auger  231  and the blower  232  are rotating, they may exert influences on the engine load. Accordingly, the control procedure goes to ST 310  where a judgment is made to determine whether or not the travel control lever  253  (FIG. 13) is in the “DRIVE” position. When the result of judgment is “NO”, this means that the travel control lever  253  is in the “STOP” position. In this condition, since the snowplow vehicle  210  is not moving in either direction, the rotating auger  231  does not give any influence on the engine load even when it is disposed in the lowermost horizontal position. Thus, the control procedure is terminated. 
     When the judgment result at ST 310  is “YES”, this means that the travel control lever  253  is in the “DRIVE” position. In this condition, since the snowplow vehicle  210  is running in either direction, the rotating auger  231  may exert negative influence on the engine load if it is disposed in the lowermost horizontal position. Thus, the control procedure further advances to ST 311  where a judgment is made to determine whether or not the direction/speed control lever  254  (FIG. 13) is in the “REVERSE” position. If the result of judgment is “NO”, this means that the direction/speed control lever  254  is in the “FORWARD” position. In this condition, since the snowplow vehicle  210  is moving forward to achieve, for example, the snow-removing operation, automatic rising of the rotating auger  231  is not necessary. Accordingly, the control procedure is terminated. 
     When the judgment result at ST 311  is “YES”, this means that the direction/speed control lever  254  is in the “REVERSE” position. In this condition, since the snowplow vehicle  210  is to be moving backward while rotating the auger  231 , the auger  231  will excessively increase engine load if it is disposed in the lowermost horizontal position. To preclude the occurrence of this problem, the control procedure goes on to ST 312  where the lift cylinder actuators  233 L,  233 R are extended for a time which is equal the operating time Td stored in the control unit  245  at ST 308 . By thus extending the lift cylinder actuators  233 L,  233 R, the auger  231  is raised to the elevated intermediate position at ST 313 . The auger  231  is thus automatically returned to the previous position, so that there is no fear of interference occurring between the auger  231  and snow when the snowplow vehicle is moving backwards. 
     In the control procedure shown in the flowchart of FIG. 15, the order or sequence of ST 309  to ST 311  may be changed. It will be appreciated from the foregoing description that the lift cylinder actuators  233 L,  233 R are operated to raise the auger  213  when at least three items of information have been received in the control units  245 . The first information item is obtained at ST 301  and represents that the set-up operation mode has been selected. The second information item is obtained at ST 311  and represents that the snowplow vehicle  210  is to be moved backward. The third information item is obtained at ST 309  and represents that the auger clutch  236  disposed between the power source or engine  235  and the snow-removing mechanism  231 ,  232  is in the “on” or engaged state. In the auto-up operation mode, the auger  231  is raised to the elevated intermediate position and not to the uppermost inclined position. Accordingly, when the auto-up operation mode is followed by the manual-down operation mode, the auger  231  can be lowered to the lowermost horizontal position in a relatively short time. This will increase the efficiency of the snow-removing operation. 
     Furthermore, according to the modified control procedure shown in FIG. 15, when the auto-up operation mode is selected, if the anger clutch  236  is in the “off” or disengaged state, the auger  231  and the blower  232  are not raised even though the snowplow vehicle is to be moved backward. There is no difficulty caused from the snowplow vehicle  210  moving backward with the auger  231  and the blower  232  disposed in the lowermost horizontal position so long as the auger  231  and the blower  232  are not operating. As a result, in the snowplow vehicle involving frequently repeated forward and reverse movements, it is possible to reduce the number of operations required to automatically raise the auger  231  and the blower  232  to the elevated intermediate position. This will reduce the number of on-off operations of the auger clutch  236  and elongate the service life of the auger clutch  236 , correspondingly. The auto-up operation necessarily reduces the load on the human operator. 
     The auger clutch  236  should by no means be limited to the belt clutch structure shown in FIGS. 12A and 12B but may include an electromagnetic clutch, a mechanical gear teeth clutch and the like. Furthermore, the power source used for driving the auger  231  and the blower  232  is in the form of an engine  235 . The engine  235  may be replaced by an electric motor. Similarly, the power source used for propelling the snowplow vehicle  210  is comprised of electric motors  226 L,  226 R. The electric motors  226 L,  226 R may be replaced with an engine. 
     In the embodiment shown in FIG. 11, the lift cylinder actuators  233 L,  233 R are designed to extend and contract at the same speed, so that the amount of upward movement of the auger  231  and the amount of downward movement of the auger  231  can be made equal to each other by determining an operating time Td of these cylinder actuators  233 L,  233 R. In the case where the speed of extension and the speed of contraction of the cylinder actuators  233 L,  233 R are different from each other, a stroke sensor (not shown) may be associated with one of the cylinder actuators  233 L,  233 R so as to determine the amounts of extension and contraction of the cylinder actuators  233 L,  233 R. 
     Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described. 
     The present disclosure relates to the subject matters of Japanese Patent Applications Nos. 2001-276075, 2001-301013, and 2001-301228, filed Sep. 12, 2001, Sep. 28, 2001 and Sep. 28, 2001, respectively, the disclosures of which are expressly incorporated herein by reference in their entireties.