Such hydraulic transmissions have been well-known and put into use in automobiles and other vehicles. For example, Japanese Laid-Open Patent Publication No. H6(1994)-42635 discloses an automotive mowing tractor which comprises a hydraulic swash plate pump for variable delivery and a hydraulic motor with fixed displacement. In this tractor, the pump, which is driven by the engine, delivers the hydraulic pressure to the motor, which rotates the wheels. Furthermore, this tractor is provided with a seesaw pedal, which is stepped and operated by the driver with a foot. When this pedal is stepped forward, the swash plate is swiveled to a "forward drive direction", and the vehicle is driven forward. Likewise, when this pedal is stepped rearward, the swash plate is swiveled to a "rearward drive direction", and the vehicle is driven backward.
This pedal is designed to automatically return to a neutral position when it is not stepped, i.e., when it is released. With the pedal at the neutral position, the angle of skew of the swash plate is zero, and the pump discharges no oil. In this condition, the rotation of the motor is locked because the pump does not allow any oil to flow. This locking is so complete that any attempt to rotate the motor, which must force the oil to flow through the pump, is futile. Thus, without operating the brake, the driver can prevent the wheels from rotating by maintaining the skew angle of the swash plate of the pump at zero and locking the rotation of the motor.
If the vehicle is on a slope, for example, and the swash plate of the pump is operated to make the skew angle of the pump to zero in order to lock the rotation of the motor, then the motor is subjected to a torque which is transmitted through the wheels of the vehicle because of the component force resulting from the weight of the vehicle that acts to move the vehicle in the downward direction parallel to the slope. In this condition, as long as there is no leak in the hydraulic circuit, which includes the pump and the motor, the rotation of the motor is kept being locked. However, if any leak occurs, then the motor will rotate slowly at the speed which corresponds to the rate of the oil leak. In this leaky condition, even if the driver tries to stop the vehicle by setting the skew angle of the swash plate to zero, the vehicle moves slowly down the slope.
This problem is not very serious if the direction of unintended slow move of the vehicle is forward, and such creeping is often observed in automobiles which are equipped with an automatic gear shift. However, if the direction of this unintended move is rearward, then the problem is serious. For example, if the vehicle moves back slowly when the driver stops the vehicle on an up-hill slope with the shift lever set at the D range (a forward drive), then it is a dangerous condition.