1. Field of the Invention
The present invention relates to a hydrostatic transmission having a hydraulic pump and a hydraulic motor fluidly connected with each other. Especially, it relates to a pressure regulation means for adjusting hydraulic pressure in a closed hydraulic circuit of the hydrostatic transmission.
2. Background Art
Conventionally, there is a well-known continuously variable transmission generally called a hydrostatic transmission or an HST which includes a hydraulic pump with a movable swash plate fluidly connected to a hydraulic motor with either a movable swash plate or a fixed swash plate, wherein the movable swash plate(s) is (are) moved so as to change the capacity of the hydraulic pump (the capacities of the hydraulic pump and motor), thereby changing the rotational speed and direction of rotational force output from the hydraulic motor. This transmission is used for various uses, e.g., for driving a vehicle.
Furthermore, as disclosed in Japanese Utility Model Provisional Publication No. 66454/92, there is a well-known HST provided with a neutral-returning spring and a shock absorber which are attached to a control arm interlockingly connected to the swash plate in the HST. By the neutral-returning spring, the movable swash plate is automatically returned to its neutral position when the control arm is released from an operational force. The shock absorber prevents a dynamic brake which happens if the automatic returning of the movable swash plate to its neutral position is sudden.
On the other hand, Japanese Patent Provisional Publication No. 9385/98 points out a problem of the external arrangement of the shock absorber, wherein the movement of the shock absorber is unstable because of the influence of variable air conditions such as air temperature and the external shock absorber prevents the apparatus from being minimized. The document also proposes such a construction that, instead of the external shock absorber, pistons, which are pushed out by the hydraulic pressure of oil discharged from the hydraulic pump when a movable swash plate of the hydraulic pump is rotated from its neutral position, abut against the control arm. Such pistons are advantageous in their stable movement and contribution for minimization of the HST apparatus.
Furthermore, the document proposes means for adjusting the hydraulic pressure in the HST circuit, wherein tips of the pistons are opened and the surface of the control arm abutting the tips of the pistons are formed with grooves so that the tip opening of each of the pistons communicates with each of the grooves until the control arm rotated from its neutral position reaches a certain angle in the vicinity of the neutral position. Consequently, when the control arm is located in the vicinity of its neutral position, the oil generating a remaining hydraulic pressure in the HST circuit is drained through the groove, thereby suppressing the shock when the vehicle stops and expanding the neutral zone of the HST so as to securely hold the vehicle stationary. On the other hand, since the tip openings of the pistons are closed by the control arm when the control arm is rotated from the vicinity of its neutral position, i.e., the expanded neutral zone, the oil in the HST circuit is not drained so as to secure the good operational efficiency of the HST. The proposed means have these advantages.
For the disposal of the hydraulic pump and the hydraulic motor in the housing serving as the oil sump, the voluminal increase of hydraulic oil in the housing, which is caused by variation of the oil temperature, must be absorbed. Generally, an oil reservoir is provided for this absorption. A tank disposed outside the housing may serve as the oil reservoir. Otherwise, a part of an inner wall of the housing may be formed into an oil reservoir.
However, the arrangement of the tank as the oil reservoir outside the housing increases the number of parts and prevents the HST apparatus from being minimized. The oil reservoir formed of the housing requires a complicated mold for forming the housing, thereby increasing the number of processes and costs.
For solving the problems, it may be proposed that, instead of an oil reservoir, an amount of the hydraulic oil in the housing is reduced so as to be received therein even when it is expanded by heating. However, in the housing, the level of the reduced hydraulic oil is lowered and the air is filled thereabove. Accordingly, if a vehicle is left on a slope with its HST in neutral and with its engine at idle, the level of hydraulic oil may become lower than the tip opening of the piston or the groove, thereby causing the air in the housing to be unexpectedly absorbed into the HST circuit through the groove and the piston.
The air mixed with the hydraulic oil in the HST circuit causes the HST to free wheel wherein it may happen that the vehicle unexpectedly descends the slope or the vehicle does not start while it ascends the slope.
For another problem, if the neutral position of the HST is inaccurate, the hydraulic pump may unexpectedly discharge a little amount of hydraulic oil even when the HST is located at its neutral position, thereby driving the hydraulic motor so as to unexpectedly move the vehicle slowly. Therefore, some means for regulating hydraulicoil is required to solve this problem. However, if such means constantly drains increased hydraulic pressure oil from the HST circuit, the transmittal efficiency of the HST is reduced while the hydraulic pressure in the closed HST circuit must be increased in response to the operation of the HST.