Patent ID: 7762371

Claim:
A pump unit for traveling a vehicle capable of transmitting a rotational power, which is operatively transmitted from a driving power source, through a pair of first hydraulic fluid lines and a pair of second hydraulic fluid lines to first and second hydraulic motor units, respectively, the pump unit comprising: first and second hydraulic pump main bodies which are operatively driven by the driving power source and which are fluidly connected to the first and second motor units through the pair of first hydraulic fluid lines and the pair of second hydraulic fluid lines, respectively; a housing for accommodating the first and second hydraulic pump main bodies; a pair of first hydraulic fluid passages and a pair of second hydraulic fluid passages which are formed in the housing and which form parts of the pair of first hydraulic fluid lines and the pair of second hydraulic fluid lines, respectively a first charge fluid passage which is formed in the housing so as to have a first end fluidly connected to a hydraulic source and second ends fluidly connected to the pair of first hydraulic fluid passages through a pair of first check valves, respectively; a second charge fluid passage which is formed in the housing so as to have a first end fluidly connected to the hydraulic source and second ends fluidly connected to the pair of second hydraulic fluid passages through a pair of second check valves, respectively; a first relief valve provided in at least a forward-movement high-pressure side first hydraulic fluid passage out of the pair of first hydraulic fluid passages which has a high pressure at a forward movement of the vehicle, the first relief valve causing the forward-movement high-pressure side first hydraulic fluid passage to be fluidly connected to a portion of the first charge fluid passage that is positioned on an upstream side of the corresponding first check valve in a flow direction of charge fluid when the hydraulic pressure in the forward-movement high-pressure side first hydraulic fluid passage exceeds a predetermined hydraulic pressure; and a second relief valve provided in at least a forward-movement high-pressure side second hydraulic fluid passage out of the pair of second hydraulic fluid passages which has a high pressure at the forward movement of the vehicle, the second relief valve causing the forward-movement high-pressure side second hydraulic fluid passage to be fluidly connected to a portion of the second charge fluid passage that is positioned on an upstream side of the corresponding second check valve in a flow direction of charge fluid when the hydraulic pressure in the forward-movement high-pressure side second hydraulic fluid passage exceeds a predetermined hydraulic pressure.