PRESSURE OIL COOLING STRUCTURE OF HST DRIVING DEVICE

PROBLEM TO BE SOLVED: To cool the temperature of pressure oil circulated in a HST driving device effectively so as to hold a smooth driving condition at all times by returning pressure oil after circulation into an oil tank, and returning it into the oil tank communicating with a pressure oil circuit. SOLUTION: An oil pressure pump 21 and a switching valve 22 are interposed in an oil pressure circuit 19, and the pipe passage 23a of a relief circuit 23 connected to a connecting pipe passage 19a and a pipe passage 22a for leading pressure oil returned from the switching valve 22 are communicated with a housing case 17. Pressure oil sucked by a charge pump Z is supplied to an oil pressure pump P and an oil pressure motor M so as to drive a HST driving device X. Driving control in a treatment part is carried out by the switching valve 22 in the oil pressure circuit 19 led from an oil tank T to an oil pressure cylinder 20. Returned pressure oil, flows into the housing case of the HST driving device X through the pipe passage 22a and the pipe passage 23a, and it is returned into the oil tank T through an oil cooler 18 connected to a drain hole 17a. It is thus possible to prevent reduction of driving efficiency of the HST driving device X so as to improve cooling effect.