Patent ID: 6860106

Claim:
A hydrostatic transmission comprising: a housing; an oil sump formed in said housing; a center section having a horizontal surface and a vertical surface disposed in said housing; a hydraulic pump having a cylinder block which is rotatably, slidably disposed on said horizontal surface of said center section; a hydraulic motor having a cylinder block which is rotatably, slidably disposed on said vertical surface of said center section; a pair of oil passages provided in said center section; a first pair of arcuate ports opened on said vertical surface of said center section and communicating with said pair of oil passages; a second pair of arcuate ports opened on said horizontal surface of said center section, wherein each of said second pair of arcuate ports has a length and a depth substantially perpendicular to each other, wherein the depth of one of said second pair of arcuate ports is shallow at a first end and gradually deepens to a second end which fluidly communicates with one of said pair of oil passages and the depth of the other of said second pair of arcuate ports is shallow at a first end and gradually deepens to a second end which fluidly communicates with the other of said pair of oil passages, whereby feed oil discharged from said cylinder block of said hydraulic pump is introduced into said one of said pair of oil passages and feed discharge oil in said other of said pair of oil passages from said cylinder block of said hydraulic motor is introduced into said cylinder block of said hydraulic pump; oil holes branched from said pair of oil passages formed in said center section, opened on an opposite surface of said center section which is opposite to, said horizontal surface of said center section; a valve casing inserted into each of said oil holes, wherein an outer portion of each of said valve casings projects from said opposite surface of said center section to said oil sump; a valve body inserted into each of said valve casings; openings provided in said respective projecting outer portions of said valve casings, wherein said openings are closed by said valve bodies and are freely opened and closed so as to constitute a check valve with said oil sump; and an oil filter disposed in said oil sump so as to cover said projecting outer portions of said valve casings on said opposite surface of said center section so that said oil filter divides said oil sump into an inner oil sump and an outer oil sump, wherein said openings of said valve casing directly communicates with said inner oil sump.