Hydrostatic transmission

A hydrostatic transmission, comprising:

• a housing, an interior space of said housing serving as a fluid sump;
• a pair of first and second fluid passages disposed in said housing;
• a hydraulic pump disposed in said housing;
• a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit as a hydrostatic transmission;
• a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit;
• a first oil filter disposed in said fluid sump for filtering fluid to be introduced into said charge fluid passage;
• a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage is open into communication with said fluid sump inside said first oil filter and said drain fluid passage is open into communication with said fluid sump outside said first oil filter while said charge fluid passage and said drain fluid passage are separated from each other, and
• a check valve intermediately provided in said drain fluid passage, wherein said check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.

A hydrostatic transmission comprising:

• a housing, an interior space of said housing serving as a fluid sump;
• a pair of first and second fluid passages disposed in said housing;
• a hydraulic pump disposed in said housing;
• a hydraulic motor disposed in said housing, wherein said first and second fluid passages are interposed between said hydraulic pump and said hydraulic motor so as to constitute a closed fluid circuit;
• a charge fluid passage disposed in said housing so as to be connected with each of said first and second fluid passages for supplying fluid from said fluid sump into said closed fluid circuit;
• a first check valve interposed between said charge fluid passage and said first fluid passage, wherein said first check valve allows only a flow of fluid from said charge fluid passage to said first fluid passage;
• a second check valve interposed between said charge fluid passage and said second fluid passage, wherein said second check valve allows only a flow of fluid from said charge fluid passage to said second fluid passage;
• a drain fluid passage including an orifice disposed in said housing so as to be connected with at least one of said first and second fluid passages so that, when hydraulic pressure in said at least one of said first and second fluid passages is increased, excessive fluid in said at least one of said first and second fluid passages is drained through said drain fluid passage to said fluid sump, wherein said charge fluid passage and said drain fluid passage are open into communication with said fluid sump while said charge fluid passage and said drain fluid passage are separated from each other;
• a relief valve provided in said drain fluid passage, wherein said relief valve closes when hydraulic pressure in said at least one of said first and second fluid passages is increased beyond a predetermined degree, and
• a third check valve intermediately provided in said drain fluid passage, wherein said third check valve allows only a flow of fluid from said at least one of said first and second fluid passages to said fluid sump.