GB1228169A - - Google Patents

Abstract

1,228,169. Hydrostatic transmission control. JOSEPH LUCAS (INDUSTRIES) Ltd. 28 Aug., 1968 [1 Sept., 1967], No. 40092/67. Headings F2C and F2D. In a hydrostatic transmission for a tractor wherein a swash-plate pump 11 is connected by supply and return ducts 16, 18 to a swash-plate motor 14 and the stroke of the pump is varied from zero to maximum in either forward or reverse under the control of a lever 38, a normally closed by-pass valve 34 is opened to interconnect the ducts 16, 18 when the lever 38 is moved from the zero stroke position shown to a neutral position 38a. The high pressure in the duct 16 (or 18) acts through a shuttle valve 33 on the right hand end of the by-pass valve 34 against the bias of a spring and liquid pressure supplied through a restricted bore in the valve 34. If during operation the pressure in the duct 16 (or 18) exceeds a predetermined value a high pressure relief valve 35 opens to vent the left end of the valve 34 which moves left to interconnect the ducts 16, 18. When the lever 38 is moved to the position 38a it opens a valve 41 to vent the left end of the by-pass valve 34 which moves left to interconnect the ducts 16, 18. Further movement of the lever 38 to the position 38b acts through a cable 45 to open a valve 46 and vent the chamber above a piston 44 whereupon a pin 43 is spring pressed into engagement with teeth 42 on the rotor of the motor 14 which is thus positively braked. The lever 38 and a pedal 39 act through a linkage 37, similar to that described in Specification 1,014,501, and cam plate 31 on control valves 25, 26 of follow-up servo's 20, 23 controlling the strokes of the pump and motor. The control initially increases the pump stroke from zero to a maximum in either forward or reverse and thereafter reduces the motor displacement from a maximum to a predetermined minimum. An input driven make-up pump 27 supplied liquid through a cooler 28, filter 29 and non- return valves 32 to the low pressure duct and also supplies liquid to the servo control through a variable throttle valve 30 which varies the speed of response of the servo system. In Fig. 2 (not shown) the valve 41 is replaced by a valve which is opened either by movement of the lever 38 to the position 38a or by movement of the pedal 39 to its extreme position.