Hydrostatic transmission control system

Claim 1

In a control system, including a brake device, for a vehicle having a hydrostatic transmission of the type which includes a main hydraulic circuit comprising a variable displacement propulsion pump having a pivotable adjusting member and including a spring-centered adjusting device, and hydraulic motor means, said control system also including a hydraulic control circuit comprising a driving engine, a control pump drivingly connected to said engine so as to rotate at a speed proportional to the speed of said engine, said control pump being in fluid communication with said main hydraulic circuit and said hydraulic control circuit, said hydraulic control circuit including fluid reservoir means in fluid communication therewith, a first or high pressure control conduit, a second or low pressure control conduit, said first and second control conduits being substantially parallel to each other and communicating with said spring-centered adjusting device, directional control valve means interposed in said first and second control conduits, differential pressure regulator valve means interposed in said first and second control conduits upstream from said directional control valve means, said differential pressure regulator valve means being continuously biased to an open position, with said differential pressure regulator valve being operatively connected to said brake device so as to be progressively movable to a closed position in proportion to the degree of actuation of said braking device, and fixed orifice means interposed in said second control conduit upstream from said differential pressure regulator valve, wherein the improvement comprises: a. a branch conduit, downstream from said fixed orifice means, for connecting said first and second control conduits; and b. adjustable orifice means interposed in said branch conduit for adjusting the differential pressure between said first and second control conduits.