Patent GB-958334-A (GB)
Title: Fluid pressure servomotor control for hydrostatic transmissions
IPC: F16H61/46
Filed: 1962-08-07
Abstract: 958,334. Hydraulic transmission of power. NEW YORK AIR BRAKE CO. Aug. 7, 1962 [Aug. 28, 1961; May 24, 1962], No. 30156/62. Heading F2C. An hydraulic transmission comprises a variable displacement pump 112 and a reversible motor 111 connected by lines 114, 115. The pump is of the swash plate type in which the swash plate is biased to a zero displacement position and is movable to maximum displacement in either direction by a double acting motor 122. Two manually actuated valves are provided in the control system, the valve 127 being a directional control valve, actuation of which determines forward or reverse drive by the motor, and a feedback valve 128 which controls the movement of the swashplate 113. A pump 119 provides an hydraulic supply which acts as a make up pump for the transmission lines 114, 115 and as a supply for the control system by supplying fluid through a conduit 131 leading to the feedback valve 128. The valve 128 comprises a valve member 143, having lands 144, 145 separated by a groove 146 and loaded by a graduated spring 151 towards the left. The valve member 143 co-operates with a movable element 138 having circumferential grooves 152, 154 from which lead radial ports 153, 155. The groove 152 is always in communication with the supply passage 131 for all positions of the element while the groove 154 is in constant communication with a conduit 129 leading to the directional control valve 127. The valve member 143 is moved manually through a rod 158a while the valve element 138 is moved through cam pins 139 on a lever 141, pivoted at 142 and rotated by the swashplate 113. The directional control valve 127 has conduits 125, 126 leading to either side of the motor 122 and a movable valve member 163 provided with lands 166, 167, 168 and ports 171, 172 on the outer sides of lands 166, 167, which open into an axial bore 169 which is in constant communication with a drive 127 to a reservoir 121. The valve 163 also controls the connection of the transmission lines 114, 115 to a conduit 137. The valve 163 is connected to a lever 195 pivoted about its centre, the ends of which are connected by springs 196, 197 to rods 203 carried by lever arms 192, 193. Rotation of one of the arms 192, 193 first acts through the springs to rotate the lever 195 to move valve 163 to the forward or reverse position after which the rods 203 engage a stop 202 to move a piston 158 thus allowing spring 151 to move the valve member 143. Stops 205 are provided for the levers 192, 193 and locking means between the levers prevent rotation of one lever when the other is being operated. Operation.-With forward and reverse levers in the position shown the valve 127 connects both sides of the motor 122 to the reservoir 121 through ports 171, 172 and bore 169 of valve member 163. The conduit 129 is connected to the reservoir through groove 154 port 155 conduit 132, and bore 169. The supply through passage 131 to valve 128, groove 152 and port 153 is blocked by land 144. Assuming that the lever 192 provides forward drive and the lever 193 reverse, if the lever 192 is actuated the valve 163 is moved to the left connecting conduits 129 and 125 and leaving conduit 126 connected to the reservoir 121. When the rod 203 contacts and moves the stop 202, the piston 158 is moved to the left and the valve member 143 moves to the left under the action of the spring 151. The land 144 moves to the left past the port 155 thus closing this port to the reservoir 121 and then opening the port to the supply 131 through groove 152 and port 153. Fluid then flows through the conduit 129, valve 127 and conduit 125 to the left hand side of the motor 122 causing its piston to move to the right and tilt the swash plate 113. Movement of the swash plate provides a feedback by rotating lever 141 about its pivot 142 thus moving the valve element 138 to the left until land 144 closes port 155 thus isolating conduit 129 from both the supply 131 and exhaust to reservoir 121. Further movement of the lever 192 increases the angle of tilt of the swashplate or movement in the opposite direction reduces this angle. For any given position of the lever 192 the angle of the swashplate may be reduced by an excess pressure in the high pressure transmission line or by excess pressure in a utilization circuit 221. Chambers 136, 136a in the valve 128 are connected to the high pressure line 114 or 115, by valve 163 and the utilization circuit 221. If the pressure in either chamber rises above a predetermined value it acts on a plunger 156, 156a to move the valve member 143 to the right to reduce the angle of inclination of the swash plate. A pilot controlled relief valve (not shown) is connected to the reservoir and controls escape of fluid from the high pressure line 114, 115 through a shuttle valve. The relief valve is only operative when the conduit 188 to valve 127 is closed by land 168, 167 i.e. when the valve 163 is in the forward or reverse position. For reverse drive, the lever 193 is actuated to first move valve 163 to the right and then move valve 143 to the left as above. Modification.-In a first modification the directional control valve is actuated separately and is provided with a detent engaged by a spring bolt to hold the valve in a neutral, forward or reverse position. The feedback valve is biased to the left in its normal position so that movement of the control valve to forward or reverse provides immediate tilt of the swash plate to its maximum inclination. The feedback valve may be actuated directly or through a fluid link which may be from the main brake cylinder of a vehicle. In a further modification the directional control valve is biased by a spring to the forward position and separate pistons, manually actuated, act through a fluid link to move the piston 158 of the feedback valve, one of the pistons of the actuator also supplies fluid pressure, when actuated, to the directional control valve to move it to reverse position prior to actuating the feedback valve. Specification 883,129 is referred to.