GUARD AS 3433274 - Caterpillar

B. Propelling System "Neutral"
1. The variable displacement pumps use a tiltable swashplate to vary displacement (output flow). The swashplate is mounted on trunnion bearings and is connected to hydraulic control (servo) cylinders. The control directs fluid to and from the servo cylinders causing the swashplate to tilt and change displacement of the pump. The swashplate can be tilted in either direction from 0 angle (neutral stop position) and provide pump flow in either direction. Fluid is forced out as the angle of the swashplate pushes the pistons into the bores.2. The output speed of the motors is determined by the amount of fluid being displaced (pumped) to them. In the neutral stop position, primary braking occurs when the pump, with its swashplate at 0 angle, blocks the flow of oil and doesn't displace any in either direction.3. Because each servo control cylinder is spring loaded, loss of control pressure to servo cylinder in neutral position will cause the swashplate to remain in neutral position. The variable swashplate must be maintained in a true neutral location to prevent reciprocation of the pistons in the pump cylinder barrel. The propulsion pump is considered to be in neutral when there is no output flow being generated by the variable displacement pump, as its shaft, internal components, and charge pump are being driven. (With no flow available to the motor, it is at rest also, so there is no transmission output.) The variable displacement propulsion pump is driven in neutral with the charge pump providing flow to keep circuit primed and makeup internal leakages.Propelling System "Forward"
C. Propelling System "Forward"
1. The pump is considered to be in a forward or power transmitting mode of operation when system flows and pressures permit the hydraulic motor output shaft to rotate.2. Moderate forward operator effort of the control handle begins transmission output by moving the spring centered control valve spool from its centered position allowing pressurized charge pump flow, or control pressure, past the spool to one servo piston. The pressurized servo piston pushes against the swashplate, overcoming the centering spring force in the opposing servo piston, causing the swashplate to rotate to a position out of neutral. Fluid in the servo piston with the spring under compression is exhausted through the control valve spool to the pump housing.Maximum displacement of the pump is attained when the control lever is moved to its extreme position. As the swashplate attains any position selected by the control lever, follow-up links, connecting the swashplate to control valve spool and lever, relocate the control valve spool to a closed position. Closing of the spool blocks the fluid between the spool and servo piston to hold the swashplate in its desired location.3. As the variable swashplate is moved from its neutral position, a reciprocating action is created by the pistons in the pump cylinder barrel, generating a flow. Flow rate is determined by length of and frequency of strokes. Flow leaving the pump cylinder barrel is identified as being high pressure.