MH3049 Material Handler Hydraulic System Caterpillar

Illustration 1	g03341557
(1) Final drive (2) Travel motor

The machine has four travel motors. Each travel motor (2) is connected to a final drive (1). Each final drive (1) is a planetary gear box and has a 60:1 ratio. The gearbox uses synthetic 80w-140 gear lube.

Each motor is equipped with integral relief valves. Relief valves have a factory setting of 44,816 kPa (6,500 psi). The travel motors have a bolt on counterbalance valve that gives the machine a hydrostatic transmission feel. The four motors are variable displacement and have shift on the fly capability.

Illustration 2	g03257876
Rear travel motor shown (3) Port for travel (4) Port for brake release (5) Port for case drain (6) Port for travel (7) Port for displacement change

Illustration 3	g03257997
Schematic showing just one travel motor (3) Port for travel (4) Port for brake release (5) Port for case drain (6) Port for travel (7) Port for displacement change

Illustration 4	g03200740
Cutaway image of a travel motor (typical example) (6) Main pressure port (8) End cover (minimum displacement) (9) Control module (10) Setting piston (11) Connecting arm (12) End cover (maximum displacement) (13) Connection module (14) Valve segment (15) Intermediate housing (16) Cylinder barrel (17) Spherical piston with laminated piston ring (18) Synchronizing shaft (19) Inner roller bearing (20) Outer roller bearing (21) Bearing housing (22) Shaft seal with retainer (23) Output shaft

The drive motors rotate when solenoid valves release hydraulic fluid which is used to turn the motor. The flow of hydraulic fluid is controlled by pilot pressure directed to the drive solenoids. When the pilot pressure decreases, the servo valve spool moves to the right hand setting chamber. When the servo valve moves right, the setting piston moves to the left and the displacement increases.

The shaft speed now decreases and the available output torque increases at a constant pump flow and system pressure.

Travel Displacement

Illustration 5	g03043777
(1) Piston motor (propel) (2) Piston pump (propel) (3) Return line (4) Pressure line

Propel A and B

The machine propel is selected with the travel pedal. The signal for travel is sent to the control module. The control module communicates the request to the master module via the J1939 Data Link. The master control module relays the signal to the propel solenoid. The propel solenoid is connected to the control valve. When the master control module opens the propel solenoid (high speed request), the swash plate on the drive motor is at a minimum angle. Under this condition, the machine will be driven by high speed and low torque. When the propel solenoid is closed, the swashplate will move to the maximum angle. Under this condition the machine will be driven by low speed and high torque.

The machine can travel at up to 5 km/h (3 mph) when in creeper mode. When the machine is in standard mode, the top speed is 12 km/h (7.5 mph). The operator can shift between the modes while the machine is in motion.

Forward Travel Operation

When the lower portion of the travel pedal is depressed, piston motors (1) rotate clockwise and the machine moves in a FORWARD direction. Pilot pressure is directed to the brake defeat valve that is in piston motor (1). Pilot oil then travels to the forward servo piston that is in piston pump (2). The oil from the pump flows through line (4) to motor (1). This causes piston motor (1) to rotate clockwise. Return oil from motor (1) flows through line (3) and the return oil flows back to piston pump (2) .

Pilot pressure will move the forward servo piston and the displacement control valve to the left for forward travel. The displacement control valve will direct a charge pressure to the pump servo piston. As a result, the piston pump (2) will upstroke in the forward direction.

As piston pump (2) upstrokes, flow increases to the piston motor (1). The piston motor (1) will turn in the forward direction. System pressure will be directed through the brake defeat valve to the left end of the displacement control valve. The displacement control valve is held in position by spring force.

Note: A brake pressure reading higher than 4826 kPa (700 psi) for more than 1 second results in the creation of the log by the control module.

Reverse Travel Operation

When the upper portion of the travel pedal is depressed, the swashplate of piston pump (2) is tilted rearward. Piston motor (1) will rotate counterclockwise and the machine moves in the REVERSE direction. Line (3) will become the pressure line and line (4) will become the return line. Pilot pressure is directed to brake defeat valve that is in piston motor (1). Pilot oil then travels to reverse servo piston that is in piston pump (2).

Pilot pressure will move the reverse servo piston and the displacement control valve to the right for reverse travel. The displacement control valve will direct a charge pressure to the pump servo piston. As a result, the piston pump (2) will upstroke in the reverse direction.

As piston pump (2) upstrokes, flow increases to piston motor (1). Piston motor (1) will turn in the counterclockwise direction. System pressure will be directed through the brake defeat valve to the left end of the displacement control valve. The displacement control valve will be held in position by spring force.

Note: A brake pressure reading higher than 4800 kPa (700 psi) for more than 1 second results in the creation of the log by the control module.