938G Series II Wheel Loader and IT38G Series II Integrated Toolcarrier Hydraulic System Gear Motor (Hydraulic Fan) Caterpillar


Gear Motor (Hydraulic Fan)
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The hydraulic fan motor is a fixed displacement gear motor that is mounted between the engine and the radiator. The hydraulic fan motor drives the radiator fan. The fan draws cool air through the radiator and the hydraulic oil cooler. Hydraulic oil for the hydraulic fan motor is provided by the hydraulic fan pump.




Illustration 1g00933631

Schematic of the Hydraulic Fan Motor

(1) Pump oil. (2) Hydraulic fan motor. (3) Tank oil. (4) Check valve. (5) Case drain.




Illustration 2g00937724

Oil Flow through the Gear Motor

(1) Pump oil. (3) Tank oil. (6) Drive gear. (7) Motor housing. (8) Idler gear.

High pressure oil from the hydraulic fan pump flows into the inlet port. The pressurized oil causes drive gear (6) to rotate. Drive gear (6) and idler gear (8) turn in opposite directions. The high pressure oil will be drawn into the gear teeth. The oil will then travel along the outside of the gears toward tank oil (3). Tank oil (3) exits fan motor (2) through the outlet port. The oil that is exiting gear motor (2) will flow through the hydraulic oil cooler and into the hydraulic tank.

Gear motors operate because of a pressure differential. The differential is between the inlet and the outlet of the motor. The pressure differential acts across the gear teeth. This creates a force that rotates the gear.

There are two paths around the gear. One path is around the outside of the gears. The other path is through the intersection of the gears. The pressure works against the area of one tooth. A gear motor has a two tooth force working in one direction and a one tooth force working in the opposite direction. The result of the force causes the output shaft to rotate.




Illustration 3g00912909

View of the Area of One Tooth

The resistance of the fan blade causes an increase of pressure at the motor inlet. This causes a higher pressure differential across the motor and across each tooth. The arrow in Illustration 3 shows the area of a tooth. The oil pressure times the area of one tooth determines the one tooth force that is developed. The tooth force that is multiplied by distance A will equal the torque that is developed. Dimension A shows the distance from the center of the shaft to the tooth.




Illustration 4g00910623

Rear View of a Gear Motor

Check valve (4) allows the reverse flow of oil through hydraulic fan motor (2).




Illustration 5g00933640

Typical View of a Makeup Valve

(1) Pump oil. (3) Tank oil. (4) Check valve.

Note: Illustration 5 shows the makeup valve during normal operation of the motor.




Illustration 6g00933659

Typical View of a Makeup Valve

(1) Pump oil. (3) Tank oil. (4) Check valve.

Note: Illustration 6 shows the movement of check valve (4) as the fan motor is going into shutdown.

Gear motor (2) is equipped with a check valve that is used for makeup oil. Check valve (4) is installed in order to eliminate cavitation of the fan motor (2). Check valve (4) will allow oil that is exiting hydraulic motor (2) to return to the motor inlet during the reduction of flow from the pump or a complete shutdown.

Fan motor (2) will continue to rotate because the resistance of the air that is being pushed through the radiator is not strong enough to stop the fan blade from turning fan motor (2). Continued rotation of fan motor (2) creates a vacuum in check valve (4). The vacuum causes a lower pressure in pump oil (1). When the pressure at tank oil (3) becomes higher than the pressure in pump oil (1), the force will open check valve (4). The oil will flow through check valve (4) and back to pump oil (1) .

Reference: For additional information about the specifications for the hydraulic fan motor, refer to the Service Manual module Specifications, "Gear Motor (Hydraulic Fan)" for the machine that is being tested.

Reference: For additional information about testing and adjusting the fan motor, refer to the Service Manual module Testing and Adjusting, "Hydraulic Fan Speed - Test and Adjust" for the machine that is being tested.

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