950G Series II Wheel Loader Caterpillar


Gear Pump

Usage:

950G II AXR


Gear Pumps




Illustration 1g01062104

The gear pump consists of the following items that are shown in illustration 1:

(1) seal retainers

(2) seals

(3) seal backups

(4) isolation plates

(5) spacers

(6) drive gear

(7) idler gear

(8) housing

(9) mounting flange

(10) flange seal

Bearings are mounted in the housing and the mounting flange on the sides of the gears in order to support the gear shafts during rotation.

Gear pumps are positive displacement pumps. They deliver the same amount of oil for each revolution of the input shaft. The pump output is controlled by changing the speed of rotation. The maximum operating pressure for gear pumps is limited to 27600 kPa (4003 psi). This pressure limitation is due to the hydraulic imbalance that is inherent in the gear pump design. The hydraulic imbalance produces a side load on the shafts that is resisted by the bearings and the gear teeth to housing contact. The gear pump maintains a volumetric efficiency above 90% when pressure is kept within the designed operating pressure range.

Gear Pump Flow




Illustration 2g01062106

The output flow of the gear pump is determined by the tooth depth and the gear width. Most manufacturer gear pumps standardized on a tooth depth and profile that is determined by the centerline distance (1.6", 2.0", 2.5", 3.0", etc.) between gear shafts. With standardized tooth depths and profiles, the flow differences within each centerline classification of pump is determined by the tooth width.

As the pump rotates, the oil is carried between the gear teeth and the housing from the inlet side to the outlet side of the pump. The direction of rotation of the drive gear shaft is determined by the location of the inlet and outlet ports. The direction of rotation of the drive gear will be to move the oil around the outside of the gears from the inlet port to the outlet port. This is true on both gear pumps and gear motors. On most gear pumps, the inlet port is larger in diameter than the outlet port. On bidirectional pumps and motors, the inlet port and the outlet port will be the same size.

Gear Pump Forces




Illustration 3g01080795

The outlet flow from a gear pump is created by pushing the oil out of the gear teeth as they come into mesh on the outlet side. The resistance to oil flow creates the outlet pressure. The imbalance of the gear pump is due to outlet port pressure being higher than inlet port pressure. The higher pressure oil will push the gears toward the inlet port side of the housing. The shaft bearings carry the majority of the side load in order to prevent excessive wear between the tooth tips and the housing. On the higher pressure pumps, the gear shafts are slightly tapered from the outboard end of the bearings to the gear. This design feature allows full contact between the shaft and bearing as the shaft bends slightly under the pressure.

The pressurized oil is also directed between the sealed area of the pressure balance plates and the housing and the mounting flange in order to seal the ends of the gear teeth. The size of the sealed area between the pressure balanced plates and the housing is what limits the amount of force that pushes the plates against the ends of the gears.

Pressure Balance Plates




Illustration 4g01080797

There are two different types of pressure balance plates that are used in gear pumps. The earlier type (1 ) that is shown in illustration 1 has a flat back. This type uses an isolation plate, a backup for the seal, a seal shaped like a 3, and a seal retainer. The later type (2 ) shown in illustration 1 has a groove shaped like a 3 cut into the back and is thicker than the earlier type. Two different types of seals are used with the later type of pressure balance plates.

Gear Pumps with Pockets




Illustration 5g01062109

Gear pumps with a housing that is machined with pockets for the gears have a radius from the pocket walls to the bottom of the pockets. The isolation plate or the later pressure balanced plate that is used in the pocket must have chamfered edges or a curved outer edge in order to fit fully against the bottom of the pocket. Using a sharp edge isolation plate, a sharp edge seal retainer or a sharp edge pressure balance plate in a housing pocket will force the pressure balance plates against the ends of the gears which will cause a failure.

Internal Gear Pump




Illustration 6g01062110

The internal gear pump that is shown in illustration 4 has a small drive gear (pinion gear) that drives a large ring gear (outer gear). The ring gear is a little larger in pitch than the drive gear. A stationary crescent is located below the pinion gear between the drive gear and the ring gear. The inlet and outlet ports are located at either end of the crescent.

When the pump rotates, the teeth of the drive gear and the ring gear unmesh at the pump inlet port. The void between the teeth increases and fills with inlet oil. The oil is carried between the drive gear teeth, the crescent, the ring gear teeth, and the crescent to the outlet port. When the gears pass the outlet port, the void between the teeth decreases and the teeth mesh. This action forces the oil out of the teeth and into the outlet port.

The internal gear pump is used as the charging pump in some large piston pumps.

Conjugate Curve Pump




Illustration 7g01062112

The conjugate curve pump (shown in illustration 5) is also called a GEROTOR pump. The inner members and outer members rotate within the pump housing. Pumping is achieved by the way the lobes on the inner member and the outer member contact each other during rotation. When the inner member and outer member rotate, the inner member walks around inside the outside member. The inlet ports and outlet ports are located on the end covers of the housing. The fluids that enter through the inlet is carried around to the outlet. This fluid is squeezed out when the lobes mesh.

A modified conjugate curve pump is used in many steering systems handmetering control unit (HMU). When a conjugate curve pump is used in the HMU, the outer gear is stationary and only the inner gear rotates.

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