950G Series II Wheel Loader Caterpillar


Piston Pump

Usage:

950G II AXR


Piston Pumps




Illustration 1g01062544

Most piston pumps and motors have common parts that use the same nomenclature.

The following pump parts are shown in illustration 1:

(1) head

(2) housing

(3) shaft

(4) pistons

(5) port plate

(6) barrel

(7) swashplate

The two designs of piston pumps are the axial piston pump and the radial piston pump. Both pumps are highly efficient, positive displacement pumps. However, the output of some pumps are fixed and the output of some pumps are variable.

Axial Piston Pumps and Motors




Illustration 2g01062422

The fixed displacement axial piston pumps and motors are built in a straight housing or in an angled housing. The basic operation of a piston pump and a motor are the same.

Straight Housing Axial Piston Pumps and motors

Illustration 2 shows the positive displacement fixed output axial piston pump and the positive displacement variable output axial piston pump. In most publications the fact that both pumps are positive displacement is considered to be understood and the pumps are refered to as fixed displacement pumps and variable displacement pumps.

In the fixed displacement axial piston pumps, the pistons move backward and forward in a line that is near parallel to the centerline of the shaft.

In the straight housing piston pump, shown on the left side of illustration 2, the pistons are held against a fixed, wedge-shaped swashplate. The angle of the swashplate, controls the distance the pistons move in and out of the barrel chambers. The larger the angle of the wedge-shaped swashplate, the greater the distance of piston movement and the greater the pump output per revolution.

In the variable displacement axial piston pump, either the swashplate or the barrel and port plate may pivot back and forth to change the angle to the shaft. The changing angle causes the output flow to vary between the minimum and maximum settings although the shaft speed is held constant.

On either pump, when a piston moves left, oil flows through the intake and fills the space left by the piston movement. As the pump rotates, the piston moves right, then the oil is pushed out through the exhaust and into the system.

Most piston pumps that are used on mobile equipment are axial piston pumps.

Angled Housing Axial Piston Pump




Illustration 3g01062425

In the angled housing piston pump shown in illustration 3, the pistons are connected to the input shaft by piston links or by spherical piston ends that fit into sockets in a plate. The plate is an integral part of the shaft. The angle of the housing to the shaft centerline, controls the distance the pistons move in and out of the barrel chambers. The larger the angle of the housing, the greater the pump output per revolution.

The output flow of a fixed displacement piston pump can only be changed by changing the input shaft speed.

Straight and Angle Housing Piston Motors

In the straight housing fixed displacement piston motor, the angle of the wedge-shaped swashplate determines the speed of the motor output shaft.

In the angle housing fixed displacement piston motor, the angle of the housing to the shaft centerline determines the speed of the motor output shaft.

In both motors, the output shaft speed can only be changed by changing the input flow to the motor.

Some smaller piston pumps are designed for pressures of 69000 kPa (10007 psi) or more. Piston pumps that are used in mobile equipment are designed for a maximum pressure of 48000 kPa (7000 psi) or less.

Radial Piston Pump




Illustration 4g01062427

In the radial piston pump shown in illustration 4, the pistons moves outward and inward in a line that is 90 degrees to the centerline of the shaft.

When the cam follower rolls down the cam ring, the piston moves outward. Atmospheric pressure or a charge pump pushes oil through the valve inlet port and fills the space left by the piston movement. When the cam follower rolls up the cam ring, the piston moves inward. Oil is pushed out of the cylinder and through the outlet port.

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