One of the oldest hydraulic pumps is the gear-on-gear design shown in Figure 8-4. As the driven gear turns, the idler gear turns in the opposite direction. At first, air trapped between the teeth and housing is moved to the outlet and forced out by the meshing teeth in the center. This starting action creates a negative pressure (vacuum) at the inlet. Atmospheric pressure then pushes oil into the pump. Now hydraulic fluid flows around the teeth and out to the circuit. Because the sealing action -- between the gear teeth and the housing, and where the teeth mesh -- has minimum clearance, when fluid is blocked, the gears stop turning.

A standard gear pump is unbalanced because there is high pressure on one side and low pressure or vacuum on the other side of the gears. This causes high bearing loads and shortened service life at pressures above 1500 psi. Some newer designs reduce this unbalance by clearing the housing (or clearance area) and only having a short sealing area. This greatly reduces bearing forces so that pressures up to 4000 psi continuous are commonplace today. However, even with this new design there is no compensation for gear or housing wear.

Gear-on-gear pumps can have more than one pumping section within a common housing. This allows for different flows or pressures to some circuits for speed and force changes.

Internal-gear fixed-displacement pumps

Figure 8-5 shows a cutaway view and the symbol for an internal-gear pump. The standard design is unbalanced and has no way to compensate for tooth or housing wear. Most pumps of this type are limited to 1000 psi or less. They are often used as transfer or supercharging pumps at low pressure due to their less efficient design. (There is a German-designed internal-gear pump that has a wear-compensating feature and a special bearing arrangement that allows it to operate continuously at up to 5000 psi and with more than 95% overall efficiency throughout its life.) Standard gear pumps start out at 85 to 90% efficiency when new. As the gears and housing wear, their efficiency deteriorates until they no longer supply enough fluid to maintain cycle time.

Gerotor fixed-displacement pumps

The newest design of a gear pump is called a gerotor (combining the words generated and rotor). A cutaway view and symbol is shown in Figure 8-6. This pump design is not common in the marketplace. At present there are only one or two manufacturers that offer this type. On the other hand, as a fluid motor it is one of the most common designs and is offered by more than 15 different companies.

A gerotor pump uses a driven gear of, say, seven teeth inside an internal-tooth gear with eight teeth. The driven gear rotates inside the internal tooth gear and they both turn in the same direction. Because of the machined shapes, the driven gear always makes contact with the internal tooth gear at different points as they rotate. As the example in Figure 8-6 shows, this allows cavities to open and close as the gears turn.

In the example, as the driven gear turns clockwise, the internal tooth gear turns the same direction, but at one tooth per revolution slower speed. This action causes cavities to form on the left hand that start reducing pressure in this area. This reduced pressure (vacuum) allows higher atmospheric pressure to push fluid into the pump and fill the forming cavities. Kidney-shaped cavities in this sector, on both sides of the teeth, accept fluid to fill them for 180° around the inlet side. As the gears continue to turn, the cavities formed on the left side start closing on the right hand side. This forces fluid through the kidney-shaped openings and to the outlet port.

Like other gear pumps, gerotor pumps are unbalanced and have no way to compensate when clearances become worn. Although a new gerotor pump starts out at 85 to 90% efficiency, it deteriorates as it runs and constantly loses volume.

Gerotor pumps also can have more than one pumping section in a common housing, again allowing for different flows or pressures to some circuits for speed and force changes.

Another point on gear pumps: their output flow cannot be varied -- except by changing them physically or running them at a different speed. The next two types of pumps are capable of changing volume while running the same speed. These pumps can also reduce flow on a pressure build-up signal and almost eliminate the need for a relief valve.

Multi-screw fixed-displacement pumps

The pump in Figure 8-7 is similar to a gear pump but uses helical gears or screws to move the fluid. The driven screw is in close fit mesh with the idler screws and all gears have minimum clearance in the housing. As the driven screw turns, the idler screws also turn and the cavities between the screws move toward the outlet. This action forms a vacuum at the inlet. Atmospheric pressure then pushes fluid into the cavities and the fluid moves to the outlet. This pump has very smooth flow -- without the pulses produced by the other positive-displacement pumps in this manual. Flow from the outlet is smooth and continuous. However, screw pumps are not highly efficient. There is a lot of bypass in the original design and as the screws and housing wear, bypass increases. This design pump often is used to supercharge other pumps, as a filter pump, or a transfer pump at low pressure.