Positive Displacement Pump FAQs: How Do Hydraulic Vane and Gear Pumps Work?

pump faq

As described in Hydraulic Pumps 101, hydraulic pumps are among the most necessary components in a hydraulic system. They convert mechanical energy into hydraulic power.

Hydrostatic pumps, or positive displacement pumps, create this power with very little quantity or velocity of the liquid, making them the subject of many questions about their functionality.

In general, positive displacement pumps work by displacing liquid by trapping it between pumping elements and a stationary casing. The design of PDPs can include gears, lobes, rotary pistons, vanes, or screws.

PDPs can be categorized as reciprocating or rotary. Rotary positive displacement pumps use rotating cogs or gears to transfer fluids rather than the backward-and-forward motion of reciprocating pumps.

To know how PSDs operate further, let’s answer a few common questions among hydraulics distributors and end-users:

How do hydraulic gear pumps work?
How do hydraulic vane pumps work?
What are the advantages and disadvantages of each?

How Hydraulic Gear Pumps Work

Gear pumps are the simplest example of a rotary positive displacement pump.

Their straightforward design can include either internal or external gears, lobes, or screws.

Regardless of design, the rotating action of teeth or grooves creates a liquid seal and suction at the pump inlet. Fluid is drawn into the pump, enclosed within the rotating teeth, and transferred to the discharge.

Image source: Michael Smith Engineers LLC

External Gear Pumps rotate, trapping fluid between the teeth and moving it from the inlet to the casing.

Internal Gear Pumps work similarly, but two interlocking gears of different sizes trap and disperse fluid as one gear rotates inside the other.

Gear pumps have these advantages:

Simple design makes them less complex, lower in cost, and easier to maintain
Creates a high pressure up to 3,000 psi
Able to pump viscous fluids, unlike centrifugal pumps
They are efficient, with low chances of leakage and contamination
Can run in two directions for loading and unloading

Disadvantages include:

Gears can not tolerate abrasive fluids
They are noisy
Their limited size cannot handle large bulk flow rates

How Hydraulic Vane Pumps Work

A vane pump, as its name implies, is a rotary displacement pump that consists of vanes mounted on a cylindrical rotor.

Vane pumps work similarly to gear pumps, creating suction and drawing fluid from an inlet with the rotation of the vanes instead of gears, compressing and discharging the enclosed fluid at the outlet.

This set of paddle-like vanes is mounted radially on a cylindrical rotor and creates the compartments that trap and transport the fluid through the system. The vanes can be sliding or flexible.

A sliding vane pump has rigid vanes that are fitted into radial slots and remain in their slots when stationary. When the shaft rotates with enough speed (about 700 rpm), the vanes travel outwards under centrifugal force, maintaining close contact with the casing wall. The vanes may also be spring-loaded to ensure contact even when the pump is stopped.

In a flexible vane pump, the rotor is made from a flexible material, shaped with several lobes that maintain contact with the pump casing. The rotor is offset and slightly larger than the casing, bending and creating compartments that expand at the inlet for suction and compress at the discharge outlet.

With the sliding action and contact with the casing, vane pumps are ideal for pumping low to medium-viscosity fluids.

Advantages of vane pumps include:

Good suction characteristics
Accurate and low maintenance
Lower noise due to smooth operation and low pulsation
No internal metal-to-metal contact
Self-compensates for wear through vane extension
Can handle thin liquids (or run dry for periods) at relatively high pressures

Disadvantages of vane pumps include:

Complex housing and many parts
Requires a shaft seal or magnetic coupling, which can be a source of leaks
Pulsation may be a problem at low speeds
Not suitable for high-pressure or high-viscosity applications
Not good for?abrasive applications

Hydraulic Parts Source is a great resource to fulfill your hydraulic pump repair and replacement needs. Visit Hydparts.com for a full list of pumps we service, or contact us to get a quote.