Types of Hydraulic Pumps

Fixed displacement types

Gear Pumps

Probably the simplest and most common used today, they are easily maintained and economic. Two basic types exist, the internal gear and external gear both are of the positive or fixed displacement pump group. External pumps use two external gears that mesh and push oil around the outside of the gear. Pressures up to 250bar are common, but cast iron designs increase this to 320 bar. Straight cut and helical spur gears give lower noise performance.

Gerotor Pumps

Internal gear or Gerotor  pump designs have one external gear rotor meshing with the inside of an internal idler gear these are commonly found in automotive oil pumps. The off-centre rotor seals to the idler and the volumes are continuously changing, passing fluid from suction inlet to outlet.  Gerotor pumps are generally found in low pressure applications where they are moderately efficient yet not too noisy.

Screw Pumps

Two helical screw form shafts intermesh inside a common housing, one shaft has a drive end. Fluid passes through this pump in a linear direction giving a fixed displacement output. Screw pumps are generally low noise due to the continuous gear contact and very reliable. Efficiencies can be low especially in increased viscosity applications.

Variable displacement types

Bent Axis hydraulic pumps

These hydraulic pumps can be either fixed or variable displacement types. The pump body houses a  rotating cylinder with pistons acting around its periphery. The pistons acts at angle to a thrust plate mounted on the shaft end. When the shaft rotates the pistons are reciprocated in turn relative to the pump body. To vary the pump displacement angle of the thrust plate is varied. This effectively changes the stroke of the piston and hence changing the amount of fluid moved for each revolution. The mechanics of this pump is highly efficient and reliable, and is often found in mobile machinery.

Axial Piston Pumps

Similar in layout to the Bent Axis pump, yet the variable displacement mechanisms are simplified. The axial arrangement of the shaft and pistons means this design is compact, efficient and economically produced. A wide variety of pressure, flow and power control functions can be fitted to ensure this pump matches the machines needs.

Radial Piston Pumps

Simple versions are fixed displacement type, but many come is a variable displacement option.  An odd number of radial pistons are arranged around a rotating shaft. This is encased within an eccentric ring. As the shaft rotates the distance between the eccentric ring and shaft centre line varies, hence the pistons move through a suction and pressure cycle. The driven shaft is often hollow and allows fluid to enter and exit the pump. The displacement is varied varying the amount of eccentricity; this is done either manually via adjustment screws or hydraulically with a piston. These are excellent for high pressure and are strong and reliable.

Rotary Vane Pumps

A good choice for low noise and reliable service, but pressure capability can be low <140bar. By and large fixed displacement designs are used, but variable designs are possible. Sliding vanes are arranged around the rotating shaft. This is within an eccentric ring, which can be adjusted. The vanes form a seal against the eccentric ring and in the rotating shaft. Easily serviced these are common for in machine tool applications.

This is an overview of the main types of hydraulic pumps used today, along with a simplified description of the mechanisms of each. A further study of each type of pump, their applications and relative merits will soon be published.