 Hydraulic pumps are used in hydraulic drive systems and
can be hydrostatic or hydrodynamic. A hydraulic pump is a
mechanical source of power that converts mechanical
power into hydraulic energy It generates flow with enough
power to overcome pressure induced by the load at the
pump outlet. When a hydraulic pump operates, it creates a
vacuum at the pump inlet, which forces liquid from the
reservoir into the inlet line to the pump and by mechanical
action delivers this liquid to the pump outlet and forces it
into the hydraulic system

 Different types of hydraulic pumps:
FIXED DISPLACEMENT TYPES
 Gear pumps
 Gerotor pumps
 Screw pumps
VARIABLE DISPLACEMENT TYPES
 Bent axis pumps
 Axial piston pumps
 Radial piston pumps
 Rotary vane 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.

 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-
center 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

 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.

 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 an shaft rotates the pistons are reciprocated
in turn relative to the le to a thrust plate mounted on the
shaft end. When 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.

 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.

 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

 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.