20
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March - April 2016
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www.machinerylubrication.com
By al an Dellinger, gPM hyDraulic consulting
Hydrostatic drives are used in a
variety of applications
throughout all types of indus-
tries. They are sometimes referred to as
hydrostatic transmissions. Anytime one or
more hydraulic motors need to be driven at
variable speeds with bi-directional
cap ability, a hydrostatic drive is often used.
Common applications include conveyors,
log cranes, mobile equipment, centrifuges,
chemi-washers and planers. Hydrostatic
drives are some of the least understood
systems because many of the components
are located on or inside the hydrostatic
pump assembly.
A schematic of a typical hydrostatic
drive is shown in Figure 1. The bi-direc-
tional, variable displacement pump
controls the direction and speed of the
hydraulic motor. This type of drive is
commonly called a closed-loop system.
Notice how the pump's two ports are
hydraulically connected to the two ports on
the motor, forming the closed loop.
Main Pump
A piston-type pump is always used in a
hydrostatic system. The pump volume can
range from zero to the maximum amount.
In Figure 2, the pump swashplate is in the
vertical position, which means the pump
output is zero gallons per minute (GPM).
HYDRAULICS
Hydrostatic
Troubleshooting
Understanding and
Systems
Figure 1. A typical hydrostatic drive