Figure 8-7.--Fluid Service Cart, Model 310.
A few of the most common types of rotary pumps
pump. When a fixed displacement pump is used in a
hydraulic system, a pressure regulator (or unloading
are discussed in the following paragraphs. All of the
valve) is used to control the amount of pressure in the
pumps discussed are gear pumps, so called because
system. The pressure regulator also unloads or relieves
they use gears to trap the fluid. This style of pump is
the pump when the desired pressure is reached. This is
ve r y s i m p l e i n d e s i g n a n d fi n d s w i d e u s e i n
to keep the pump from working against a load when the
low-pressure hydraulic systems. A gear pump delivers
hydraulic system is at maximum pressure and not
a constant volume of fluid at any given rpm.
functioning. During this condition, the pressure
SPUR TOOTH PUMPS.--The gear-type rotary
regulator bypasses the fluid from the pump back to the
pump shown in figure 8-8 is called a spur tooth pump. It
reservoir. The pump, all the while, continues to deliver
consists of two meshed gears that revolve alongside
a fixed volume of fluid during each cycle. In this type
each other in one housing. The driving gear is turned by
of system you also need a relief valve to bypass
a drive shaft, which engages the power source. The
pressure if the regulator or unloading valve fails.
clearances between the gear teeth and the pump
housing are very small.
The inlet port is connected to the fluid supply line,
Rotary pumps operate by means of rotating parts
and the outlet port is connected to the pressure line. In
that trap the fluid at the inlet (suction) port and force
the pump shown in figure 8-8, the drive gear rotates
it through the discharge port into the hydraulic
c o u n t e r c l o c k w i s e a n d t h e d r ive n g e a r r o t a t e s
system. Gears, lobes, and vanes are commonly used
clockwise. As the teeth pass the inlet port, fluid is
as elements in rotary pumps. Rotary pumps are of the
fixed displacement type.
trapped between the teeth and the housing, and is
carried around the housing to the outlet port. As the
There are many types of rotary pumps that are
teeth mesh again, the liquid between the teeth is
classified in various ways. They may be classified by
displaced into the outlet port. This action produces a
shaft position (vertically or horizontally mounted); by
positive flow of liquid into the system. A shear pin or
the type of drive (electric motor, internal combustion
shear section is incorporated in the drive shaft to
engine, etc.); by manufacturer's name; or by service
protect the power source or reduction gears if the pump
application. However, rotary pumps are usually
classified by the types of rotating elements they use.
fails because of excessive load or binding of parts.