The function of the
hydraulic system power pump is to change mechanical horsepower to hydraulic
horsepower.
Types of Power Pumps
There
are two types of power pumps, a gear pump and a piston pump.
Gear
pumps have efficiencies that average about 70-80% overall efficiency, where
overall efficiency is defined as:
Gear pumps move fluid based upon the number of gear teeth and the
volume spacing between gear teeth.
Principles of Operation:
Gear type pumps are ideal when working with pressures up to 1500
lb./sq.in. As mentioned previously, the volumetric efficiency of gear pumps
depends upon the number of teeth, the engine speed and the tooth area.
As the liquid comes from the reservoir, it is pushed between the gear teeth.
The oil is moved around to the other side by the action of the drive gear
itself and sent through the pressure line. What makes the oil squeeze in
between the gear teeth? gravity and the pressure head. To prevent leakage of
oil from the high to the low pressure side from occurring, you can make the
gears fit better.
You might want to increase
the pressure used to move the fluid along. However,
the higher the pressure,
the higher the friction loading
on the teeth.
Friction will develop heat which will expand the gears
and cause the pump to seize (parts will weld together and gears will stop rotating). In order to stop this, you can have the pump case, the
gears, and the bearings made out of different materials, (e.g., steel gears [1-1/2 inch thick], bronze bearings, aluminum
casing). Normally, the gear speed is higher than the engine speed (normally 1.4
times the engine speed).
Oil can leak over and under the gears. To prevent leakage, you can
press the bearings up against the gears. This decreases seepage
but this decreases
the mechanical efficiency when
friction increases. Even though oil acts as lubricant, seizing can occur when
oil is drained from the hydraulic system.
As mentioned previously we can push the bearing up against the gear to decrease leakage. As F increases, M decreases , thus, the gear and bushing increase in friction and mechanical efficiency decreases. when you increase the pressure on inlet side of the pump, leakage will increases around the gear. to reduce the leakage , you must push the bearing and gear closer, causing an increase in friction . that in why inlet pressure over 1500lb/sg, are not used.
Advantages of Gear Pump
- High pressure
- High speed
- Relative quite operation
- Easy to maintain .
