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The oil supply to the check valves can be made available by gravity if an
oil tank or reservoir is positioned above the check valve inlet. Another
method is to provide oil under a low charge pressure to the relief valve
with a positive displacement gear pump. A check valve (See Fig. 9) usually
consists of a ball and seat positioned between two ports. As a directional
control, it has a free flow and no flow direction. Flow through the seat
will push the ball away, flow to the ball pushes the ball against the seat
so pressure forces it to seal the passage.
Click for a closer view Fig 9
POSITIVE DISPLACEMENT PUMPS
In order to provide sufficient pressure and flow volumes for lift systems
and for charging hydrostatic transmission systems, pumps which
deliver a positive displacement of oil are required.
Gear type pumps, either with internal, or external designs are most
commonly applied. The internal gear pump (Fig. 10) consists of an inner
gear turned by the drive shaft and a larger outer gear, within a closely
fitted housing. The two gears are not concentric so as they rotate,
pumping chambers open and close. Inlet and outlet parts aligned with the
gear face allow oil to be drawn into the chambers, and compressed as the
chambers diminish in size at the outlet ports.
External gear pumps have two gears meshed within a close fitting housing.
One of the gears is turned by the power source, and the other is turned by
its mesh with the first gear. (Fig. 11.) Each tooth provides a pumping
chamber transporting fluid from the inlet to the outlet. The mesh of
the gear teeth forces the oil to be expelled through the pump outlet. In
this manner, the pumps can produce a positive charge of fluid.
Click for a closer view
Resistance to the flow of oil produces
pressure necessary for the system to function. If the amount of resistance
could not be controlled, it could become so high that the weakest
component containing the oil would burst, or the energy source would be
stalled. Every hydraulic circuit that utilizes a positive
displacement pump therefore requires a relief valve to protect against
excessive pressure. If a load cylinder were stalled or reaches the end of
its stroke, for example, an alternate path must be provided for the pump
output. The relief valve allows the oil to be returned to the system
supply or reservoir.
A simple relief valve (Fig. 12. ) is
similar to the check valve in construction. The component that acts as a
valve may be a round ball, or a poppet that has a smooth concentric
surface to act upon a concentric seat area. The important part of the
valve is the relief spring. It determines the pressure of the system by
the force it applies to the valve. This force is called the cracking
On most relief valves the cracking pressure
can be adjusted to some degree with screw type mechanisms, or by placing
adjusting shims above the spring. We must caution that raising the relief
pressure should not be attempted without proper test equipment to gauge
the hydraulic system, nor should it be changed if other pump or actuator
components are failing to perform at their rated capacities.
Positive displacement gear pumps that are part of
hydrostatic transmission charge systems, or provided for the specific
purpose of hydraulic lift circuit availability, can be utilized to perform
many labor saving tasks on small tractors. Remember Pascal’s law says that
pressure “acts with equal force on equal areas.” Lift cylinders of both
single acting and double acting design are used as actuators.
[Source: Simplicity publication, Hydraulic
Systems Training Information, #840172, Principles and Operation of Tractor