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SYSTEM MAINTENANCE
Regular filter maintenance, oil change intervals, and installation of the
correct fluids will provide the best service life to all hydraulic
systems. Mechanics repairing and maintaining equipment must protect
against rust, corrosion, dirt, oil deterioration and other adverse
conditions detrimental to hydraulic systems. Due to the high pressures
that can be attained, safety precautions should always be observed in
operating and testing hydraulic circuits. Long lines and hoses need to be
secured to prevent wear and chaffing that could lead to ruptures. Regular
inspection of hydraulic runs will reduce down time failures.
Hydraulic oil flowing through fittings, valves, and cylinders, and being
compressed in hydrostatic transmissions or lift circuits will result in
heat being generated. This is normal in operation and component design is
such that heat can be dissipated during use. Excessive generation of heat,
or arrest of thermal conductance in system parts leads to breakdown of the
fluid in viscosity and chemical make up. Normal maximum temperature. for
hydrostatic transmissions are determined by their manufacturers.
CAUSES FOR EXCESSIVE HEAT
Hydraulics that display excessive heating should have all cooling
components inspected first. Fans used to move air across heat exchanges or
cooling fins and tubes must not be restricted by debris. Shrouds and
baffles designed to direct air flow should be in place. Cooling surfaces
should be kept clean. Dirt and oily residue act as insulating material,
restricting the passing of heat away from the oil. Any restriction in the
lines or fittings will cause the oil temperature to rise as it is pumped
through the restricted area. Tubes that are pinched or crushed, hoses that
are kinked, and damaged or undersized fittings are all likely to generate
heat. Air trapped in the oil also causes heat as it is compressed in the
circuit. The air bubbles will explode in the compressed oil damaging any
component surface they are in contact with. All lines and fittings must be
kept tight to prevent air from entering the system.
HYDRAULIC CIRCUITS
A typical hydraulic lift system is shown in (Fig. 17).
The tractor gear transmission case provides the oil supply. Intake is away
from the case bottom to prevent particles that have settled from entering
the system. Large diameter intake lines keep restriction to a minimum. The
external gear pump driven by the tractor engine pumps the oil to the
pressure set by the relief valve. The oil is directed to the single acting
cylinder by the control valve, or returns to the gear case through the
relief valve. Oil released from the cylinder also returns to the gearcase
when the implement is
lowered.
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Click for a closer view
 Fig 17
Hydrostatic transmission charge pumps may
also provide a source of oil flow and pressure that can be utilized to add
lift systems to smaller tractors. (Fig. 18) describes the flow path. The
reservoir may be the tractor gearcase, or a simple tank. Intake is
positioned off of the bottom. The filter traps particles before the oil
passes through vital components. Most charge pumps are of the internal
gear design. Pressure to the charge check valves is controlled by the
charge relief valve. When the set pressure is reached, the oil flows to,
and through the open center lift control valve, and returns to the
reservoir.
Click for a closer view
 Fig 18
Activating the control valve directs flow
to the cylinder, enabling the pump to generate pressure up to that set by
the implement relief valve. Oil on the opposite side of the piston flows
through the control valve and returns to the reservoir. When the piston
reaches the end of its stroke, or is restricted by an excessive load, the
implement relief cracking pressure is reached allowing the oil to return
to the reservoir.
IN SUMMARY
The purpose of this manual is to provide basic information on the
application of hydraulics in our industry. It only scratches the surface
of knowledge available about the subject. Public libraries, and equipment
makers are excellent sources of information to increase your knowledge on
this technology
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[Source: Simplicity publication, Hydraulic
Systems Training Information, #840172, Principles and Operation of Tractor
Hydraulic Systems]
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