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
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
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.
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.
[Source: Simplicity publication, Hydraulic Systems Training Information, #840172, Principles and Operation of Tractor Hydraulic Systems]