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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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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.

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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

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

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