Valves used to control working cylinders are defined as directional control valves. Most use a spool contained in a machined casting to provide the flow paths required for cylinder response. In (Fig. 16) the flow paths through a simple spool valve are shown when applied to a double acting cylinder.
Spring loaded centering devices are incorporated to return the spool to the hold position when control levers are released from the extend or retract modes.
Some common variations allow
for a ˜float™ position which allows oil to flow freely between the piston
and rod end of the cylinder when the spool is extended and held by a
positioning detent. Valve blocks may have ports positioned so several
valves may be stacked together to provide control for complex requirements
such as back-hoes. The system relief valve may also be positioned after
the control valve in the circuit which will allow flow under pressure to
be routed to other hydraulic devices. Control linkages may be designed to
provide positioning of the piston in the cylinder such as with a three
point hitch on some agricultural type tractors.
OIL SUPPLY AND FILTRATION
An oil supply reservoir must be considered as more than a storage tank to hold the fluid until demanded by the hydraulic system. It will dissipate heat from the oil, and settle particle contamination from the oil. It may be a simple container made specifically to hold the required volume of oil. More often, space and application of the oil are shared with other tractor functions, most often transmission gears. The quantity of oil must be such that when all cylinders are filled to their capacity, the intake opening remains submerged sufficiently to prevent air from entering the hydraulic system. Remember, air is compressable so is detrimental to operation. Positive displacement pumps also are most efficient when the level of the intake is as near as possible to the pump inlet. Venting of the reservoir to the atmosphere is necessary as fluid levels vary in use. If a vacuum is allowed to form in the tank, the pump may starve for oil, resulting in loss of lubrication. The ventilating opening should be baffled or filtered to avoid contaminating the fluid.
The components required for hydrostatic principles to be applied depend upon close tolerances between parts. Contamination of the oil by foreign materials or chemicals would cause early damage and inefficient operation of the hydraulic system. Strainers or filters incorporated into the circuit between the reservoir and pump intake are most often utilized. Strainers will remove coarse materials to the size of its micron rating. (One micron is 39 millionths of an inch.) A strainer with a 25 micron rating would stop particles .0099 in size. Absorbent filters utilizing resin impregnated papers have higher micron ratings, and are capable of absorbing soluble contaminants. Filters or strainers positioned before the pump must allow free flow of the oil to prevent starvation.
[Source: Simplicity publication, Hydraulic Systems Training Information, #840172, Principles and Operation of Tractor Hydraulic Systems]