Tractor Hydraulics - The Basics

Hydraulics was one of the greatest inventions for helping man compound the work he can do. Its amazing how a little floor jack can lift tons and tons of weight with just the flick of a handle. Whats even more amazing is that all the principals of hydraulic theory can be wrapped up in such a small package. This same package applies to any hydraulic system from the largest bulldozer to the oldest and smallest tractor. This short series will take a look at the basic layout of a simple hydraulic schematic and then give some maintenance tips for long operation.

All hydraulic systems start with oil. Oil is the lifeblood of the beast. Since oil is a liquid it cannot be compressed. That means it will flow and transmit power virtually anywhere. As with any fluid this form of power transmission needs to be channeled and told where to go. In simple terms that is what a hydraulic system does. The oil is the lubricant and vehicle for transmission of this power. Hydraulic oil is not just any oil. It is formulated to withstand a wide variety of temperatures and has additives that control its reaction to the rubber seals and plastic parts found in the components of the system. A wrong oil type can cause o-ring swelling and dissolve seals. Use only hydraulic oil recommended for your application.

First thing we want to start with is a can to hold the oil. Actually well call it an oil reservoir. This container can be a rectangular box mounted to the side of a loader or welded to a log splitter. It can also be the transmission casing of a typical tractor. A specially formulated Hydrotrans fluid performs functions for the transmission and hydraulic needs. The reservoir contains the hydraulic fluid and protects it from the outside elements. It will usually have an air breather screwed onto the filler cap, which permits air to circulate in and out of the reservoir as the fluid level changes and might also have a dipstick to measure the level of oil.

Lets next look at the hosing one might find on a hydraulic system. Typically there are three types of hoses or conduits for directing the oil to where you want it to go. The low-pressure hoses are usually a larger diameter nylon reinforced rubber. These are found as the supply lines to the pump or a return line from the spool valve. Smaller diameter wire reinforced hoses usually have pressed pressure fittings on the ends. These are high-pressure lines that carry the oil under extremely high pressure to various parts of the machine. They go from the pump to the spool valve and from the spool valve to the cylinders and back. Depending on the system pressure these can be steel mesh reinforced. A pressure around 1000 psi will generally have one coating of wire reinforcement while a 3000 psi line will have a double wire mesh. A rubber coating for protection from rubbing and weather penetration then surrounds the mesh. The pressed on fittings are installed at the hose supplier although reusable fittings can be purchased at a higher cost. When using reusable fittings make sure they match the hose size diameters inside and outside, as they dont always conform to the crimp style hose size. One last type of hose is the rigid style. This consists of steel line with pipe or tubing fittings made onto the line. These are usually used for very tight elbows and angles that would crimp a normal hose. They are also used for lengthy runs where proper mounting can keep them from vibrating and rubbing against other components.

From the reservoir the oil flows though a low-pressure hose to the pump. The pump is powered by any number of sources. A front-end loader will usually have a pump attached by a PTO shaft to the crankshaft pulley of the engine. A typical tractor will have an internal drive running a pump mounting in the transmission casing that runs the three point hitch. A log splitter will have a small five-horse engine with a coupling attached to the pump. Either way some power source will run the pump. A pump can be of a number of varieties ranging from a piston style with an adjustable swash plate to a simple vane type. Many tractors, though, use a simple gear type pump. The oil comes in the larger of the two openings, enters the gear tooth cavity, and then is expelled when the gears mesh, thus forcing oil into a smaller high-pressure line. The tolerances are very close on these pumps and it is not wise to disassemble one unless the tools and gauges are present to do adequate service.

After the oil is exhausted from the pump it travels through a high-pressure supply line to a spool valve. A spool valve is a fancy name for a switch that controls fluids. Oil is directed through the spool valve to the selected circuit of choice. If nothing is selected then it flows through the valve back to the reservoir. When a lever is actuated the oil flows through the valve and into the now opened circuit of choice, usually to a hydraulic cylinder performing some function. As the oil flows under pressure to one side of the selected cylinder the ram extends or retracts depending on the selection. Oil from the other side of the cylinder flows back through its high-pressure hose, through the spool valve and back to reservoir. When the spool-actuating lever is pulled the other direction the process is reversed and the hydraulic cylinder actuates in the opposite direction. A spring detent keeps the shifting lever in a neutral position when not in use. A spool valve can contain any number of individual spools each controlling its own circuit. A log splitter, for example, will have one spool operating one cylinder, the ram that splits the wood. A backhoe can have six or more functions operating off one spool valve assembly. Manufacturers of heavy equipment design spool valve assemblies to accommodate any number of circuits and provide for bolt on attachments much like adding another couple of slices of bread to the spool sandwich. For the most part all the spools perform the same identical function: to direct the oil someplace else.

Loose ends found on hydraulic systems include oil coolers, pressure relief valves and oil filters. These are installed at various locations in the system depending on the manufacturer. Some relief valves have an adjustment on them but it is best not to disturb the setting, as it is factory set to flow in harmony with the rest of the system. Adjusting the setting to a higher psi rating can do damage to other parts of the system along with overstressing rated hose pressures. Other servicing components will be discussed in the next part of the series.

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