by Curtis Von Fange

Tractor Hydraulics
The Basics

Hydraulics was one of the greatest inventions for helping man compound the work he can do. Itís amazing how a little floor jack can lift tons and tons of weight with just the flick of a handle. Whatís 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 weíll 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.

Letís 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 donít 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.

by Curtis Von Fange

Hydraulic Basics - Part Two

In the last entry to this series we gave a brief overview of hydraulic system theory, its basic components and how it works. Now lets take a look at some general maintenance tips that will keep our system operating to its fullest potential.

The two biggest enemies to a hydraulic system are dirt and water. Dirt can score the insides of cylinders, spool valves and pumps. Water will break down the inhibitors in the hydraulic oil causing it to emulsify and lose its lubricating features. Scoring of cylinder walls and breakdown of internal seals will result. It is imperative to keep these two problem areas in check.

The primary way that dirt can enter the system is through the air breather on the reservoir. As the cylinders in the hydraulic circuits are activated the oil level in the reservoir goes up and down. Air rushes in and out of the reservoir through the air breather in order to compensate for the various fluid levels. Unfortunately, this air is often contaminated with air borne dust and debris from the excavating process that the tractor is doing. The air breather is supposed to screen out this dust by trapping it in between layers of oil saturated filter material. Many times these filters are not cleaned or checked for holes or cracks thereby permitting extremely dirty air to penetrate the system. Always keep the breather assemblies clean and intact because, many times, this is the only form of protection from the dusty atmosphere.

Another way that dirt can penetrate the system is through careless repairs. Broken hoses are a main contributor to this cause. As with most farm equipment dirt, grease, and debris tends to collect around the areas that have the most oil leakage. Hose end fittings are notorious for collecting such globules of junk. When a hose breaks the usual fix is to unscrew the fitting remove the bad hose and then stick on a new hose. Many times the disconnected hoses are turned upward to keep oil from oozing out while the new hose is made at the dealer. It is during these times that small pieces of debris and dirt tend to fall off of the surrounding areas and into the open hose ends. Some folks even try to collect the drained oil and return it unfiltered to the reservoir. All of these practices can permit unwanted and potentially damaging foreign material to enter the system.

When a hose breaks it is good practice to isolate the hose ends first. Removing the hose supports so the ends can be swung away from the tractor body can do this. Then wire brush or bristle brush the ends to remove all the loose ends and debris. Remove the hose ends, drain the remaining hoses into a bucket, and then wrap them with a clean rag held in place by a twisty tie or a rubber band. Use a bungee cord to hold the hose off to the side while waiting for the new hose to come to the jobsite. When reinstalling it might be a good idea to blow some clean air through the new hose just to be sure no debris from the manufacturing process dropping inside. Reattach the hose ends and then re-secure to the tractor with the corresponding mounting brackets. It is important to always use these brackets when they are provided because it keeps the hoses and tubes from vibrating and wearing holes in them.

When examining the tractor for routine maintenance it is wise to check the hosing for certain wear characteristics. As mentioned in the last series the hydraulic hoses are a divided into high and low-pressure applications. On older hoses the outer rubber covering will, over time, react to the sunís ultraviolet rays along with the heat generated from the system use. They will begin to crack and eventually peel thereby exposing the wire braiding underneath. Water penetration will start to rust the braid and safe-operating pressures will be drastically reduced. Look for these cracking and peeling signs on the hoses. Also look vigorously for vibration and moving fittings that will rub through the outer hose skin in quite short order. On low pressure hoses look for gobs of grease, dirt and sharp debris that tends to accumulate on them since they are often housed behind the tractor cowlings and sheet metal.

Other areas to check include the fitting ends. Occasionally these ends will loosen up and cause oil seepage. If the oil leaks out it could also let air and dirt in under the right circumstances. Check but donít over-tighten. Keep the oil cooler clear of dirt and debris. This area also tends to attract junk like a magnet. Reverse blowing of the cooler fins with compressed air or a low-pressure hose will keep the cooler functioning properly. Many systems now have oil filters on them. Keep these serviced on regular intervals as recommended by the manufacturer. In dirty and dusty conditions in never hurts to do it more often.

Water is the other big enemy to the hydraulic system. It usually comes into play from the humidity found naturally in the atmosphere. When the equipment sits idle the reservoir is not always what one would think as full. It might be full by the dipstick but may only be halfway full by reservoir capacity. It is like this because each double acting cylinder on the machine is divided into two chambers. These two chambers have different oil capacities. The power stroke cycle carries the most oil and generates the most power while the retraction stroke, with the ramrod in it, has less oil in the corresponding chamber. This is why the oil level vacillates so wildly in the reservoir and also explains why the capacity is larger than showing full on the dipstick. This also presents a problem in high humidity areas. The half full reservoir tends to have more exposed steel areas that can generate more surface area for condensation. The moisture collects over time to present quite a problem. Equipment that is used for many hours at a time will tend to evaporate out the moisture because of the heat generated by the hydraulic use. On the other hand, equipment that sits for long periods or is not used actively where it never warms up to an extended operating temperature will collect large quantities of moisture. Some water will be held in suspension by the moisture retardant additives in the oil. But excess water will emulsify the oil and cause internal damage to seals because the oil has lost some of its lubricating qualities. The point is to keep the oil changed according to the manufacturerís specs. But if the hydraulics are used very little and never seem to reach a good operating temperature it might be wise to do it more often.

The tolerances found in hydraulic pistons and spool valves are extremely close. Even the smallest particle of dirt can score and/or groove the finely ground steel parts. Creeping cylinders and lost power invariably results. Taking the time to inspect the hydraulic system for dirt and water penetration will save mountains of dollars in repairs and will ensure that the hydraulic system on your tractor gives a long and valuable life.

by Curtis Von Fange

Hydraulic Basics - Part Three

Hydraulic Cylinder Anatomy

Letís make one more addition to our series on hydraulics. Iíve noticed a few questions in the comment section that could pertain to hydraulic cylinders so I thought we could take a short look at this real workhorse of the circuit.

Cylinders are the reason for the hydraulic circuit. They take the fluid power delivered from the pump and magically change it into mechanical power. There are many types of cylinders that one might run across on a farm scenario. Each one could take a chapter in itself to introduce and explain. There are vane type cylinders that give rotary motion. Many newer tractors have these as part of their power steering units. Another rotary actuator is the hydraulic motor found on logging winches and the like. Probably the most common, though, is the simple piston-type cylinder that we will focus on in this entry.

There are two major kinds of piston-type cylinders in common farm use. The first is the single acting cylinder. These cylinders give force in only one direction. Oil under pressure, admitted at only one end of the cylinder, raises the load. An outside force such as gravity or a spring is needed to retract the rod into the cylinder casing. Common examples of this type might include the hydraulic cylinder on a tractor three point hitch. The second type is a double-acting cylinder. This type can apply pressure for the cylinder in either direction depending on the oil flow. It provides hydraulic power for extension and retraction like on a front-end loader bucket.

To help understand a little better how a cylinder works lets mentally create a simple image of comparison. Picture a single acting cylinder as a hollow piece of pipe with a sealed cap on the bottom end. At the bottom of the pipe a hole is drilled and threaded and a hose installed that will let oil flow in and out. Inserted into the pipe is wafer-like piston that has rubber or plastic seals around its circumference that keeps oil from seeping past it. This piston slides up and down the bore of the pipe. Through the center of the piston is a smaller diameter rod that is tightened with a nut on one side with the rod extending out the open pipe on the other. Screwed down on the open side of the pipe is another cap with a hole in it that lets the rod through and keeps the rod properly aligned with the pipe and piston. It is also lined with a rubber wiper seal to prevent oil seepage and/or rain penetration. Somewhere at the top of the tube there might be a hole that lets air in and out of the pipe depending on the piston movement. This is a simplified hydraulic cylinder image. As the oil is pumped through our oil supply line it pushes the piston up the bore and creates mechanical action at the end of the rod. A spring or gravity action from the raised load retracts the rod and piston. Oil returns to the reservoir.

A single acting cylinder is quite similar to a ram cylinder, but they are different. A ram-type cylinder does not have a piston in the cylinder bore. The rod itself is the piston and the seals at the end of the cylinder bore are o-rings placed in inverted grooves that are part of the cylinder housing, not seals contained in a removable cap. Its action is the same as single acting cylinder but it tends to have less surface area for the oil to push against since the rod is generally much smaller in diameter than a typical piston. It does have some advantages though. The rod is usually bigger and resists bending due to side loads. The packing is generally easier to reach and replace. No air vent is needed since oil fills the whole inner chamber of the cylinder housing. Once again spring action or gravity is necessary for retraction.

Double acting cylinder construction is quite similar to single acting. Picture our example above and then, instead of a ventilation hole at the end of the cylinder housing simply picture a threaded hole with another oil hose attached. Thatís about the only difference. Oil that is pumped into this other hole will act on the reverse side of the piston and cause the rod to retract in the opposite direction. Granted, there may be a different piston configuration thereby accommodating additional seals or wipers and the end cap may also have additional seals but the basic makeup is the same.

Because the piston rod takes up space in the cylinder bore and piston it should be understood that the force provided in the retraction stroke of the cylinder will be somewhat less than the expansion stroke. These types of double acting cylinders are called Ďunbalancedí cylinders. On a balanced cylinder, perhaps found on some power steering cylinders on tractors, there is a rod attached to each side of the piston. This provides for equal pressure area on the piston and results in equal force delivered from the cylinder in either direction.

There are many types of caps that can be found on cylinder ends. Some are simple screw on types, others have snap ring retainers that hold the seal assembly in place. Others have an internal groove ring with only an access hole on the side of the cylinder housing for removal. Some can be rusted in place while others are a head scratching enigma in figuring out how to remove. The tech manuals or parts schematics can help to identify the type you might be working with. Some tips might include the following. The one that has the small access hole in the side is common to Massey Ferguson industrial units. It has a square piece of soft stock that wraps around the external seal configuration and matches it to the cylinder bore and holds it in place. Take a small screwdriver and start the end of the stock out of the cylinder side while gently rotating the end cap with a pipe wrench. It will roll out of the bore as the cap end twists. Make sure not to force it or screw it the wrong way as the other end, a small ninety degree hook into the cap, piece will surely twist off. Many front end loader cylinders have an internal snap ring that holds the cap configuration in place. By removing pressure off of the cylinder this cap can manually be recessed into the cylinder bore thus exposing this elusive internal snap ring. Pop it out and the cap will then come off.

One might make a note that when replacing the seals in the cylinder cap one will usually have to remove the piston and disassemble it in order to remove the rod so the cap can be slid off the rod end. Watch for nicks and sharp edges when doing this because they will cut any seals that are passing over them. A fine file or emery paper will help smooth them out. Also make sure to use a locking nut on the piston rod end that definitely locks in place. It can be quite annoying to have the rod come loose from the piston because the nut was not locked in place.

Take care when disassembling a cylinder to make sure to observe the direction of any seals or wipers. Proper installation necessitates the correct direction when reassembling. Minor grooving in the cylinder bore can sometimes be removed by using a cylinder hone. Make sure and use a lubricating solvent like mineral spirits when honing. Also donít remove too much material as it will cause leakage in the newer seals. Make sure and inspect the piston rod for straightness. A bent rod will cause very slow or erratic actuation of the cylinder even when it is not hooked up to anything. (Thatís for you Tim, with your JD sickle bar.) A bent rod can only be replaced with another chrome coated stock item.

Reassembly of the unit should be done under an exceptionally clean environment. Use care when reapplying seals and o rings. Use the same type of lubricating oil as the system requires during reassembly. When installing the piston and cap end onto the cylinder it is quite helpful to use a piston ring compressor to compress the seals without damaging them.

Remember, this is only a brief outline for understanding and replacing cylinder components. Many newer applications contain other features like stroke control devices, pop off valves, cushions, protective thermal relief valves and the like. If there is any question about what kind of project you are working with then it would be wise to consult with a shop that works frequently on hydraulic cylinders and equipment.