Hydraulic Cylinder Anatomy|
Curtis von Fange
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.
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