How Do Hydraulic Cylinders Work?


Hydraulic cylinders haven’t really changed a lot over the years. The manufacturing processes are much more streamlined and the tolerances are much tighter, but for the most part cylinders are still the hard working push/pull tools they have always been. These things have literally shaped the world around us. Anything that gets lifted, pushed, hauled, dumped, dug, crushed, drilled or graded has gotten that way by some truck, crane, dozer or tractor using a hydraulic cylinder. But how do hydraulic cylinders work?

The amazing amount of force a cylinder exerts is due to the simple mechanical principle of pressure exerted on the surface area of the piston. Simply put, the larger the diameter of the cylinder, the more it will lift. The formula for this is Area X PSI (Pounds per square inch) = Force.

Simple hydraulic cylinder diagram

The PISTON is inside the cylinder, the diameter of which is known as the BORE. OK, Technically, the the bore is the inside diameter of the tubing but this difference is of minor significance. The piston needs a piston seal to keep the pressure from bypassing to the other side, which allows it to build the required pressure (If a cylinder isn’t lifting the force it should, the piston seal is probably worn).

The piston is attached to the ROD (or shaft) of the cylinder, usually with the rod passing through the piston and attached with a large nut on the opposite end. To correctly calculate the pulling force of a cylinder, the surface area of the rod must be subtracted from the formula. The rod is probably the hardest worked component in the whole system. The rod is the largest single chunk of steel in the cylinder, unpainted and exposed to all the elements. It has to be extremely strong (to resist bending), exceptionally hard (to resist corrosion and pitting), and smooth as silk (to keep the rod seals intact to prevent leakage of fluid and pressure). The STROKE of the cylinder is the total travel possible from the fully retracted length and the fully extended length of the rod.

The GLAND or “head” of the cylinder is the part that the cylinder rod extends and retracts through. It contains the rod seals & the wiper seal which keeps contamination out of the cylinder.

The BUTT is the base or “cap” end. This end usually can use a variety of attachment points. Speaking of attachments, how do cylinders attach to their implement? Usually by using a CLEVIS, CROSS TUBE, PIN-EYE or TANG.

Most commercially available cylinders are double-acting which means they have ports on each end and they are used to push AND pull. These are easy to convert to single-acting (push or pull only) by using a simple breather device to allow the air on the unused side to be expelled.

So that’s pretty much all there is to hydraulic cylinders and how they work! They really are simple devices, capable of doing tremendous amounts of work. If you have anything to add, questions or corrections, feel free to contact me or leave a comment!

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  1. Are double acting cylinders supposed to hold the their position when stopped moving in either direction under weighted conditions. hb

  2. They should, but this is more of a function of the valve than of the cylinder. If the valves work ports are “blocked” in neutral (also call a cylinder spool) then yes, the cylinder should hold. A “motor spool” valve leaves it’s work ports open in neutral, and a “float spool” opens both work ports to the tank, so either of these types of valves will allow the cylinder to drift. Also, if the seals are bad in the cylinder, it will bypass fluid which will make it drift.

  3. the bucket on my skidsteer drifts. I replaced the sealsand wipers on both cylinders but one of the pistons on one of the cylinders was scratched. The cylinder felt smooth and did not appear damaged. I was told a scratched piston would not hurt because the seals are in contact with the cylinder not the piston itself. Without tearing this cylinder apart again is there a way to check the cause of the drift….could it be the control valve and not the piston seals? The bucket of the skidsteer has 2 cylinders, can one of the cylinders be faulty and why doesn’t the other cylinder hold up the bucket? I am leaning towards a bad control valve but I was told that I have a bad piston causing the drift. What is your take on this?

  4. It could very well be the valve. If the seals in the valve are leaky, it would allow the fluid to bypass in the neutral position and therefore the bucket cylinders would drift.

    One way you can check the hydraulic cylinder seals itself is to remove the hose from the retract side, and put it into a bucket. If you fully extend the cylinder and hold it there, the valve should bypass over the relief and once all the fluid is out of the line, It should stop. If the hydraulic fluid never stops coming out of the retract port, then it is bypassing the cylinder seals and you know there’s a problem with the hydraulic cylinder. Otherwise, I would troubleshoot hydraulic valve.

  5. To find the volume of the cylinder, you must calculate the area of the piston, then multiply by the stroke. Then just factor in the flow rate of the pump! If your cylinder hold 1 gallon of fluid and you have a 16 GPM pump, you would fully extend the cylinder in about 3.75 seconds.

    (Piston area)X(Stroke)/GPM

  6. Thanks for sharing this post, it tells us how do work Hydraulic Cylinders?…Hydraulic cylinders get their power from pressurized hydraulic fluid, which is typically oil. A hydraulic cylinder is the actuator or “motor” side of this system. The “generator” side of the hydraulic system is the hydraulic pump which brings in a fixed or regulated flow of oil to the bottom side of the hydraulic cylinder, to move the piston rod upwards.

  7. wel we have cotton bailing hydraulic press !! the problem m geting is dat the main ram does not come back to original position or do not setle down completely !!!wat wil be the main cause behind it

  8. Hi i am having a crane of 45ton capacity.the problem i am facing is that first of all my lifting cylinders drift down by itself i send the valves to get repaired,now the position is lifting cylinders hold themselves and their is no drifting but when load is applied it drifts down.
    i have checked that their is no leakage of seals.
    pls tell me other solution

  9. It’s possible that there is leakage at the valve. The best way to hold cylinders in place is to use a “lock valve” or double pilot operated check valve. This valve would lock the cylinders in both directions and would only open when pressure is applied to the other side.

  10. I bought a batwing mower with heavy wings. The mower came (new) with a medium-sized cylinder (I don’t know the exact diameter) on each wing, but with only a quarter-inch hydraulic hose going to each cylinder. My tractors that do well handling other hydraulic equipment do not have enough hydraulic power to lift the batwings without me getting off the tractor and lifting on the wings to help them get started up. I know that lift power is supposed to be proportional to the diameter of the cylinder, but is it possible that the small, quarter-inch, hydraulic hoses on the cylinders are hindering the lift of the wings?

  11. Hi, Francis. It’s not likely to have anything to do with the hoses. It won’t affect the pressure that gets to the piston. If you didn’t buy it new, it could be that the cylinder seals are worn, allowing fluid it pass the seals and not generate enough force to lift the wings.

  12. I’ve got a 1964 JD500 backhoe, frontloader that has a few serious leaks from the cylinders. I just need to know the easiest way to replace the seals, but mostly, what do the components even look like, how many parts to it and does that outer ring turn loose with a special tool?

  13. Ross, I assume that it’s leaking from the shaft seal? Welded cylinders can be put together a few different ways. Usually it’s got a threaded head which screws out & you can use a spanner wrench to unscrew it, but sometimes they have a locking cotter pin which has to be removed & then the head will “pop” out. Once you get the head off out of the cylinders, you will need to remove the “piston nut” which holds the piston onto the shaft. With the piston nut off, you can slide the rod out of the head & access the rod & wiper seals which would line the inside diameter of the head (or gland).

    Since it’s over 40 years old, it may be a beast to disassemble. Make sure not to damage the rod while trying to loosen the piston nut & be mindful of the threads on the head gland.

    Another option might be to replace the cylinders with some reasonably priced standard hydraulic cylinders and modify them to fit your loader.

  14. Hi, I have an old ram type hydraulic pipe bender which we changed the oil seal due to constant leakage, but since then it has refused to hold pressure again and consequently, dose not bend what it’s supposed to bend again.what do you think is the problem ?

  15. Adam, it may need a new piston seal. It is possible that the seal was nicked when it was taken apart, or just worn out. Usually if a cylinder won’t hold pressure, it’s a seal issue.

  16. It is said that a cylinder (positioned vertical with a load acting on top from the annulus side) will not drift if piston seal is leaking inside, provided that there is no external leak from the ports and rod-seal, and the DCV is zero-leak. They say it will drift only a small amount until pressure on both sides equalize. What do you think about this.
    What is the cylinder is installed with load hanging down on annulus side?
    Best regards

  17. The hydraulic cylinder will drift initially, but hard to say what a “small amount” of drifting is. The reason the pressure would eventually equalize is because there is more fluid on the base end of the cylinder than on the rod end. So given that there is no other place for the fluid to go, the volume differential would cause the pressure to rise on the rod end until equalization. If the reverse is true, and the load is pulling away from the base end, the pressure would never equalize, as the fluid essentially has an open reservoir to escape to, since the rod end holds less fluid than the base.

  18. This is very useful post about the working of Hydraulic cylinders. The best thing about these cylinders is that they are easy to convert to single-acting by using a simple breather device.

  19. We need something to move a steel pipe about 30 ft long in and out through another larger diameter pipe. I wished to know if any hydraulic rams can be used to move 30 ft of pipe of diameter lets say 2 -in. Any assistance would be appreciated.

  20. We have a situation where we have hydraulic cylinders actuated lid to open and close the lid. However, in close position or as we call “park up position” we have rods exposed while rod retracts back in cylinders when we want to open the lid. We need to open the lid intermittently and this leaves rod exposed most of the time.
    I’m wondering what sort of covering protection can I get for the rods to avoid dust, corrosion etc. damaging them when exposed.

  21. if i gonna modify from a manual bottle car jack to become an electric hydraulic jack, what do i need to pump the fluid? please help me for the school work.

  22. i am having a 100ton hydraulic rubber bale pressing machine. it is a dual pump system. a high pressure pump and a low pressure pump. it is suppose for the low pressure pump to works until achieving 500psi, the the activating the high pressure pump until achieving 3000psi. but the machine just able achieving 700psi. what has gone wrong?

  23. If it’s an older system, it could be that the hydraulic cylinder is internally worn and the fluid is bypassing the seals. It could also be that the hydraulic unloading valve is malfunctioning preventing the switch to high pressure.

  24. I have an old ford 4500 with a backhoe and front loader the cylinders in the front that roll the bucket have been rebuilt but they will not keep the bucket up what could this be?

  25. hi ihave aproblem with my excavator .every things working well eccept boom lifting ..when itry to lift the boom itdoesnt respons with abig sound in the main hydraulic pump…ichecked every thing but still didnt find the problem…pls ineed ur help

  26. my backhoe front bucket,2 cylinders one leaked ,had it repaired,now. it does not lower. I try to lower it goes up,push up goes up and down,will not function on the up and down lever,do not know what it will do,push up or down goes in reverse

  27. my john Deere loader bucket is in the down position and cant lift it cause the starter is out can I lift the bucket up high with a crane to open the side motor screens doors to get in?

  28. You should be able to do that if you hold the valve in the up position while you try to lift it. Or disconnect the hoses (careful, with the fluid)

  29. I have a hydraulic log splitter with a 5 horse Brigs turning the pump. Ram works great on the out stroke but when on the return stroke it bogs the engine and chokes it out? I’m scratching my head. Any idea the problem?

  30. Can somebody find a solution….A single rod cylinder with piston having area ratio 2:1 on the free end ,rod end respectively if supplied with a hydraulic oil pump pressure simultaneously with no loads connected …first of all will it move…extend or retract…at what speed?

  31. if i know only the weight of an object & distance to be moved

    what is the formula to design a hydraulic cylender and
    what is the force and time to taken to travle please
    healp me to design a hydraulic cylender for my project


  32. I have recently acquired an old Hydromac skidsteer that has a slightly smaller [1/8″] ram on one side though different style end caps both are the same size on the outside. Could this have been an appropriate replacement? Both cylinders seem to be in good condition with no leaks so could I install an adjustable inline pressure regulator on the larger side to compensate for the smaller size?

  33. I’m glad you talked about how it is pretty easy to convert a double acting cylinder to a single acting type. I have been looking for different cylinders, but I need one that is single acting. It’s good to know that you can change them with a breather device.

  34. Thanks! Also, depending on the application, make sure the seals will work ok in a single-acting setup. If it has simple “o-ring & backup seals” you might have trouble with it not wanting to come down.