Repairing hydraulic cylinders may look like a reasonably simple task from the outside looking in but without the right tools and techniques, something as simple as repairing a hydraulic ram can become a nightmare.
At FluidKraft, we have seen and worked on just about every type of cylinder on the market, everything from small 4 in 1 Bucket cylinders to Skip Bin truck cylinders we have seen it all and have learnt a trick or two about quick and efficient repairs.
We decided to share some of the knowledge we gathered over the last decade and answer some of the most common questions we come across when people attempt to repair hydraulic rams themselves.
Be sure to read on and learn how to repair cylinders like the pros do.
What Causes a Hydraulic Cylinder to Leak Down?
Also known as cylinder bypass, this is can be caused by a number of different factors. Firstly, your hydraulic cylinder piston seals could just simply be old and over time start to reduce their effectiveness and wear to the point where there is not enough preload on the hydraulic seals to maintain a 100% positive seal.
Another cause of cylinder bypass could be internal damage to the seals, piston, barrel or other internal components.
This can be caused by many different factors, the most common ones we come across are, Piston nut loosening to the point of coming away from rod, wear strip and bearings could be worn and causing a metal on metal situation (not ideal in most cases), Dirty hydraulic oil, another hydraulic component failing and metal particles flowing into cylinder and casing damage to cylinder internals, worn/out of round cylinder barrel.
If the hydraulic cylinder barrel becomes scored, bypass of the seals will almost certainly occur, similar if there is damage to the piston.
Sometimes we see hydraulic cylinder bypass caused by barrel ‘bulging’ in the centre and therefore creating a larger diameter in the centre of the barrel only allowing oil to travel around the piston and not hold the load required in that section of stroke.
If the cylinder has over centre (also known as; load holding, crane valves, anti-drop) valves these may not be adjusted right. We strongly recommend that if you suspect this that you engage a competent person to check and adjust them, if not done correctly damage to the cylinder, machine parts and/or personal injury could occur.
How Do I Know if My Hydraulic Cylinder is Leaking?
The first and most simple way is to clean down the cylinder very well and look for any possible signs that something could be abnormal. External hydraulic oil leaks, dings/damage to the barrel OD that could be affecting the cylinder barrel ID, damage to the rod, bent, scored, broken or cracked weld around clevis/pin eye. These are all telltale signs that further investigation is required.
Any of the above usually means removing the cylinder from the machine and stripping down for inspection. It is very important that all the hydraulic cylinder internal components are thoroughly checked and measured as a diameter difference of only 0.2mm can cause grief.
There are also ways of checking to see if a cylinder is bypassing on the machine before deciding to remove the cylinder.
How Do You Stop a Hydraulic Cylinder From Leaking?
This depends on a lot of factors, if we keep it straight forward and say that all the parts of the hydraulic cylinder are in serviceable condition then a simple hydraulic seal replacement should do the job.
You can source seals from the OEM (Genuine Seal kit) of aftermarket seals can be sourced from various different wholesalers around the world.
In most cases, you will always need to remove the cylinder from service, strip completely, clean and measure so you can rebuild it. This should stop a hydraulic cylinder from leaking if all the parts are in serviceable condition.
How Do You Remove Air From a Hydraulic Cylinder?
Depending on the design and the type of hydraulic system this can be answered in a number of ways. If we choose a standard type double-acting cylinder, oil in bottom for extend, oil in the top rod side for retract, these cylinders whether they be in a closed or open loop hydraulic system should be able to self-bleed air as the return flow and air goes back to the hydraulic tank it will breathe to the atmosphere. In some cases, like if the hoses or pipework to the cylinder is extremely long or full stroke of the cylinder is not possible you could end up with air inside the cylinder. Air is not ideal in hydraulic systems and care should always be taken to minimise this.
In all cases, if you suspect there to be air in the hydraulic system please take some time to learn how your system operates and how it was designed before jumping in with spanners. Safety is paramount in this type of hydraulic maintenance and bleeding cylinders can be very dangerous to an untrained person.
How Do You Safely Remove a Damaged Hydraulic Cylinder?
First things first, SAFETY. Take time to inspect the machine and study the area in which you need to work. Understand the hydraulic system you are working on to a level where you know without question if there is any stored energy present (accumulators, heavy load on cylinders, machine parked on a hill etc), any danger that could hurt yourself or any other person.
Ensure the machine you are working on is tagged out and isolated so others cannot come and start the machine while you are working on it.
Once you have the machine locked out and you are certain there is no stored energy left in the lines you will need to then remove both ends that fix the cylinder to the piece of hydraulic equipment. Handy Hint; Be sure to remove the hoses/pipe work and plug both the cylinder ports and the hoses/pipework first..
Be sure that there is no pressure in the cylinder. If the cylinder clevis pins are tight to move it could be due to the fact that the load is still acting on the cylinder. It can also mean your clevis pins are seized and could require engineering assistance to remove.
Removing a cylinder from a hydraulic system can be a messy job, ensure you contain any hydraulic oil spill that could occur. Plug and cap both the hydraulic hoses and/or pipework along with the ports on the cylinder to ensure no oil is spilt in transit.
Once both the rod end and base end hydraulic pins have been removed the cylinder can now be removed.
Please note, in between steps may be required depending on size, mounting, location on the machine and application of the cylinder
How to Disassemble a Hydraulic Cylinder?
Take a moment to inspect the hydraulic ram and obtain a good understanding of how the cylinder has been assembled.
If we pick a main boom cylinder to suit a CAT 301.7D. This cylinder is quite small and can be easily removed from the boom assembly by one person.
If this same cylinder is from a CAT 335F the handling and logistics just from the physical size of the cylinder need to be considered and can be a major part of the cylinder repair process
This is where very specialised equipment is required for the stripping and assembling stages. Most times these larger type cylinders need to be loaded on a hydraulic strip bench so the rod, gland and piston can be pulled apart with hydraulic force. In these cases, the piston nut will require slow and controlled hydraulic force to be removed and reinstalled safely.
The first step is ensuring you have the means to be able to hold the cylinder barrel safely. Glands can be retained to the cylinder barrel in a number of different ways, the threaded external barrel, threaded internal barrel, bolted to the barrel, retaining wire and circlip to name a few.
Occasionally we come across glands that are welded and need to be machined off. Most of the welded type glands render the cylinder BER (Beyond Economical Repair)
Depending on the gland type you may need a socket set, in hex drive, C-Spanner, Pipe chain wrench or a just simply and pipe wrench. The latter options are not favourable due to the fact that chain wrench’s and pipe wrenches can easily damage cylinder components due to their design if delicate care is not taken.
Once the gland has been removed from the barrel the rod will need to be secured either in a vice or specialised bench clamp so the piston nut can be removed. Care is required so as not to damage the chrome rod surface as damaging this surface can cause damage and premature failure of seals.
To remove the seals from the piston and the gland a set of seal picks is the best option. They come in a variety of shapes and styles and make light work of stiff, brittle and old hydraulic seals.
Some cylinders are very large to work on a standard type bench and require the mechanical advantage of a hydraulic strip bench.
Modern-day hydraulic strip benches incorporate gland removal, piston nut removal, hydraulic disassembly and assembly of the rod into the barrel, hydraulic nut tightening and hydraulic nut loosening, testing and reporting all from the one machine
How Do You Remove A Hydraulic Cylinder Piston Nut?
After you have removed the rod assembly from the barrel you will most definitely need to remove the piston nut or piston (if the piston is threaded and retains itself). Pistons can be retained by either a nut, threaded piston nut (the piston is threaded and retains itself), roll pin, Clip, circlip or welded direct to rod (not removable)
A simple socket set or spanner will quickly loosen standard nuts along with Pin punches, ball-peen hammer, circlip pliers. Stubborn or hard to remove piston nuts can be carefully heated to a measured temperature to release any Loctite or rust that is not assisting with the removal process.
Careful consideration must be taken we removing glands and piston as more often than not there may be a locking safety type design incorporated to assist the thread from loosening or coming off the rod. These locking designs must be removed prior to any type of nut, thread removal technique. They could be in the form of a scotch key, grub screw, set screw with ball, LOCTITE or similar and steel ring tab just to name a few.
The safest way to remove large hydraulic cylinder piston/piston nuts from a rod assembly is by the use of a hydraulic disassembly bench that incorporates a hydraulic nut cracker. These are purpose-built and designed for hydraulic repair workshops that professionally rebuild hydraulic cylinders daily.
If it’s anything other than a nut that retains the piston onto the rod there may be a need to use a custom tool or spanner to loosen. At times we need to manufacture/fabricate tools to be able to adapt to our strip bench.
How Do You Repair Hydraulic Cylinder Scratches?
At times cylinder rods can get damaged by external objects as the chrome rod extends out of the barrel on extension and this can leave it prone to damage.
Depending on how bad the damage is, we can repair it in 3 different ways, we can polish if they are not too deep and the chrome is still intact, we can strip off the old chrome and electroplate (re-chrome) the damaged rod or we can manufacture a totally new rod from scratch.
If the barrel of the cylinder is scratched, scored, marked then honing of the barrel should always be considered after inspection of the surface in the barrel. If the cylinder barrel has any defects, dings, scores, wear marks/shiny spots these areas need to be carefully investigated and honed to clean up.
The most standard and universal type piston seal designs usually allow for a barrel that is 0.305mm over size on diameter. If the barrel damage is excessive there are a couple of options, manufacture, and machine up another barrel on a lathe and use as much of existing parts off the old barrel as possible or purchase another barrel from an OEM (Original Equipment Manufacturer).
You want to hone the barrel oversize until it cleans up and manufacture a new oversized piston. This last option might sound unconventional but it is extremely time and cost-efficient. Most seal designs can be stretched to within a specified limit.
Honing of hydraulic cylinder barrels can be done by either manual hand honing or on a specialised hydraulic cylinder hone machine with the appropriate tooling and stones. Specialised hydraulic cylinder hones are large and require a fair level of operator knowledge the get the best performance and surface finish on the cylinder barrel.
How Do You Repack a Hydraulic Cylinder?
To be able to reseal a cylinder most of the time it will need to be stripped down and the seals replaced. The main sealing of a hydraulic cylinder rod to the barrel is by either a v-pack, o ring, standard u-cup or energized u-cup seal.
There are also piston seals, wear rings, o rings and backups, glide bushes, buffer seals and other seals depending on the application, size, oil type, oil temperature and working conditions of the cylinder. Hydraulic seals can also be engineered and manufactured out of various different materials to suit specific applications.
Things to Consider When Reassembling a Hydraulic Cylinder?
- The main point to consider is the safety of yourself and the people around you. Hydraulic cylinders can be an energy source and with modern hydraulic systems incorporating load holding (over centre) valves and hydraulic accumulators quite commonly, actuators (cylinders, motors, final drives) can become an outlet for stored energy under the right conditions.
- Attempting to remove a load-holding valve on a cylinder that has not been removed correctly can cause the oil remaining inside the cylinder to pressurize. Removing the valve under pressure can cause serious personal injury and/or damage to cylinder components.
- If you are unsure about any of the above, I strongly suggest you request the service of a skilled and competent person to undertake your repair
How Do You Fill a Hydraulic Cylinder?
The system design and cylinder type will determine if you need to bleed and/or pre-fill the hydraulic cylinder or not.
An open-loop hydraulic system generally allows air to return back into the hydraulic reservoir and breath out through the breather cap during a standard test operation. A few operations of complete extension and retraction of the hydraulic cylinder should completely remove the air from the cylinder and lines.
Keep in mind If there is a really long run of hose/tube the air may not make it back to the hydraulic reservoir and may need to be manually exhausted from the system.
With a single-acting type ram (Hoists) like that found on a truck tipper and dog tipper trailers, these only have one port for the oil to enter and flow back to the hydraulic tank.
Air can get trapped inside and cause staggered operation or jumpy and jerky raising and lowering. This is due to atmospheric air we breathe being compressible. (hydraulic oil is not compressible)
These hoists usually have a bleed nipple at the top of the cylinder to assist with the air bleeding process.
How Do You Test a Hydraulic Cylinder?
There a few situations where you would want to test a hydraulic cylinder
One, if the cylinder has just been rebuilt and before installing onto the machine. Testing here could save extra labour and downtime if the cylinder is still leaking or causing concern.
This can be done by adapting to a hydraulic system and creating conditions similar to or the same as the cylinder was designed for. This is what we call bench testing a cylinder.
If you suspect the cylinder to be bypassing, dropping, leaking down then you can test this by installing gauges on both sides of the cylinder ports and looking for a pressure drop of the working port.
If the cylinder is bypassing internally the oil will be able to bypass around the piston and travel back to the hydraulic tank. This would cause the system to not be able to reach the maximum pressure on that circuit. This is called on machine testing.
Testing for bypass on a bench can be done by fully extending the cylinder, turn off the pump and then remove the retract hose. Once the retract hose has been removed turn on the pump and operate the valve to extend the cylinder. Look for oil bypassing the piston and coming out of the retract port.
Once you have done this to can repeat this on the opposite port to check for retraction bypass.
It is also important to inspect mid-stroke as this is generally where the cylinder will be doing most of its work. Extend the cylinder to halfway ensuring both sides of the cylinder are full with oil. Remove the extension hose and cap off with a high-pressure plug.
Apply hydraulic pressure to the retract hose and if the cylinder rod is bypassing the rod will try to extend. This is due to the effective working area on the extend side of the piston being larger than the rod side. Equal hydraulic pressure in the system with the piston seals bypassing will cause the cylinder rod to extend.
You may just have a leak that you are trying to locate and need to test to find it. Clean the cylinder thoroughly and operate to the maximum system pressure that the cylinder was designed for. Inspect all areas of the cylinder. Cylinder ports, gland seals, rod seals, all welds on the barrel, along the barrel.
Some leaks can be very small and can be difficult to locate so be very diligent in your inspection so as not to overlook anything.
(Take care not to run any part of your hand or body around any hoses, adaptors or cylinders while you are operating or testing as hydraulic fluid injection can occur if there is a leak.
This is caused by oil travelling at high velocity which can pierce through human skin and enter your bloodstream. This can cause serious injury and/or death so please take extra care when testing for leaks on any type of hydraulic component.
If you are unsure about any of the above recommendations or procedures, I strongly suggest you request the service of a skilled and competent person to undertake your repair)
Related questions
Will a Hydraulic Cylinder Work With Air?
In a word yes you may be able to get the cylinder to move but the big advantage that hydraulic oil has over air is the working pressures and also the medium used to convey. Compressed air is just that, atmospheric air that has been compressed into a confined space and stored and directed for use.
It can be compressed and therefore only effective up to a certain pressure. Compressed air also has a high content of water present and this is not good for metal components (corrosion)
Hydraulic oil, on the other hand, is not compressible therefore becomes a good medium to use for the transfer of energy. Hydraulic oil also acts as lubrication for metal components.
The components used for pneumatic (Air) system and a hydraulic (Oil) system are very different in there construction and design, therefore as a general rule cannot be interchanged for use on either system.
What is the Main Difference Between Pneumatic Cylinders and Hydraulic Cylinders?
The main difference here is the working pressure of the two systems. Pneumatics generally run in the pressure range of 100-150 psi whereas a hydraulic system can run up to 10,000psi
Obviously, the construction of pneumatic cylinders and hydraulic cylinders is vastly different. If you take a close look at a pneumatic cylinder and of its construction, you will notice the material is usually aluminium.
Aluminium has good anti-corrosion properties but its tensile strength is very poor. The same piece of aluminium in a steel construction would be able to handle a lot higher pressure but would be susceptible to corrosion from any water or atmospheric conditions.