A hydraulic cylinder is the actuator or ‘motor’ side of a hydraulic system – they are sometimes referred to as the ‘muscles’; making light work of lifting, lowering, moving or ‘locking’ heavy loads.
The ‘generator’ side of the system is the pump which brings in a fixed or regulated flow of oil to the bottom side of the cylinder – to move the piston rod upwards. Hydraulic cylinders transform the pressure and oil flow in a hydraulic system into work or mechanical force. They are used where linear motion is required to move something.
Also known as ‘hydraulic jacks’, ‘hydraulic rams’ or ‘actuators’, they convert fluid power into mechanical energy. A hydraulic cylinder differs from a hydraulic motor as it carries out a linear (translatory) rather than rotary movement, hence the term “linear motor”.
Used at high pressures, hydraulic cylinders produce large forces and precise movement; they are therefore constructed of strong materials, such as steel that is capable of withstanding the large forces involved.
There are primarily two styles of hydraulic cylinder construction used in industry: tie rod and welded body cylinders. Beyond this, other broad types of cylinder design include: telescopic, plunger, differential, re-phasing and single and double-acting hydraulic cylinders.
Hydraulic cylinders are usually double-acting: oil under pressure can be applied to either side of the piston to provide movement in either direction. Single-acting cylinders are sometimes used where the weight of the load is used to return the cylinder to the closed position.
Hydraulic cylinders enable more flexibility in design and structure when transferring force between two different points. Different sized cylinders make it possible to create a system that can pull, push and lift weights; bends and corners can be incorporated into the system design – useful if there are real space constraints.
However, a hydraulic cylinder should only be used for linear pushing and pulling. No bending moments or side loads should be transmitted to the piston rod or the cylinder. For this reason, a cylinder should ideally be connected by using a single clevis with a spherical ball bearing. This allows the cylinder to move and allow for any misalignment between it and the load it is pushing.
Rephasing Hydraulic Cylinders
Two or more cylinders can be installed in ‘parallel’ or ‘series’ – with the bores and rods sized such that all rods extend and/or retract equally when flow is directed to the first, or last, cylinder within the hydraulic system. This is known as ‘rephasing’ hydraulic cylinders.
In ‘parallel’ applications, the bore and rod sizes are always the same, and the hydraulic cylinders are always used in pairs.
In ‘series’ applications, the bore and rod sizes are always different, and two or more hydraulic cylinders may be used. In these applications, the bores and rods are sized such that all rods extend or retract equally when flow is applied to the first or last cylinder within the system. This hydraulic synchronisation of rod positions eliminates the need for a flow divider in the hydraulic system, or any type of mechanical connection between the cylinder rods.
Cylinders come in many forms; we can find the one best suited to your needs.
Telescopic Hydraulic Cylinder
The length of a hydraulic cylinder is the total of the stroke, the thickness of the piston, the thickness of bottom and head and the length of the connections. Often this length does not fit in the machine. In that case the piston rod is also used as a piston barrel and a second piston rod is used.
These kind of hydraulic cylinders are called telescopic cylinders. If we call a normal rod cylinder single stage, telescopic cylinders are multi-stage units of 2, 3, 4, 5 and even 6 stages. In general telescopic hydraulic cylinders are much more expensive than normal cylinders. Most telescopic cylinders are single-acting (push). Double-acting telescopic cylinders must be specially designed and manufactured. You can find more technical information in the next section below: ‘HOW CYLINDERS WORK’.
Tie Rod Style Hydraulic Cylinder
Tie rod style hydraulic cylinders use high strength threaded steel rods to hold the two end caps to the cylinder barrel. This method of construction is most often seen in industrial factory applications. Small bore cylinders usually have 4 tie rods, while large bore cylinders may require as many as 16 or 20 tie rods in order to retain the end caps under the tremendous forces produced.
The National Fluid Power Association (NFPA) has standardised the dimensions of hydraulic tie rod cylinders. This enables cylinders from different manufacturers to interchange within the same mountings. Tie rod style cylinders can be completely disassembled for service and repair.
We can provide you with standard and custom-made hydraulic cylinders, including tie rod hydraulic cylinders, to suit your specific application, requirements and operating parameters; just complete our Bespoke Cylinder Design Form and one of our hydraulic engineers will get back to you promptly.
Welded Body Hydraulic Cylinder
Welded body hydraulic cylinders have no tie rods. The barrel is welded directly to the end caps; the ports are welded to the barrel; the front rod gland is then usually threaded into or bolted to the cylinder barrel. This allows the piston rod assembly and the rod seals to be removed for service.
Welded body cylinders have a number of advantages over tie rod style cylinders: welded cylinders have a narrower body and often a shorter overall length enabling them to fit better into the tight confines of machinery. Welded cylinders do not suffer from failure due to tie rod stretch at high pressures and long strokes. The welded design also lends itself to customisation. Special features are easily added to the cylinder body; these may include special ports, custom mounts, valve manifolds, and so on. The smooth outer body of welded cylinders also enables the design of multi-stage telescopic cylinders.
Welded body hydraulic cylinders dominate the mobile hydraulic equipment market in applications such as construction equipment (including excavators, bulldozers) and material handling equipment (fork lift trucks and tail lift gates). They are also used in heavy industry such as cranes, oil rigs, and large off road vehicles in above ground mining.
Piston Rod Construction: The piston rod of a hydraulic cylinder operates both inside and outside the barrel, and consequently both in and out of the hydraulic fluid and surrounding atmosphere.
Metallic Coatings: Smooth and hard surfaces are desirable on the outer diameter of the piston rod and slide rings for proper sealing. Corrosion resistance is also advantageous. A chromium layer may often be applied on the outer surfaces of these parts. However, chromium layers may be porous – attracting moisture and eventually causing oxidation.
In harsh marine environments, the steel is often treated with both a nickel layer and a chromium layer – often 40 to 150 micrometre-thick layers are applied. Sometimes solid stainless steel rods are used. High quality stainless steel such as AISI 316 may be used for low stress applications. Other stainless steels such as AISI 431 may also be used where there are higher stresses, but lower corrosion concerns.
Ceramic Coatings: Due to shortcomings of metallic materials, ceramic coatings were developed. Initially ceramic protection schemes seemed ideal, but porosity was higher than projected. Recently the corrosion resistant semi-ceramic Lunac 2+ coatings were introduced. These hard coatings are non-porous and do not suffer from high brittleness.
Lengths: Piston rods are generally available in lengths which are cut to suit the application. As the common rods have a soft or mild steel core, their ends can be welded or machined for a screw thread.
Hydraulic cylinders, also known as ‘hydraulic rams’, get their power from pressurised hydraulic fluid, normally hydraulic oil…
The hydraulic cylinder consists of a cylinder barrel, in which a piston connected to a piston rod moves back and forth. The barrel is closed on each end by the cylinder bottom – also called the cap end – and by the cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber, cap end, and the piston rod side chamber, ‘rod end’. The hydraulic pressure acts on the piston to do linear work and motion.
Flanges, trunnions, and/or clevises are mounted to the cylinder body. The piston rod also has mounting attachments to connect the hydraulic cylinder to the object or machine component that it is pushing.
A hydraulic cylinder is the actuator or ‘motor’ side of the 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. The piston pushes the hydraulic oil in the other chamber back to the reservoir. If we assume that the oil pressure in the piston rod chamber is approximately zero, the force on the piston rod equals the pressure in the hydraulic cylinder times the piston area (F=PA).
The piston moves downwards if oil is pumped into the piston rod side chamber and the hydraulic oil from the piston area flows back to the reservoir without pressure. The pressure in the piston rod area chamber is (Pull Force) / (Piston Area – Piston Rod Area).
Parts of a Hydraulic Cylinder
A hydraulic cylinder consists of the following parts:
Cylinder Barrel: The cylinder barrel is mostly a seamless thick-walled forged pipe that must be machined internally. The cylinder barrel is ground and/or honed internally.
Cylinder Bottom or Cap: In most hydraulic cylinders, the barrel and the bottom are welded together; this can damage the inside of the barrel if done poorly. Therefore, some hydraulic cylinder designs have a screwed or flanged connection from the cylinder end cap to the barrel. In this type the cylinder barrel can be disassembled and repaired in future.
Cylinder Head: The cylinder head is sometimes connected to the barrel with a sort of a simple lock (for simple cylinders). In general however, the connection is screwed or flanged. Flange connections are the best, but also the most expensive. A flange has to be welded to the pipe before machining. The advantage is that the connection is bolted and always simple to remove. For larger hydraulic cylinder sizes, the disconnection of a screw with a diameter of 300 to 600 mm is a huge problem as well as the alignment during mounting.
Piston: The piston is a short, cylinder-shaped metal component that separates the two sides of the cylinder barrel internally. The piston is usually machined with grooves to fit elastomeric or metal seals. These seals are often O-rings, U-cups or cast iron rings. They prevent the pressurised hydraulic oil from passing by the piston to the chamber on the opposite side. This difference in pressure between the two sides of the piston causes the cylinder to extend and retract. Piston seals vary in design and material according to the pressure and temperature requirements that the hydraulic cylinder will see in service. Generally speaking, elastomeric seals, made from nitrile rubber or other materials, are best in lower temperature environments while seals made of viton are better for higher temperatures. The best seals for high temperature are cast iron piston rings.
Piston Rod: The piston rod is typically a hard, chrome-plated piece of cold-rolled steel which attaches to the piston and extends from the cylinder through the rod-end head. In double rod-end hydraulic cylinders, the actuator has a rod extending from both sides of the piston and out both ends of the barrel. The piston rod connects the hydraulic actuator to the machine component doing the work. This connection can be in the form of a machine thread or a mounting attachment such as a rod-clevis or rod-eye. These mounting attachments can be threaded or welded to the piston rod or, sometimes, they are a machined part of the rod-end.
Rod Gland: The hydraulic cylinder head is fitted with seals to prevent the pressurised hydraulic oil from leaking past the interface between the rod and the head. This area is called the rod gland. It often has another seal called a rod wiper which prevents contaminants from entering the hydraulic cylinder when the extended rod retracts back into the cylinder. The rod gland also has a rod bearing. This bearing supports the weight of the piston rod and guides it as it passes back and forth through the rod gland. In some cases, especially in small hydraulic cylinders, the rod gland and the rod bearing are made from a single integral machined part.
A hydraulic cylinder should be used for pushing and pulling only. No bending moments or side loads should be transmitted to the piston rod or the cylinder. For this reason, the ideal connection of a hydraulic cylinder is a single clevis with a spherical ball bearing. This allows the hydraulic actuator to move and allow for any misalignment between the actuator and the load it is pushing.
Regular hydraulic cylinder maintenance is crucial to the upkeep and performance of your system.
1. First things first – keep your hydraulic oil clean
Many hydraulic failures occur because of hydraulic oil contamination. This is important to prevent damage to your system and components. If hydraulic oil is contaminated, it must be thoroughly cleaned as soon as any contamination has been found. It is also best to fit high-efficiency filters and change them when necessary.
If a filter has a build-up of debris, a differential pressure gauge can be installed to give you a good indication as to when they need to be changed. This means that you will be preventing a bypass, where the oil flows through the system unfiltered.
2. Undertake regular maintenance
Regular maintenance on your hydraulic cylinder is important for optimal performance. Make sure you keep a close eye on the condition of the cylinder rod for any signs of corrosion, pitting or wear. Too much moisture can cause corrosion; whether this is from the air or from within the hydraulic fluid itself. If water contamination occurs within the fluid, it can cause major failure of the component or system – the oil therefore needs to be thoroughly cleaned as soon as possible. Corrosion also encourages wearing of the seals, due to increased friction; pitting causes a similar problem.
Misalignment is often the main cause of uneven wear within a cylinder. Premature wearing of the bearing seals and rod can be caused by side loads allowing the rod to rub on one side of the bearing. Hydraulic cylinder maintenance here can often be as simple as a re-chrome or polish to repair a corroded or damaged rod. If a cylinder rod is beyond repair, a replacement is the best course of action. However, before you re-install the new rod, it is important to first remove the problem that originally caused the damage and make the new cylinder more cost-effective.
3. Rotate or alternate cylinders
As with other hydraulic components, if the risk of downtime is not an option, it is always a good idea to hold another cylinder(s) either as a ‘critical spare’ (see below) or that you can alternate usage between regularly. This break between uses keeps the cylinders in good shape, despite possible particle contamination and harsh operating conditions. It is also wise to undertake maintenance on these cylinders when they are removed from the system, so that they can be repaired if necessary.
Before re-assembling a cylinder, you may want to first replace all of the seals. Inspection of the cylinders may also give you an idea as to the condition of the entire system. For example, if there is a consistent varnish throughout your system, it could mean that the oil temperature is too high and it needs to be altered.
4. Maintain the accessories of your cylinder
Each separate part that makes up your hydraulic cylinder is just as important as the cylinder itself. For example, if a pivot pin or clevis is worn or damaged, this could cause slop and play within the joints of the cylinder. This leads to misalignment and wear or more permanent damage.
*Critical spares analysis – an analysis of system critical hydraulic components and recommended spares; so that you can understand which of your hydraulic components are most susceptible to failure and the impact that this may have on safety, production and systems performance… and just as importantly the cost and lead times involved in sourcing replacements.
Hydraulic cylinder testing is crucial in order to maintain optimal performance.
In testing, measurements of any leakage need to be taken and to do this, the typical method is to pressurise the cylinder at the end of stroke. This hydraulic cylinder testing technique is known as the ‘end-of-stroke bypass test’.
The only major disadvantage to this test is that, as the name suggests, the only seals tested are the ones at either end of the cylinder – where there isn’t a lot of deterioration of the component. Realistically, the centre of the tube is most likely to be prone to more damage, therefore that’s where the test needs to be performed.
In a double-acting cylinder, a ‘mid-stroke bypass test’ is a suitable method to test the piston seals. One of the issues with this technique is how to control the cylinder; a hydraulic engineer has to hold the component mechanically to keep the cylinder in the centre. Even a smaller sized cylinder can generate 6 to 10 tonnes of force, so this method is very dangerous. With larger cylinders, this task is impossible.
The only test that is appropriate for this is something called ‘hydrostatic testing’. This is a test that is performed after the piston has been held hydraulically in any position that is required. The pressure intensification that is created helps to keep the piston in one position along the entire cylinder to make sure that all the seals are functional and there is no leakage. The only consequence this might have is if the pressure is allowed to build up at the piston end of the cylinder with the valve closed, the pressure at the rod end will to too great, which could cause disastrous failure.
In this case, a pressure-relief valve must be used in the rod end when carrying out the hydrostatic test. This is vitally important for not only the protection of the machinery but also for the personal safety of the engineer carrying out the test. To perform this kind of hydraulic cylinder testing, the following steps must be carried out:
- Ensure the cylinder is secured so that it is controlled;
- Clean hydraulic fluid must fill both sides of the cylinder – do this through the service ports;
- Connect all components – directional control valve, relief valve, ball valves and gauges;
- Remove any air from the cylinder by using the directional control valve and stroking the cylinder numerous times;
- Place the piston rod mid-stroke and close the ball valve;
- Direct the fluid flow to the side of the cylinder where the rod is located;
- Increase the setting on the relief valve until the pressure of the cylinder is seen on the gauge;
- Close both the ball valve and the directional control valve;
- Record the pressure of the cylinder from on both gauges and keep an eye on any changes.
Our hydraulic cylinder repair service.
A hydraulic cylinder repair is a relatively simple process, as they are less complex than other hydraulic components; in many cases a well-repaired cylinder is indistinguishable from a brand new unit. We offer a hydraulic cylinder repair service, in addition to repairs on pumps, motors, power packs and other types of hydraulic equipment.
Internal and external leaking is the most common reason for repairs on hydraulic cylinders. But, by disassembling and closely inspecting the damage to the cylinder, you may be able to see other problems that aren’t immediately obvious.
Our service includes a free assessment, where we will strip down your faulty unit and diagnose the full extent of work needed – providing a quotation and details of how many parts can be salvaged. We may conclude that a repair is not viable, or that your repaired equipment may be unreliable or prone to poor performance in future. If this is the case then we will always contact you to explain why and to quote for a replacement new unit so that you can choose what you want to do.
When assessing your equipment our team may also be able to provide recommendations on how to improve your system’s design and efficiency, allowing you to reduce running costs and minimise the likelihood of any future downtime and hydraulic repair work. We can also undertake a “critical spares analysis” identifying those key system components that we recommend you hold as spares.
There are different types of hydraulic cylinder repair:
Piston seal: the piston seal can become defective if it is distorted, eroded or missing and this is usually a cause of an oversized barrel or bulge during operation. At this point, replacement of the barrel or cylinder is required. If you replace the piston seal only and not the barrel, whilst this is a quicker method of repair, it is a short-term fix. More maintenance would be required later on.
Rod seal: A distorted rod seal can be caused by excessive wear of the guide bush or if the rod is bent. The consequence of this is the rod weight pushing on the seal, causing failure. If you replace the rod seal without identifying and correcting the underlying issues, then again, the fix will only be short-term.
Rod: In most cases, the chrome on the rod can be inspected and if it is shiny on one side and dull on the other, the rod is bent. The straightness of the rod should always be checked when repairing the cylinder. Bent rods can be corrected by using a press. Whilst it is possible to prevent damage to the hard-chrome plating during this process, if the chrome is spoiled, it must either be re-chromed or the entire cylinder must be replaced. A damaged chrome surface decreases the efficiency and service life of the rod seals. If the rod only has some minor scratches, they can be polished out using fine emery paper.
Piston: in light-duty applications, pistons within the cylinders are designed with aluminium or cast iron, and are in constant direct contact with the cylinder bore whilst in operation. Some scratching on the outer surface of the piston poses no risk to the cylinder’s function, as long as the nominal bore diameter does not exceed the minimum diameter of the piston. A good tool to use to check the length could be a micrometer.
Taking the necessary precautions for safe hydraulic cylinder storage.
Prevention is better than cure. Fact. So here are some tips to help ensure that your hydraulic cylinder storage procedures are up to scratch:
When you store your cylinders, keep them in a clean, dry area, and preferably indoors wherever possible – even if you think your cylinder is too large or long for indoor storage. Really try to find or create the space somewhere – it will be worth it in the long run!
Keep all of the exposed metal covered and protected. The internal surfaces of the cylinder can be smothered with grease, while a special tape, that has been infused with oil, should be wrapped around the exposed chrome on the rod. The rod must be retracted before the tape is applied though, if not the tape could damage the rod seal.
If there is no oil in the cylinder, it will most probably have been replaced by air. Problems then arise if the air isn’t dry and the environment temperature decreases and reaches dew point, causing moisture to form inside the cylinder. This, in turn, could lead to rust and pitting, which can cause serious issues if the cylinder is in operation. Not only does it decrease the effectiveness of the cylinder, but it can also significantly reduce the life of the cylinder.
If you thoroughly clean the inside of the cylinder and then fill it with clean oil, this problem can be avoided, but care must be taken due to the ambient temperature. This depends on when the cylinder is put into storage and then taken out again to be installed in a system. For example, if your hydraulic cylinder storage takes place over winter, and it is then taken out again to be reinstalled in the summer, the ambient temperature of the oil in the cylinder is going to change. Oil can be filtered into a cylinder during storage if you take the following precautions:
- Ensure that the temperature changes within the environment won’t have the outcome of static pressure that surpasses the operating pressure of the cylinder;
- Only put oil in the cylinder when it has been fully retracted and through the rod-port. By doing it this way, you avoid a hazardous pressure increase;
- Service port plugs or blanks are good to use and they are rated for the operating pressure of the cylinder;
- A warning needs to be placed at each service port;
- Make sure that there is a way of checking and release any pressure before these services port blanks are detached.
Finally, make sure that the cylinder doesn’t experience any pressure intensification when exposed to alternating temperatures once being installed onto a system after a long period of storage.
Hydraulic cylinder seals are commonly found in applications where components are exposed to liquids.
They play a vital role within the hydraulic system, as they enable the transfer of liquid power into linear motion. The hydraulic cylinder seals are manufactured from a variety of materials including rubber and polyurethane. Known for their durability under appropriate application settings, these seals can, however, fail or cause serious problems / damage if settings are not correct. Oil contamination, high temperature, chemical erosion or inadequate installation can all cause hydraulic cylinder seals to fail leading to damage in the wider system.
Oil Contamination: contamination within the system is a main cause for the hydraulic cylinder seal failure. Hydraulic oil contamination increases the risk of internal leakage as well as reducing the control of flow and pressure in valves, which could eventually waste horsepower and overheat.
Contamination can form from hydraulic oil, environmental exposure, system wear, the manufacturing process and servicing. Contamination can get into the system when the rod retracts, so it is best to consider the installation of a rod wiper. If hydraulic oil is filtered appropriately, this oil contamination can be prevented. If you start to notice a scratched rod and cylinder surface, extreme seal wear and an oil leak, it may be the result of contamination and your oil needs to be thoroughly cleaned as soon as possible.
High Temperature: If hydraulic cylinder seals look hard and brittle, or the seal lip or body is eroding, the temperature of the oil in the cylinder may be too high. High temperature for a long period of time may result in the seal losing functionality, either by excessive compression or damage to the seal material. This could be due to the seals not being manufactured with the right materials, too much friction or close proximity to the source of heat.
Chemical erosion: If the inappropriate material is used within the hydraulic cylinder seal, it could lead to a serious chemical breakdown, or could change the compounds of the oil altogether. This causes chemical attacks from the additives in the oil on the non-compatible material which can reduce the seal’s elements. A result of the chemical breakdown is the loss of the seal lip or too much swelling or shrinkage. A more obvious sign of a chemical breakdown can be discolouration of the hydraulic cylinder seals.
Inadequate installation: One of the main reasons for hydraulic failure is inadequate installation. It’s vitally important that when you are installing a seal that: the equipment must be thoroughly cleaned, the hydraulic cylinder seal cannot be damaged by scratches and that the entire system is sufficiently lubricated. If the seal gland is too tight or a gland is obstructing a seal lip, this could also cause problems. A seal that has been installed upside down can be avoided through care and attention during installation.