A Hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment (engineering vehicles), manufacturing machinery, and civil engineering. Uninformed people sometimes call these devices "pistons", but that term is simply incorrect.
- 1 Operation
- 2 Parts of a hydraulic cylinder
- 3 Hydraulic cylinder designs
- 4 Piston rod construction
- 5 Distribution of Forces on Cylinder Components
- 6 Hydraulic Cylinder Repair
- 7 Cylinder mounting methods
- 8 Special hydraulic cylinders
- 9 References
Hydraulic cylinders get their power from pressurized hydraulic fluid, which is typically 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 one end by the cylinder bottom (also called the cap) and the other end by the cylinder head (also called the gland) where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder into two chambers, the bottom chamber (cap end) and the piston rod side chamber (rod end / head end).
Flanges, trunnions, clevises, Lugs are common cylinder mounting options. The piston rod also has mounting attachments to connect the cylinder to the object or machine component that it is pushing / pulling.
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 hydraulic cylinder, to move the piston. The piston pushes the oil in the other chamber back to the reservoir. If we assume that the oil enters from cap end, during extension stroke, and the oil pressure in the rod end / head end is approximately zero, the force F on the piston rod equals the pressure P in the cylinder times the piston area A:
Retraction force difference
For double-acting single-rod cylinders, when the input and output pressures are reversed, there is a force difference between the two sides of the piston due to one side of the piston being covered by the rod attached to it. The cylinder rod reduces the surface area of the piston and reduces the force that can be applied for the retraction stroke.
During the retraction stroke, if oil is pumped into the head (or gland) at the rod end and the oil from the cap end flows back to the reservoir without pressure, the fluid pressure in the rod end is (Pull Force) / (piston area - piston rod area):
where P is the fluid pressure, Fp is the pulling force, Ap is the piston face area and Ar is the rod cross-section area.
For double-acting, double-rod cylinders, when the piston surface area is equally covered by a rod of equal size on both sides of the head, there is no force difference. Such cylinders typically have their cylinder body affixed to a stationary mount.
Parts of a hydraulic cylinder
A hydraulic cylinder consists of the following parts:
The main function of cylinder body is to hold cylinder pressure. The cylinder barrel is mostly made from a seamless tube. The cylinder barrel is ground and/or honed internally with a typical surface finish of 4 to 16 microinch. Normally hoop stress is calculated to optimize the barrel size. The piston reciprocates in the cylinder.
Cylinder base or cap
The main function of the cap is to enclose the pressure chamber at one end. The cap is connected to the body by means of welding, threading, bolts, or tie rod. Caps also perform as cylinder mounting components [cap flange, cap trunnion, cap clevis]. Cap size is determined based on the bending stress. A static seal / o-ring is used in between cap and barrel (except welded construction).
The main function of the head is to enclose the pressure chamber from the other end. The head contains an integrated rod sealing arrangement or the option to accept a seal gland. The head is connected to the body by means of threading, bolts, or tie rod. A static seal / o-ring is used in between head and barrel.
The main function of the piston is to separate the pressure zones inside the barrel. The piston is machined with grooves to fit elastomeric or metal seals and bearing elements. These seals can be single acting or double acting. The difference in pressure between the two sides of the piston causes the cylinder to extend and retract. The piston is attached with the piston rod by means of threads, bolts, or nuts to transfer the linear motion.
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 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.
The cylinder head is fitted with seals to prevent the pressurized oil from leaking past the interface between the rod and the head. This area is called the seal gland. The advantage of a seal gland is easy removal and seal replacement. The seal gland contains a primary seal, a secondary seal / buffer seal, bearing elements, wiper / scraper and static seal. In some cases, especially in small hydraulic cylinders, the rod gland and the bearing elements are made from a single integral machined part.
The seals are considered / designed as per the cylinder working pressure, cylinder speed, operating temperature, working medium and application. Piston seals are dynamic seals, and they can be single acting or double acting. Generally speaking, Elastomer seals made from nitrile rubber, Polyurethane or other materials are best in lower temperature environments, while seals made of Fluorocarbon Viton are better for higher temperatures. Metallic seals are also available and commonly use cast iron for the seal material. Rod seals are dynamic seals and generally are single acting. The compounds of rod seals are nitrile rubber, Polyurethane, or Fluorocarbon Viton. Wipers / scrapers are used to eliminate contaminants such as moisture, dirt, and dust, which can cause extensive damage to cylinder walls, rods, seals and other components. The common compound for wipers is polyurethane. Metallic scrapers are used for sub zero temperature applications, and applications where foreign materials can deposit on the rod. The bearing elements / wear bands are used to eliminate metal to metal contact. The wear bands are designed as per the side load requirements. The primary compounds for wear bands are filled PTFE, woven fabric reinforced polyester resin and bronze
There are many component parts that make up the internal portion of a hydraulic cylinder. All of these pieces combine to create a fully functioning component.
- Cylinder base connection
- Internal Threaded Ductile Heads
- Head Glands
- Polypak Pistons
- Cylinder Head Caps
- Butt Plates
- Eye Brackets/Clevis Brackets
- MP Detachable Mounts
- Rod Eyes/Rod Clevis
- Pivot Pins
- Spherical Ball Bushings
- Spherical Rod Eye
- Alignment Coupler
- Ports and Fittings
Single acting vs. double acting
- Single acting cylinders are economical and the simplest design. Hydraulic fluid enters through a port at one end of the cylinder, which extends the rod by means of area difference. An external force or gravity returns the piston rod.
- Double acting cylinders have a port at each end, supplied with hydraulic fluid for both the retraction and extension.
Hydraulic cylinder designs
There are primarily two styles of hydraulic cylinder construction used in industry: Tie rod style cylinders and welded body style cylinders.
Tie rod 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. Tie rod style cylinders can be completely disassembled for service and repair.
The National Fluid Power Association (NFPA) has standardized the dimensions of hydraulic tie rod cylinders. This enables cylinders from different manufacturers to interchange within the same mountings.
Welded body cylinder
Welded body 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 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 customization. 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 such as construction equipment (excavators, bulldozers, and road graders) and material handling equipment (forklift trucks, telehandlers, and 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 an hydraulic cylinder operates both inside and outside the barrel, and consequently both in and out of the hydraulic fluid and surrounding atmosphere.
Wear and corrosion resistant surfaces are desirable on the outer diameter of the piston rod. The surfaces are often applied using coating techniques such as Chrome (Nickel) Plating, Lunac 2+ duplex, Laser Cladding, PTA welding and Thermal Spraying. These coatings can be finished to the desirable surface roughness (Ra, Rz) where the seals show optimum performance. All these coating methods have their specific advantages and disadvantages. It is for this reason that coating experts play a crucial role in selecting the optimum surface treatment procedure for protecting Hydraulic Cylinders.
Cylinders are used in different operational conditions and that makes it a challenge to find the right coating solution. In dredging there might be impact from stones or other parts, in salt water environments there are extreme corrosion attacks, in off-shore cylinders facing bending and impact in combination with salt water, and in the steel industry there are high temperatures involved, etc.... It is important to understand that currently there is no single coating solution which successfully combats all the specific operational wear conditions. Every single technique has its own benefits and disadvantages.
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.
Distribution of Forces on Cylinder Components
The forces on the piston face and the Piston Head Retainer vary depending on what Piston Head retention system is used.
If a circlip (or any non preloaded system) is used, the force acting to separate the Piston Head and the Cylinder Shaft shoulder is the applied pressure multiplied by the area of the Piston Head. The Piston Head and Shaft shoulder will separate and the load is fully reacted by the Piston Head Retainer.
If a preloaded system is used the force between the Cylinder Shaft and Piston Head is initially the Piston Head Retainer preload value. Once pressure is applied this force will reduce. The Piston Head and Cylinder Shaft shoulder will remain in contact unless the applied pressure multiplied by Piston Head area exceeds the preload.
The maximum force the Piston Head Retainer will see is the larger of the preload and the applied pressure multiplied by the full Piston Head area . It is interesting to note that the load on the Piston Head Retainer is greater than the external load, which is due to the reduced shaft size passing through the Piston Head. Increasing this portion of shaft reduces the load on the Retainer. 
Side loading is unequal pressure that is not centered on the cylinder rod. This off-center strain can lead to bending of the rod in extreme cases, but more commonly causes leaking due to warping the circular seals into an oval shape. It can also damage and enlarge the bore hole around the rod and the inner cylinder wall around the piston head, if the rod is pressed hard enough sideways to fully compress and deform the seals to make metal-on-metal scraping contact.
The strain of side loading can be directly reduced with the use of internal stop tubes which reduce the maximum extension length, leaving some distance between the piston and bore seal, and increasing leverage to resist warping of the seals. Double pistons also spread out the forces of side loading while also reducing stroke length. Alternately, external sliding guides and hinges can support the load and reduce side loading forces applied directly on the cylinder.
Hydraulic Cylinder Repair
Hydraulic cylinders form the most integral part of many hydraulic systems. It is a common practice to dissemble and rebuild an entire device in the case of hydraulic cylinder repair. Inspection of the leakage issue and scrutinizing cylinder parts (especially the seals) is helpful in recognizing the exact problem and choosing on the repair options accordingly. Steps involved in the repair of hydraulic cylinders:
Disassembly of the cylinder parts
First of all, you should place the cylinder in a suitable location, which has sufficient space to work. If you are working in a cumbersome space, it will be difficult for you to keep track of opened up parts. After bringing the cylinder to an appropriate spot, open the cylinder ports and drain out all the hydraulic fluid. The cover of cylinder can be removed by unscrewing the bolts. Once you take off the cover, remove the piston by loosening the input valves.
Finding out the problem
Once the piston is completely removed, you will be able to see multiple seals on different parts that are connected to the piston rod . First of all, you need to examine the piston rod to see if there is any damage. If the shaft of the rod has bends or if the Cylinder Bore has scratches, then get them repaired at a professional repair shop. If the damage is permanent, then you can order and manufacture new piston rod for your hydraulic cylinder. Sometimes, piston seals get greatly damaged, distorted, or broken. Such damaged seals can cause leakage of hydraulic fluid from the cylinder leading and lower the overall pressure. When such events occur, you know that these seals need to be replaced.
Repairing or replacing damaged parts
The parts of the hydraulic cylinder that are distorted ( piston rod, rod seal, piston seal and/ or head of rod), need to be either repaired or completely replaced with new parts. The seals can be repacked with the help of a hydraulic cylinder seal kit. These kits will have seals and suitable o-rings. Remember the size and type of old seal while removing them and fix the new ones accordingly. Make sure that you handle the new seals with utmost care so that they do not get damaged in any way.
Rebuilding the Hydraulic Cylinder
Before reassembling all the parts of your cylinder, you should clean and dry the cylinder barrel completely. Also clean the piston rod, shaft, and other parts of the cylinder. Get the broken and damaged seals repacked. Then assemble the parts back on the piston rod. The assembly needs to be done in a reverse order. Once you have assembled all the parts back, put the rod into the soft-jaw vise and screw back the bolts onto the piston rod.
If the parts of the hydraulic cylinder are severely damaged, then it is advisable to replace them with new parts with the help of a professional repair expert. Trying to replace/ repair too many parts on your own can lead to faulty reassembly. By following the above steps, you can accomplish the task of hydraulic cylinder repair. Make sure that you prevent ingress of moisture or dirt after assembly of the parts is done.
Cylinder mounting methods
Mounting methods also play an important role in cylinder performance. Generally, fixed mounts on the centerline of the cylinder are best for straight line force transfer and avoiding wear. Common types of mounting include:
Flange mounts—Very strong and rigid, but have little tolerance for misalignment. Experts recommend cap end mounts for thrust loads and rod end mounts where major loading puts the piston rod in tension.
Side-mounted cylinders—Easy to install and service, but the mounts produce a turning moment as the cylinder applies force to a load, increasing wear and tear. To avoid this, specify a stroke at least as long as the bore size for side mount cylinders (heavy loading tends to make short stroke, large bore cylinders unstable). Side mounts need to be well aligned and the load supported and guided.
Centerline lug mounts —Absorb forces on the centerline, but require dowel pins to secure the lugs to prevent movement at higher pressures or under shock conditions.
Pivot mounts —Absorb force on the cylinder centerline and let the cylinder change alignment in one plane. Common types include clevises, trunnion mounts and spherical bearings. Because these mounts allow a cylinder to pivot, they should be used with rod-end attachments that also pivot. Clevis mounts can be used in any orientation and are generally recommended for short strokes and small- to medium-bore cylinders. 
Special hydraulic cylinders
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 kinds of cylinders are called telescopic cylinders. If we call a normal rod cylinder single stage, telescopic cylinders are multi-stage units of two, three, four, five or more stages. In general telescopic 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.
A hydraulic cylinder without a piston or with a piston without seals is called a plunger cylinder. A plunger cylinder can only be used as a pushing cylinder; the maximum force is piston rod area multiplied by pressure. This means that a plunger cylinder in general has a relatively thick piston rod.
A differential cylinder acts like a normal cylinder when pulling. If the cylinder however has to push, the oil from the piston rod side of the cylinder is not returned to the reservoir, but goes to the bottom side of the cylinder. In such a way, the cylinder goes much faster, but the maximum force the cylinder can give is like a plunger cylinder. A differential cylinder can be manufactured like a normal cylinder, and only a special control is added.
Position sensing "smart" hydraulic cylinder
Position sensing hydraulic cylinders eliminate the need for a hollow cylinder rod. Instead, an external sensing "bar" using Hall Effect technology senses the position of the cylinder’s piston. This is accomplished by the placement of a permanent magnet within the piston. The magnet propagates a magnetic field through the steel wall of the cylinder, providing a locating signal to the sensor.
A note about popular terminology
In the United States, popular usage refers to the whole assembly of cylinder, piston, and piston rod (or more) collectively as a "piston", which is incorrect. Instead, the piston is the short, cylindrical metal component that separates the two parts of the cylinder barrel internally.
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