Tutorial on Cylinders

Hydraulic cylinders convert the energy produced from a hydraulic pump into a linear mechanical output so that they can perform useful work.

Hydraulic cylinders are sometimes also referred to as hydraulic rams, hydraulic jacks, linear hydraulic actuators, and hydraulic actuators. These terms are all synonymous although the terms "hydraulic ram" and "hydraulic jack" are usually applied to short stroke, single acting cylinders with large diameter piston rods. Hydraulic cylinders are the muscles of machinery.

Hydraulic cylinders are so named because they consist of a piston that moves through a smooth round cylinder or tube. This cylindrical tube must be sealed at both ends with end plates. The end plates are also called end caps or cylinder heads. The piston is firmly connected to a shaft called a piston rod that exits the cylinder through a hole in one end cap. This is called the rod end. The opposite end of the cylinder is called the cap end or the blind end (because it does not have an eye for the rod to stick out).

The cylinder end caps also usually contain the ports where hydraulic fluid is admitted into the cylinder.

The piston rod is the working end of the cylinder and is usually fastened to a load that must be moved. The opposite end of the cylinder body is called the cap end or blind end. It is usually attached to a surface which the actuator pushes against although a large variety of mountings are available that can be mounted at various positions over the body of the actuator.

The cylinder barrel or tube is usually made from high strength seamless steel tubing that has been honed or skive roller burnished to a fine finish on the inside diameter. This will provide a smooth surface for the hydraulic piston to slide through. The tube must be of sufficient thickness to contain the hydraulic pressure that will be used. In a welded body hydraulic cylinder, the barrel must also provide the mechanical strength and rigidity to support the loads that the body of the actuator will see. This is especially true when a mid trunnion mount is attached to the center of the cylinder barrel.

 

The amount of distance that a hydraulic cylinder is able to push the piston rod is called the cylinder stroke or travel.

The amount of force that a cylinder is able to produce is directly related to the area of the piston to which the hydraulic fluid is exposed and the pressure of the hydraulic fluid. The larger the piston area, the more force is produced. The higher the pressure of the hydraulic fluid, the more force is produced. This amount of force is also called the cylinder capacity. It is measured in Pounds of force (lbsf), Tons of force, Newtons of force (N), or Kilograms (Kgf) of force. This force capacity is determined using the basic mathematical formula F=PA, where F= Force, P= Pressure, and A= Area. this equation is easily manipulated to determine the correct size of a cylinder for a required force, or the hydraulic system pressure required to produce a force for a given cylinder.

It should be kept in mind that the effective piston area on the rod end of a hydraulic cylinder is reduced by the area of the piston rod. The piston rod area must be subtracted from the total piston area to find the effective area of the piston being used during the retraction stroke. The force of a cylinder produced while retracting (or pulling) will always be less than the force produced when the cylinder is extending. The larger the diameter of the piston rod, the greater effect it will have in reducing the force of the cylinder in retracting.

Hydraulic cylinders are specified by bore size, stroke, mounting style, rod diameter, and pressure rating. Other details include seal material, temperature rating, materials of construction, cushioning, and more.

Hydraulic rod cylinders are often shown in machine diagrams by the following standardized ISO symbol:

Hydraulic cylinders are designed to be either single acting or double acting.

A single acting hydraulic cylinder is the most simple and least expensive design. Pressurized hydraulic fluid is pumped into the cylinder at one end only and pushes the piston in one direction, usually to extend the rod. Once the work is accomplished, the oil is depressurized and diverted back to the oil reservoir. The piston is returned to its retracted position by an external force such as gravity or a compressed return spring. The piston of a single acting cylinder requires only one seal to contain the pressurized hydraulic fluid on the one side of the piston.

A double acting cylinder is more complicated as it uses pressurized hydraulic fluid to both extend and retract the piston rod. It thus requires fluid ports at both ends of the actuator in order for the oil to be directed onto both sides of the piston. The piston must therefore be equipped with seals that will contain the pressure from both sides. The hole from which the piston rod extends out of the one end of the actuator must also be sealed to prevent fluid pressure loss. This area is often called the rod gland. It also consists of a bearing surface to guide and support the piston rod as it moves back and forth. Another seal called a rod wiper is usually installed here to clean the rod as it re-enters the cylinder. The rod wiper excludes contamination that might damage the inner seals or the oil quality. Under extreme conditions a special hardened, heavy duty rod wiper called a rod scraper can be used to remove sticky contaminants from the piston rod as it re-enters the cylinder.

There are two basic design styles of hydraulic cylinders: the welded cylinder and the tie rod cylinder.

The welded body hydraulic cylinder is built by welding the steel end caps to a heavy gauge steel tube. The rod gland is usually bolted or threaded to a flange welded to the rod end of the cylinder. This provides access to the inner workings of the actuator for disassembly and repair. The cap end of a welded cylinder, also sometimes called the blind end, is not removable.

Above: A typical welded body hydraulic cylinder.

Tie rod cylinders use high strength rods to hold the end caps onto the cylinder barrel. Small bore hydraulic cylinders, from 1 inch to 8 inch bore, usually have four tie rods holding the ends caps together. Large bore cylinders may require as many as 24 tie rods in order to retain the huge internal forces being generated.

Above: A typical tie rod construction hydraulic cylinder.

Welded body hydraulic cylinders have the advantage of being more compact is size than the tie rod design. Without the external tie rods and the wide heads to retain these rods, they are narrower in profile. Welding the heads to the barrel also reduces their overall length compared to tie rod cylinders.

Tie rod cylinders suffer from some disadvantages due to their design. At high pressures, the tie rods stretch slightly. On long stroke cylinders this stretch may add up to such an extent that the tube detaches from the end caps and a loss of pressure and fluid volume occurs. Long stroke tie rod cylinders also require intermediate heads to be installed along the body to support the tie rods from sagging. Finally, certain mounting styles can cause the end caps of a tie rod cylinder to misalign again causing a loss of fluid pressure and volume.

Other cylinder designs have been conceived that incorporate the barrel crimped onto the end caps or using retaining rings inside tie tubes. These styles lack strength and durability and are thus usually limited to low pressures (less than 500 psi) and small sizes (less than 4 inch bore).

Other cylinder designs that are sometimes used are double rod end cylinders, multistage cylinders, tandem cylinders, and telescopic cylinders.

Double rod end cylinders have piston rods extending out both end caps. Thus when the piston is pushed down the barrel, one piston rod extends while the other is retracting.

This design is sometimes used when the actuator is mounted by the two rod ends and the load is fastened to the cylinder body. The load thus traverses back and forth with the cylinder body.

Another reason for using this design is to equalize the areas or volumes on both sides of the piston. The result is that the extend and retract speed and the forces produced in both directions will be exactly the same.

A double rod end cylinder also enables the user to attach devices to the back end of the cylinder to adjust the stroke of the actuator or to measure or indicate the distance travelled.

Sometimes cylinders are assembled to provide several discrete, exactly repeatable strokes without the need for complicated feedback control systems. This can be accomplished by integrating two or more cylinders together to produce multiple stages of extension.

One multi-stage configuration consists of mounting two cylinders back to back by their rear heads. Thus when one cylinder (A) is extended, one discrete travel is produced. When the other cylinder (B) is extended, another discrete travel is produced. If the two cylinder bodies used are of different strokes, 4 discrete travels can be produced from the two actuators (0, A, B, and A+B).

If yet a third cylinder (C) is added to the assembly by attaching it to the rod end of cylinder B, AS many AS 8 discrete travels can be produced (0, A, B, A+B, C, A+C, B+C, A+B+C). Three cylinders are usually the limit to this assembly style due to cost, overall length, and weight considerations. Any more position requirements can be usually be provided more effectively by a feedback control system.

Another multi-stage cylinder configuration is produced by having cylinders mounted in sequence "nose to tail". In this arrangement a cylinder piston rod extends through the read head of another cylinder and pushes the piston of the front cylinder forward a discrete amount. In fact, numerous stages can be produced this way. Again, the disadvantages are cost, weight, and a build up of overall length.

Tandem cylinders are closely related to multistage cylinders but are used to produce a force multiplying output effect rather than a number of discrete stroke outputs. A tandem cylinder arrangement is often used when a large bore cylinder can not be fitted into a narrow space. Thus two or more cylinders mounted end to end are combined to push along the same line of force.

Often cylinders are required to produce an long output travel from a very small retracted space. This would not be able to be accomplished using a standard rod style cylinder. Instead a series of hydraulic tubes nested like sleeves that telescope within each other are used to provide a long total output travel. Telescoping style cylinders are available in designs using 3, 4 or as many as 6 stages or sleeves. Travels of up to 500 inches can be provided.

Most telescopic cylinders are single acting cylinders. They are usually retracted using gravity. A typical example of a single acting telescopic cylinder is that which is used to tilt the dump body on the back of a dump truck.

Telescopic cylinders can be built as double acting cylinders too. These are much more complex and more expensive. They must be used carefully as the internal pressures produced if they are not retracted properly can burst the seals.

Telescoping cylinders must be carefully designed into each application. Because they often have such a long extended length, the application must ensure that the side loads exerted upon the cylinder bodies do not cause the unit to buckle and collapse.

Never the less, telescoping hydraulic cylinders are a very effective solution to many actuator requirements that are not effectively solved using any other method or style of mechanism.

Hydraulic telescopic cylinders are often shown in machine diagrams by the following standardized ISO symbol:

Hydraulic cushioning can be provided to decelerate the cylinder load as it approaches the end of stroke. Cushions will prevent the piston from banging into the end caps with high impact forces. If not effectively cushioned, the resultant slamming could damage the piston and reduce the service life of the actuator.

Cushions are in fact small diameter pistons that enter a small receptacle machined into the end caps. As the cushion piston enters the cushion sleeve, the flow of hydraulic oil leaving the cylinder is restricted. This thereby slows down the speed of the actuator.

Care must be taken with the use of hydraulic cushions when combined with heavy loads. A very large moving mass will produce extremely high pressures within the cushion when the hydraulic flow is suddenly restricted. This spike in pressure may destroy the cylinder seals. In these situations, some other form of end of stroke deceleration should be incorporated into the machine design.

The addition of end of stroke cushioning may add to the overall length of a cylinder body and should therefore be considered before the design of any equipment is finalized.

Hydraulic cushions may not be available on the rod end of hydraulic cylinders with large diameter piston rods as the space required to accommodate the cushion sleeve is not available in the end cap.

Cylinder heads with cushions usually have a built in check valve that allows the free flow of hydraulic fluid into the cylinder so that the speed of the cylinder will not be limited when the direction of travel is reversed. Cylinders with adjustable cushions will have needle valves mounted in the heads so that the flow of fluid leaving the cushion can be adjusted and the amount of deceleration fine tuned for the application.

Hydraulic actuators can be fitted with a wide variety of mounting styles to enable a designer to fit the cylinder into a machine. Mountings include fixed style mounts that hold the cylinder body rigidly in place and pivoting mounts that allow the body of the actuator to move with the machine components that it pushes.

Fixed mounting styles include front and rear flanges, threaded side tapped mounts, and foot mounts. These allow an actuator to be fasten to a flat surface such as a table or a steel plate.

Great care must be taken when using such fixed mounts so that the load does not exert side forces on the piston rod. Side loads on the piston rod will cause wear on the rod bearing, the inside diameter of the barrel, the outside diameter of piston, the rod seals, and the piston seals. A cylinder that encounters excessive side load forces will experience a short service life and require expensive repairs and machine downtime. Side loading may also cause high friction, binding and erratic cylinder movement.

The worst case scenario for a cylinder employing a fixed mount is when the load itself is being guided such as on a linear bearing or track. Any misalignment between the cylinder and movement of the load will produce side loads.

Above: A hydraulic cylinder with spherical eye mounts on both ends allowing the actuator to pivot and twist as it moves thus avoiding side loads.

A cylinder experiencing side loading will exhibit a dull surface finish on one side of the piston rod and a standard polished finish on the other side. Likewise the rod bearing, the piston and the inside diameter of the barrel will show asymmetrical wear patterns.

Pivoting mounts include rear pivot and rear clevis mounts, spherical eye mounts, and trunnion mounts. This mounts allow for misalignments by enabling the actuator to pivot or swing through an arc. Spherical mounts, although more expensive than clevis mounts, are especially effective as they also allow the cylinder to rotate slightly around its longitudinal axis.

The piston rod must be sized to accommodate the forces that the hydraulic actuator will produce. For example, a long stroke actuator with a small diameter piston rod, may fail in service if the rod buckles due to encountering forces that exceed its column strength.


Finally, a large diameter piston rod will result in very high retraction speeds and reduced retraction forces. The high retraction speed is caused by the reduced volume on the rod end. The pump will take much less time to fill that small volume than the large volume on the cap end of the cylinder. The large rod reduces the effective area of the piston on the rod end resulting in smaller retraction forces.

With these points in mind, it is important to size a hydraulic cylinder piston rod properly for each application.

The speed of advance of the piston rod is adjusted by controlling the flow of hydraulic oil entering or leaving the cylinder. This is accomplished using valves. In simple systems that have one set speed of advance, the required speed is set by manually adjusting a needle valve mounted on the oil return line from the cylinder. This valve restricts the flow of oil exiting the cylinder thereby controlling the speed of advance. This needle valve often has an integral check valve allowing a free flow of oil in the opposite direction. Thus the cylinder can be rapidly retracted without restriction or it can have another valve set to control the retracting speed.

In more complex systems requiring constant cylinder speed adjustments, this is accomplished using the hydraulic fluid directional control servo valves. These valves allow an infinite adjustment of the volume of oil flow to the hydraulic actuator thus controlling cylinder output velocity. These valves are available with either manual control, usually by levers, or electronic control for interfacing with computers.

The primary advantage of hydraulic cylinders is that they enable large amounts of power to be applied to machinery in areas located remote from the large heavy source of power generation. This source of power is usually an electric motor, a diesel engine, a turbine, or, in very simple systems, a hand or foot pump.

Another advantage of hydraulic actuators is the incredible power to weight ratio and power to size ratio that they possess. Even a small hydraulic actuator weighing only a few pounds can exert a tremendous force. For example, a 2 inch bore, 4 inch stroke actuator, will have an outside diameter of less than 2.5 inches, an overall length of only about 12 inches, will weigh only about 10 pounds, but will produce over 6000 pounds (3 tons) of force when pressurized to 2000 psi.

Other advantages of hydraulic actuators include infinitely variable speed control, infinitely variable positioning, ability to hold large masses in place, and automatic overload protection. Hydraulic systems are rugged and are able endure difficult conditions. The technology is mature and used worldwide.

By combining hydraulic actuators with other mechanical devices including linkages, gears and valves, the applications are virtually limitless and confined only by ones ingenuity and imagination. Linear travel can be converted to rotary or oscillating motion. Travel and forces can be multiplied. Positioning and velocities can be controlled, changed and held.

At Hyco Canada we are ready to assist you in the design of custom hydraulic cylinders to suit your exact application requirements.

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