An Introduction To Hydraulic Cylinders Types , And Selection

An Introduction To Hydraulic Cylinders Types , And Selection 

What are hydraulic cylinders?

Cylinders allow hydraulic systems to apply linear motion and force without mechanical gears or levers by transferring the pressure from fluid through a piston to the point of operation.

Hydraulic cylinders are at work in both industrial applications (hydraulic presses, cranes, forges, packing machines), and mobile applications (agricultural machines, construction equipment, marine equipment). And, when compared with pneumatic, mechanical or electric systems, hydraulics can be simpler, more durable, and offer greater power. For example, a hydraulic pump has about ten times the power density of an electric motor of similar size. Hydraulic cylinders are also available in an impressive array of scales to meet a wide range of application needs.

Selecting the right cylinder for an application is critical to attaining maximum performance and reliability. That means taking into consideration several parameters. Fortunately, an assortment of cylinder types

Classification and definitions

All hydraulic linear actuators have a fluid filled volume contained within the body of the

device, the tube or barrel, and a component which moves within the cylinder called a piston

or piston rod. When an actuator is moving to increase in length ie. the piston is moving out

of the cylinder body, it is said to be extending. When the piston is moving into the cylinder

body it is said to be retracting. In the simplest devices the piston is a cylindrical rod which

transmits the output force directly to the load

These devices are also given the name ram, plunger or displacement cy linder. They may be used in heavy duty industrial applications egometal roIling mills but there are also many smaIl diameter and short stroke devices which fit this category. A device like this, which can only move under the action of supplied fluid in one direction (extend) is also caIled single acting. Such a device requires an additional force to return the piston/plunger in the retract direction when the fluid pressure is reduced, this could be provided by a gravity load . There are also cylinders with a return stroke operated by a spring which may be either internal or external. A seal set is required around the piston rod as indicated and it is the cross sectional area of the rod which determines the cylinder performance since there is a significant clearance within the cylinder bore. An alternative type returns the piston under the action of a fluid flow and is caIled a double acting cylinder. A double acting device has a rod connected to the load which is a significantly smaIler diameter than the cylinder to aIlow a ring area between the outside diameter of the rod and the cylinder bore for fluid to act on the other side of the piston (which is the fuIl bore diameter). This gives an area exposed to fluid on each side of the piston but these are different, and this type is sometimes referred to as an unequal area or single ended actuator. This type requires some sealing between the piston outside diameter and the cylinder bore as weIl as on the piston rod. The term jack can be used to describe a single-acting cylinder employed for lifting or 'jacking' actions but it is also used to describe double-acting cylinders in certain industries

(eg. in the aircraft industry).

The two devices so far described are the most commonly used. The other basic variation in geometry gives a design with a rod at both ends. This may be caIled a through-rod, double rodded, double ended or equal area actuator. The rods may be the same diameter giving equal ring areas each side ofthe piston and equal force capability in both directions It also gives 'two-bearing' support for the rod at each end ofthe cylinder and hence greater rigidity. Some have different rod sizes, usually used to operate control switches. It requires an additional sealing arrangement for the extra rod. 

1- Single-Acting Cylinder. This cylinder only has a head-end port and is operated hydraulically in one direction. When oil is pumped into a port, it pushes on a plunger, thus extending it. To return or retract a cylinder, oil must be released to a reservoir.

A plunger returns either because of the weight of a load or from some mechanical force such as a spring. In mobile equipment, flow to and from a single-acting cylinder is controlled by a reversing directional valve of a single-acting type

2- Double-Acting Cylinder. This cylinder must have ports at the head and rod ends. Pumping oil into the head end moves a piston to extend a rod while any oil in the rod end is pushed out and returned to a reservoir. To retract a rod, flow is reversed. Oil from a pump goes into a rod end, and a head-end port is connected to allow return flow. The flow direction to and from a double-acting cylinder can be controlled by a double-acting directional valve or by actuating a control of a reversible pump

3-Differential Cylinder. In a differential cylinder, the areas where pressure is applied on a piston are not equal. On a head end, a full piston area is available for applying pressure. At a rod end, only an annular area is available for applying pressure. A rod’s area is not a factor, and what space it does take up reduces the volume of oil it will hold. Two general rules about a differential cylinder are that

 • With an equal GPM delivery to either end, a cylinder will move faster when retracting because of a reduced volume capacity.

• With equal pressure at either end, a cylinder can exert more force when extending because of the greater piston area. In fact, if equal pressure is applied to both ports at the same time, a cylinder will extend because of a higher resulting force on a head end.

4-Nondifferential Cylinder. This cylinder has a piston rod extending from each end. It has equal thrust and speed either way, provided that pressure and flow are unchanged. A nondifferential cylinder is rarely used on mobile equipment.

5- e. Ram-Type Cylinder. A ram-type cylinder is a cylinder in which a cross-sectional area

of a piston rod is more than one-half a cross-sectional area of a piston head. In many cylinders

of this type, the rod and piston heads have equal areas. A ram-type actuating cylinder is used mainly for push functions rather than pull.

6- a telescoping, ram-type, actuating cylinder, which can be a single- or double acting type. In this cylinder, a series of rams are nested in a telescoping assembly. Except for the smallest ram, each ram is hollow and serves as a cylinder housing for the next smaller ram. A ram assembly is contained in a main cylinder housing, which also provides the fluid ports. Although an assembly requires a small space with all of the rams retracted, a telescoping action of an assembly provides a relatively long stroke when the rams are extended.

7- Piston-Type Cylinder. In this cylinder, a cross-sectional area of a piston head is referred to as a piston-type cylinder. A piston- type cylinder is used mainly when the push and pull functions are needed.

A single-acting, piston-type cylinder uses fluid pressure to apply force in one direction.In some designs, the force of gravity moves a piston in the opposite direction. However, most

cylinders of this type apply force in both directions. Fluid pressure provides force in one direction and spring tension provides force in the opposite direction.

Most piston-type cylinders are double-acting, which means that fluid under pressure can be applied to either side of a piston to provide movement and apply force in a corresponding direction.

This cylinder contains one piston and piston-rod assembly and operates from fluid flow in either direction. The two fluid ports, one near each end of a cylinder, alternate as an inlet and an outlet, depending on the directional-control valve flow direction. This is an unbalanced cylinder, which means that there is a difference in the effective working area on the two sides of a piston. A cylinder is normally installed so that the head end of a piston carries the greater load; that is, a cylinder carries the greater load during a piston-rod extension stroke

8- Cushioned Cylinder. To slow an action and prevent shock at the end of a piston stroke, some actuating cylinders are constructed with a cushioning device at either or both ends of a cylinder. This cushion is usually a metering device built into a cylinder to restrict the flow at an outlet port, thereby slowing down the motion of a piston.

9- Lockout Cylinders. A lockout cylinder is used to lock a suspension mechanism of a tracked vehicle when a vehicle functions as a stable platform. A cylinder also serves as a shock absorber when a vehicle is moving. Each lockout cylinder is connected to a road arm by a control lever. When each road wheel moves up, a control lever forces the respective cylinder to compress. Hydraulic fluid is forced around a piston head through restrictor ports causing a cylinder to act as a shock absorber.

When hydraulic pressure is applied to an inlet port on each cylinder’s connecting eye, an inner

control-valve piston is forced against a spring in each cylinder. This action closes the restrictor

ports, blocks the main piston’s motion in each cylinder, and locks the suspension system.

Basic equations

There are two fundamental sets of equations relating the actuator dimensions to the variables associated with the fluid itself, and these determine the actuator force and the speed of movement of the piston relative to the cylinder. These equations are developed for the double-acting, single-ended cylinder and substitution of the appropriate areas will give the equivalent equations for the other types. The actuator diameter is the principal dimension determining the full piston or cylinder bore area and together with the rod diameter in a double acting device determines the ring or annulus area on the rod side of the piston. Manufacturers usually specify the actuators with a table for the cylinder bore

and rod diameters dp, and dR receptively , frequently with the relevant cross sectional areas , denoted Ap AND AA in figure above then AA can be calculated from the piston and rod diameters

AP must be greater than AA by ratio 1.2 to 2 

UE=Q/AP

where

UE : actuator extended speed (m/s)

Q:  flow rate should be known m³

UR=Q/AA

where

UR: retract direction the speed

actuator performance

the difirence is return flow between extended and retract moition is flow out from cylinder after extended 

QOUTE = UE×AA=Q×AA/AP

for retract stroke return flow is

QOUTR=UR×AP=Q×AP/AA

the force on piston rod during extended is

Fe=PP×AP

In a single acting device this will also be the extend force available with the working area either that of the piston as above or in the case of the true displacement cylinder of Figure  the rod area should be used. In a double acting device if return line pressure is included producing a pressure on the annulus area of pA, then the force balance becomes:

the power output from an hydrau;ic cylinder is

POWER=F×U

where

F= force         (KN)

U = PISTON SPEED (m/s)

SO

POWER= F×L/t    (kw)

where

L= STROKE LENGTH             (m)

t= time to complete the stroke   (s)

The hydraulic power necessary to provide this will be greater. There will be a friction loss in the actuator and pressure losses in the flow. These can be estimated or calculated more accurately if the actuator and circuit details are known. These are discussed below under the Selection & peiformance heading

Maintenance.

 Hydraulic cylinders are compact and relatively simple. The key points to watch are the seals and pivots. The following lists service tips in maintaining cylinders:

a. External Leakage. If a cylinder’s end caps are leaking, tighten them. If the leaks still do not stop, replace the gasket. If a cylinder leaks around a piston rod, replace the packing. Make sure that a seal lip faces toward the pressure oil. If a seal continues to leak, 

b. Internal Leakage. Leakage past the piston seals inside a cylinder can cause sluggish movement or settling under load. Piston leakage can be caused by worn piston seals or rings or scored cylinder walls. The latter may be caused by dirt and grit in the oil.

NOTE: When repairing a cylinder, replace all the seals and packings before reassembly.

c. Creeping Cylinder. If a cylinder creeps when stopped in mid stroke,  Another cause could be a worn control valve.

d. Sluggish Operation. Air in a cylinder is the most common cause of sluggish action. Internal leakage in a cylinder is another cause. If an action is sluggish when starting up a system, but speeds up when a system is warm, check for oil of too high a viscosity (see the machine's operating manual). If a cylinder is still sluggish after these checks, test the whole circuit for worn components.

e. Loose Mounting. Pivot points and mounts may be loose. The bolts or pins may need to be tightened, or they may be worn out. Too much slop or float in a cylinder’s mountings damages the piston-rod seals. Periodically check all the cylinders for loose mountings.

f. Misalignment. Piston rods must work in-line at all times. If they are side-loaded, the piston rods will be galled and the packings will be damaged, causing leaks. Eventually, the piston rods may be bent or the welds broken.

g. Lack of Lubrication. If a piston rod has no lubrication, a rod packing could seize, which would result in an erratic stroke, especially on single-acting cylinders.

F

h. Abrasives on a Piston Rod. When a piston rod extends, it can pick up dirt and other material. When it retracts, it carries the grit into a cylinder, damaging a rod seal. For this reason, rod wipers are often used at the rod end of a cylinder to clean the rod as it retracts. Rubber boots are also used over the end of a cylinder in some cases. Piston rods rusting is another problem. When storing cylinders, always retract the piston rods to protect them. If you cannot retract them, coat them with grease.

i. Burrs on a Piston Rod. Exposed piston rods can be damaged by impact with hard objects. If a smooth surface of a rod is marred, a rod seal may be damaged. Clean the burrs on a rod immediately, using crocus cloth. Some rods are chrome-plated to resist wear. Replace the seals after restoring a rod surface.

j. Air Vents. Single-acting cylinders (except ram types) must have an air vent in the dry side of a cylinder. To prevent dirt from getting in, use different filter devices. Most are self cleaning, but inspect them periodically to ensure that they operate properly. Cylinder mounting methods

Hydraulic seal selection

To select the correct hydraulic seal for your application, please consider the following steps.

Step 1: Seal function The seal you require will usually be for the rod, ram, gland or piston — and you may also require wipers or bearing strips

Step 2: Sealing method Depending on your application, you may require single, double or multi-lip sealing

Step 3: Operational parameters The maximum pressure of the application should be considered along with the temperature range in which it will operate

Step 4: Material compatibility To determine that the seal materials are compatible with your system media

Step 5: Operational features consider and check availability of

your seal for the following

  Endless: this is our preferred method of supply, as it provides the highest sealing integrity.

  Split-type: for easier and quicker installation during plant maintenance

Step 6: Sizing the seal Refer to the section drawings for the required housing measurements,

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