Hydraulics Index

Hydraulic Cylinder

 Introduction Types of Cylinder Construction Cushioning Telescopic Mounting

Introduction

Hydraulic Cylinders are used to convert the pressure of hydraulic fluid to high force linear movement.    At one end of the range of cylinder applications are small single acting cylinder of 10mm dia and 10 mm stroke, available for jig positoning and clamping.    At the other end rams are available providing forces of many 1000 tonnes.    The note sbelow are outline in nature to provide a basic understanding of the type and construction of hydraulic cylinders.   Links are provided to hydraulic cylinder manufacturers with sites providing much more detailed information.

Basic Calculations for obtaining the stroke force and speed of hydraulic cylinders are found on my webpage. Hydraulic Calcs

For many applications the hydraulic actuator which provides the linear movement is a cylinder.    This provides motion by extented or retracting the motion of a piston, or ram, within a cylinder.    The movement of the piston /ram out of the cylinder is usually the power stroke .    For long stroke cylinders the limitation of the power stroke will be its strength of a member as a strut.    Generally the retraction motion is not loaded.

The piston ram velocity, under normal design loading conditions, is simply the fluid flow rate divided by the cross section area of the side of the pistion under pressure.

The force provided by the piston is simply the cross section area of the piston/ram face under pressure x the fluid pressure.

It is clear that when the piston is being retracted the area under pressure is reduced by the diameter of the piston and therefore the force available is reduced and the speed is increased... both factors are normally desireable

Types of Hydraulic Cylinders

The simplest linear actuator is a Ram.    Rams are generally used in applications for moving vertical heavy loads although horizontal Rams are used when suitable guides rods are provided to guide the motion.    Ram-type cylinders are available for long strokes and are used on jacks, elevators and automobile hoists.

Hydraulic cylinders are further identified as

Single acting cylinders

 Symbols according to BS ISO 1219-1

Single acting cylinders are pressurized at one end only while the opposite end is vented to the atmosphere or tank.    They are usually designed in such a way that a device such as an internal spring retracts them.

Double acting cylinders.

 Symbols according to BS ISO 1219-1

Double acting cylinders are the most commonly used hydraulic cylinders.    The pressure can be applied to either port providing force in extending and retracting.    The force in extending the cylinder is generally greater than the retracton force.    This difference in effective area is in turn caused by the area of the rod that reduces the piston area during retraction.    Since more fluid is required to fill the piston side of the cylinder during extension, the operation is understandably slower.    During the retraction operation, the same amount of pump flow will retract the cylinder faster because of the reduced fluid volume displaced by the rod.

Note : A special form of double acting cylinder is available with the rod protruding from both ends of the cylinder.     This arrangement provides the same force and speed in both directions.

Construction

The cylinder consists of five basic parts: two end caps (a base cap and a bearing cap) with port connections, a cylinder tube ( barrel ), a piston and the rod itself.     This basic construction provides for simple manufacture as the end caps and pistons remain the same for different lengths of the same diameter cylinder. The end caps can be secured to the barrel by welding, through tie rods or by threaded connections.  The end caps are generally made of cast iron or cast aluminum and incorporate threaded entries for ports.

The inner surface of the cylinder tube is very smooth to prevent wear and leakage.    Generally, a seamless drawn steel tube machined/honed to an accurate finish is used.    In applications where the cylinder is used infrequently or involves possible contact with corrosive materials, stainless steel, aluminum or brass may be used as the cylinder material.

Pistons are usually made of cast iron or steel.    The piston not only transmits force to the rod, but must also include a bearing surface to maintain its position and stability and a dynamic seal to ensure positive separation between the high and low-pressure sides.     Piston seals are generally used between the piston and tube.    Occasionally piston seals are not used when special design factors require and or a small degree of fluid leakage is permitted.    Sometimes special dynamic seals are used to provide a bearing surface or the piston may be made of brass or surfaced with brass (or bronze) which is then honed to a finish, similar to that of the tube bore.

The surface of the cylinder rod is exposed to the atmosphere when extended is therfores liable to suffer from the effects of knocks, dirt, moisture and corrosion.    When the cylinder is retracted, contaminants of the likes of dirt, moisture and corrosive materials may be drawn back into the barrel, causing problems inside the cylinder.    Heat-treated chromium alloy steel is often used for added strength and corrosion protection, although the thin layer of oil also provides a degree of protection.     I

In order to remove surface dust and contaminants, cylinders are fitted with a wiper or scraper seals inserted in the end cap.    In dusty atmospheres, external rubber bellows may also be used to prevent the entry of dust.

In order to prevent the leakage of fluid on the high-pressure side along the rod, an internal sealing ring is fitted behind the bearing.    In some designs, the wiper seal and the bearing and sealing rings are combined to form a cylinder head cartridge assembly to simplify maintenance.    The rod is generally attached to the piston via a threaded end.    To prevent leakage along the rod a static O-ring around the rod is fitted on the rod where the piston is located. the main features of a typical double acting cylinde is shown, crudely in the figure below.

Cylinder cushioning

End caps have to withstand shock loads at the extreme ends of piston travel.    These loads arise not only from fluid pressure, but also from the kinetic energy of the moving parts of the cylinder and load.    These shock loads can be reduced with cushioning systems built in the end caps.    The principle of operation of the cushioning is shown in the sketch below.  When the Piston aproaches the end of its stroke the flow to the output port is restricted by a projection on the piston entering the discharge route. The flow is then through small hole and via a needle valve which provides control of the cushioning effect.    To allow smoothe full force movement of the piston away from the end cap a non-return valve is included.

Symbols according to BS ISO 1219-1 for double acting cylinder with adjustable cushioning at both ends

Telescopic cylinder

The length of an hydraulic cylinder required to provide a sufficient stroke is sometimes too muuch for the space available.     The installation length of a closed cylinder being is the total of the stroke plus the total length of the thickness of the piston, and ends     For these cases telescopic cylinders are used .     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 are specially designed and manufactured.

Mounting

The mounting of the cylinder on the equipment is a very important factor.    The mounting can arranged to support the cylinder along the line of force e.g flange mounting - this providing a rigid mounting. The cylinder can also be mounted outside the line of force e.g foot mounting .    In this case the strength of the mounting should be sufficient to cater for the imposed moment.    The mounting may be along the line of force allowing angular movement e.g trunnion of clevis mounting.     This type of mounting is best when the force moves in an arc.

A selection of different mounting options is shown in outline below.

The Piston attachment also includes many variations allowing different degrees of rigidity.

A selection of different mounting options is shown in outline below.

 Useful Links Ramko hydraulic Cylinders.. Cylinder manufacturer Helipebs Controls.. Cylinder manufacturer Eaton .. A supplier of hydraulic equipment..

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