Hydraulic Actuators:Cylinders

POWER CONVERSION IN HYDRULIC SYSTEM

Objectives Describe the construction and operation of basic

hydraulic cylinders, limited-rotation actuators, and motors.

Compare the design and operation of various types of hydraulic cylinders.

Select appropriate cylinder design options available for mounting hydraulic cylinders and reducing hydraulic shock.

Compare the design and operation of various types of hydraulic motors.

Objectives Contrast the operation of fixed- and variable-speed

hydraulic motors. Describe the construction and operation of a basic

hydrostatic transmission. Size hydraulic cylinders and motors to correctly meet

system force and speed requirements. Interpret manufacturer specifications for hydraulic

cylinders.

ACTUATORS Hydraulic systems are used to control & transmit power

A pump driven by prime mover (electric motor) creates flow of fluid.

An actuator is used to convert the energy of the fluid back into mechanical power

Amount of output power developed depends upon the flow rate, pressure drop across the actuator & its overall efficiency

TYPES OF ACTUATORS

Linear actuator (hydraulic cylinder)

Provides motion in straight line

Linear displacement depends on stroke length (length of actuator)

Usually referred to as cylinders, rams (single acting cylinders) or jacks (cylinder used for lifting)

Rotary actuators (Hydraulic motors)

Produces continuous rotational motion

Pump shaft is rotated to generate flow, a motor shaft is caused to rotate by fluid being forced into the driving chambers

ACTUATORS Semi rotary actuators

Produces non-continuous rotational motion Limited to less than one revolution (

SINGLE ACTING CYLINDER

OPERATION OF SINGLE ACTING CYLINDER

OPERATION OF SINGLE ACTING CYLINDER

Produces linear motion in one direction

Consists of cylinder (barrel), piston, piston-rod (ram) & inlet port at piston end or blank end (other end is known as rod end)

Cylinder is machined to high surface finish (honing)

Fluid enters through inlet port into piston end or blank end pressure build up-force generation on piston-movement of piston EXTENSION or FORWARD STROKE

RETRACTION or RETURN by compression spring or under the influence of gravity (only in case of vertical mounting)

Usage of seals Piston seal (imparts clearance between piston & cylinder for clear movement), Rod seal or End seal (prevention of leakage from cylinder), Bearing (supports piston rod) & Wiper (prevention of dirt/dust entry into the cylinder)

Drain hole helps in removing the leaked oil across piston seal

Widely used in hydraulic systems

DOUBLE ACTING CYLINDER

DOUBLE ACTING CYLINDER

DOUBLE ACTING CYLINDER

Produces linear motion in two directions

May be single rod ended or double rod ended

Piston is connected to smaller diameter piston rod

Fluid pressure acts on either side of piston alternatively

Both sides of piston has oil ports

Parts of double acting cylinder

Fluid enters through left port causing extension stroke while when it enters through right port causes retraction stroke, for present case

For a given pressure double acting cylinder (single rod type) exerts greater force when extending than when retracting

CONSTRUCTIONAL FEATURES OF CYLINDER

CONSTRUCTIONAL FEATURES OF DOUBLE ACTING CYLINDER

Five basic parts Base cap & Bearing cap with port connections, a cylinder barrel, piston & piston rod

End caps are secured to barrel through welding or threaded connection

Smooth inner surface of barrel (seamless drawn tube) to prevent wear & leakage

Pistons (separates high & low pressure zones) are usually made of cast iron or steel

Function of End seal, bearing & wiper seal

DOUBLE ROD CYLINDER

DOUBLE ROD CYLINDER

Consists of two rods

It is of double acting type

Suitable for extracting work from both sides at a time

For the same rod diameters force remains same

Used when required to exert equal forces & speed in both directions

This type of cylinder is center mounted and is normally used when the same task is performed at either end on staggered cycles

TANDEM CYLINDER

TANDEM CYLINDER Also known as combination cylinder

Two separate pistons are mounted on same rod

Two double acting cylinders are connected in series

Suitable for higher force (2 cylinders twice force) generation with smaller cylinders

High volume of oil is required to drive the cylinders

TELESCOPIC CYLINDER

TELESCOPIC CYLINDER Multiple cylinders mounted concentrically within one another

Suitable for longer strokes with shorter retraction

Operates on displacement principle

Stop rings limit the movement of each section

When the cylinder extends, all the sections move together until the outer section is prevented from further extension by its stop ring

Remaining sections continue out-stroking until the second outermost section reaches the limit of its stroke and so on until all sections are extended, the innermost one being last of all

For a given input flow rate, the speed of operation will increase in steps as each successive section reaches the end of its stroke,

For a specific pressure the load lifting capacity reduces for each successive section

Example : high lift fork truck, tilting of truck dump bodies

Hydraulic Cylinders Actuators are the components used in a hydraulic

system to provide power to a required work location

Cylinders are the hydraulic system components that convert fluid pressure and flow into linear mechanical force and movement

Hydraulic Cylinders A basic cylinder consists of:

Piston

Piston rod

Barrel

Hydraulic Cylinders Parts of a typical cylinder

Hydraulic Cylinders The piston forms sealed, variable-volume chambers in

the cylinder

System fluid forced into the chambers drives the piston and rod assembly

Linear movement is produced

Hydraulic Cylinders Seals prevent leakage between:

Piston and cylinder barrel

Piston rod and head

Barrel and its endpieces

Wiper seal, or scraper, prevents dirt and water from entering the cylinder during rod retraction

Hydraulic Cylinders Various seals are used in a cylinder

Hydraulic CylindersRod wipers prevent

contamination from entering on rod retraction

IMI Norgren, Inc.

Hydraulic Cylinders Cylinders are typically classified by operating principle

or by construction type

Single-acting or double-acting

Tie rod, mill, threaded end, or one piece

Hydraulic Cylinders Single- and double-acting cylinders

Single-acting Double-acting

Hydraulic Cylinders Single-acting cylinders exert force either on extension

or retraction

They require an outside force to complete the second motion

Hydraulic Cylinders Double-acting cylinders generate force during both

extension and retraction

Directional control valve alternately directs fluid to opposite sides of the piston

Force output varies between extension and retraction

Hydraulic Cylinders Effective piston area is reduced on retraction due to the

rod cross section

Hydraulic Cylinders Volume is reduced on retraction

Hydraulic Cylinders External tie rod bolts are used to secure the ends on

the tie-rod cylinder design

Commonly found on heavy industrial machines

External tie rods increase chance of damage and promote accumulation of dirt

Hydraulic Cylinders Tie-rod cylinder

Hydraulic Cylinders Mill cylinders

Yates Industries, Inc.

Hydraulic Cylinders Threaded-end cylinder

Bailey International Corporation

Hydraulic Cylinders One-piece cylinder has the cylinder barrel welded to

the ends

Produces a compact actuator

Cost effective to manufacture

Cannot be serviced (throwaway)

Hydraulic Cylinders Hydraulic ram is commonly used in hand-operated

jacks

Rod is basically the same diameter as the inside of the cylinder barrel

Large-diameter rod is more rigid under load, but cylinder can generate force in only one direction

Hydraulic CylindersTypical hand-operated

jack

Hydraulic Cylinders Telescoping cylinders are available for applications

requiring long extension distances

Rod is made up of several tubes of varying size nested inside of the barrel

Each tube extends, producing a rod longer than the cylinder barrel

Typical example is the actuator that raises the box on a dump truck

Hydraulic Cylinders Telescoping cylinders

Star Hydraulics, Inc.

Hydraulic Cylinders Cylinders often use hydraulic cushions

Provide a controlled approach to the end of the stroke

Reduces the shock of the impact as the piston contacts the cylinder head

Hydraulic Cylinders Cylinder cushioning device

Hydraulic Cylinders A variety of mounting configurations are used to

attach the cylinder body and rod end to machinery

Fixed centerline

Fixed noncenterline

Pivoting centerline

Expected cylinder loading is the major factor in the selection of the mounting style

Hydraulic Cylinders Head-end flange mount

Hydraulic Cylinders Fixed-noncenterline mount

Hydraulic Cylinders Pivoting-centerline, clevis mount

Hydraulic Cylinders Pivoting-centerline, trunnion mount

Hydraulic Cylinders The force generated by a cylinder is calculated by

multiplying the effective area of the piston by the system pressure

Hydraulic Cylinders Effective cylinder piston area

Hydraulic Cylinders Force generated during the extension of a double-

acting cylinder with a single-ended rod is calculated as:

Ef = Sp Pawhere:Ef = extension forceSp = system pressurePa = piston area

(Calculations require consistent units of measure in these formulas)

Hydraulic Cylinders Force generated during the retraction of a double-

acting cylinder with a single-ended rod is calculated as:Rf = Sp (Pa Ra)where:Rf = retraction forceSp = system pressurePa = piston areaRa = rod area

Hydraulic Cylinders Speed at which the cylinder extends or retracts is

determined by:

Physical volume per inch of cylinder piston travel

Amount of fluid entering the cylinder

Effective area of the piston is used to calculate the volume of the cylinder per inch of piston travel

Hydraulic Cylinders Extension speed of a double-acting cylinder with a

single-ended rod is calculated as:Es = Fr (Cg Pa)where:Es = extension speedFr = flow delivery rateCg = cubic inches in one gallonPa = piston area

Hydraulic Cylinders Calculate retraction speed of a double-acting cylinder

with single-ended rod as:Rs = Fr [Cg (Pa Ra)]where:Rs = retraction speedFr = flow delivery rateCg = cubic inches in one gallonPa = piston areaRa = rod area

Hydraulic Cylinders Flow rate to produce a desired extension or retraction

speed is calculated as:Fr = (Ea Cs) Cgwhere:Fr = system flow rateEa = effective piston areaCs = cylinder speedCg = cubic inches in one gallon

Hydraulic Cylinders Hydraulic cylinder manufacturers provide detailed

specifications concerning:

Construction

Physical size

Load capacity

Hydraulic Cylinders This information includes basic factors such as:

Bore

Stroke

Pressure rating

Other details, such as service rating, rod end configurations, and dimensions

Hydraulic Cylinders Typical manufacturers catalog page

Bailey International Corporation

Limited-RotationHydraulic Actuators Limited-rotation devices are actuators with an output

shaft that typically applies torque through approximately 360 of rotation

Models are available that are limited to less than one revolution, while others may produce several revolutions

Limited-RotationHydraulic Actuators Most common designs of limited-rotation actuators

are:

Rack-and-pinion

Vane

Helical piston and rod

Limited-RotationHydraulic Actuators Rack-and-pinion limited rotation actuator

IMI Norgren, Inc.

Limited-RotationHydraulic Actuators Vane limited-rotation actuator

Limited-RotationHydraulic Actuators Helical piston and rod limited-rotation actuator

Limited-RotationHydraulic Actuators Limited-rotation actuators are used to perform a

number of functions in a variety of industrial situations

Indexing devices on machine tools

Clamping of workpieces

Operation of large valves

Limited-RotationHydraulic Actuators Limited-rotation actuators are used in this robotic arm

IMI Norgren, Inc.