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.