Fluid PowerHydraulic Cylinders

Tim Hague

Nick Burdg

Function of Hydraulic Cylinders

The main function of a cylinder is to convert hydraulic power into linear mechanical force.

This force performs work or transmits power.

Overview

Types of Cylinders Construction and Operation Cylinder Ratings Formulas for Application Cylinder Features Installation and Trouble Shooting

Types of Cylinders

Ram Single Acting Telescopic Spring Return Double Acting

Ram Cylinders

The simplest single acting cylinder One fluid chamber Exerts force in only one direction Mounted mostly vertically The cylinder retracts by the force

of the load due to gravity

Ram Cylinders

Ram cylinders are most commonly used in elevators, jacks, and automobile hoists.

Can also be used on a scissors lift like the one on the loading dock of KL.

Single Acting Cylinders

Acts much like a ram cylinder Main difference is that the single acting

cylinder uses a piston. The leakage flow that goes past the piston is ported the tank.

Telescopic Cylinders

Mostly a single acting cylinder Series of rod segments called sleeves, most

common to only have 4 or 5 sleeves in each cylinder

The sleeves work together to provide a longer stroke

The maximum force is at the collapsed position The speed will increase at each stage, but will not

allow as much force.

Telescopic Cylinders

•Most commonly seen in high reach Fork Lifts

Spring Return Cylinders

Considered a single acting cylinder Pressure applied to the cap end pushes the spring down as

the rod is extending When the pressure is removed the spring force allows the

cylinder to retract The drain is in the spring chamber and allows the leakage

flow past the piston seal

Double Acting CylindersTypes

Basic double acting (differential cylinder)

Double rod cylinder (nondifferential cylinder)

Tandem cylinder

Double Acting Cylinders

Most common type of cylinder Pressure is applied to both rod end and

cap end The majority of the cylinders are basic.

This means that there is unequal areas at either end.

Double Acting Cylinders

Rod extension is slower because has a larger area, but allows a greater force because of the bigger area.

Retract is faster because of the smaller area, but the force allowed is smaller because of the smaller area.

Basic Double Acting Cylinder

Double Acting CylindersDouble Rod Cylinder

Nondifferential type cylinder Same areas at both ends of the cylinder if the rods are the

same size Provides equal force, and equal speed in both directions

Double Acting CylindersTandem Cylinder

Two pistons in line with a common rod This allows you to have a greater forces without

increasing the size of the cylinder bore Tandem cylinders are used in places where there is

insufficient space to increase the size of the cylinder bore.

Cylinder Construction

Cylinder Actuation

Fluid routed to and from cylinder through ports in each of the heads

Cylinder retracts from pressure to rod end port and the other port connected to tank

Cylinder extends from pressure to cap end port and rod end port connected to tank

Cylinder Mounting

Main function to anchor the cylinder Mounting methods include tie rod, bolt

mount, flange, trunnion, side lug and side tapped, and clevis

Tie rod is most common

Cylinder Mounting

Cylinder Mounting

Cylinder Ratings

Ratings include size specifications and pressure capability

Size specifications - piston diameter (bore) - rod diameter - stroke length Pressure capability - pressure rating established by manufacturer - rating limits maximum force capability

Formulas For Applications

V (in/min) = GPM x (231 / Area (in²)) Q (flow) = Area (in²) x V (in/min) / 231 F (lbs) = Pressure (psi) x Area (in²) P (psi) = Force (lbs) / Area (in²)

Effects of Cylinder Performance for Changes of Flow, Size, and Pressure

Data for Different Cylinder Sizes

Cylinder Features

Basic size and pressure ratings are not the only things that define a cylinder

Features such as: - seals - cushions - stop tubes - rod spacers - ports - bleed ports -limit switches

Seals

Cast iron is most common piston seal Long service life is most important characteristic Rubber-like materials are the most common rod seal Rod wiper or scraper keeps foreign material from entering

the cylinder and the hydraulic system

Cylinder Cushions

Used to slow down the piston at the end of its stroke

Basic elements include the plunger, adjustable cushion orifice, and a check valve

Stop Tubes

Metal collar that fits over piston rod next to piston Used primarily on long stroke cylinders Provides better cylinder rod support Majority of applications do not require a stop tube

Tie Rod Spacers

Tie rod spacers and center supports improve rigidity of long stroke tie rod cylinders

Spacer keeps proper position and prevents excessive deflection Tie rod center support has side mounting lugs and serves as

additional mounting location

Ports

Internal or external opening in a cylinder or a valve Designed to allow the passing of fluid into or out of

the component Wide variety of port type configurations Straight thread O-ring fittings are highly

recommended for leak-free connections Poor installation of tubing or hose to port causes

leakage

Bleed Ports

Usually cylinders bleed air when ports are vertical on top

Bleed ports are necessary to remove trapped air when the ports are installed on the bottom of the cylinder

Desirable on high speed, high performance, heavy load applications

Limit Switches

Signal rod position to a control circuit or a safety circuit to limit end of travel

Common types -mechanical = mechanical actuation of electrical switch when switch is

activated by lead angle on a hardened cylinder cushion - proximity = activated when metal cushion passes close to magnetic

pickup of the switch * becoming increasingly popular due to simplicity

Cylinder Installation and Troubleshooting

Cylinder application design » hydraulic engineers

Installation » hydraulic technician Proper installation and maintenance is crucial

to all components to achieve max efficiency Recognizing and controlling potential

problems is purpose of troubleshooting

Design Problem

A dump truck to lift 5 ton. Specify a cylinder.

Velocity in retract is 15 in/sec

Stroke = 18 in

P = 2000 psi

References

Eaton-Vickers Text, Industrial Hydraulics Manual 4th Edition 2001

www.parker.com www.airlinehyd.com www.caterpillar.com www.duramastercylinders.com