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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