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.
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 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 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)
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