Designing with hydraulic cylinders on mobile machinery

Mobile machinery is rife with hydraulic cylinders, particularly for use on buckets, shovels, booms and other tools and attachments on machines like backhoes, excavators, telehandlers, cranes, balers, skid steer loaders, dump trucks and more. These machines and attachments use hydraulic cylinders to push, pull, lift and lower loads that other might otherwise be impossible to move with lesser force technologies.

Cylinders are responsible for converting hydraulic power into linear motion to do work or move a load by applying pressure to the cylinder’s piston. These somewhat simple devices usually feature a basic construction, including the aforementioned piston inside a cylindrical or rectangular tube or body, end caps and housings, as well as necessary bolts, nuts, plugs, bearings, rod seals and wipers, and more.

The two most common designs used on mobile equipment are welded-tube and tie-rod cylinders. Welded cylinders are constructed of a heavy-duty tube that is closed off with a welded-on cap. Mounting anchors, including trunnion, side lug and side tapped, ped, clevis, bolt, and flange types, are usually attached to the welded cap. A threaded gland on the rod end allows these cylinder types to be easily repaired. The simpler, but less durable tie-rod cylinder is designed with bolts and tie rods that hold two castings on each end of the barrel together. Also easy-repairable, this cylinder design is common on agriculture equipment and is often used on NFPA or ISO cylinders.

To determine a cylinder’s mechanical force, one must know the diameter of the piston and use the following formula to give you the linear force of a cylinder. First, convert the piston diameter into area by using πr2 then multiply area by your chosen pressure unit.

Differential cylinders are the most common hydraulic cylinder design. Because they feature a rod in just one end, the force created when retracting is not equal to that created when extending. In retraction, a differential cylinder has less area for pressure to work upon because of the space taken up by the rod. Consequently, a differential cylinder will retract with more velocity than it extends, given equal flow at either service port. Once again, this effect is a result of the space taken up by the rod—in this case, filling up the reduced volume more quickly. The force reduction resulting from the rod location is inversely proportional to the flow increase from the same cause. For example, if the area of the cap side of the piston is twice that of the area from the rod side of the piston, it is referred to as a “two-to-one” cylinder. This cylinder will extend with twice the force than in retraction, and retract in half the time as extension.

Differential cylinders are normally double-acting, i.e., they are powered in retraction as well as extension. A differential cylinder can be powered in just one direction, if required, which is called single-acting. A single-acting cylinder is manufactured in various forms, and normally when a differential cylinder is used as single acting, it will have a breather at the non-powered port to avoid trapped air. Single-acting cylinders are sometimes spring loaded, and are important if the mass of the load isn’t enough to push the fluid back through valves alone. Springs can be added to either the cap or rod end of a cylinder, enabling spring-extend or spring-retract functions respectively.

Single-acting cylinders can also be rams, which are cylinders consisting of just a rod in a tube. Pressure pushes the ram up or out, but often requires energy to retract. This energy can come from the mass of the load, but sometimes is generated from springs or other devices that are part of the machine itself, rather than inside the cylinder.

Another common cylinder species is the telescopic variety. These cylinders use multiple-sections that are both bodies and rods, and can extend far longer than their retracted length would suggest. They are relatively compact and can usually extend twice their retracted length.

Telescopic cylinders have as many as seven stages or more, providing a compact footprint with a surprisingly long stroke length. A cylinder with 120 in. of stroke only needs 50 in. of retracted space, while a traditional cylinder will always be longer than whatever its stroke length is.

Image courtesy of Aggressive Hydraulics