By Josh Cosford
Hydraulic cylinders are used in countless machinery applications: Excavators, loaders, balers, telehandlers, cranes, man-lifts, drill-rigs, dump trucks—not to mention operating booms, arms, lifts, platforms, buckets, and platens. Hydraulic cylinders are the most effective and efficient method of pushing, pulling, lifting and lowering.
Cylinders convert hydraulic energy into linear pushing or pulling mechanical force by applying pressure to the piston area of the cylinder. The mechanical force generated is proportional to the combined factors of both the piston area and the pressure applied to that piston area. Cylinder manufacturers describe their pistons in diameter, so it is necessary to break out your Grade 7 math and use to first convert diameter to area. Multiply area by your chosen pressure unit (be careful with units and ensure they jibe with the units for area), and the result will be the linear force of the cylinder. The formula is Pressure x Area = Force.
With a differential cylinder (the most common type, with a rod in one end only), it is important to consider that force is not equal when retracting as 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.
Cylinders are created with various manufacturing techniques, but the two most common types are tie-rod and welded-tube. A tie-rod cylinder uses cast or machined ends, a common tube between them, and then tie rods to pull the cap end towards the rod end. Tie rod cylinders are easy to repair, and depending on their duty rating, can be anything from inexpensive farm-duty tie-rod cylinders, to NFPA or ISO cylinders, which are expensive and heavy when optioned for high pressure.
The welded-cylinder uses a heavy-duty tube with a cap welded to it, and often the mounting treatment welded to the cap. The rod end usually has a threaded gland, so they are repairable as well. Welded-type cylinders are the most common in mobile applications, and typically have a favorable price to quality ratio.