What is a hydraulic cylinder? It seems like such an elementary question, especially considering the depth of fluid power related material on the various WTWH and Design World websites. However, as a person whose job deals with fluid power on the daily, even I can forget how important the fundamental lessens in hydraulics are.
A hydraulic cylinder is a linear actuator powered by hydrostatic energy. A linear actuator is a mechanical device engineered to create force in a straight line, either pushing or pulling. Hydrostatic energy is the motivation behind the cylinder’s ability to create linear force, and is created (i.e., converted) by the hydraulic pump. Hydraulic energy is a combination of pressurized fluid and the volume or rate the fluid moves at, which is expressed as pressure and flow respectively.
A simple ram-type cylinder consists of a cylindrical tube (hence, “cylinder,”) and a steel rod inserted and sealed in that tube. Pressurized fluid is pumped into the cylinder, where pressure upwards of 3000 psi or more acts upon the circular bottom of the tube. The force of the pressurized fluid will push the rod out of the cylinder as long as the mass on the rod is less than the force created against the rod by the fluid. It should be noted that force not flow creates motion; the creation of movement due to flow is a popular misconception requiring explanation another day.
Often times, a hydraulic cylinder has a piston attached to the bottom of the rod, and this piston is sealed against the barrel. A piston allows the rod to be smaller in diameter than barrel, opening up area and volume on the rod side of the cylinder. Because hydrostatic energy can act on either side of the piston, it can now both push and pull, unlike a ram which can only push. A caveat with these “double-acting” cylinders is that the rod volume and area is reduced, creating a differential between the rod and piston sides. These “differential” cylinders will retract with less force because of the smaller piston area, but retract with more velocity because of the reduced volume.
Hydraulic cylinders are used on every application requiring linear force, or by taking advantage of levers and booms, they are used on bending, tilting and arcing applications as well. They are used on presses, shears, clamps et al, for industrial applications, and on booms, buckets, lifts et al, for mobile applications. Hydraulic cylinder applications are countless, and because of the benefits of hydraulics, especially on mobile machinery, they will continue to thrive for decades to come.