When hydraulic or pneumatic systems must produce linear motion, cylinders become their most important component by converting fluid pressure and flow to force and velocity.
The linear motion and high force produced by cylinders are big reasons why designers specify hydraulic and pneumatic systems in the first place. One of the most basic of fluid power components, cylinders have evolved into an almost endless array of configurations, sizes, and special designs. This versatility not only makes more-innovative designs possible, but makes many applications a reality that would not be practical or possible without cylinders.
The most common cylinder configuration is double acting (Fig. 1). Routing pressurized fluid into the rod end of a double-acting cylinder causes the piston rod to retract. Conversely, routing pressurized fluid into the cap end causes the rod to extend. Simultaneously, fluid on the opposite side of the piston flows back into the hydraulic reservoir. (If air is the fluid medium, it usually is vented to the atmosphere.)
1. Cutaway view shows key features of a typical cylinder—in this case, a double-acting welded design for hydraulic service.
Because the area of the rod-end piston face is smaller than the cap-end area, extension force is greater than retraction force (assuming equal fluid pressures). Because total cylinder volume is less with the piston rod fully retracted (because of rod volume) than when it is fully extended, a cylinder retracts faster than it extends (assuming equal flow rates).
Single-acting cylinders accept pressurized fluid on only one side of the piston; volume on the other side of the piston is vented to atmosphere or returns to tank. Depending on whether it is routed to the cap end or rod end, the pressurized fluid may extend or retract the cylinder, respectively. In either case, force generated by gravity or a spring returns the piston rod to its original state. A hydraulic jack for vehicles represents a common application of a single-acting, gravity-return cylinder.
Read more: Cylinders: The Basics and More