xists an almost endless array of sizes, profiles, and capacities supplied by numerous manufacturers, cylinders fall into two basic operational categories:
The most common cylinder configuration is single acting (see the illustration). Pressurized fluid is routed to only one side of the piston causing extension of the plunger. When the hydraulic fluid is allowed to exhaust through the port, the weight of the load retracts the plunger. An optionally supplied internal spring can also provide forced retraction.
Spring return retraction of single-acting cylinder
To retract, small cylinders are outfitted with steel springs to pull the plunger back once the oil pressure is released. This is called "spring return".
Load return retraction of single-acting cylinder
Larger cylinders retract through the weight of the load pushing the plunger back. This is called "load return."
Routing pressurized fluid through two ports provides hydraulic action bilaterally to the piston. This requires the connection of a fluid circuit to the cylinder in order to accommodate the entering and leaving fluid streams. Double acting cylinders are used in applications that require force to both push and pull.
What are some of the specialized parts of a cylinder?
Most cylinders are equipped with some, if not all, of the following specialized components:
- The plunger wiper located at the cylinder top and plunger interface cleans the plunger during retraction and thereby reduces hydraulic fluid contamination and minimizes seal-damaging foreign matter.
- A stop ring located at the plunger to piston interface, or in some models at the top dead cylinder bore interior, prevents over-extension of the plunger and absorbs unintended lateral forces caused by off-center load application.
- Side bearings located at the interior top and bottom of the plunger stroke support the plunger against side forces that can be produced when eccentric (off-center) loads are encountered.
What other equipment is used with a cylinder?
Depending on the application, a host of auxiliary components can be required to complete a task. The motive force is supplied by an external pump powered by hand, electricity, air, or gasoline driven. A simple connection between port and pump or a circuit is attached to the cylinder in order to convey fluid. Normal fluid power accessories like control valves, hoses, gauges and fittings complete the hydraulic circuit. In some instances an accumulated fluid volume is necessary; this is provided by a fluid storage tank, often referred to as a reservoir. See the diagram entitled Typical Circuit for a Hydraulic Cylinder Jack to get a diagrammatic view of some of these components.
How do I pick the right cylinder to fit my job?
Choosing the right cylinder for your application boils down to answering a list of simple questions:
- Will your job require a pushing or lifting action or a pulling action, or possible both? Single acting cylinders extend the plunger when pressurized; double acting cylinders extend and also retract the plunger when pressurized thus providing force in two directions.
- How much clearance exists between the item you wish to manipulate, i.e. the load, and surrounding stationary structures such as floors or walls? If space is abundant then a standard profile cylinder will be satisfactory. If space is limited then a low profile, ultra-low profile, or even a flat (pancake) profile cylinder may be in order.
- What is the amount or distance of movement that will be necessary? This will dictate the required stroke of the cylinder.
- After lifting, will the manipulated item (load) need to be suspended for an extended period? If the ancillary hydraulic accessories will not be permanently available to the job, then a cylinder with a locking collar (nut) should be selected so that the load can be indefinitely supported mechanically.
- Will the cylinder be relatively stationary during the course of the job, or will its frequent relocation and positioning be necessary? If portability is important, then consideration should be given to lightweight m