Hydraulic cylinders come in various different types and are useful in countless applications, though most cylinders do share a single primary purpose. They ultimately create a linear action that moves something attached to the cylinder. Visually inspecting the cylinder's appearance and its operation is a good way to determine what type of cylinder it is. Hydraulic cylinders can be very large and powerful for heavy-duty industrial applications or much smaller for a host of mobile uses. Many other visual clues can give you more information about a cylinder.
Check the number of fluid ports on the cylinder. Pressurized fluid can push on either end of a double-acting cylinder to move the piston rod. Fluid pushing on the rod end of the cylinder will retract it, and fluid pushing on the cap end will extend it. Since fluid has to flow to both sides of a double-acting cylinder, it will have two fluid ports. A spring forces either the retraction or extension of a single-acting cylinder, so it only has one fluid port.
Look at both ends of the cylinder. The cylinder is a non-differential cylinder if you see rods extending from both sides. Most double-acting cylinders will extend with more force than they retract, because fluid pushes on a larger surface area than on the rod end.Non-differential hydraulic cylinders have equal surface areas on both ends, because the rod takes an equal amount of the piston's space on each side. These cylinders are used when extension force and speed on each type of the cylinder have to be equal.
Watch the cylinder extend. Most cylinders extend fully in one action, but a telescoping cylinder is made from two or more telescoping sections that extend successively. The cylinders don't take up much space, but their telescoping rods can extend further than non-telescoping cylinders. Telescoping cylinders are used to create a longer range of motion than other cylinder types.
Study the cylinder's extension for am extremely small range of motion. This indicates a short stroke cylinder. A short-stroke piston creates a lot of force with a very small rang of motion, because its piston has a substantially larger diameter than its rod end. You can also identify a short-stroke piston by its short length and large barrel radius.
Look at the rod diameter. A cylinder with a rod outside diameter equal to or nearly equal to its interior piston diameter is a ram cylinder. Single-acting ram cylinders make good hydraulic jacks, but ram cylinders can also be double-acting cylinders.
Examine the cylinder's barrel construction. Mil-duty cylinders have thick, heavy duty barrels. End caps usually bolt through the barrel of a mil-duty cylinder instead of screwing onto the barrel or snapping into place like lighter duty cylinders. Non-replaceable cylinders have very different constructions. They are thin-walled cylinders of mostly aluminum construction. They are designed to be thrown out instead of being rebuilt.