View A Video on Hydraulic Cylinders - A Quick Introduction
Hydraulic cylinders are actuation devices that convert the hydraulic
energy of pressurized fluids into the mechanical energy needed to
control the movements of machine linkages and attachments. This energy
conversion generates linear force and motion. While single acting
cylinders are available, most are double acting hydraulic cylinders,
meaning that they can move in both directions along a horizontal,
vertical or any other predetermined plane.
Along with directional divides, hydrolic cylinders can be separated into categories based on cylinder or barrel type. Welded hydraulic cylinders and tie rod hydraulic cylinders are the most common variations. The latter, tie-rod barrels, are often used for heavy duty hydraulic cylinders as they are reinforced by several external rods which bear the majority of the applied load. Welded cylinders, however, are small hydraulic cylinders commonly used in construction, agricultural and other industries where powerful cylinders must be fitted to compact machines. Smaller, lighter designs do not result in a diminished capacity as illustrated by high pressure hydraulic cylinders which are often light weight and smaller than many other models. Specialized barrels, such as hydraulic rams and telescopic cylinders, are also widely available. Cylinders of any body type use hydraulic pistons to lift, turn, tilt, press, steer, pull and push heavy machine components and any attached loads. This strenuous work necessitates they be made of durable materials. As even stainless steel hydraulic cylinders can eventually corrode or become damaged, replacement cylinders are also available to increase the longevity of hydraulic systems.
Also called redesigned or re-engineered cylinders, replacement components can be used for more than repairs and are often integrated as operational upgrades for pre-existing machinery. As industries continue to grow, so do the required capabilities of hydraulic cylinders and the industrial equipment and machines of which they are a part. Hydraulic cylinders are extremely variable, allowing their use in a number of different industries. Agriculture, construction, oil and gas, manufacturing, military, machining, automotive, aviation, robotics, aerospace and waste removal industries all make use of these devices which greatly increase efficiency and mechanical capabilities as the output force is much higher than the initial force applied. Although the mechanism itself can provide this force in only a linear motion, clevis fittings attached to the end of the piston rod allow the attached devices to move in an angular motion. This significantly broadens the usability and applicability of hydraulic cylinders. Lifts, material handling equipment, snowplows, brakes, power steering, excavators, dozers, back hoes, cranes, elevators, forklifts, jacks, dump trucks, spacecraft, ships, planes and even advanced robotic arms utilize the power of hydraulics.
Despite their impressive conversion of kinetic energy into mechanical energy, basic hydraulic cylinders are relatively simple devices. A round, rectangular or oval tube shaped barrel makes up the main body of the cylinder that houses and connects all of the components. At one end of this barrel is the cylinder cap, which closes off and seals the non-moving end of the cylinder. The cylinder head closes off the other end, but has a round seal through which the piston rod may move in and out. Double acting cylinders have a cylinder head on both ends and no end cap. The piston face is a metal disc-like part that fits exactly in the cross section of a cylinder barrel, dividing the chamber into two smaller compartments. The piston is essential to the generation of linear motion by means of hydraulic fluid pressurization. Attached to this is the piston rod. The rod is housed partially within the barrel, but extends beyond the body through the cylinder head and is attached to the machine components which must move. Each compartment within the barrel also has a port through which high pressure hydraulic fluid is introduced and through which un-pressurized fluid is returned to a reservoir. Tubing and a pressure vessel are needed to store and transport the fluid. When pressurized hydraulic fluids are introduced to the vessel, it presses upon the piston and engages the attached rod. Direction is determined by what side of the piston meets with the pressurized fluid. Fluid above the piston will retract the rod, while fluid below it will cause it to extend. The introduction of various amounts of pressurized hydraulic fluid on either end controls the movement of the piston, rod and attached load. Several seals are placed around the piston head, flow valves and cylinder head to ensure that fluids do not leak into, out of or from one compartment to the other causing a loss of pressure and decreased functionality.
Not only the seals, but all cylinder components must be made of durable materials that can withstand the friction and heat created by the use of the hydraulic cylinder. The barrel, seals, piston and rod must also be compatible with the hydraulic fluid which is generally a mineral, oil, ether or water composite. Stamping or extrusion processes are used to produce seals which are made of nitrile rubber, viton, polypropylene, brass or stainless steel depending on the application. The pistons are made of brass, steel, stainless steel, aluminum, cast iron, or bronze. Piston rods and cylinders are made of these same materials, though are produced by different manufacturing processes. Cold rolling is used to manufacture the rods, which are often hard chrome plated to provide protection from corrosion and wear. Cylinders are made using a few possible techniques. Welding uses a single sheet of metal, while other techniques, such as CNC machining combine different finished components to create the final piece. In either case, the interior surface of the barrel must have a micro-smooth surface, allowing the piston to move cleanly through the body with minimal energy lost to friction. Selecting the proper hydraulic cylinder for a specific application involves more than manufacturing technique, body material and fluid. Further considerations include maximum operating pressure, stroke, bore size and rod diameter among others. As the work force generated by pressurized hydraulics can vary significantly, it is important to understand system requirements before choosing a specific model.
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Types of Hydraulic Cylinders
- Double acting hydraulic cylinders use hydraulic pressure to actuate the rod to extend and retract in both
- Heavy duty hydraulic cylinders are designed for high pressures, high flows and rugged environments.
Heavy duty cylinders are particularly suited to demanding industrial
and mobile applications.
- High pressure hydraulic cylinders have significantly smaller and lighter designs than
standard cylinders, which saves substantial weight and space in equipment.
High pressure hydraulic cylinders are used in applications that need
high forces and short or medium strokes, such as material testing and
- make the appartus that changes hydraulic fluid into mechanical power.
- Hydraulic pistons are short, cylinder shaped discs housed within cylinder barrels in order to compartmentalize the enclosed space within hydraulic cylinders.
- Hydraulic rams are the large output pistons.
- Hydrolic cylinders are devices that convert pressured fluid into mechanical power. Hydrolic is a common mispelling of hydraulic.
are designed for a multitude of applications in which a large force
is required in both the push and pull directions.
are used in many applications, such as snowplows, construction equipment,
personnel lifts and material handling equipment.
- Replacement cylinders are manufactured and installed in older equipment with outdated cylinders. Replacement cylinder manufacturers offer products that will update machinery with the newest cylinder technology.
use hydraulic pressure to actuate the rod in only one direction.
- Small hydraulic cylinders can have strokes of less than an inch and are used in applications that require extreme precision.
- Stainless steel hydraulic cylinders are linear actuators designed specifically for highly corrosive environments as well as those where hygienic cleaning is essential to industrial processes.
are two-way hydraulic cylinders that provide a faster starting stroke
and a subsequent, more powerful working stroke.
- Telescopic cylinders have multiple stages which enable longer strokes to be achieved, while
utilizing less space.
have their head gland threaded on and the threads are protected by an
- Tie rod hydraulic cylinders use one or more steel rods, which are installed on the outside diameter
of the cylinder housing, to provide extra stability. Cylinder tie-rods
commonly bear a large portion of the applied load.
- Welded hydraulic cylinders are made of a heavy duty, smooth welded housing for increased stability.
Most housings of hydraulic cylinders are made of multiple parts, but
not in the case of welded cylinders.
Hydraulic Cylinder Terms
A container in which fluid is stored under pressure. Accumulators have
some type of limit in the loading mechanism for maintaining pressure.
- Also called a bleed
valve, or a device commonly used on hydraulic cylinders to remove pressurized fluid
and air from the system.
- The inside diameter of
- End closures of hydraulic
that completely covers the bore area.
- Cap, plug or cover
for the fluid passage of hydraulic cylinders.
adjustable, mechanical or hydraulic device that is used to limit the
stroke of hydraulic cylinders.
- A device which meters
the rate of fluid used in hydraulic cylinders.
- The cavity of a stuffing
box used within hydraulic cylinders.
- The end closure of hydraulic
that covers the differential area between the bore area and the piston
- A type of seal consisting
of an elastomer in the shape of a doughnut. O-rings are usually mounted
in a groove on hydraulic cylinders for sealant purposes.
- A cylindrical member
forming the internal element of assemblies in hydraulic cylinders that transmits or
receives motion by a connecting rod. Fluid acts with the piston to convert
pressure energy into linear motion within hydraulic cylinders.
unit of measurement of pressure within hydraulic cylinders.
- A device found within
hydraulic cylinders that turns mechanical
energy into either fixed or variable hydraulic energy.
- The large output piston
of a hydraulic cylinders press.
- A device
that directs the rod end discharge to the piston side of hydraulic
the speed of hydraulic cylinders. Regenerative circuits can be incorporated into
a directional control valve as the fourth position on hydraulic
- A design
that allows the use of two or more hydraulic cylinders in a series. This design
automatically synchronizes the position of hydraulic cylinders at the end of every stroke
within the hydraulic cylinders.
- A device that connects
the bore area to the head of hydraulic cylinders.
- Any cylindrical-shaped
part of hydraulic cylinder components that controls the flow passing through the
component in accordance with its movement.
- The linear movement
of a valve spool or hydraulic cylinder rod that establishes the limits of motion.
- A small chamber
in found within hydraulic cylinders that is compressed around a reciprocating shaft or piston
to form a seal.