The ultimate guide to hydraulic cylinders

Hydraulic cylinders, also known as ‘hydraulic rams’, get their power from pressurized hydraulic fluid, normally hydraulic oil.

The hydraulic cylinder consists of a cylinder barrel, in which a piston connected to a piston rod moves back and forth. The barrel is closed on each end by the cylinder bottom – also called the cap end – and by the cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber, cap end, and the piston rod side chamber, ‘rod end’. The hydraulic pressure acts on the piston to do linear work and motion.

Flanges, trunnions, and/or clevises are mounted to the cylinder body. The piston rod also has mounting attachments to connect the hydraulic cylinder to the object or machine component that it is pushing.

A hydraulic cylinder is the actuator or ‘motor’ side of the system. The ‘generator’ side of the hydraulic system is the hydraulic pump which brings in a fixed or regulated flow of oil to the bottom side of the hydraulic cylinder, to move the piston rod upwards. The piston pushes the hydraulic oil in the other chamber back to the reservoir. If we assume that the oil pressure in the piston rod chamber is approximately zero, the force on the piston rod equals the pressure in the hydraulic cylinder times the piston area (F=PA).

The piston moves downwards if oil is pumped into the piston rod side chamber and the hydraulic oil from the piston area flows back to the reservoir without pressure. The pressure in the piston rod area chamber is (Pull Force) / (Piston Area – Piston Rod Area).

The ‘generator’ side of the system is the pump which brings in a fixed or regulated flow of oil to the bottom side of the cylinder – to move the piston rod upwards. Hydraulic cylinders transform the pressure and oil flow in a hydraulic system into work or mechanical force. They are used where linear motion is required to move something.

Also known as ‘hydraulic jacks’, ‘hydraulic rams’ or ‘actuators’, they convert fluid power into mechanical energy. A hydraulic cylinder differs from a hydraulic motor as it carries out a linear (translatory) rather than rotary movement, hence the term “linear motor”.

Used at high pressures, hydraulic cylinders produce large forces and precise movement; they are therefore constructed of strong materials, such as steel that can withstand the large forces involved.

There are primarily two styles of hydraulic cylinder construction used in industry:?tie rod and?welded body?cylinders. Beyond this, other broad types of cylinder design include telescopic, plunger, differential, re-phasing, and single and double-acting hydraulic cylinders.

Hydraulic cylinders are usually?double-acting: oil under pressure can be applied to either side of the piston to provide movement in either direction.?Single-acting?cylinders are sometimes used where the weight of the load is used to return the cylinder to the closed position.

Hydraulic cylinders enable more flexibility in design and structure when transferring force between two different points. Different sized cylinders make it possible to create a system that can pull, push, and lift weights; bends and corners can be incorporated into the system design – useful if there are real space constraints.

However, a hydraulic cylinder should only be used for linear pushing and pulling. No bending moments or side loads should be transmitted to the piston rod or the cylinder. For this reason, a cylinder should ideally be connected by using a single clevis with a spherical ball bearing. This allows the cylinder to move and allow for any misalignment between it and the load it is pushing.

Telescopic Hydraulic Cylinder

The length of a hydraulic cylinder is the total of the stroke, the thickness of the piston, the thickness of bottom and head and the length of the connections. Often this length does not fit in the machine. In that case the piston rod is also used as a piston barrel and a second piston rod is used.

These kinds of hydraulic cylinders are called telescopic cylinders. If we call a normal rod cylinder single stage, telescopic cylinders are multi-stage units of 2, 3, 4, 5 and even 6 stages. In general, telescopic hydraulic cylinders are much more expensive than normal cylinders. Most telescopic cylinders are?single-acting?(push).?Double-acting?telescopic cylinders must be specially designed and manufactured.

Tie Rod Style Hydraulic Cylinder

Tie rod style hydraulic cylinders?use high strength threaded steel rods to hold the two end caps to the cylinder barrel. This method of construction is most often seen in industrial factory applications. Small bore cylinders usually have 4 tie rods, while large bore cylinders may require as many as 16 or 20 tie rods to retain the end caps under the tremendous forces produced.

The National Fluid Power Association (NFPA) has standardized the dimensions of hydraulic tie rod cylinders. This enables cylinders from different manufacturers to interchange within the same mountings. Tie rod style cylinders can be completely disassembled for service and repair.

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We can provide you with standard and custom-made hydraulic cylinders, including tie rod hydraulic cylinders, to suit your specific application, requirements, and operating parameters.

Welded Body Hydraulic Cylinder

Welded body hydraulic cylinders?have no tie rods. The barrel is welded directly to the end caps; the ports are welded to the barrel; the front rod gland is then usually threaded into or bolted to the cylinder barrel. This allows the piston rod assembly and the rod seals to be removed for service.

Welded body cylinders have a few advantages over tie rod style cylinders: welded cylinders have a narrower body and often a shorter overall length enabling them to fit better into the tight confines of machinery. Welded cylinders do not suffer from failure due to tie rod stretch at high pressures and long strokes. The welded design also lends itself to customization. Special features are easily added to the cylinder body; these may include special ports, custom mounts, valve manifolds, and so on.?The smooth outer body of welded cylinders also enables the design of multi-stage telescopic cylinders.

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Welded body hydraulic cylinders dominate the mobile hydraulic equipment market in applications such as construction equipment (including excavators, bulldozers) and material handling equipment (forklift trucks and tail lift gates). They are also used in heavy industry such as cranes, oil rigs, and large off-road vehicles in above ground mining.

Piston Rod Construction

The piston rod of a hydraulic cylinder operates both inside and outside the barrel, and consequently both in and out of the hydraulic fluid and surrounding atmosphere.

Metallic Coatings

Smooth and hard surfaces are desirable on the outer diameter of the piston rod and slide rings for proper sealing. Corrosion resistance is also advantageous. A chromium layer may often be applied on the outer surfaces of these parts. However, chromium layers may be porous – attracting moisture and eventually causing oxidation.

In harsh?marine?environments, the steel is often treated with both a nickel layer and a chromium layer – often 40 to 150 micrometer-thick layers are applied. Sometimes solid stainless-steel rods are used. High quality stainless steel such as AISI 316 may be used for low stress applications. Other stainless steels such as AISI 431 may also be used where there are higher stresses, but lower corrosion concerns.

Ceramic Coatings

Due to shortcomings of metallic materials, ceramic coatings were developed. Initially ceramic protection schemes seemed ideal, but porosity was higher than projected. These hard coatings are non-porous and do not suffer from high brittleness.

Lengths

Piston rods are generally available in lengths which are cut to suit the application. As the common rods have a soft or mild steel core, their ends can be welded or machined for a screw thread.

Rephasing Hydraulic Cylinders

Two or more cylinders can be installed in ‘parallel’ or ‘series’ – with the bores and rods sized such that all rods extend and/or retract equally when flow is directed to the first, or last, cylinder within the hydraulic system. This is known as ‘rephasing’ hydraulic cylinders.

In ‘parallel’ applications, the bore and rod sizes are always the same, and the hydraulic cylinders are always used in pairs.

In ‘series’ applications, the bore and rod sizes are always different, and two or more hydraulic cylinders may be used. In these applications, the bores and rods are sized such that all rods extend or retract equally when flow is applied to the first or last cylinder within the system. This hydraulic synchronization of rod positions eliminates the need for a flow divider in the hydraulic system, or any type of mechanical connection between the cylinder rods.

Cylinders come in many forms; we can find the one best suited to your needs.

Reprinted from Hydraulics OnLine website.