Pascal’s law shows that:

Force = Pressure x Area

Within a hydraulic system, the pressure is given by a HPU (Hydraulic Pumping Unit). The area is the size of the hydraulic piston. These two components determine the force that can be exerted by the cylinder.

Therefore, the weight that can be pushed or lifted by a hydraulic cylinder is equal to the pressure provided by the pump multiplied by the size of cylinder rod.

Most pumps have a standard range; usually from 3000 psi to 10,000 psi (210 to 690 Bar). The average tends to be at the lower end of the range; around 210 Bar. A pump can lift an infinite amount of weight, depending on how big the cylinder is. Increasing the PSI may require adjusting the seals, valves and cylinder design to cope with increased pressure.

The pump also dictates the speed at which the cylinder can operate, as it provides hydraulic fluid at the necessary flow rate.

Hydraulic cylinder designers need to think about the scale and size of a hydraulic cylinder. A cylinder that has an excessive reach poses the risk of buckling – meaning that the rod and barrel have to be engineered to be larger. However, this increase in area would have a bigger force, potentially over engineering the fabrication and making it heavier.

The cylinder is first designed by a team of hydraulic cylinder engineers, taking into account the use of the hydraulic cylinder and the environment that it will be in. This dictates materials, coatings, seals and other specialist parts. Hydraulic systems are designed in compliance with any external legislation that relates to the industry that it is being designed for. The offshore industry in particular has a number of requirements, which can design in compliance with.

Once the design has been finalised, the machine shop engineers start producing the cylinder. Cylinder rods and other parts are bored to minutely accurate measurements using highly specialised machinery. Cylinder rods are chromed or coated with specialist coatings such as Corex.

The fitting department puts all the components together (rod, pistons, tubes and seals). Purchasing from a wide variety of high-quality manufacturers,not tied to any one product and are able to choose the most appropriate part for each cylinder. The fitting department assembles the cylinder, ready for inspection and testing.

The in-house inspection department offers rigorous testing of all hydraulic cylinders before they are approved for the client. They perform pressure, pull and load tests as required. Apex are able to facilitate all third-party inspection agencies that may be required for offshore work, including DNV-GL, ABS and Lloyds Register.

Parts are spray painted in house. They can be painted to any required standards. Paint standards for offshore hydraulic cylinders are particularly specialised, with Norsok providing a range of that is typically approved by inspection agencies. At the point of painting, company logos or details can be added.

The cylinder tube (or barrel) is made of steel; usually carbon steel. There are a range of different steels available in terms of strength, with higher pressures requiring stronger steels and thicker cylinder walls. In places where it cannot be painted (such as food preparation areas) it would be made of stainless steel to avoid it being prone to corrosion. The cylinder barrel can be coated in a variety of paint or coatings, with company logos often displayed here.

The internal diameter of the tube usually needs no protection with paint or chrome because the hydraulic fluid protects it from corrosion. However, if the hydraulic fluid is a corrosive material (such as water) the inside of the tube will be coated as well as the rod.

The cylinder rod is the only external part that can’t be painted. Because of this, it has to be protected in other ways. In order for the cylinder to work well the coating has to be completely smooth. The cylinder rod needs to be highly resistant to pitting, corrosion or wear. Any cracks can scrape on seals, causing contamination to enter the hydraulic fluid and leading to eventual hydraulic system breakdown. Therefore, the materials and coating of the hydraulic cylinder is of the utmost importance. Most commonly, the cylinder rod is made of steel or stainless steel and is then coated with Hard Chrome Plating (HCP). However, a number of even more hard-wearing coatings have been developed, including COREX – a specialised coating developed which can be up to ten times less porous than HCP, with hardness of up to 1400Hv; double that of HCP. Extreme environments may require different materials, such as Inconel, which is particularly corrosion resistant.

The piston is attached to the piston rod and moves up and down the cylinder bore, pushed by the hydraulic fluid. It provides the area used to create the hydraulic force. It is vital that no hydraulic fluid can pass over the internal piston.

Most hydraulic fluids are oils, with a huge number of different types available. Different hydraulic systems, pumps and valves will require a different viscosity of fluid. Sometimes, in applications where any flammable liquid poses a risk, the hydraulic fluid may be water based.

Both ends of the hydraulic cylinder need a mounting interface; one at the base and one at the head. The mounting interface depends entirely on the customer requirements and application of the fabrication.
Different mountings include:

• Pins
• Threaded connections
• Flanges
• Trunnions
• Spherical bearings
• Foot mounted
• Bolted

The hydraulic tube needs to be sealed at both ends to stop oil leaking and keep the pressure in. One end of the tube is sealed off with a base. However, the other end needs to be open to allow the rod to move in and out of the tube. At this end the tube is sealed with a neck gland that allows the rod to move in and out. The neck gland material would be chosen during design, with seals on both the rod and the tube.

Hydraulic fluid needs to enter and exit the cylinder via ports, with one port at either end of the cylinder tube and the hydraulic piston between the two ports. These must be secure, as a weak port can cause a dangerous leak of hydraulic fluid under intense pressure.

Seals are used throughout the hydraulic cylinder. They can be made of a wide variety of different materials, depending on the function and type of hydraulic cylinder. They need to be hard wearing, able to survive multiple repetitions of the rod moving in and out of the barrel, removing any contamination.

Hydraulic cylinder designers will select the right seal for the cylinder application, taking multiple factors into account. Cylinders that operate at very high temperatures will require seals that are not prone to melting, and so they may select a material such as Viton. At the other end of the spectrum, cylinders working at freezing temperatures will need a polyurethane material that won’t crack in the severe cold.

Zurcon and PTFE seals may be chosen for circumstances where seals are required to withstand the friction associated with very quick repetitions, such as in a factory environment.

Specialist seals can also be designed with back up rings (for seals working under intense pressure) and seals with an extremely tight closure (for very thin hydraulic fluid that can leak out of standard seals).

Valves and pumps are connected to the hydraulic cylinder via feed pipes and hoses. These are highly specialised, having to work under extreme pressures. They also need to be able to resist corrosion, so will be made of a material suitable for the outside environment as well as the relevant hydraulic fluid.

Filtration is vital to the hydraulic system. Hydraulic fluid must be kept entirely free of contaminants in order to ensure that seals are not damaged, and the system remains working well. There are a number of filters throughout the hydraulic system and depending on the environment of the machinery, these will need to be regularly changed. Some highly contaminated locations, such as factories, may benefit from bypass filtration units, which allow the filters to be changed without stopping the machinery. The filtration system can be isolated from the rest of the hydraulic system – the filters can be changed quickly and easily and then the system reattached. This has been found to increase the lifespan of the hydraulic system without the need for expensive services.