Selecting seals for bespoke hydraulic cylinders
Steven North of Apex Hydraulics shares some of the questions that designers need to consider when choosing a seal for hydraulics cylinders.
Seal selection for hydraulic cylinders is a complex process, where multiple factors must be considered. Hydraulic cylinders are used in so many different applications and environments that there are almost unlimited combinations of factors, complicating the process of seal selection. The seal that a designer chooses is a very significant part of the hydraulic cylinder design.
Identifying the best seal is vital to the correct operation and longevity of a hydraulic cylinder, thereby massively increasing the lifespan of the system. So let’s look at some of the typical questions asked during the seal selection process.
Does the seal need to be easy to replace? In some cases, hydraulic cylinders can weigh many tonnes and be within a fabrication that weighs much more. Bringing such a cylinder back into a workshop for replacement of a seal is impractical; sometimes it would require removing the factory roof and hiring cranes to lift the cylinder. When seals have been built into applications such as this, it is important to choose seals that are designed to be able to be changed on site, so the entire fabrication doesn’t need to be removed.
What temperatures will the cylinders be working at? Some seals will be working under particularly low temperatures or very high temperatures. Other hydraulic cylinders will experience regularly fluctuating temperatures, or even have to endure sparks from nearby operations. The seal needs to be chosen in response to the temperature that it will be working at. There are large numbers of specialist seals, made to cope with extreme temperatures. For high temperatures, materials such as Viton, with a very high melting point would be used. Some seals may be in freezing temperatures and have to resist without cracking or splitting. Low temperatures may require a resistant polyurethane seal.
Where will the cylinder be used? The environmental conditions where the cylinder is used will have a huge impact on the seals that are chosen. Cylinders placed on the sea bed need to keep very high pressures of sea water out, while keeping oil in. Lots of land based environments, such as plant based machinery may be prone to very
high oil contamination levels and the seals will need
to be especially durable to keep dirt and other contaminants from entering the system.
What speed will the cylinder be working at? Most seals are designed to be used at the relatively slow speed strokes that are typical of many hydraulic cylinders. Some hydraulic cylinders, however, such as those used in factory production lines or on piling rigs, need to move more quickly. Special seals that are able to cope with the friction and heat of these speeds have been designed for this, made of robust, hard wearing materials such as PTFE and Zurcon.
What hydraulic fluid will the cylinders be using? Different applications will require the use of different hydraulic fluids. Cylinders working in potentially combustible environments, such as in deep mining applications, will need to use a non-flammable hydraulic fluid. Hydraulic fluids used subsea and in other precarious eco systems, will need to be non-damaging to the environment.
Synthetic hydraulic fluids have been developed to be non-flammable and non-detrimental to the environment; but these often have other properties that can cause standard sealing materials such as nitrile rubber to break down over time, or swell. In these cases, seals will need to be a resistant material such as polyurethane or Viton.
The viscosity of the hydraulic oil will also be very relevant when determining the best seal for the hydraulic system. At pressure, thin oil can be pushed through gaps that thick oil can’t, therefore requiring an energised seal, which is composed of a seal within a seal working like a spring to push outwards, creating a very tight closure.
What pressure will the hydraulic system be working at? The working pressure of the hydraulic system will greatly affect the choice of cylinder that a designer will make. At low pressures energised seals will be used to stop oil leakage. At high pressure, seals with precise back up rings will be required to stop extrusion.
Are there any special requirements for the seal? In some cases, there are additional circumstances that require very specialist seals to be used. Hydraulic cylinders placed on the sea bed may be inactive in an extended position for many years, before being required to retract again. In these cases, there may be a build-up of marine growth that a seal is required to clean off.
Specialised rod wipers have been developed, based on ice scrapers, that are able to scrape off a build-up of marine growth. These are extremely precise in measurements, fitting the rod perfectly and cut and sharpened like a knife, to be able to get under the marine growth and be strong enough to scrape it off. Secondary to the rod scraper would be a polyurethane wiper to remove any other contaminants that have been missed as well as helping to keep out subsea pressures.
Selecting the right seal for your application: Your bespoke cylinder designer will be able to work closely with you to analyse all the different factors of speed, temperature, pressure, oil type and application, and suggest the best seal selection for your design.
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