Hydraulic systems are often found in dirty environments where the components are liable to contamination. So the correct choice of sealing in an hydraulic system is crucial.
Contamination can enter an hydraulic system in any number of ways, such as through faulty piping and dirty filters.
In addition to contaminants, some equipment may be working in an environment that it was not intended for, or place loads on seals they were never designed to take.
If the hydraulic fluid does become contaminated, the first items that will be damaged are the sealing elements within the hydraulic cylinder.
In a cylinder, there are three primary sealing points: the piston seal, whose job is to energise the piston movement, the rod seal and the wiper seal. If the piston seal starts to wear, then the system will lose efficiency. If the rod seal gets damaged due to wear or particle contamination, then the cylinder will start to leak visibly. The role of the wiper seal is to prevent damage to the rod seal, especially from external contamination.
Before selecting a seal for a given cylinder, it is important to understand some basic parameters about the system. It is important to know the maximum and minimum pressure and temperature the seal will work in, as well as the peak loading pressure, and the environment.
There is a great number of different seal designs on offer. In double acting pistons the seal takes pressure from two directions. So an acceptable solution is a three piece seal comprising a rubber element with two bearing rings. Where extrusion of the seal might prove a problem, designers should specify a seal with extrusion rings. This type of seal is primarily a rubber energised seal. For higher pressures a seal with a rubber fabric mix in the build should be specified.
Where bearing rings are used they need to be placed away from the seal in order to allow the cylinder to withstand higher side loads.
As designers demand that equipment works faster and harder, seal manufacturers have turned to different materials to help out. Newer composite seals that use either PTFE, or a derivative of PTFE with a filler, extend system life. PTFE also eliminates the stick slip problem encountered with rubber seals.
With a single acting seal in a heavy duty application, designers are faced with making a system choice based on temperature, pressure and environmental characteristics.
The rod seal must be able to cope with the same conditions as the piston seal. Here the preferred choice for heavy duty applications is a U-ring fabric construction with a rubber header moulded onto it. In operation, when the pressure in the system increases, the U-ring lips are forced apart.
A polyurethane seal is able to withstand heavy duty conditions. It has good wear characteristics and can withstand temperatures up to 110 C without any problems. What is more, they do not suffer from stick slip, and work faster than traditional seals.
There is also a lot of rod wiper choice. In a simple push-fit arrangement, a rod wiper is simply pushed into the cylinder housing and a metal reinforcement ensures that it stays in place. Some other types require the seal to be inserted into a blind groove. For more heavy duty requirements where large amounts of contamination are found, a scraper design is used to remove any ingress on the cylinder rod.
Although O rings have been around for years, designers are moving away from them to the quad ring or a NU-LIP seal. The main advantage of the newer design is that it has four sealing lips, as opposed to only two on an O-ring. The quad ring cannot suffer from any type of roll, and has the benefit of extra sealing lips. It can also have a special coating like PTFE applied to it to reduce friction.
Cylinder bearing ring materials are also an important part of the system design. A large number of PTFE based designs are now available, all with different fillers, such as glass, bronze, or graphite, in addition to new woven designs that have hard bearing and low friction characteristics. Because of the higher load capabilities of the bearings, such designs do not need such a high surface area, leading to smaller cylinder designs and lower cost.
As hydraulic systems developers move away from mineral to synthetic oils, this too will affect the type of seals used in the system. Rubber based seals deteriorate quickly in these conditions, so polyurethane will come into its own as a replacement. Polyurethane has a cost benefit too. It is about half the cost of the traditional rubber fabric seal, will last longer, and yet will work in an average environment of 2500psi.
In the future, look for a better range of polyurethane and more types of PTFE seals that will allow designers to provide faster, simpler and best of all, less expensive hydraulic equipment.