Sealing is a critical factor in the proper function of hydraulic applications, as leakage from the cylinder or across the piston, along with the ingress of unwanted contaminants, decrease the lifespan and efficiency of the entire application. Not only must hydraulic seals prevent leakage, but they must also be capable of withstanding high pressures, extreme temperatures and transverse forces within the cylinder itself.
In any hydraulic machine system, motion is produced by the action of pressurised fluids (gas or liquid). Although production techniques continue to evolve by becoming more efficient and streamlined, the fundamental principle behind hydraulics has remained the same essentially. Fundamentally, hydraulic systems are a push or pull mechanism that enable the machine to perform its function. For example, heavy machinery uses hydraulic fluid to lift hefty loads and the more efficient the hydraulic system, the less effort is required to lift a given load.
A piston is a part of the hydraulic system located within the cylinder (which is normally gas and liquid tight). The primary function of the piston is to transfer power by ejecting fluid from or compressing fluid into a cylinder. The force exerted by the piston and cylinder is directly proportional to the pressure per square unit (cm or inch) exerted by a fluid acting upon it. In other words, the more pressure exerted per unit of surface area, the more the piston will move and, therefore, the more work will be done for a given load.
The diameter of the cylinder is known as its bore, and its edges are sealed in such a way that the fluid does not pass to the other side of the piston. In any hydraulic system if pressure drops, one of the first areas to assess is the wear of the piston seal. The Piston itself is attached to a rod which is held in the centre of the bore by a nut on the other side of the piston, consequently push and pull calculations must take into account the mass of the rod. The stroke of the piston system is the distance as measured by a fully extended rod.
Piston Seals or Piston Rings are internal to the cylinder head and seal against the cylinder bore, preventing fluid from flowing across the cylinder head. This allows pressure build up on one side of the piston, making the cylinder extend or retract. Piston seals are dynamic seals, typically single-acting and double-acting seals. In single-acting applications, the outside diameter lip is designed to seal against the cylinder bore, whereas the inside diameter lip is designed to seal the gland. Double-acting pistons contain pressure on both sides of a piston without leakage. They allow maximum mechanical effort to the moving the piston along the bore of a cylinder.
OTHER HYDRAULIC SEAL DESIGNS
There are numerous sealing products, technologies and techniques to keep oil in and contaminants out of the hydraulic system, each providing unique benefits. However, in some harsh applications, the features of individual seals do not fulfil the demanding requirements for the equipment and a sealing system is required.
Sealing systems use a combination of specific seals that work together to offer superior performance. High-pressure rod end sealing systems normally consist of four elements: the wiper, the rod seal (or main seal), the buffer seal (or secondary seal), and the wear (or guide) bands. The piston sealing system consists of a main seal combined with wear bands. The four most commonly used materials for sealing applications are polyurethane (PU), Nitrile (NBR), fluorocarbon (FKM / Viton), and polytetrafluoroethylene (PTFE).
• Contain system fluid from escaping to atmosphere
• Must provide sealing function at low and high pressure
• Require excellent extrusion and wear resistance
• Should provide good pump-back capability for lubricating oil film
• Often must withstand up to 6000 psi
• Provide aggressive wiping force to prevent mud, water, dirt and other contamination
• Allow lubricating oil film to return to system on inward stroke
• Protect main sealing elements, thus increasing life of seals
• Are often made from Polyurethane, which offers high abrasion resistance
• Are often used as a linkage pin grease seal
• Prevent contact between metal parts in the cylinder
• Centre rod and piston from housing elements
• Increase seal life
UK Seals offer an extensive selection of hydraulic seal styles, each of which provide specific functions based on the machines you use. Our range includes: Piston Seals, Wipers, Rod Seals, Pressure Seals, Single and Double Acting Piston Seals, Rod Composite, Piston Composite, Fluid Seals, Guide Rings, Wear Rings, Vee Packs, Chevron Seals, Internal and External Distributor Seals, PTFE Back Up Rings (anti-extrusion rings).
NON-STANDARD HYDRAULIC SEALS
We appreciate that many custom appliances require a non-standard solution, which is why we place great emphasis on matching our hydraulic seals to your specification.
Contact UK Seals’ Technical team to discuss the best sealing solution for your process. Call us on +44 (0)1388 722817 or send us an email to firstname.lastname@example.org.