Hydraulic Seals

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There are two main types of hydraulic seals: static, which seals within a confined space like a gasket, and dynamic, which is exposed to movement on its inner diameter, usually around a shaft of some kind. Dynamic hydraulic seals, also called shaft seals, include rod seals and piston seals, which are hydraulic cylinder seals that keep pressurized liquids contained within the cylinder and contaminants and air from getting inside. These seals are made with a lip that provides superior sealing while being exposed to linear movement. Lip Seals are made of many different elastomeric materials, including polymers, plastics and rubbers, and they are effective in providing a seal in revolving machinery and equipment. These are called rotary seals. Rubber seals, particularly Teflon seals, are generally the most widely used because of their strength, durability and resistance to chemicals, moisture and high temperatures. Bonded seals are composed of an elastomeric seal that is adhered to a metal washer for high pressure applications. Many hydraulic systems use oil as hydraulic fluid. They use oil seals (or metric oil seals outside of the United States) that are made of resistant materials like Teflon, silicone, polyacrylate or a fluroelastomer that can handle continuous exposure to oil. Without sealing devices, hydraulic systems would completely fail and cause the machinery they power to become unusable. While they are reliable and last for a long time, regular maintenance checks ensure the seals' extended performance.

Various types of seals are used in applications that have constantly moving equipment, such as the rotating or reciprocating shafts and cylinders that are an essential part of hydraulic and pneumatic systems. Devices that use seals include hydraulic rams, presses and mixers, brake devices, valves, clean rooms, conveyors and test equipment. The most common place to find these seals, however, is in a hydraulic cylinder. Hydraulic cylinders use water, oil or other pressurized fluids to facilitate the movement of heavy machinery. Construction equipment, agricultural vehicles and forestry machinery all use hydraulic systems to power their moving parts. The seals are located in the piston, on the cylinder head or on the rod shaft. They are used to prevent the hydraulic fluid from flowing across the piston, from leaking past the interface between the rod and the head and to keep it from leaking to the outside of the cylinder. They also provide a degree of lubrication and decrease metal-to-metal contact and friction. Wear rings are another type of sealing device, as well as rod wipers. They are used to prevent contaminants from entering the cylinder when the extended rod retracts back.

The distinctive types of sealing positions and directions that are used for hydraulic and pneumatic seals are axial seals, rod or internal seals, external or symmetric seals and piston seals. The sealing direction is important for hydraulic seals and pneumatic seals in mechanisms that use axial movement, such as cylinders and pistons. Because the action may be single or double, a seal can be used for one or two directions. One of these types, the rod seal, is radial and is press fitted into the housing bore. Another, the axial seal, has the function to axially seal an adjoining housing or other mechanical component. When purchasing hydraulic seals, there are some important factors to consider, such as the diameter of the housing bore and the inner or outer seal, the dimension of the outer shaft and the axial or radial cross section needed for the application. Also, it is important to consider certain operating parameters like the working temperature, vacuum rating and highest working speed and pressure. There are many types of hydraulic seals for different uses in applications. Some of the industries that make use of these seals include medical and pharmaceutical products development, waste disposal, aerospace manufacturing, marine products manufacturing, food and chemical processing, automotive manufacturing, defense contracting, nuclear power and pulp and paper mills.

Hydraulic seal manufacturers generally fabricate seals from rubber or metal, and in some cases leather or felt. Some of the rubber materials used to create seals are nitrile, silicone, natural rubber, butyl and styrene butadiene. Metals used to make the washers in bonded seals include stainless steel, aluminum, brass, bronze and carbon steel, all of which are often galvanized or plated for extra strength and protection against oxidation. Most elastomeric seals are made via extrusion, while Teflon seals are sintered in an oven from powder form, since Teflon cannot melt. Those that are bonded to metal washers use a bonding chemical to create a permanent and strong bond between the rubber material and metal. Different kinds of hydraulic seals exist for numerous purposes and applications. Oil seals are common hydraulic seals. They are used to retain oil and other lubricants in rotary applications and prevent leaks. Many of these operate with a flexible lip that rubs against the shaft or housing to provide a tight seal against leakage. Mechanical seals are used in applications that have a rotating shaft under a state of high pressure, temperature and speed. These hydraulic seals are used to prohibit fluid leakage for such mechanisms as agitators, pumps, mixers and cryogenic seals. Hydraulic seal manufacturers create many products that are application specific and specially designed to prevent leaks under very particular circumstances.

Images Provided by Allied Metrics Seals & Fasteners, Inc.

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Hydraulic Seal Types

• Balanced seals are seals that are designed to reduce the effects of hydraulic pressure in the seal compartment.
  
• Bellows seals use a formed or welded bellows to supply a secondary sealing and spring loading.
  
• Bidirectional pressure seals uniformly seal in response to pressure from both directions. Bidirectional pressure seals are also known as "double balanced seals" or "reversible balanced seals."
• Bonded Seals are seals that involve metal materials bonded to rubber materials in order to combine the strength of metal with the elasticity of rubber.
• Dynamic seals stop leakage past components that are in relative motion.
  
• Isolators are a type of bearing seal used in place of oil seals to supply greater reliability.
• Hydraulic cylinder seals are tools used to retain the pressurization of fluids and to prevent the contamination of hydraulic cylinders.
• Lip seals are versatile and high-performance seals used for uniform, low-pressure sealing.
  
• Mechanical seals prohibit fluid leakage around the rotating shaft in a state of extreme pressure, temperature and shaft speed. Typical uses for mechanical seals are for agitators, pumps, mixers and cryogenic seals.
  
• Metric oil seals are available in a wide variety of sizes, all measured in metric units.
  
• Oil seals and grease seals are seals with flexible lips that, when placed to a housing or shaft, prohibit the escape of fluids or prevent contaminants from entering. Certain grease and oil seals are equipped with a spring to aid in maintaining contact of the seal with the shaft.
  
• O-ring seals are sealing rings that have a circular cross section that is either solid or hollow.
• Piston seals are tools used to reduce friction between a piston and the cylinder in which it operates; they also retain hydraulic fluid pressure in piston cylinders. 
• Rod seals are tools used to retain pressure in hydraulic cylinders as well as to prevent the contamination of those cylinders. 
• Rotary seals are tools used to protect rotary systems from becoming contaminated by dirt and other hazards; they are also used to help maintain fluid pressure in rotary cylinders.
• Rubber seals are seals that are made from rubber materials, such as nitrile, viton, natural rubber and hypalon, among others.
• Shaft seals are tools that help to prevent the contamination of hydraulic shafts. They are also used to help retain fluid pressure in hydraulic systems.
• Teflon® seals are seals that have been made from Teflon® or are coated with Teflon®. Teflon® is a plastic material that supplies improved lubrication properties, resistance to wear and added strength.

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Hydraulic Seal Terms

Abrasion - The surface loss of a material caused by friction.

Abrasion Resistance - A material's resistance to the surface loss caused by frictional forces.

Adhesion - A state in which two surfaces are bonded together through interfacial forces.

Axial Movement - Motion along an axis or parallel to the center line of a shaft.

Bellows - A component made from corrugated plastic or rubber around a shaft that has the ability to stretch.

Bill of Material - A list of all materials needed to manufacture a certain product.

Bloom - A change of appearance in rubber, resulting from a solid or liquid material moving towards the surface. Bloom resembles surface dust.

Brittleness - The tendency of a material to crack under deformation.

Conductive - A material's property of being able to conduct/transmit electricity.

Cross Section - The view of an o-ring when cut at a right angle to the axis, which shows its interior construction.

Excluders - Used in a hydraulic or pneumatic cylinder to exclude or scrape the rod clean.

Flow - A material's ability to move in a mold during the molding process.

Friction - The resistance to motion when two surfaces are in contact.

Oxidation - Occurs when a material's surface reacts with oxygen, usually causing a change in the appearance or texture of the surface.

Plug - A conical rubber component pressed into a hole or tube to finalize a seal.

Porosity - The measure of small holes in a material.

Resilience - A material's ability of returning to its original state after deformation.

Shrinkage - The tendency of a material to contract while cooling.

Spacer - A ring with flat sides to provide specific dimensional spacing between two components.

Viscosity - The resistance of a material to flow under stress.

Washers - Round, flat rings used as spacers, gaskets or slip devices under the head of a bolt.

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