Seals (or packings) are deformable materials sandwiched between the mating surfaces to close of small gaps. They retain fluid under pressure within the system and keep foreign matter out. They prevent the metal-to-metal contact of sliding surfaces. They are made from polymer materials that are flexible (rubber) and soft (plastics). The elastomeric quality enables them to flex more efficiently.
A single molecule consisting of a group of similar or dissimilar atoms is known as ‘mer’ or ‘monomer. When conditions are right, several monomers can link together chemically to form long, chain-like structures (polymerization process). The resulting macromolecules incorporating thousands of monomers are known as polymers. Polyethylene, rubber, plastics are examples of polymer-based materials. According to the way the molecules are arranged in polymers, they can be of amorphous or crystalline type.
Amorphous polymers are composed of long and twisted molecular chains that are non-symmetrical. The intermolecular forces are weak in amorphous polymers. All rubbers (elastomers) are amorphous at room temperature. When a compressive load/stretching force is applied to the elastomeric material, the entangled molecular chains uncoil and straighten them. When the stress is removed, the chains tend to coil up again, reverting to their normal state of entanglement.
Crystalline polymers are composed of orderly arranged molecules. The intermolecular forces are strong in crystalline polymers. The orderly arranged molecules of polymers are rigid. Most plastics are either crystalline or semi-crystalline.
Seals are liable to encounter potentially harmful service and environmental conditions such as extreme pressure and heat/cold. Other ingredients must be added to enhance its physical and chemical properties. These ingredients may include fillers (to reinforce the material), cure activators and accelerators (to increase cure speed), plasticizers (to aid flexibility), and pigments (for colourization).
Properties of Polymers
The number of molecules (the chain length) and thus the molecular weight has a significant impact on the polymer’s physical properties. That is, polymers with high molecular weights are essential in the formulation of tough materials meant for applications with severe operating conditions. Physical Properties of polymers include hardness, tensile strength, tear resistance, abrasion resistance, compression set, and resilience. When the polymer is heated, its inter-molecular forces decrease. The seal material must be chemically compatible with the system fluid.
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Measurement of Hardness
Hardness is measured with a portable instrument called ‘Shore durometer’. It utilizes a cone indenter point loaded by a calibrated spring to gauge the resistance of the test specimen of seal material to indentation. The penetration depth (d) of the indenter under the load determines the hardness of the specimen. There are two Shore scales used for measuring the hardness.
Shore A Scale
The Shore A scale is used for testing soft elastomers (rubbers). Its loading force is 822 g, and it uses a lighter spring and a 350 angle indenter point.
Shore D Scale
The Shore D scale is used for testing hard elastomers (plastics). Its loading force is 4536 g, and it uses a stiffer spring and a sharp 30° angle indenter point.
When the Shore A durometer is pressed against a flat elastomer, the indenter point is forced back against the spring. The force reflects on the gauge with an arbitrary scale of 0 to 100. Harder substances generate large durometer numbers. The most common hardness range for seal materials for hydraulic systems is from 50 to 80 Shore A. The shore D durometer accurately measures the hardness of materials that are harder than 90 Shore A.
Terms and Definitions
Compression set is the amount by which the seal material remains short of its original shape after being released by the compressive load. This parameter is, usually, expressed as a percentage of its original dimension. A seal often hardens and assumes the gland shape due to compression set.
Extrusion refers to the flow of a part of an O-ring into the clearance between two mating metal parts when subjected to high pressure. The extruded portion of the seal is liable to be nibbled away from the low-pressure side. The continuous biting away of the extruded part can lead to a complete seal failure. Anti-extrusion devices can be used to avoid extrusion.
Factors Affecting Seal Performance
System and device parameters that affect the seal performance are the pressure, temperature, the speed of movement of dynamic sealing surfaces, the quality and finish of mating surfaces, and the humidity. The properties of the seals are also affected by their exposure to oxygen, ozone, and sunlight.
Seals are subjected to operating pressures as well as the shock pressures. Further, excessive pressure can cause seal extrusion and pressure spikes can deform the seals. The result of repeated deformations is the premature wear of the seal materials.
Lower temperatures may harden the seals and make them brittle. At higher temperatures, the seal materials may become too soft to withstand the applied pressure and are susceptible to extrusion.
As the speed of a moving part at the contact surface of a seal increases, the fluid film between the sealing surfaces breaks and, as a result, more friction is produced.
Improperly finished metal surfaces generate greater friction. Smooth surfaces lack the necessary cavities to hold the lubricating fluids. The purpose of all surface finishes is to provide surfaces that inflict least wear to seals. The range of ideal average roughness (Ra) of the working surfaces in hydraulic systems is 0.5 to 0.6 μm.
Oxygen, Ozone & Sunlight
Oxygen, especially along with heat, causes the hardening of the seal. Ozone and sunlight are capable of causing breakage of the polymer chains.
Classification of Hydraulic Seals
A static seal is used in between stationary parts in hydraulic devices to seal high-pressure fluid. A dynamic seal is used in-between parts, where there is reciprocating or rotary motion. In reciprocating seal applications, a seal slides back and forth within its gland. In rotary seal applications, a seal moves radially within its groove.
O-ring is probably the most commonly used static seal. In high-pressure systems, static seals may be configured with backup rings to prevent excessive compression of the seals and seal extrusion.
Hydraulic seals for applications involving the oscillation or slow rotation with surface speeds less than 15 metres/min are usually classified as dynamic seals. Hydraulic seals for applications involving high-speed rotation with surface speeds greater than 15 metres/min (0.8 ft/s) are usually classified as high-speed rotary seals.
- Based on butadiene and acrylonitrile copolymer
- Excellent abrasion resistance, high tensile strength, and high resilience
- Good compatibility with petroleum-based fluids
- Limited resistance to heat
- Most widely used for U-cups, Lip seal, and V-packing
Viton (Fluorocarbon Rubber or FKM)
- Carbon backboned polymers, highly fluorinated
- Excellent heat resistance, with thermal stability up to 262 0C
- Compatible with a broad range of fluids
- Low compression set and excellent ageing characteristics
- Used for U-cup seals, lip seals, V-packing, and wipers
- Made from silicon, oxygen, and carbon
- Offers good resistance to compression set, at high temperatures
- Highly resistant to sunlight, ozone, oxygen, and moisture
- Mainly used as static seals
- Used for O-rings, gaskets, and special seals
Ethylene Propylene Rubber (EPR)
- Offers excellent resistance to heat, ozone, and UV light
- Not suitable for petroleum-based fluids
- Used for ester-based fluids, such as Skydrol
- Used for U-cup seals, lip seals, and V-packing
- Formulated from copolymers of ether- or ester-based urethanes
- An excellent choice for petroleum-based fluids
- Good mechanical properties, such as high resilience and high tensile strength
- Good resistance to extrusion, abrasion, tear, oxidation, and oil swell
- Maintains the stability of shapes of sealing edges
- Provides long service life
- Formulated from synthetic rubber and fluorine
- Very high heat resistance and excellent mechanical properties
- Low water absorption and good chemical resistance
- Good low-temperature properties and creep resistance
- Used for backup rings, anti-extrusion rings, guide ring bushes, and scrapers
P T F E (Teflon) Group
PTFE (polytetrafluoroethylene)-Teflon -is a fluoro-plastic distinguished by excellent resistance to chemicals
- Offers excellent resistance to most chemicals
- Very low coefficient of friction
- Operates over a broad range of temperatures
- Tendency to creep, depending on the temperature and load
- Used for backup rings, V-packing, O-rings, rotary seals, and gaskets
15% Glass-filled/60% Bronze-filled PTFE
A virgin PTFE is fortified with glass fibre and/or bronze to retain their toughness and flexibility, reduce their thermal expansion, and, improve their wear strength
- Excellent chemical inertness and high heat resistance
- Low-temperature flexibility and low running friction
- Higher resistance to extrusion as compared to virgin PTFE
- Preferred for high power hydraulic applications
- Used for making piston seals, rod seals, and wipers
Seal Materials and their Temperature Ranges