Seals - Types and Selection Considerations
A seal is a device which when installed in a gland formed by two mating parts inhibits the flow of a fluid (gas or liquid).
There are many kinds of seals: all metal, all rubber, plastic, or combinations of these (composite and compound). A composite seal is made of metal or plastic and rubber bonded together whereas in a compound seal the metal and rubber are fitted together.
Over the years many configurations of seals have been developed for both general and specific applications. From V-Rings to X-rings to the shaft seal, the geometry's of the seal cross section have proliferated. However, the most common and widely used seal is the 0-ring, a simple doughnut shaped device of great versatility.
As many seal shapes have been developed over the years as have a wide variety of elastomeric compounds. These are primarily synthetic rubber formulations of Nitrile (BunaN), Ethylene Propylene, Silicone and Fluorosilicone, and the Fluorocarbons (Viton TM and Fluorel TM).
These compounds are the result of service requirements for a rapidly increasing variety of challenging environments, which include watches, syringes, engines, sprinklers, disc drives. A vast range of products incorporate seals. And yet, a few principles allow the understanding of the successful function of the seal in an incredible array of applications.
Basically, three primary choices must be made when specifying a seal: the type of seal (the shape), the size or dimensions of the seal, and the material of the seal.
Because the 0-ring is the most common, versatile, and simplest elastomeric seal, the following discussion will use the synthetic rubber 0-ring as the illustrative example of basic sealing concepts.
An 0-ring is a doughnut-shaped object, or torus, that functions as a seal upon compression ("squeeze") between the mating surfaces of the cavity walls or "gland" into which the 0-ring is installed.
Manufactured from rubber-like materials ("elastomeric compounds"), two opposing surfaces of the 0-ring (either top and bottom, or inner and outer) are squeezed between gland walls, creating zero clearance within the gland. This provides an effective seal, blocking the flow of liquids or gases through the gland's internal passage.
All seals fall within one of two categories. Figure 1 shows the two basic applications for 0-rings:
- Static - contained within two NON-MOVING gland walls..."statically" as in face seals.
- Dynamic - contained within MOVING gland walls..."dynamically" as in piston or rod seals.
The 0-ring is the only type of seal commonly used in both situations.
Prior to ordering 0-rings, the Cross Section and Inside Diameter dimensions of the seal must be known.
When considering the optimal cross section of a seal the amount of "squeeze" needed in the application is very important. To obtain the correct amount of "squeeze" for optimum 0-ring sealing, careful consideration must be given to the size of the 0-ring in relation to the size of the gland.
Referring to Figure 1 once again, it can be seen that in either "static" face seals, or "dynamic" piston and rod seals, the 0-ring is being squeezed slightly within the gland.
As Figure 1 clearly demonstrates, 0-ring squeeze may occur in one of two possible directions:
- Axial - squeezed on the top and bottom surfaces of the 0-ring..."axially" as in face seals.
- Radial - squeezed on the inner and outer surfaces of the 0-ring..."radially" as in piston or rod seals.
Virtually every gland has a slight gap ("diametrical clearance") between the two mating surfaces forming the gland's internal cavity. It is important, therefore, for the 0-ring cross section to be larger than the gland depth, allowing seal squeeze to block the diametrical gap and prevent leakage. For that reason calculation of the appropriate Internal Diameter of an 0-ring is critical.
The inside surface of the 0-ring will be riding on the bottom of the piston groove. To provide a complete seal, the 0-ring's Internal Diameter (hole diameter) must be smaller than the piston groove diameter, so that the 0-ring will fit snugly in the groove and not roll or wobble. Therefore, the internal diameter of the 0-ring will be slightly stretched in the application. This stretch should be a minimum of 1 to 2%, and should not exceed 5% in most applications. You can calculate 0-ring I.D. according to the following formula:
Tolerance stack up is an especially important factor in seals. In a sealing application, the tolerances of all of the parts in contact with the seal must be considered in order to create an effective seal. The accumulation of these different tolerances results in tolerance stack up.
For example, in a face seal or gasket type application, careful consideration must be given to the tolerance of the 0-ring in relation to the tolerance of the groove, or glandular space, in which the 0-ring is being installed. Or, in a piston or rod seal application, tolerances for the 0-ring, piston and bore must be carefully considered. These tolerances will affect the 0-ring compression necessary to create an effective seal. Too little or too much compression or squeeze will result in seal failure.
In an application utilizing a standard size 0-ring, it is usually possible to compensate for tolerance stack-up in any number of ways including specification of a larger 0-ring. In microminiature applications there is little room for such compensation.
After you have determined the type of seal to be used, and the initial 0-ring sizing, you will next have to select the appropriate 0-ring material. It is important to consider ALL design factors, such as the necessity of the material to resist the degrading effects of chemical attack, pressure, heat, cold and friction.
Just as the 0-ring is the most widely used type of elastomeric seal, compounds of the Nitrile family are the most widely used material. Because they exhibit a good overall balance of properties, are easy to process, and are economical, Nitrile compounds are where many seal designers begin the material selection process.
The first and foremost consideration for 0-ring material selection is resistance of synthetic rubber compounds to degradation by exposure to specific chemicals. The results of chemical "attack" may be dissolution of the compound or excessive shrinking/swelling of the seal. In fact, the limited chemical resistance properties of natural rubber, such as poor resistance to petroleum oils, were the primary reason for the development of the many synthetic rubber compounds used in the manufacture of 0-rings today.
The first step in material selection, therefore, is to match your application's chemicals with the 0-ring material that offers the best resistance. Chemical compatibility tables are available from leading seal manufacturers.
Pressure affects 0-rings by tending to force the 0-ring to one or the other side of the gland. As pressure increases, the 0-ring cross section becomes distorted and is pressed against one wall of the gland, blocking the diametrical clearance gap between the mating surfaces forming the gland itself.
If the 0-ring cannot resist increasingly high pressure, part of the 0-ring will be forced ("extruded") into the diametrical gap, causing tearing, pre-mature failure and leakage.
Methods commonly used to prevent 0-ring extrusion under high pressure include: reducing the diametrical clearance (gap dimension), increasing 0-ring hardness, specification of a different 0-ring material, and the use of one or more backup rings to close the clearance gap and provide support for the 0-ring.
Low pressure sealing - In a low pressure sealing application, system pressure does not cause a noticeable increase in the seals sealing force. Because of this, all sealing potential must come from the ability of the seal to deform and create a barrier by compression of the seal. Surface finish on both the seal and gland are critical to an effective seal.
Excessive heat, over time, degrades 0-ring materials physically and/or chemically, which may render them nonfunctional as seals.
Excessive cold contracts 0-ring materials decreasing squeeze within the gland to a point where the 0-ring no longer has sufficient contact with the gland surface to produce a seal. Further reduction of temperature, without proper material selection to resist the effects of cold, results in additional 0-ring shrinkage and possible leakage. Extreme cold further affects 0-rings by making them brittle and less flexible.
The effects of "Breakout Friction" can cause excessively high hydraulic pressures to develop, which is an important material consideration in dynamic (moving) applications.
These high pressures are required to dislodge seals adhered to the gland walls when part movement is intermittent.
In continuously moving applications, excessive 0-ring "Running Friction" can cause heat to build up within the 0-ring material itself, causing swelling, which causes more heat to develop, which causes more swelling, eventually resulting in seal extrusion and failure.
As you consider availability and cost when selecting your material, it is important to keep in mind that the more expensive compounds do not incorporate all of the properties of the cheaper ones. Each compound has it's own unique strengths and weaknesses. Choosing a more expensive compound does not necessarily give you more strengths, just different ones. For example, the economical Nitrile may give you a better performance than the more expensive Viton TM in many situations.
For optimum sealing performance, correct 0-ring selection is the direct result of a number of design considerations.
These considerations include:
- Chemical Compatibility
- The ability to resist Pressure, Temperature Extremes and Friction
Because of the complexity of these interacting forces, it is critical for your seal to be rigorously tested in the actual application.
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