vighnesh rubbers

This section provides an overview of the most common types of the O-Ring sealing applications. All O-Ring sealing applications can be classified as either static or dynamic.

- Static Seals: In a truly static seal, the mating gland parts are not subject to relative movement (except for small thermal expansion or separation by fluid pressure), as contrasted from seals in which one of the gland parts has movement relative to the other. There are two basic directions of compression: axial and radial. There are also applications which combine both axial and radial compression (crush seals). However, to achieve the best results the compression should be applied in one direction only to allow the O-Ring expansion.

- Static axial seals: In this application the compression is on the top and bottom of the O-Ring, which is similar to a flat gasket. Static axial seals are typically utilized in face seal applications, as show:

Fig.: Typical static axial seal

- Static radial seals: This application means that the compression is between the inside diameter and the outside diameter of the O-Ring. Typical static radial seals are cap seals and plug seals.

- Reciprocating Seals: In a reciprocating seal, there is relative reciprocating motion (along the shaft axis) between the inner and outer elements. This motion tends to slide or roll O-ring, or sealing surface at the O-ring, back and forth with the reciprocal motion. O-Rings used in reciprocating applications are called piston or rod seals.

- Oscillating Seals: In an oscillating seal, the inner and outer components of the groove move in an arc around the axis of the shaft, first in one direction and then in the opposite direction, usually intermittently, with no more than a few turns in each direction. The most common application for oscillating O-Ring seals is in valves.

- Rotary Seals: In a rotary seal, either the inner or outer member of the sealing elements turns (around the shaft axis) in one direction only. This applies where rotation is reversible but does not allow for starting and stopping after brief arcs of motion which is classed as an oscillating seal. Examples of a rotary seal include sealing a motor or engine shaft, or a wheel on a fixed axle.

Other types of O rings used

- Seat Seals:

In a seat seal, the O-ring serves to close a flow passage as one of the contact members. The motion of closing the passage distorts the O-ring mechanically to create the seal, in contrast to conditions of sealing in previously defined types. A sub-classification is closure with impact as compared with non-impact closure. Examples of a seat-seal include and O-ring as a “washer” on the face of a spiral threaded valve, a seal on the cone of a floating check valve, and a seal on the end of a solenoid plunger.

- Pneumatic Seals: A pneumatic seal may be any of the previously described types of O-ring seals but is given a different classification because of the use of a gas or vapour rather than a liquid. This has a vital affect on the lubrication of the O-ring and thus influences all moving (or dynamic) seal installations. A further point is that pneumatic seals may be affected by the increase in gas temperature with compression. Note that the seal should be defined as “pneumatic-rotary” etc. for complete identification.

- Vacuum Sealing: A vacuum seal confines or contains a vacuum environment or chamber. The vacuum seal may be any of the previously defined types (except a pneumatic seal) and as in the case of “pneumatic seals”, both terms applicable to the seal should be given for complete identification. This classification is given primarily because, in most cases, the leakage tolerance is less than for pressure seals. In addition, the problem of pressure trapped between multiple O-rings, which increase the load on a single O-ring, does not apply. Multiple O-rings are useful in a vacuum seal.

- Cushion Installation: Such an application requires that the O-ring absorb the force of impact or shock by deformation of the ring. Thus, forcible, sudden contact between moving metal parts is prevented. It is essentially a mechanical device. An example is the use of an O-ring to prevent metal-to-metal bottoming of a piston in a cylinder. The O-ring must be properly held in place as otherwise it might shift and interfere with proper operation of the mechanism.

Select the hardness:  The hardness of 70 Shore A is most common and it gives a good sealing performance. If the pressures is higher >5MPa a hardness of 90 Shore A can be chosen to prevent extrusion of the O-Ring. For pressures of more than 10 MPa (100 bar) we recommend the use of Back-Up rings. Of course, depending on the application, the material, cross-section and the clearance are important.

Select the O-Ring dimensions:  The dimension of the O-Ring should be properly diagnosed keeping in mind two main things:

Selection of the inside diameter (d1): Axial installation (static) when the pressure is from the inside, the outside diameter of the O-Ring should be 1-2% larger than the outside groove diameter. When the pressure is from the outside, the inside diameter of the O-Ring should be 1-3% smaller than the inside groove diameter. Radial installation (static and dynamic) for Outer sealing (piston seal) the O-Ring inside diameter should be equal or max. 5% smaller than the inside groove diameter. And for Inner sealing (rod seal) the O-Ring inside diameter should not differ much, if at all, from the rod diameter, the exception being the O-Ring used as a rotary seal. In this case the O-Ring inside diameter needs to be 3% larger than the rod diameter.

Selection of the cross-section (d2):

In general an O-Ring with a larger cross-section generally has a better compression set, less swell and accepts larger tolerances.
Disadvantages are the larger supporting structure and increased friction. Consequently a smaller cross-section is usually preferred for dynamic applications.

This chart matches the general rubber properties required in most O-ring applications with the capabilities of commonly used elastomers. Since no one elastomer is rated ''excellent'' for all properties, compromises are sometimes necessary when selecting elastomers for a specific O-ring application. Start with most critical properties to narrow your choices

E = Excellent G = Good F = Fair P = Poor

Comparison of Common Seal Types
A number of common seal types, T-Seals, U-Cups, V-packing and other devices, have been, and are still used for both dynamic and static seals. When compared with an O-ring seal, these other seal types may show one or more design disadvantages which might be overcome by use of an O-ring. As an aid in assessing the relative merits of an O-ring seal, Table lists several of the important factors that must be considered in the selection of any effective seal geometry.

1- Metal inserted Oil Seal:
It is useful where housing may have a greater coefficient of expansion than the metal case oil seal. It is suitable for general sealing applications.

2 - Open type Oil Seal:

The metal encased on the outer diameter and on the non fluid sealing side ensures a better resistance to thermal expansion when compared to rubber cased metal oil seals or rubber oil seals.

3 - Closed type Oil Seal:

Is covered with both side metals, the complete
metal encased ensure strength for seal. Especially for heavy application.

4 - Rotary shaft seal (a):

It is somewhat similar to metal inserted oils seal but with an additional dust lip which is provided to avoid entry of light to medium dust and dirt.

5 - Rotary shaft seal (b):

It is similar to Open type Oil Seal but with an additional dust lip which is provided to avoid entry of light to medium dust and dirt.

6 - Rotary shaft seal (c):

Is similar to Closed type Oil Seal but with an additional dust lip which is provided to avoid entry of light to medium dust and dirt.