O-RINGS

O-Rings are torus-shaped (i.e. doughnut-shaped) objects made from elastomeric compounds such as natural or synthetic rubber, and are used to seal mechanical parts against fluid movement (air or liquid).

O-Rings perform their sealing action by deforming to take the shape of their cavity, after being oversized to guarantee a predetermined interference fit.The seal is designed to have a point contact between the o-ring and sealing faces. This allows a high local stress, able to contain high pressure, without exceeding the yield stress of the o-ring body. The flexible nature of o-ring materials accommodates imperfections in the mounting parts. They can withstand fluid pressures upto 100 bar (1500lbs/in2) provided such seals are properly designed and applied. The pressure ratings of the dynamic O-ring seal can be enhanced by using suitable back-up rings. The back-up rings may be of acetal resin, PTFE or Fabric reinforced rubber.

An O-Ring is specified by its inner diameter, cross-section diameter, material hardness/durometer (typically defined by the Shore A hardness), and material composition. "Vijay seals" hold stocks of most commercially available O-Rings in both Metric and Imperial sizes, available in the following materials: - Nitrile Butadiene, Silicone, Poly Tetra Fluro Ethylene (PTFE), Neoprene, Viton (Flurocarbon), Polyurethane, Ethylene Propelene Diene Monomer (EPDM) etc.

O-Ring Characteristics:

The simple geometry is the main characteristic of an 0-Ring which, in conjunction with proper elastomer selection results in a low cost, easy to use and efficient sealing system. Elastomeric materials, when compressed, react like a high viscosity fluid which transmits applied stress in every direction; consequently, the 0-Ring serves as a barrier, blocking the leak paths between the sealing surfaces. Sealing is always achieved through a positive compression or squeezing action, resulting in a deformation of the 0-Ring cross-section. The most important sealing characteristic of an 0-Ring is its resistance to compression set or residual deformation.

Advantages of O-rings:

They seal over a wide range of pressure, temperature and tolerance.
No critical torque on tightening, therefore unlikely to cause structural damage.
Ease of service, no smearing or retightening.
O-rings normally require very little room and are light in weight.
In many cases an O-ring can be reused, an advantage over non-elastic flat seals and crush-type gaskets.
The duration of life in the correct application corresponds to the normal ageing period of the O-ring material.
O-ring failure is normally gradual and easily identified.
Where differing amounts of compression effect the seal function (as with flat gaskets), an O-ring is not effected because metal to metal contact is generally allowed for.
They are cost-effective.

Field of applications:

Static Seals:
In a truly static seal, the mating gland parts are not subjected 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.

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 the O-ring, or sealing surface at the O-ring, back and forth with the reciprocal motion. Note: O-ring seals are generally not recommended for reciprocating installations in which the speed is less than one foot per minute.

Oscillating Seals:
In an oscillating seal, the inner or outer member of the seal assembly moves in an arc (around the shaft axis) relative to the other member. This motion tends to rotate one or the other member in relation to the O-ring. Where the arc of motion exceeds 360°, as in multiple turns to operate a valve handle, the return arc in the opposite direction distinguishes the oscillating seal from a rotary seal.

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.

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.

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 vapor 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.

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. 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.

Crush Installation:
This use of an O-ring is a variation of the static seal. The O-ring is crushed into a space having a cross-section different from that of a standard gland - for example, triangular. While it is an effective seal, the O-ring is permanently deformed and therefore generally considered non-reusable.

Rod Wiper Installation:
In this case, the O-ring is used to keep a reciprocating shaft or rod clean to prevent damaging an O-ring seal located inboard from the wiper. The wiper O-ring does not necessarily seal. If there is a possibility of trapping liquid between the wiper and sealing O-rings, the space between the two must be vented. This installation is effective on actuating cylinders of machinery used in dirty, dusty areas.

O-rings as Drive Belts:
O-rings make superior low-power drive belts.

Failure modes of O-rings:
O-ring materials may be subjected to high or low temperatures, chemical attack, vibration, abrasion, movement and the materials are selected according to the situation. O-ring materials exist which can tolerate temperatures as low as -20° C to about 250° C or higher. At the low end nearly all engineering materials will turn rigid and fail to seal, at the high end the materials will often burn or decompose. Chemical attacks can degrade the material or cause it to swell.