O - Rings
O-rings are precision sealing elements of circular cross-section, preventing unwanted loss of liquids or gases. The sealing effect of the O-ring is achieved by compressing the circular profile of the ring in the groove. If operating pressure is still applied to the ring, the sealing effect of the OK is increased to a certain extent. Depending on the operating conditions and the working environment, the appropriate material, hardness and size are selected. The most commonly used material is NBR with a hardness of 70 Shore A.
Successful application and good sealing is dependent on the correct choice of these parameters:
If possible, choose the O-ring with the largest cross section that is still structurally possible for the application. Such an OK seals better on the seating surface and compensates much better for possible unevenness.
Static sealing
For static sealing, the inner diameter of the ring d1 should be equal to the inner diameter of the groove when sealing from the outside or the diameter of the piston rod when sealing from the inside.
There are two types of compression in static sealing:
- Radial compression - (valves, fittings, hydraulic and pneumatic cylinders, etc..) If the sealing joint is equal to zero, pressures up to 500 bar or more can be sealed. For high or pulsating pressures, the use of retaining rings is recommended.
- Axial compression - (sealing of flanges, lids, etc... ) Care must be taken to ensure that the sealing surfaces are firmly seated so that they cannot disengage even under pressure and an undesirable joint cannot form. With this optimum design of metal parts, pressures of 1000 bar or more can be sealed.
Dynamic sealing
For the selection of the right O-ring, similar rules apply for hydraulics and pneumatics. There are limits to the fitting of O-rings for dynamic sealing, as the need for lubrication under rising pressure is made more difficult by slippery movement. This increases abrasion, creates local overheating and results in rapid wear. It should therefore be noted at the outset that for dynamic seals the O-ring is not the ideal sealing element and in many cases other sealing elements such as sleeves should be chosen.
Hydraulics - The type of lubricant, the number and length of strokes, and the quality of the metal surfaces of the components are critical to the life of the ring. The pressure should not exceed 10 MPa and the movement should not exceed 0.3 m/s. O-rings are therefore not suitable for long stroke and high speed cylinders.
Pneumatics - Although lower pressures are achieved in pneumatics, lubrication is not optimal compared to hydraulics.