Running Surface Finish and Fits
A close fit between the dynamic parts and a superior surface finish of the sliding parts is of special importance to the proper sealing and long service life of O-Rings.

Sealing Clearance

Nom. Ø	Clearance
up to 10mm	H9 / e8
10 to 60mm	H8 / e8
> 60mm	H7 / f7

For allowable clearance values this table specifies the ISA fits that should be adopted dependent on the nominal diameters.

The roughness of the parts sliding against the rubber should be between 0.2 and 0.8m (Ra) = 8 - 32 Ru. A surface finish finer than 0.15 µm (= 6Ru) should be avoided as it will be difficult to maintain a lubricating film.

The allowable sealing clearance depends on the pressure to be sealed and the hardness of the O-Ring compound being used. At elevated pressures a part of the O-Ring will be forced into the sealing clearance. Soft compounds can be severely damaged by this extrusion. So, dependent on the pressure to be sealed, the amount of sealing clearance is bound to the maximum values.

The diagram below represents roughly the connection between hardness, pressure and clearance. At pressures over 100 kg/cm² it is always advisable to support the O-Ring with a backup ring.

The following tables specify the allowable values at the pressures and the O-Ring hardnesses stated for the cross sections according to AS 568A. Dimensions are in millimetres.

Hardness 70-80° Shore A
Cross Section	1.78   2.62   3.53   5.33   7.00
Pressure	Clearance
35 bar (515 psi) 70 bar (1029 psi) 100 bar (1470 psi) 150 bar (2205 psi) 175 bar (2572 psi)	0.20   0.25   0.30   0.36   0.41 0.15   0.18   0.23   0.25   0.30 0.10   0.13   0.15   0.18   0.20 0.05   0.08   0.10   0.10   0.11 0.03   0.04   0.05   0.06   0.06

Hardness 90° Shore A
Cross Section	1.78   2.62   3.53   5.33   7.00
Pressure	Clearance
35 bar (515 psi) 70 bar (1029 psi) 100 bar (1470 psi) 150 bar (2205 psi) 175 bar (2572 psi) 200 bar (2940 psi) 350 bar (3675 psi)	0.36   0.41   0.46   0.51   0.51 0.30   0.36   0.41   0.46   0.46 0.25   0.30   0.36   0.38   0.41 0.20   0.23   0.25   0.30   0.30 0.15   0.18   0.20   0.25   0.25 0.13   0.15   0.18   0.20   0.20 0.08   0.10   0.13   0.15   0.15

It is important to mount dirt wipers to prevent plungers and rods from wearing quickly. If the running surface is too rough, a satisfactory service life for the O-Ring cannot be expected. Lubrication and the right running surface are critical.

Surfacing of the Groove and Clearance
O-Rings are generally installed in non-adjustable grooves, the dimensions of which are such that the O-Rings have a diametrical compression of 10-20 %. The groove width should be such that the oval deformation plus a certain amount of swelling can be accepted. The groove can be more easily turned when its sides are slightly conical. The bevel should be no greater than 5°. Groove surface finish should be 1.6 - 3um (64-125 Ru).

Lubrication
For dynamic applications, the O-Ring requires thorough lubrication in order to ensure a long service life. Wear is quickly induced by dry running. O-Rings also require lubrication when used in pneumatic equipment, for example, by filling the groove with a grease of high melting point. Mineral and vegetable oils and fats must not be used when Ethylene Propylene O-Rings are fitted.