Bore Seals And ‘O’ Rings For Reciprocating Shafts

Bores are sealed with ‘O’ rings' sited in grooves, (figure 1). The ‘O’ rings' are usually of toroidal shape (i.e. circular cross section) and are regulated by BS 1806 and BS 4518.

Figure 1. ‘O’ Ring bore seals.

Square section rings are not regulated by standards, and vary from very economical types cut from tube stock to precision mouldings. Square sections and other specialised sections can have advantages, for example in resisting twisting or rolling during reciprocating motion.

The dimensions and cross section of both the groove and ‘O’ Ring are critical. The considerations of the dimensions of the recess in which the ‘O’ ring is located are identical to those described above for ‘O’ rings in static joints. However, a clearance is necessary for reciprocating shafts in their bores and anti-extrusion rings placed on the low pressure side may again be necessary. Use of rubber of a higher hardness will also help to prevent extrusion; for example, at 14MPa the maximum radial clearance for a 70 IRHD ‘O’ ring is about 0.2mm, whereas for a 90 IRHD ‘O’ ring it is about twice this value. The most useful general hardness range is 75 ± 5 IRHD.

There are some applications in which the shaft may vibrate inside the housing. In such cases it is essential for the ‘O’ ring to retain its elastic properties, hence the Tg (glass transition temperature) of the material at the particular frequency must be considered.

Resistance to wear is also often required. As this cannot be guaranteed by any standard test, in critical cases some form of rig running, perhaps involving temperature cycling, will be needed.

Assembly and location are equally critical, the ring is normally assembled to the stationary shaft by pushing over a tapered thimble, taking great care not to twist it in the process. Use of assembly lubricants is generally undesirable and not recommended, although a little soapy water is usually safe.
Reciprocating Seals

Reciprocating seals should ideally operate under lubricated conditions. The lubrication is normally provided by a very thin film of the fluid being sealed. For pneumatic applications this is problematic. Possible solutions are the introduction of an oil mist, the use of grease packed assemblies or the use of special low friction elastomers. More flexible seals, based on deformable lips which are ‘energised’ into contact by the pressure differential, can be advantageous in keeping the friction low when the pressure is low.

Wear of seals is likely to be accelerated by the ingress of external contamination. This can be controlled by the use of scrapers or wiper seals. Harder materials, such as polyurethane, nylon or polyacetal are suitable, but they should have low friction, high wear resistance and ideally the ability to act as support rings, or bearings, helping to control lateral displacement of the shaft under side loads.

Similar considerations also apply to the anti-extrusion rings used in high pressure seals, for example for hydraulic rams.
Sealing of Rotating Shafts

Rubber lip seals. (figure 2), provide a very compact and simple solution to sealing rotating shafts. However, their use is restricted to low pressure applications, (less than 0.1MPa) and moderate temperatures, up to 150°C.

Figure 2. Rubber lip seals.

For higher temperatures and pressures, mechanical seals based on sliding contact between the faces of two rigid rings, or stuffing boxes, based on compression of soft packing material against the shaft, should be used.

The lip seal is loaded onto the shaft by a combination of the force exerted by the circumferential metal garter spring, the elasticity of the rubber and the fluid pressure. The garter spring is necessary to impart sufficient dynamic recovery. To minimise wear, the contact pressure should be as low as possible consistent with maintaining the seal. Construction material and surface finish will also affect wear rates. Shaft surface should be smooth to 0.25- 0.5µm.

Rapid seal wear will also occur if there is no fluid to form a hydrodynamic film, if the fluid pressure is too high thus raising the contact pressure, or if the shaft is poorly finished or of inhomogeneous material such that the surface may become pitted.

A high thermal conductivity shaft is advantageous.

The lip angles are unequal, the inside lip angle being the greater. This asymmetry is important for the function of the seal, there apparently being a tendency for liquids to be pumped from the low angle side towards the high angle side.

The pumping action may be enhanced by moulding helical ribs onto the outside face of the lip. The direction of the pumping action of such ribs may depend on the direction of rotation of the shaft, although some rib designs are effective whatever the rotation direction. Typical elastomers used in lip seals are listed in Table 1.

Table 1 (Part A). SAE list of elastomers in common usage for lip seals for oil.

ASTM D1418
Letter Desig’n


Synonyms or Trade Names


Chemical Makeup


Mode of Failure


Rel. Mat’l Cost

NBR


Nitrile
Buna-N


Copolymer of butadiene and acrylonitrile


Usually hardening


1.0

XNBR


Carboxylated Nitrile


Copolymer of butadiene and acrylonitrile with additional acid eater groups


Usually hardening


1.2

EAM (unofficial)


Varmac


Copolymer of ethylene, methylacrylate and a monomer to facilitate vulcanisation


Usually hardening


2

ACM


Acrylate PA


Copolymers of various acrylates and a monomer to facilitate vulcanisation


Wear and/or hardening


2.5

VMQ


Silicone


An organo-siloxane polymer with substituent vinyl and methyl groups


Usually reversion


6

FKM


Fluoro-elastomer
Viton
Fluorel


Copolymer of vinylidene fluoride and prefluoro-propylene


Usually set, with varnish buildup


20

HNBR


Highly saturated nitrile


NBR with double bonds removed through saturation process


Set, varnish buildup


15

Table 1 (Part B). SAE list of elastomers in common usage for lip seals for oil.

ASTM D1418
Letter Desig’n


Comments

NBR


Versatile good processing.
Acrylonitrile contents range from 18% (good low temperature) to 50% (good oil and heat resistance). Many options for cure systems. Unsaturated.

XNBR


Additional cross linking sites improve some physical properties. Processing impaired. Unsaturated.

EAM (unofficial)


Essentially saturated. Fair low temperature properties. Fair oil resistance. Fair oil resistance. Poor to fair processing. Few cure and viscosity options available.

ACM


Essentially saturated. Recent versions have fair (-40°C) low temperature properties. Few cure system options.

VMQ


Saturated. Low tear strength, thermally stable and heat resistant in the dry state. Good low temperature properties. Swell high in petroleum fluids, but reversible with temperature. Easy processing.

FKM


Saturated. Resistant to a variety of fluids. Fair low temperature properties. Thermally stable and heat resistant without reversion. Poor to fair processing. Some viscosity variations available.

HNBR


Good abrasion, oil and EP additive resistant. Good high and low temperature performance.

Compact glass filled PTFE seals may be used at higher temperatures than accessible for elastomeric lip seals. Their mode of operation is, however, more akin to viscoseals, their effectiveness depending on the pumping action of a helical groove.

Rubber face seals, serving as both a lip seal and a centripetal deflector, can be a useful alternative. No garter spring is needed, so the contact pressure and hence friction is lower than for a garter seal.