TechArchive

Original article date: September 1997

German company FAG has become the first bearing manufacturer to recommend the use of adhesives for bearing assembly in its own literature.

Roller bearings are usually secured into a housing or onto a shaft be means of an interference fit, which is achieved by press-fitting or shrinking the components. The push-out force subsequently needed to release the bearing depends upon component contact area and surface finish, and is typically that of the press-in force.

Manufacturers of industrial adhesives, however, claim that the strength of an interference fit can be increased by evenly applying a retaining adhesive over the contacting areas of the components of the interference fit. Typically, this method can increase push-out force by a factor of six. Applications to substantiate this claim include:

• Bearings in aircraft landing gear and helicopter rotor systems
• Bronze rings into wheel bearings
• Deep-groove ball bearings bonded to disk spindles
• Trunnions into roll tubes of printing machines
• Stator bushes and gear wheel sleeves

Provided that continuous operating temperatures to not exceed 150C, retaining adhesives can be considered for all interference fits. Moreover, should manufacturing tolerances or assembly methods preclude an interference fit, the use of a retaining adhesive will still enable bearings to be secured with a strength comparable to that attained by an interference fit.

Anaerobic adhesives have been used since 1953. They self-cure rapidly when confined between metal surfaces (such as in the joint between bearing race and housing or shaft) by deoxygenation and the catalysis of metal ions. When cured, the adhesive forms a three-dimensional polymeric structure similar to a thermosetting plastic.

The main methods of retaining (or bonding) cylindrical assemblies, such as rolling bearings, are:

• positive contact (involving keys, tapered assemblies and splines)
• frictional contact (clamping and interference fits)
• material contact (brazing, welding and adhesive bonding)

The load-carrying capacity of adhesive-enhanced interference fits depends upon the strength of the press- or shrink-fit and the strength of the adhesive bond. Loctite 603 is an example of an excellent adhesive for retaining rolling bearings. Because of its resistance to light oils, surfaces to which the adhesive is to be applied do not need cleaning with chemical solutions. A wipe with a clean cloth is sufficient. A single-component adhesive, Loctite 603 can be applied directly onto surfaces being bonded, from the bottle, via a syringe or any other metering device.

Handling strength is usually achieved within minutes and at 20C, full cure is achieved after about 12h. This time can be reduced by heating the relevant components. For example, a temperature of 100C will reduce the time to achieve full cure to 1h. Above 60C, the strength of the adhesive decreases gradually.

Bearing races which are solely retained by an adhesive are usually immune to radial stress caused by expansion or contraction. The only causes of reductions in radial clearance are major changes in temperature. Cylindrical surfaces retained by adhesives require a much rougher surface finish than those secured by an interference fit.

To prevent adhesive being scraped off during assembly, the part carrying the adhesive (shaft, hub or housing) should have its leading edge chamfered by 15-30deg. This will also ensure good spread (wetting) of the adhesive over the surfaces.

Adhesive-retained bearings can be removed without difficulty using ordinary extractors. Heat can also be applied to ease disassembly. Prior to refitting or replacing a bearing, old adhesive must be removed and surfaces to be bonded lightly abraded with a fine emery cloth.

Advantages

• Simple straightforward design, without the need for circlip grooves, or the need for intricate machining
• Faster machining at lower cost; tolerances can be eased and surface finish relaxed
• Easy assembly: a slip-fit eliminates stress caused by pressing or shrinking and does not affect or distort bearing clearances
• Differences in production tolerances are compensated for
• Special assembly tools and methods, such as presses, heating, freezing, are not required
• Fast dismantling without damage to bearing, shaft or housing, using ordinary extractors
• No fretting corrosion or corrosion from tension cracks
• The adhesive totally fills the gap between mating parts to seal and isolate the joints, and prevent micro-movements

Disadvantages

• Care must be taken when applying adhesive to ensure that it does not migrate into bearings
• Adhesive must be allowed to cure (1-12h) before retained parts are operational
• Large adhesive gaps between parts can affect accuracy and alignment
• Special centring methods may be necessary to ensure alignment
• Lower strength at high operating temperatures

• Loctite
• Tel: 01707 921000
• Fax: 01707 821200
• Contact: Colin Chapman

• FAG Bearing
• Tel: 01952 208100
• Fax: 01952 290049

September 1997