TechArchive

This article was originally written in the period 1995-2000

Assembling cylindrical parts with adhesives requires sophisticated bonding technology. Retaining compounds join circumferential assemblies.

Design benefits of adhesives

• Eliminate bulk needed for high friction force
• Augment or replace press fits
• Reduce machining costs
• Combine materials for the best properties of each
• Eliminate distortion when installing drill bushings
• Produce more accurate rigid assemblies
• Increase strength of heavy press fits
• Help bearings and bushings find alignment points
• Help dissipate heat through the assembly
• Eliminate fretting corrosion and seizure
• Help join hard and soft parts without damage
• Provide a seal when essential
• Eliminate stress in parts

There are four types of assemblies which transmit loads in torsional, radial or axial directions from a shaft to a hub. These are: positive drive (such as spines); friction drive (such as press fits); welding and soldering; and adhesive bonding.

Welding, soldering and adhesive bonding are sometimes grouped together and referred to as interfacial drive assemblies. For adhesively bonded cylindrical assemblies, designers distinguish between two types of cylindrical assemblies:

Bonded slip fits:-the cured adhesive transmits the load or torque;

Bonded interference fits (bonded shrink and press fit):-the friction and the adhesive transmit the torque or the load. Bonded shrink fits (heated slip fits during assembly which cool to form interference fits) are also in this category.

As with welding and soldering, bonded slip fit assemblies create an ideal interfacial connection. By changing from a liquid to a solid, adhesives fill the joint. The liquid fills the surface roughness of the parts, filling the inner space between substrates. After cure, the adhesive becomes an absolute interfacial connection.

The major advantages of adhesives for structural assemblies is that they require less severe machining tolerances and no securing parts. Components are assembled quickly and cleanly and transmit high forces and torques, including dynamic forces. Adhesives also seal, insulate and prevent micromovements, so that neither fretting corrosion nor stress corrosion cracking can occur. The assembly can be taken apart easily after heating reduces the adhesive strength.

Bonding simplifies the design, production and assembly of parts with drilled holes that must be in exact alignment: the adhesive compensates for misalignment within certain limits. Shafts and bearings can also be assembled without the stress of misalignments, increasing their service life. This is important when multiple bearings are used with wide spacing between the bearings, Mounting bearings with adhesives also allows use of thinner castings or smaller diameter shafts.

As much as possible, adhesive joints should only transmit compressive and shear forces. If peel forces occur under stress, they should be absorbed by suitable structural measures. Experiments indicate that the optimum distribution of stress on the adhesive is achieved when the ratio of hub width to shaft diameter is approximately 1:1.

Sources of surface contamination should be avoided, because they can reduce dramatically the maximum levels of retaining strength predicted.

Despite the maximisation of bonding surface areas, the overall weight of bonded assemblies is generally lower, since components no longer need to be designed to sustain heavy press or shrink fit forces. Fasteners, collars, pins and lockings are often no longer needed for splined assemblies. This allows lighter, more creative designs.

In practice, stresses in cylindrical assemblies are not evenly distributed over the whole length of the joint. Spikes occur on the load side at the beginning of the joint area. Designers can create favourable geometries to deal with such stress spikes.

When bonding shrink-fit assemblies, however, care must be taken in the geometrical design to assure that stress spikes are not reduced while also reducing the hub pressure in the joint area. This is very important for reaching maximum local transmission.

It is important to avoid wiping or pushing away the applied adhesive during the assembly of parts. Components must be correctly positioned at the outset. Corrections made during the cure process destroy polymer chains that have already formed, reducing the final strength of the joint. After cure, corrections are no longer possible.

Free guidance on the power of adhesives

Even thinking about adhesives can be, for the majority of engineers and designers, a step into the unknown. Just how reliable are adhesives? How strong are they? How will they age? Why will they not bond? And Why? Which type of adhesive is best? How is it applied?

These are typical questions which every adhesive manufacturer has to answer repeatedly. But the education all adhesive companies have had to provide if they were to sell their products has of necessity been fragmented and piecemeal.

The Loctite Worldwide Design Handbook – a handbook of 463 pages – explains in exacting detail every aspect of the technology and techniques of adhesives and adhesion. Much of the content evolved from the seven worldwide Loctite research, development and engineering laboratories.

The introductory section, for example, explains and illustrates the theory and molecular mechanism of anaerobic, cyanoacrylate and uv-curing adhesives. Seven separate sections guide potential users through the methodology of bonding or sealing all commonly used engineering components.

More pages list the properties, values and characteristics of 87 different types and grades of adhesives. And other section details methods of application and dispensing from simple, manual methods to sophisticated robotic systems.

The first 5000 engineers, designers and students requesting it receive it free. Long gone, so now, each book will cost (UK pounds)10.00.

Worldwide Adhesives Design Handbook Loctite (UK)

• Loctite
• Tel: 01707 821000
• Fax: 01707 821200