Achieving best clamp load retention performance in extreme conditions

Published:  09 May, 2019

Threaded assemblies are the most important detachable parts used in machinery construction, installation and repair. Relaxation of tension and self-loosening is common for any machine but when it is operating in harsh environments, the need to minimise this deterioration with effective thread locking is even more important. Various methods are available but what are their respective strengths and failings?

Mechanical devices such as split pins and tab washers are only suitable for preventing the loss of nuts and bolts. Friction devices give some resistance to vibration but do not perform well under extreme conditions. Locking devices, such as tooth flanged and ribbed flanged bolts, nuts and washers are effective at preventing self-loosening but can damage the contact surface, need larger flange-bearing surfaces and are expensive.

Increasingly, thread locking adhesives are taking the place of traditional mechanical locking devices because they deliver the greatest all-round efficiency, especially when the going gets tough. As well as preventing unwanted movement, loosening, leaks and corrosion, they resist vibration. They are free-flowing liquids or semi-solid adhesives which completely fill the gaps between mating threads of fasteners and joints.

To prove the performance of its thread locking system against other methods, Loctite submitted its products for independent testing in a transverse shock and vibration machine in which pneumatic hammers force relative movement of the stressed parts. At the same time, the existing pre-stress force was continuously measured for the number of load cycles. Characteristic clamp load retention curves of the various locking methods were then compared.

While a similarly favourable load cycle performance was provided by the ribbed flange bolt, it does have other disadvantages. It’s expensive, requires a relatively large amount of space for the flange bearing and causes unavoidable damage to the surface of the clamped parts around the bolt bearing.

A bolt with a saw-toothed flange also performed well but its teeth penetrated the bearings surface of the clamped material. The head and the nut were damaged during loosening, limiting their possible application to parts with hardened surfaces that cannot be reliably connected.

As well as performing well, the Loctite thread lockers are able to cut costs by replacing expensive special locking bolts or nuts, allowing less costly standard assemblies to be used.

Loctite thread locking adhesives are available in varying viscosities and strengths to meet the needs of a wide range of applications. They are single-component and semi-solid adhesives that cure at room temperature to a hard, solid thermoset plastic when applied between steel, aluminium, brass and most other metal surfaces.

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