Threadlocking that's robust enough for tunnel boring
Threadlocking adhesives are increasingly taking the place of more traditional locking devices because they deliver the greatest all-round efficiency. As well as preventing un-wanted movement, loosening, leaks and corrosion, they resist vibration. They also fill the gaps between mating threads to lock threads and joints.
These attributes have made anaerobic adhesive threadlocking essential to the reliability of the tunnel boring machines chosen to excavate the extension of the Crossrail Line through London. In common with other giant tunnel boring machines (TBMs) manufactured by Herrenknecht, these machines were constructed using Loctite 243 to lock fittings which have to resist massive pressure as the machines eat into the sub-soil.
Threaded assemblies are important detachable parts used in machinery construction, installation and repair but relaxation of the tension and self-loosening through vibration frequently compromises their long term efficiency. To minimise this deterioration various types and methods are available and each has its 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 add to absolute elasticity and/or increase friction but will not ensure permanent threadlocking under dynamic loads. Locking devices, such as tooth-flanged and rib-flanged bolts, nuts and washers are effective at preventing self-loosening but are expensive and need large flange-bearing surfaces.
When used on their own, or in combination with other methods, threadlocking adhesives provide distinct and wide-ranging performance advantages and they are available in varying viscosities and strengths. They are typically easy flowing, single component liquids and semi-solid adhesives that cure at ambient temperature to a solid, thermoset plastic when applied between steel, aluminium, brass and most other metal surfaces.
Low strength formulations allow disassembly with standard hand tools and are ideal for adjustment screws, calibration screws, meters and gauges and generally for thread sizes up to M80. Medium strength provides a greater degree of permanence and is good for machine tools, presses, pumps, compressors and gearboxes. The high strength products are used where frequent dismantling is unlikely and may require localised heat for removal, as do the wicking compounds that are widely used for pre-assembled fasteners.
Another group of threadlocking adhesives that are well worth noting are those in the health and safety category. These products are suitable for the lion's share of applications but their formulation doesn't require them to carry any hazard symbols, risk or safety phrases.
The Loctite 243 threadlocking adhesive used by Herrenknecht for the Crossrail TBMs falls into the medium strength category. It is suitable for all metals including passive substrates such as stainless steel, aluminium and plated surfaces and allows disassembly with hand tools for servicing.
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