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The best clamp load retention performance

The best clamp load retention performance

Threaded assemblies are the most important detachable parts used in machinery construction, installation and repair but relaxation of tension and self-loosening frequently compromise long term efficiency. To minimise this deterioration various types and methods of threadlocking 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, threadlocking adhesives are taking the place of traditional mechanical locking devices because they deliver the greatest all-round efficiency. As well as preventing unwanted movement, loosening, leaks and corrosion, they resist vibration. They are easy-flowing liquids or semi-solid adhesives which completely fill the gaps between mating threads of fasteners and joints.

The best way to evaluate a threadlocked connection is to test its behaviour under load cycles in a dynamic test machine; the lower the loss of bolt tension, the more reliable the assembly. So to determine the performance of its Loctite threadlocking system against other methods, Loctite submitted its products for testing on a transverse shock and vibration machine, similar to the Junker system.

Pneumatic hammers force relative movement of the stressed parts. At the same time the existing pre-stress force is continuously measured for the number of load cycles. Characteristic clamp load retention curves of various locking methods can then be compared. The Loctite liquid threadlocking system has the best clamp load retention performance among those tested. Whilst most mechanical methods failed this test, this doesn't indicate these methods are not useful up to a certain point. However, when their functional operation is compared with their cost, they are difficult to justify.

Whilst a similar favourable load cycle performance is demonstrated by the surface-compacting ribbed flange bolt, it does have other disadvantages. It's expensive, requires a relatively large amount of space for the flange bearing surface and causes unavoidable damage to the surface of the clamped parts around the bolt bearing surface. A bolt with a saw-toothed flange also performs well but its teeth penetrate the bearings surface of the clamped material. Bearing surfaces of the head and the nut are damaged during loosening, limiting their possible applications and parts with hardened surfaces that cannot be reliably connected.

As well as performing well, liquid threadlockers cut costs by replacing expensive special locking bolts or nuts so less costly standard assemblies can be used. Although assembly costs may vary depending on whether the adhesive is applied by hand or by dispensers, this has a minor effect on the magnitude of the total costs. The results show adhesive threadlocking is a less costly method with cost advantages accruing from the simultaneous sealing effect and the use of through holes instead of blind holes.

Loctite threadlockers 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.

Five basic types are available. Low strength formulations allow disassembly with standard hand tools and are ideal for adjustment screws, calibration screws, meters, gauges and for thread sizes up to M80. Medium strength provides a greater degree of permanence and is good for machines tools, presses, pumps, compressors and gearboxes. High strength products are designed to be very difficult to disassemble with standard hand tools, however removal is possible after applying localised heat and removing whilst hot.

Generally the threadlocking products are applied to the thread before assembly of the fastener but wicking grades are also available for pre-assembled fasteners, instrumentation and carburettor screws. The semi-solid Loctite threadlockers are available in medium and high strength formulations and suitable for thread sizes up to M50. Other noteworthy members of the Loctite range are two 'health and safety' threadlockers, Loctite 2400 and Loctite 2700 that carry no hazard symbols, risk or safety phrases.  

In the automotive industry, where there is no margin for error, Loctite threadlocking adhesives are commonly used. For example, Local Motors uses these products for two very different applications: the Rally Fighter, a high performance, off-road racing car; and Strati, a 3D printed, electrically powered road car comprising just 49 parts.

Making sure the Rally Fighter could withstand racing in extreme conditions without constant maintenance was a huge challenge. So was eliminating from Strati many of the 25,000 or more parts found in a typical road car. Off-road racers like the Rally Fighter are subject to jarring bumps and vibration that can shake loose even the sturdiest of nuts and bolts. By applying Loctite threadlockers to those fasteners it gave Local Motors the confidence that once the company's mechanics tightened a bolt or screw, it would stay tightened, even under the demanding conditions of off-road racing.

Similarly, every one of Strati's 49 parts is critical, so making sure those nuts and bolts stay tight was also essential to realising Local Motors' concept for this groundbreaking vehicle. From concept to driving was accomplished in just four months and the company credits Loctite with helping to make that possible.

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