Fastening & Joining

7-2000

Design Application Feature

Keeping it all together

Tom Shelley compares and contrasts some of the more commonly used techniques that prevent nuts and bolts from coming undone

Ever since Archimedes invented his screw for moving water – and others adapted it to threaded fasteners – engineers have been taxed by the problem of ensuring that nuts and bolts, once done up, do not unexpectedly become undone.

Over tightening, with a length of steel pipe thrust over the end of a spanner to increase leverage, is not the answer because it is liable to result in crack initiation and bolt or stud failure.

Many companies specialise in solutions to this problem, some based on adhesives and some mechanical. New ideas come to our attention at Eureka almost every month.

Nuts and bolts stay tight because the bolt becomes slightly stretched and applies a clamping force. But there are many ways in which the two can loosen: if the bolt expands due to heat; if the rough surfaces of the contacting bolt head nut and washer become smooth; or if the surface pressure at the bearing surface of the bolt and nut exceeds the compressive strength of the material.

The other way that threaded fasteners become undone is cyclic vibration, moving the joined metal parts laterally with respect to each other in such a way as to undo the fastener. The most common solutions to the problem are to maintain bolt tension with a spring washer or similar device, or drastically increase the friction of the nut on the bolt shaft or the bolt or stud in its hole. An alternative solution, although more expensive, is the growing family of mechanical devices that tighten themselves when forces and movements try to undo them.

Stick to it

Loctite’s rang of threadlocking adhesives have become so ubiquitous that many engineers refer to such compounds as ‘Loctite’ whether they are made by that company or not. The company says that compared with mechanical locknuts, a drop of threadlocker is cheap, even when taking into account the cost of cleaning components prior to application.

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The company supplies adhesives that cure to form relatively weak bonds, so that assemblies can be disassembled with normal tools, and adhesives classed as ‘permanent’. In its Worldwide Design Handbook, the company shows a photograph of 5/16 inch UNC steel studs being locked into an aluminium cylinder head using Loctite 270 at McLaren Mercedes. It says that by applying this high strength threadlocker to both male and female threads, the studs remain firmly in place even when the cam shaft bearing nuts are released.

UK-made AJett is one of a new breed of adhesive and sealant brands with a range that covers threadlocking, retaining, gasketing, pipe sealing and bonding. The company says that it uses updated chemical technology for its products to make them more versatile, flow and work better, less wasteful and competitively priced compared to older brands.

The AJett threadlocking range comprises four main products: low, medium and high strengths plus a penetrating product that wicks into already tightened threads. All self-cure within the joint. All four are single component, room temperature curing products that comply with existing US military specifications. They are also oil tolerant, and as long as the application is above 5C, have no need for an activator or primer for locking plated parts. They can be used on steel, stainless steel, brass and plated items. While AJett 128 is a high strength threadlocker for permanently locking studs, the other three products can be undone with hand tools, even after years of assembly. Service temperatures are from -50 to +150C and breakaway torque strengths are from 3Nm for product 106 Low Strength, through 20Nm (120 Medium Strength) to a high of 36Nm for AJett 128. In the UK, AJett products are available through CGP Chemicals ( www.cgpchemicals.co.uk ).

The disadvantage with threadlocking adhesive it that it can only be used once. After disassembly, new adhesive must be applied before reassembly.

Wires, pins and split pins are all commonly employed to ensure nuts do not become undone, but all add to both component and assembly cost. Car wheel bearing nuts and aircraft jet engines are two of the most common applications, both instances where repeated disassembly and reassembly are required, yet absolute integrity, guaranteed by visual inspection, is essential.

A lower cost, and more ingenious device is available from George Emmott (Pawsons).

The application was to improve the performance of a range of control valves for the processing, petrochemical and oil and gas industries. Conventional locking washers above and below the handle could not accept the reversing movement of the handle when operating the valve, which made the lever and nut loosen with consequent gland leakage. Whatever the locking device, it had to permit adjustment to make up for normal operational wear. And a further requirement was that the operating mechanism should be visible and not need either training or written instructions.

The solution adopted was a self-locking washer made of spring temper stainless steel, with spring loaded flanges that would flatten to allow the nut to be tightened in one direction only. The flanges then spring back into position, preventing any movement in the opposite direction.

A comparison of undo-able and re-usable lock nuts is made by US company Stafast Products on its website ( http://stafast.com ).

Its Belleville washer nuts are made with excess material below the tapped hole of the nut that is unthreaded. This material is fed through the washer and then riveted to hold it in place. The nut portion is able to rotate freely relative to the washer. The relative large area Belleville washer element maintains bolt clamping force while spreading the load over a larger surface area and helping lock the nut in place when tightened.

The same company also makes toothed flange lock nuts with similarly integral and free-to-turn lock washers. Serrated flange nuts have ratchet teeth on their undersides to resist loosening. The flange bases can additionally span oversized clearance holes. Double serrated nuts have ratchet teeth on both sides so they can be either way round when screwed on by automatic machinery. Nylon lock nuts include nylon inserts, which increase nut friction and seal threads in the same way as threadlocking compounds, but are reusable.

The company claims that its locking T-nuts achieve undo resistance at lower cost than threadlocking compounds. It uses what it describes as a crimp lock feature. An indentation in the barrel causes thread interference when a bolt is threaded into the bolt, making it more difficult to work loose.

More ingenious, if slightly more expensive solutions include the Disc-Lock and Nord-Lock products. The Disc-Lock Safety Wheel Nut is split into two sections, both of which have interlocking rising cams, which are joined together to form one assembly. When subject to vibration or road shock, the interlocking cams of the wheel nut attempt to rise against each other. As the angle of the cams is greater than the pitch angle of the thread on the stud the nut locks and will not come loose. (more information can be found at www.disc-lock.com). A similar approach forms the basis of the Nord-Lock double washer, sold in this country by Primatronic.

Other large suppliers of lock nuts that have come to our attention are: Nu-Screw & Nut in London (because of its range of products); Jing Fong Industry in Taiwan (which is likely to offer lower prices); and the Slip-On Lock Company in the US (because of the way its nuts can be slipped onto long threaded rods).

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