The right track to secure fastening

Written by: Tom Shelley | Published:

Tom Shelley reports on current state of the art with regard to fasteners that resist extreme vibration yet can be repeatedly re-used

There are now mechanical fasteners that can resist the most severe vibration forces, yet can be undone and replaced any number of times without loss of performance.

There is also a plethora of lock washers that work well, provided they are not re-used too many times. Unfortunately, there are an equally large number of fasteners that are said to resist vibration, but are not as effective as many believe.

Nuts on bolts have been coming undone ever since the start of the Industrial Revolution. At one time on the railways, there were gangers whose job it was to constantly patrol a length of track and look for loose bolts and other problems. In more recent times, such work has passed to maintenance companies.

Unfortunately, traditional bolts and nuts, even with an additional half nut lock nut, cannot permanently resist the vibration forces of a set of points being struck repeatedly by steel wheels attached to several hundred tons of train travelling at 100mph or more. In the investigation that followed, it was found that maintenance engineers had deliberately damaged bolt threads with a chisel in a desperate attempt to stop nuts coming undone. However, even though such actions make it harder to undo nuts with a spanner, the unfastening torque is still much less than that applied by trains.

The solution that the British railway industry has now taken on board is a bolt and nut combination that was developed for the Japanese Railway industry 25 years ago and is exemplified by their application to a railway point stretcher bar proudly exhibited in the entrance hall of Staytite in High Wycombe.

David Cartledge, the company's director of sales demonstrated how the 'Hard Lock' nut uses an eccentric cone on an inner nut that locks hard against a conical recess on the inside of an outer nut. If the inner nut starts to try to undo, the combination quickly wedges together even more firmly. Both nuts freely spin down the bolt until locked together. The outer nut includes a flange to prevent a socket being applied to the two nuts together. Hand tightening is enough to initiate the locking effect.

Cartledge also showed videos of performances of various fasteners on a Junkers Vibration test machine, which uses a load cell and two transverse moving plates clamped by the nut and bolt under test. An eccentric cam moves the plates relative to one another at 12.5Hz for a maximum of 120s = 1500 cycles (Some sources only test to 1000 cycles). The test is so aggressive that if the joint remains tight, bolts are likely to fatigue fail if they are cycled for longer periods, but it is an excellent test of a fastener's ability to withstand extreme vibration. Most bolt and locknut combinations fail to retain clamp load. Needless to say, the Hard Lock retains more than 80% and also endures that other popular vibration test, US National Aerospace Standard 3350. This test requires bolted joints to stay tight despite being violently vibrated at 1780 cycles per minute for 17 minutes.

Another fastener system that Cartledge spoke of with glowing praise is the Nord-lock system which he described as working, "Fantastically on a bolt in a hole." This system uses two washers, each with inclined cams on one side, with an inclination greater than that of the screw thread and a series of radial teeth on the other.

The washers are installed in pairs, cam face to cam face. When the bolt and/or nut is tightened, the teeth grip and lock the mating surfaces, and any rotation of the bolt or nut is blocked by the wedging action of the cams. When undone, there is a noticeable click as the cams disengage. The only problem for use on a nut and bolt combination, is that one needs to use two pairs, one under the bolt head and the other under the nut. This adds to cost.

The American Disc-Lock company offers a very similar looking product, except that it has a few ridges instead of a lot of teeth on the outsides of the washer combination. The same company also offers the Disc-lock locking nut, which the company describes as "The world's safest wheel nut." This too uses a system of inclined cams. The two nut elements are attached to each other, with a tubular extension on one nut with a thread formed in it which is folded over to prevent the two parts coming apart. To remove it, the makers say that the spanner head or socket has to be placed over both nut elements.

Engineer's bodges with nuts and bolts have been taking place for years, but are generally a very bad idea. Nuts and bolts are designed to be tightened to torques that have been carefully calculated by the designers to give optimum results, even in the light of known imperfections in tightening equipment.

Sarah Edmonds of Nord-Lock says: "The torque figures we quote are guidelines for a standard joint and represent the preload to achieve a point in the region of 75% of the yield of the bolt. Even the most accurate torque tightening equipment can have up to a 20% tolerance and therefore 75% is used to prevent any stretching of the bolt". The worst thing one can do, we are told, is to tighten to a proper torque and then give an extra quarter turn for good measure. This will stretch the bolt or wheel stud to where it will start to yield, very likely initiating a crack that will grow under fatigue loading, especially in a salt on the road environment where corrosion is enhanced at the growing crack tip.

This is assuming that the bolt or stud is good material. Cartledge observed that: "The bolt should always be stronger than the nut. The locking quality of the bolt depends on the thread of the bolt".

But what of all the nuts with deformed slits and/or cuts or internal teeth that are sold as anti vibration nuts? Knowing that Staytite makes its own anti vibration nut with a stainless steel insert with two teeth, we asked Cartledge for an opinion. He said that they all lose more than 90% of their clamp load in the Junkers test, but are bought on the principle that, "If it's hard to get it on, it should not come off". Having said that, none of them should completely unscrew, unless the vibrating bolted surface continues to press against the nut. Staytite nuts have long been used on Triumph motorcycles and by the Ford Motor Company.

This leaves serrated washers, nuts with nylon inserts and Spiralock's thread form. The serrated washers do all work to some extent, although some of them do quite a lot of damage to mating surfaces. One good example of this type is the serrated family of Bellville washers sold by International Engineering Company that incorporate inherent spring force to keep them in contact.

Nylon insert nuts, when tested on the Junkers machine, retain about 20% of their initial clamp torque because of the effects of vibration induced heat in the plastic insert which moulds it into the threads. Plastic insert nuts should only be used once.

Spiralock's technology on the other hand, properly known as the Preload Locking Internal Thread Form, does work as claimed. It uses a female thread in which the bottom of the 60 degree Vee shape is truncated by a ramp. The combination of tension distributed along the length of the bolt, the elimination of the gap between bolt and nut threads and the ramp significant increases resistance to fastened joint loosening. It is much loved by NASA and said to be key to the reusability of the Space Shuttle engines.

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