Ever since the bolt was invented it has been important to prevent its self-loosening. Under static loads the bolt will be kept in place thanks to the self-locking effect caused by frictional force under the head/nut and in the thread exceeding gliding forces in the pitch of the bolt. Anders Keife reports.

Bolted joints exposed to dynamic loads or pure vibrations are very likely to loosen if movements under the head/nut occur. This problem forces designers invent different kinds of bolt securing methods. Mentioning all known securing systems would lead to a very long list.
The main principles for bolt securing are:
* Design the joints with a large ratio between clamping length/diameter. The bolt will be relatively slim which makes it more flexible and the surface contact under the head/ nut will be maintained. Furthermore the bolt has to rotate longer before the clamping force will be lost. This method is an expensive design. Extension sockets and of course long bolts have to be used.
* Castle nut or the head/nut secured with a tab washer or locking wire. These securing systems involve relatively complicated assembly and disassembly. Unfortunately they cannot provide firm locking and part of the bolt force is lost.
* Increase of friction under the head/nut or in the thread. Locking nuts with increased friction in the thread have the advantage that they cannot completely loosen by themselves. Unfortunately, the increase of friction is not effective at high bolt tension. Non-free-spinning threads makes assembly laborious.
* Friction towards the material is increased by providing heads/nuts with serrated surface, often in combination with a flange in order to increase the surface contact and the friction force. This method usually requires a considerably higher torque. Upon assembly and mainly disassembly a strong scratching towards the material will occur. However, a large serrated surface will prevent the bolt/nut from loosening.
* Friction washers with serrations work as described above, however, provide easier storage of fasteners.
* To use adhesives in the threads has become a very common method. It requires minimum storage of fasteners but does have some other requirements. In order to obtain an effective locking effect the surfaces need to be cleaned before assembly. Service and maintenance can be troublesome since disassembly is not easy and cleaning of the surfaces is a must before reassembly. Using adhesives results in a considerable increase in friction with great variation in clamp force.
Applying adhesives in the threads is an ineffective method when service is required. Furthermore, a curing time is required to provide an effective locking effect. At an emergency repair the adhesive may not be available.

Make the bolt self-locking

Nord-Lock is a pair of washers with wedge locking action according to DIN 25 201. This method uses tension instead of friction. On one side the washers have cams with a greater rise than the pitch of the bolt.
In addition, there are radial teeth on the opposite side. The washers come pre-assembled in pairs cam face on cam face. When the bolt and/or nut is tightened, the teeth grip and seat the mating surfaces. The washer is locked in place, allowing movement only across the face of the cams. The tension created makes the bolt/nut self-locking.
Vibration tests using the Junker method and test results from the German TUV confirm the practical experiences that Nord-Lockâ does provide a secure locking method for bolted joints exposed to dynamic loads and vibrations. A bolt, which has to increase tension in order to loosen, cannot possibly loosen itself.
Nord-Lock AB has been producing the washers since 1982. The company perform a customer service by checking torque - load ratios for most kind of joints and run Junker tests in the range of M3 - M27 bolt size. The joint consists of two plates with a needle bearing in between. The bolt tension is measured during the vibration test. When opening a bolt locked with Nord-Lockâ there is a small increase of tension during the cam overriding. Even at very moderate preloads the Nord-Lockâ washers provide a secure and vibration proof locking.
The test diagram (Fig. 2) shows the bolt tension during vibration testing of some common fasteners. The tension will drop completely for non-secured nuts and most of the tension will be lost for nuts with added friction to the thread or with spring washers. Some of these may hold better but it is all depending on the friction. A lubricated thread makes the nut turn loose quicker than when the surfaces are dry.
All vibration tests show a positive result when using Nord-Lock. Some of the initial tension gets lost due to settlements since the test is rather brutal with an amplitude of ±0.3mm. The locking function proves itself when the nut is being un-tightened with a wrench. A clear increase of tension is registered. Vibration tests with the same bolt and mating material as an actual case provide a good proof of how safe the joint will be even under worst circumstances (vibrations, dynamic and shock loads).
A test on UNF 5/8 bolts with and amplitude of ±1mm is so brutal that tab washers will open due anti-clockwise turning of the bolts and there is a complete loss of clamp load. The same test with Nord-Lockâ washers shows only a minor loss of clamp load caused by settlements in the thread but there is no off-turning motion.
The Nord-Lock washers can be used on long holes as well. The edges of the long hole can be deformed by using the standard dimension of washers with a regular nut. This combination of fasteners leads to slightly greater settlements than if a washer with larger outer diameter is used. Therefore it's recommended to use the special washers with a larger outer diameter in combination with a flanged bolt/nut.
To assemble a bolt on powder painted surfaces has become common. Even here, Nord-Lock secures safely. Comparative Junker vibration tests with painted and unpainted surfaces show a limited difference of settlement.
Using a standard dimension washer with a regular nut on a painted surface the preload in the joint will decrease slightly due to settlements not only in the thread but also in the layer of paint, however, the locking effect remains positive. Using a washer with a larger outer diameter in combination with a flanged bolt/nut will minimise the settlements.

Advantages

For machinery, which requires regular service and maintenance, Nord-Lockâ is the optimum solution of bolt securing. Bolted joints secured with Nord-Lockâ are easy to untighten with general tools and mounting elements can easily be lubricated before reassembly.
Normally lubricated bolts loosen easier than if the friction is higher. Lubricants in combination with Nord-Lock do not jeopardise the security. The bolt is not only safely locked but also easy to open and the fasteners are re-usable.
When securing a stud bolt it is normally locked in the taped hole with an adhesive and the nut is often of jamming type. This method is complicated. Difficult repair work has to be done if the bolt should break from fatigue. A stud bolt with the same pitch in both ends will not turn by itself and the nut can simply and safely be secured to the material with Nord-Lock and any repair job will be easy.
Thanks to the fact that Nord-Lock washers do not rely on friction they will not loose their locking effect due to lubrication. Lubrication is always important to reduce friction upon assembly. With less friction, less torque is required to obtain the desired preload. Besides that the spread of the preload will be minimised by more controlled friction.
The washers are available in through hardened steel either yellow zinc chromated, Dacromet or Delta coated and also in stainless steel 316. The stainless steel washers can be used at temperatures up to 500ûC without any reduction in performance. This is an important factor for many applications such as turbines and engines.
The washers come in pre-assembled pairs, which makes assembly easy and safe. The standard dimension range for steel washers is M3 to M130 and for stainless steel M3 to M80. They lock standard and high grade bolts.
Ultra Dynamics in the United Kingdom is a manufacturer of rail retarder systems used in marshalling yards worldwide. These hydraulic retarders are at set distances along the track to bring the train to the correct speed for that part of the marshalling yard. Due to the fact that the retarders are exposed to high/low frequency vibrations, shock loads, pressure loads and daily temperature changes, Nord-Lock washers were chosen as the standard fastener. Friction style fasteners were ruled out, as the dynamic loads cause transverse movements in the stud thread and under the nut leading to severe loss of friction making the risk of slackening too high.
In order to achieve an optimum of clamping force, lubrication of the threads and nuts is necessary. With measures for lower friction, the locking methods based on increased friction will loose their effect. Using Nord-Lock, which locks by tension, in combination with lubricants will provide a safe joint with high tension.
Another Nord-Lock user is VAE railway technology in Zeltweg, Austria, a leader in the construction of high speed switch points. Stable and precisely set tracks allow high speeds up to 250km/h.
The tracks are being equipped with high speed switch points. One location for Nord-Lock is the switch driver responsible for the changing of points and the quick positioning of the switch tong. This tong absorbs extreme dynamic loads, transverse movements in the bolt head and/or nut leading to a severe loss of friction. Nord-Locks wedge-locking effect safely and economically solves the problem. The geometric design of the fasteners used properly in a joint guarantees self-locking regardless of friction. The bolt securing system was chosen after stringent tests were carried out by different engineering bodies for use in the cab, the chassis and the passenger compartments.
Alstom DDF in France is installing Nord-Lock washer to secure bolts on structural and traction connections in order to achieve maximum safety. The washers have proven to solve loosening problems on rails, switches and signal systems in Europe and in the US.

Enquiry No 16

Anders Keife is with Nord-Lock, Malmo, in Sweden. www.nord-lock.com