Anders Keife examines aspects of fastener technology, including a bolt securing system that does not use any traditional technique to secure the fastener.

It would not be overstating the case to say that fastener technology receives scant attention from many engineers. Bolts and nuts are looked upon as trivial hardware, but if the fasteners allow the joint to fail, expensive warranty claims or maintenance costs may result. This is especially true when the joints are subjected to dynamic loads where fatigue can cause the bolts to loosen or break.

Nord-Lock AB produces a bolt securing system that does not use any traditional technique to secure the fastener, and yet offers a safe locking system. Nord-Lock utilises tension instead of friction to secure a bolted joint. The proven wedge locking method meeting DIN25201 is superior to any traditional method.

The Nord-Lock bolt securing system consists of a pair of washers with cam faces on one side with a cam angle a greater than the thread angle b. In addition, there are radial teeth on the opposite side. The washers are installed in pairs, cam face to cam face.

When the bolt and/or nut is tightened the teeth grip and seat the mating surfaces. The Nord-Lock washers are locked in place, allowing movement only across the face of the cams. Any loosening movement of the bolt/nut is blocked by the wedge effect of the cams. When untightening sliding always occurs on the cam faces of the washers thereby minimising deformation of the mating surface.

When faced with a problem joint, it is not surprising that the design engineer will not have an answer when asked about the clamp load. Calculations must be based on the existing conditions and these are very vague. If all the parameters are not exact the torque calculation will be unreliable. Examples of these parameters are:

• Thread condition.
• Hardness of the mating surfaces.
• Extra friction from a ‘locking’ fastener.
• Type of mating material (steel, aluminium, etc).
• Oil or grease on the thread.
• Moisture on the surfaces.
• Type of bolt head (flanged, regular or serrated).
• Surface coating of the bolt.
• New or used fastener.
• Friction coefficient of thread, nut and bolt head.

Control over the clamp load in a bolted joint is vital. To achieve that the friction on the bolt should be low which in turn will result in a minimum of clamp load deviation.

Some of the many reasons for machine failure due to bolts coming loose or breaking are:

• In principle, the bolt is designed to hold due to its own friction in the thread and under the head. This works during static conditions but not when there are high dynamic forces. Especially transverse dynamic loads are dangerous.
• The clamping length could be too short so that any motion in the bolt will result in a severe loss of tension.
• When re-using bolts without lubrication, the friction can be very high, resulting in a very low clamp load.
• Most adhesives will increase the friction resulting in a low clamp load.
• Loss of clamp load due to bearing surface being too small leading to fatigue failures.

It often happens that an engineer chooses a high-grade bolt instead of a standard grade for a joint that has had bolt failures. Unfortunately, the surface treatment on high-grade bolts compared with normal electro zinc-plated may change the friction conditions which should be taken into consideration when calculating the torque figures. In addition, there is a risk of over-loading the base material of the joint. Even with a higher torque value, the clamp load may be too small due to the higher friction. The joint becomes more expensive but not safer.

Other bolt securing systems include:

Locking nuts: A survey of bolt securing systems will show a large number of different locking nuts, which try to hold onto the thread through increased friction. During vibration tests with transverse motions according to the Junker Principle, meeting the German standard DIN65151, there is a loss in preload of 80 to 90percent at a given amplitude.

Any of these nuts show more or less the same result. The pitch of the thread always makes the nut turn at a certain amplitude of transverse motions. The greater the amplitude and the shorter the clamping length, the quicker they start turning loose.

Tab washers, safety wire and castellated nuts: These methods are very expensive and labour-intensive, plus they will not provide a firm locking since the tab, wire, or pin is of soft steel. When using tab washers there is always a risk of moving the nut/bolt during assembly, resulting in a loss of preload. Furthermore, if the tab is not exactly parallel with any of the sides of the bolt head, it may start rotating when exposed to vibrations or dynamic loads. Even insignificant rotation of the bolt/nut may cause a significant loss of preload.

Spring washers: It is a misunderstanding that spring washers have a locking effect. Most spring washers only add a minute amount of spring action (up to 10percent of the capacity of a grade 8.8bolt).

Adhesives: All laboratory tests with adhesives have shown a significant increase of friction when tightening, resulting in large deviation of preload. The amount of friction varies depending on the type of adhesive and coating of the bolt.

Unfortunately, it seems as if many engineers know very little about torque/load ratios when using adhesives. Since the added friction is concentrated on the thread it also increases the torsion stress in the bolt while tightening. High torsion stress in bolted joints is undesirable since it considerably reduces the strength of the bolts. Well-lubricated bolts sustain up to twice as much tension during tightening than bolts with an adhesive applied to the thread. Tests by tightening to yield point will verify this alarming fact.

In order to achieve a desired preload with a minimum of deviation, fasteners need to be lubricated – a process that has no undesirable effect on the Nord-Lock Bolt Securing System. The wedge locking technique does not allow any loosening movement in the bolt, thereby, maintaining the clamp load. Lubricating of fasteners will make tightening easier. Utilise the maximum bolt tension, enhance their corrosion resistance property and minimise torsion stress. 

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