Bolted joints became the centre of attention after recent findings that they are often used to as little as 30percent of their capacity. Moreover, a failed critical bolted joint could lead to expensive warranty claims or maintenance costs.

Control of the clamp load in a bolted joint is vital. However, when faced with a problem joint, it is not surprising that the design engineer will not have an answer if asked about the clamp load. Torque calculations must always be based on the existing conditions that often are very vague. Unless all parameters are correct, the calculation will be unreliable. Examples of parameters are:

• Thread condition of the fasteners.
• Hardness of contact surface.
• Material (steel, aluminium, copper, etc.)
• Extra friction from a locking fastener.
• Extra friction from an adhesive.
• Lubricant on the thread.
• Type of bolt head (flanged, regular or serrated).
• Surface coating of the bolt.
• New or reused fastener.

Major automotive companies test incoming fastener batches on the actual material of any specific bolted joint to obtain the torque/load relationship and its deviation. Their engineers therefore know the clamp loads in the joints. However, smaller and medium sized companies are usually not in possession of sophisticated bolt testing laboratories.

Torsional stress

During tightening, bolts are subjected to both tensile and torsional stress. The total stress in a bolt can be calculated using the formula.

Total stress = √ σx2 + 3τxy2

In order to maximise the desired tensile stress (σx) it is vital to minimise the torsional stress (τxy). Tensile stress (clamp load) is achieved when the bolt is axially elongated.

Unwanted torsional stress (twisting) in bolts arises during tightening due to thread friction. High thread friction increases torsional stress and causes yielding at lower clamp load levels than normal.

A lubricant is necessary to minimise the malign torsional stress. However, many commonly used bolt-locking systems are based on increased thread friction (deformed nuts, adhesives etc). To minimise thread friction and concurrently safely secure the joint has often incorrectly been considered impossible.

The use of locking systems that increase thread friction is the single most common reason why the full capacity of bolted joints is not utilised. The following example illustrates the problem.

Applying an adhesive significantly increases thread friction during tightening. The red graphs (Fig. 2) show tightening of bolts with adhesive on the threads. Due to increased thread friction and torsional stress, only half as much clamp load is obtained before reaching the yield points (marked with x).

When tightening the same bolts lubricated, which is illustrated by the green graphs, almost twice as much clamp load is obtained. The diagram clearly shows why low and uniform friction during tightening is necessary to ensure that the bolts’ full capacity can be used.

Under static load the achieved clamp load in a joint is maintained. However, bolted joints exposed to dynamic loads or vibrations are likely to gradually loosen. Even if some of the common bolt locking methods (such as serrated washers, adhesives or deformed nuts) work fairly well when the dynamic loads are lenient, only the Nord-Lock Bolt Securing System has proven fully reliable when conditions are extreme.

Furthermore, its locking function is not lost by lubrication which means that thread friction and thereby also the torsional stress can be minimised.

Nord-Lock’s bolt securing system consists of a pair of pre-assembled washers. The washers have a cam angle ‘α’, which is greater than the thread pitch ‘β’. In addition, there are radial teeth on the opposite sides of the washers (Fig.3).

The washers are always installed in pairs, cam face to cam face. When the bolt 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 attempt of the bolt/nut to rotate loose is blocked by the wedge effect of the cams.

Nord-Lock AB has developed well equipped in-house laboratories where clients get the opportunity to put joints from their own applications to the test. In simulations of real-life conditions torque-load ratios are measured and Junker vibration tests are performed. In a Junker vibration test (meeting DIN65151) bolted joints are subjected to transverse movements while a load cell continuously measures the bolt tension.

The Junker test is used to compare different bolted joint configurations and is a first step in selecting the best technical solution to prevent bolt loosening. The Junker test is often considered a worst-case scenario and bolted joints performing well in this test normally function flawlessly in real life conditions.

Fig.4 demonstrates that many commonly used bolt-securing devices show limited locking performance when exposed to vibration.

Wedge-locking effect

Bolted joints secured by Nord-Lock only lose some initial preload due to normal settlements between the contact surfaces. Nord-Lock’s wedge-locking effect is verified by the increase in clamp load during untightening.

For machineries requiring regular service and maintenance, Nord-Lock is the optimum solution. Bolted joints secured with Nord-Lock are easily assembled and disassembled and no special tools are needed. Because the Nord-Lock bolt securing system uses tension instead of friction to secure bolted joints the locking function is not affected by lubrication. The use of a good lubricant is recommended in order to reduce torsional stress, minimise clamp load deviation and to protect against corrosion.

Since the clamp load deviation is very low when tightening lubricated fasteners secured by Nord-Lock, joints can always be safely locked at the highest possible preload level.

Robin Isacsson is an Applications Engineer with Nord-Lock, Malmo, Sweden.

www.nord-lock.com