Harlen Seow looks at a various methods for evaluating bolt securing solutions
Bolts and nuts are often looked upon as trivial hardware, even though the outcome of bolt failure can be very costly. The most common result of bolt failure is material damage and expensive downtime.
In effect, the failed bolt can cause substantial damage also on the company's image and brand. In a worst case scenario, personnel injury may result. Nord-Lock states that a good bolt securing solution should provide maximum safety and reduce the overall costs - in fact so effectively that the locking system will pay for itself and ultimately slashes costs.
There are various methods for evaluating different bolt securing solutions. A common test is the Junker vibration test according to DIN65151. This method exposes the joint to transverse movements, underneath the bolt head and is seen as the worst possible case for bolt assemblies. Nord-Lock's international team of sales engineers meet clients locally with a Junker demonstration tool, which illustrates the difference between Nord-Lock and regular friction-based locking methods in just a few seconds (400 load cycles, as seen in Fig.1). The comparative test shows that, when bolted joints are exposed to vibration, most locking systems will allow rotation of the fasteners, resulting in a complete or significant loss of clamp load.
The Junker test is a first step in selecting the best technical solution to prevent bolt loosening. The test is often considered a worst-case scenario and bolted joints performing well in this test normally function flawlessly in real life conditions. The Nord-Lock washers are designed to create a wedge effect and the proof of this can be seen by the increase in tension during un-tightening (Fig.2). This wedge effect prevents the bolt from vibration or shock induced loosening. Bolted joints secured by Nord-Lock only lose some initial preload due to normal settlements between the contact surfaces.
Various studies have been done in Nord-Lock's test laboratories. During regular testing of customers' applications, it has been noticed that illogical measures have been taken with the purpose of achieving higher bolt security. For example, it often happens that an engineer chooses a higher-grade bolt to replace a failed bolt even though calculations confirm the adequate strength of the bolt. Increasing the bolt grade will not only change the friction condition but also the stiffness in the joint and both must be taken into consideration. Furthermore, the higher grade bolt will make the joint more expensive but does not necessarily increase the security.
Here are seven considerations for cost effective bolt securing:
1. Is the locking method based on friction? When tightening a non-lubricated bolt, 90 per cent of the torque is needed to overcome friction, whereas only 10 per cent will result in clamp load. Adding friction-based locking methods reduces clamp load at a specific torque. If the application is exposed to dynamic loads it may not withstand vibration induced loosening.
2. Use a method that allows lubrication for your critical joints. Lubrication reduces friction, facilitates reuse and improves corrosion resistance. It also minimises torsion stress, allows you to utilise the bolt's full capacity and achieving the desired preload with less efforts. Many 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, but with the tension-based wedge locking of Nord-Lock this can be done with lubrication.
3. Do I have control over the clamp load? The existing conditions needed for calculating torque are often vague. Examples of parameters are thread condition, hardness and material of the contact surfaces, friction condition of locking fasteners or adhesives, lubricant on the thread, type of bolt head (flanged, regular or serrated) and surface coating of the bolt. Control over the clamp load in a bolted joint is vital for critical applications. With Nord-Lock, the clamp load can be predicted by utilising the torque/load ratio and torque guidelines are available online for standard bolt qualities. Since it is possible to lubricate when using Nord-Lock washers friction can be minimised.
4. Is the locking method sensitive to temperature variations? Methods using chemicals or nylon inserts can change their locking characteristics when the temperature is elevated or falls below freezing point. It is recommended to use locking methods that have the same temperature characteristics as regular bolts and nuts.
5. Does the method increase assembly time? Traditional solutions like locking wire and tab washers require much skill from the engineers/fitters and such assemblies are very time consuming. Chemical locking requires a curing time which results in increased assembly time. Locking systems that increase thread friction (ex. nylon insert nuts) will prolong the assembly time since a tool is required throughout the thread length, not only during tightening.
6. What are the maintenance and reuse characteristics? The reason why bolted joints are so widely used is that they have great holding power and dismantling is easy. For most applications, good maintenance routines are essential for the durability and high efficiency of the application. An absolute requirement for a cost effective bolt securing solution is that dismantling is quick and easy. Independently of the reuse routines, it is beneficial if both the fasteners and the looking device can be reused. Nord-Lock washers are reusable in most cases.
7. Does the locking method require any special tools or fasteners? A good solution should allow you to use standard fasteners and tools. This should be observed when comparing costs between different locking methods. Nord-Lock's solution can replace a previous large mix of locking devices and thus reduce the number of items to stock, order and refill. Although Nord-Lock recommends the use of a torque wrench, any wrench can be used.
Nord-Lock offers a wide bolt securing range, for sizes from M3 to M130. Washers are available in steel with zinc flake coating and in stainless steel A4. Special alloy washers for high temperature, chloride or salt water applications are also available. Nord-Lock works for joints with short clamp lengths and stud bolts, designs which are often considered difficult to secure. Special washers, the so-called sp washers, designed for soft underlying surfaces, painted surfaces, large or slotted holes are part of Nord-Lock's standard range.Nord-Lock has been put through extensive testing and achieved approvals from leading authorities such as TÜV (Technical Inspection Association in Germany) and DIBt (German Institute for Building Technology). Nord-Lock also holds a type approval from DNV (Det Norske Veritas).
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Harlen Seow is with Nord-Lock International AB is Malmo, Sweden. www.nord-lock.com