Are your bolts preloaded properly?

The joining of components or structures is an essential feature of engineering and the practice of bolting components is ubiquitous. Despite its widespread appeal and usage, however, bolting is not a trivial exercise, and disregard of the fundamentals or lack of awareness of the subtleties associated with bolting, has led to some catastrophic failures. This Tech Tip discusses some of these subtleties associated with preload in a little more detail.

A characteristic parameter in bolting concerns the pre-load which is achieved during installation.  A bolted joint includes not only the bolt and nut but also the members/components that the bolt is being used to join/affix (and any washers or gaskets being used).  This whole system when installed needs to be considered as a complete structure, in which the bolt and members share the applied load.  During preloading the bolt is generally loaded in tension and the members which are being bolted together, carry a corresponding compressive loading.  The whole system is in equilibrium, and small changes in the bolt length and plate thickness system accommodate the forces.  If an external tensile load is applied to the members of a properly preloaded joint, the compressive loading induced in the members during preloading is reduced as the members move away from each other.  However, as the stiffness of the bolt is typically significantly lower than that of the members (unless they are gasketed or in poor contact) the tensile load in the bolt changes by only a very small degree (typically in the region of 10% -18% depending on the diameter and length of the bolt).  This is particularly important in cyclic loading (fatigue) situations, where only a small proportion of the applied cyclic load is carried by the bolt and the largest change in load is in the members, which do not readily fatigue because of their large dimensions and the compressive nature of the load induced during pretensioning.

In complex loading situations it is vitally important that the tensile loads that are imposed on loaded bolting situations are fully consistent with the design requirements and that the loads achieved during pretensioning.  Although torque tensioning methods are very practical and relatively easy to apply they are highly dependent on any damage to the load bearing surfaces of the thread and nut, contamination, and the nature of the lubrication which all contribute to changes in the frictional loads that need to be overcome during tightening.  Even in optimal conditions only a small percentage of the applied torque (typically in the region of 10%) actually preloads the bolt and under conditions of severe wear or corrosion a significant amount of the torque employed to preload the bolt will be reacted by friction.  This reduction in pre-load can result in the bolt coming loose (especially in situations where vibration is experienced) and the members separating.  Once the joint separates the bolt caries the full cyclic load and failure by fatigue often ensues. 

The challenge of achieving the optimum preload is not easily solved, as it is difficult to measure the tensile loads induced in the bolt during pretensioning without very careful measurement of bolt elongation, modifying the bolt or using strain gauges.  Another option that affords a relatively high degree of accuracy is to calibrate the preloading technique used by means of a measuring system that relates the actual tensile force in a bolt to the parameter used to achieve preload (e.g. the applied torque required to achieve the required pre-load).  One of the best known and widely used of these devices are the so called ‘Skidmore machines’ manufactured by Skidmore-Wilhelm.  During calibration representative bolts are preloaded in a Skidmore machine to establish what torques or power pack pressures are required to generate the necessary preload in the bolt.  If this is done for a series of three to five bolts a mean torque/pressure to achieve the required preload can be established.  This mean torque can then be used to tighten the bolts in the structure with a much higher chance of achieving the requisite preload.  This relatively simple calibration procedure incorporates the features and characteristics of both the bolt and lubricant employed, and allows some of the variance associated with changes in friction and surface damage to be mitigated.  Although this technique provides a significant improvement in the accuracy of the preload achieved, the friction characteristics of the bolted members are not included (unless a washer or a representative sample of the member is used during the calibration process) and this can cause some loss of accuracy.  It is also difficult to calibrate the preload achieved when the bolt (screw) is tightened into a threaded hole as the friction lost in the thread can typically not be calibrated.  However, the Skidmore approach to characterising the bolt loads is arguably one of the best options for improving the accuracy of preload in an industrial environment. 

Based on our long standing interaction with Skidmore Wilhelm and the historical success the use of their products has afforded our clients, Origen has decided to strengthen the relationship and become a distributor for Skidmore machines in South Africa.  We trust that this association will benefit you and will make it easier for you to access these products which have been shown to improve reliability and reduce the risk of failure in a number of situations.