TECHNICAL ARTICLE AS PUBLISHED IN
The Journal January 2018 Volume 136 Part 1
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The science of bolt technology
This article explores the technical requirements of the simple, but highly sophisticated componentry; the bolt.
INTRODUCTION Safety is at the heart of every infrastructure company’s operating practice, with both public and workforce safety the subject of continued vigilance. With this focus and with the everincreasing demands for 24 hour working and shorter possessions, rail fasteners, including the nut and bolt, play an understated but vital role in the sophisticated world of asset management., The nut and bolt will always have a key role despite the ongoing move to continuous welded rail (CWR). Every PWI engineer understands that the use of bolts in track is essential. Equally they will understand that they are components that are infinitely variable, that may need maintenance or replacement and in some circumstances may appear to fail. Rail personnel are engaged day in, day out on track with bolts and all the attendant safety and cost issues. This simple mechanical product contributes hugely to the delivery of a safe and operationally effective infrastructure. The retention of proper pre-load in permanent way fasteners and the prevention of them fracturing or even falling out, impacting safety and operational effectiveness, is an issue which continues to exercise engineers’ and operators’ minds. The key to their safe and effective use on the permanent way is an understanding of bolt design, pre-load, torque and clamping force and the effects of friction, vibration and settlement upon performance. The use of a locking system which is straightforward to assemble, disassemble and reassemble, which maintains fastener preloads in the face of high levels of transverse vibration, and is cost -effective at system level, is recommended both for new and upgraded permanent ways. See figures 1, 2 and 3.
AUTHOR: David G. Vile
Technical Director Tracksure
MATERIAL SPECIFICATION AND DESIGN In the UK the most common material for imperial size threaded fasteners used in track, has been as per specification BS 64:1992: ‘Normal and high strength steel bolts and nuts for railway fishplates’. The specification relates to 2 grades of bolt; normal, non-heat treated with no head markings and high strength, heat treated with the head marked “V”. As a result of metrication the international fastener industry uses the International Standard ISO EN 898-1 2013: ‘Mechanical properties of fasteners made of carbon steel and alloy steel -- Part 1: Bolts, screws and studs with specified property classes – Metric coarse thread and fine pitch thread’. The standard does not specify the manufacturing process to attain the mechanical properties but broadly speaking the threaded fasteners are produced as follows:
Figure 1: P&C bolted frog
Machined (on standard lathe) • no material limitations • no upper diameter limits • lower cost in low quantities Rolled threads • tensile strength 5-10% higher • fatigue strength higher • production rate higher • lower cost in high quantities Coarse Threads (BSW, UNC, ISO Metric) • less likely to seize • less likely to strip female mating thread
Figure 2: P&C stock rail chair bolts
Fine Threads (BSF, UNF, ISO Metric) • stronger (larger tensile stress area) • less likely to loosen (lower helix angle) The Standard identifies 10 classes of material ranging from ultimate tensile strength of 300 MPa to 1200 MPa. The bolt head must be marked, along with manufacturer and batch numbers, with 2 numbers separated with a point indicating UTS MPa ÷ 100 and 0.2% Proof Stress MPa ÷ 100 %. Nuts are similarly classified and marked per ISO 898-2 with one number indicating UTS in MPa ÷100.
Figure 2: P&C stock rail chair bolts 42
High strength (V) Classification 8.8 and above are referred to as high tensile bolts and the steel is hardened and tempered to achieve higher proof stress. Class 5.8 and 10.9 to ISO EN 898-2013 low carbon steels are broadly equivalent to BS 64 grades Normal and V respectively.
BOLT CLAMP FORCE The importance of adequate axial preload has been understood by the aero and auto industries since the early 20th century. Sufficient clamp load is required to avoid joint separation, maintain joint stiffness and thereby minimise the dynamic forces acting on the bolt, to ensure adequate bolt fatigue life. In these cases, the bolts are taken to 100% proof load or yield point tightening. However, if the joint is serviced, the bolts must be replaced. See table 1.
In standard torque tightening procedures 75% of the 0.2% Proof Stress load is achieved, significant increase in clamping force can be achieved by selecting a higher tensile strength bolt, this provides for a stronger bolted joint in tension however, it is at a loss of ductility, resistance to bending and tolerance to settlement, see figure 5.
Table 1: Material properties table for commonly found class of bolts in track
Figure 4: Uni-axial tensile loading
Figure 5: Comparison of Stress â€“ Strain Curves Material Class 5.8 and 10.9
Table 2 43
Torque, friction and clamp load Variation in the Coefficient of Friction (μ) between a pair of sliding surfaces is the chief source of uncertainty in the prediction of preload in bolted joints. μ is a property of 2 materials, their surface condition and any lubricant. Figure 6 shows that the amount of torque available to stretch the bolt and generate clamp load is only about 12% of that applied. The rest is absorbed by overcoming surface friction of the nut contacting surface and thread friction. The full equation is a very complex one and the industry relies on empirically derived data to calculate the thread friction coefficient factor K. The relationship between applied torque and the tension created is calculated using the equation: T=K×D×F where T = torque, K = nut friction factor, D = bolt diameter and F = bolt clamp load. This equation is called the short-form or simplified equation; K is published in look up tables issued by the fastener industry. In a free running mild steel threaded fastener K is typically listed as = 0.12 lubricated and 0.18 dry. Recommended torque values are also published to achieve the required 75% of 2% proof load clamp force for each thread diameter. Only tables that confirm this should be used.
Figure 7: Adjustment switch double helical spring compressed to controlled spring height
It should be noted that when using prevailing torque, vibration resistance nuts, this equation is not applicable and reference to the supplier of the nuts should be made for advice on how the correct bolt clamp load can be achieved. This is rarely consistent, if the nut and bolt are reused and often they will be weathered, or the threads and prevailing torque feature distorted.
PRE-LOADING THREADED FASTENERS Table 2 details the approximate accuracy in achieving required torque and clamp load using various methods of tightening.
NON-PRELOADED JOINTS In track, there are specific requirements for joints which are closed to ensure proper placement of the components, but not so tight as to prevent limited motion, i.e. sliding in an expansion fishplate joint or to control spring load in an adjustment switch, see figures 7 and 8. Typically, the bolts are tightened to 30-50% of the preloaded or tight joint fishplates to allow for rail movement due to thermal expansion. With such low clamp force there is little resistance to vibration loosening and a method is needed to ensure correct placement of the joint components is maintained. See settlement image.
Figure 8: Expansion joint Fishplate Settlement is due to wear or brinelling of the mating surfaces due to component movement in the joint and the indentation of a softer metal by a harder one, due to the local compressive stress exceeding the yield stress of the softer metal. In a bolted joint this effect may be noticed during assembly, when more than
one torque application is required to achieve a stable preload. In service, additional train axle loads may cause deeper indentation and loss of bolt preload. In this case, it will be necessary to adjust the joint after a short period in service by re-tightening the fastener to re-establish desired clamp load.
VIBRATION LOOSENING Vibration is a well-known cause of bolt loosening. Both transverse/bending, as well as axial vibration forces are generated in the joint by passing trains and nuts that are not resistant to vibration forces will self-loosen due to nut rotation, causing the loss of bolt clamp load and strength in the bolt. It has been shown that prevailing torque systems, e.g. stiff-nuts, nylon inserts and anaerobic adhesives, may offer some limited resistance to nut self-loosening, but it has been shown that bolt preload can still be progressively lost in the case of combined axial and transverse vibration. Locking devices which rely on retaining pre-load, e.g. wedge lock washers, cannot prevent this serious failure mode, as once settlement occurs the bolt preload is lost. The engagement of the two washers is only effective if the indentation of the abutment surfaces of washer to bolt head and washer to stock rail or chair both hard surfaces, requires the washer to be even harder to create the indentations. Any joint movement would cause an attrition of the mating surfaces and subsequent loss of bolt clamp load. Other methods of thread deformation using twin nuts have limitations in terms of their serviceability and effectiveness when reused.
SUMMARY For all those involved in the challenge of design, installation and maintenance of bolted joints on the permanent way, the challenges are significant. Vibration loosening and settlement will reduce clamp force in any joint, more so in applications where it is not possible to achieve the correct pre-load in the first instance. An accurate application of torque and hence bolt clamp force is vital to attempt to achieve the structural strength of the bolted joint, but in practical reality it is variable when working with limited possession time, older damaged threads with high friction resistance and multifarious other operational challenges.
Figure 9: Typical bolt head failure due to combined bending and torsional overload, the propagation of the initial crack took a period to reach the final rupture hence the corrosion following the crack. High tension class 12.9 bolt.
However, an awareness of the technical challenges of maintaining bolted joints in track, is the precursor to enhanced safety and improved operational practice. Added to increased understanding and awareness, there are additional technologies and processes available that complement the infrastructure managerâ€™s expertise. In safety critical and/ or maintenance intensive applications this is a very progressive way of mitigating the operational challenges of failing joints. We are facing demands for greater utilisation and of course a safer environment for our staff and customers. The maintenance of the mechanical integrity of bolted joints has a vital role to play in meeting these objectives.
Figure 10: Typical first engaged thread failure due to torsional over load, instantaneous failure during torque tightening/re-torque tightening. High tension 10.9 class bolt.
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Great insight for those people who use bolts in critical applications