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Distributor's Link Magazine Summer 2018 / Vol 41 No3


154 THE DISTRIBUTOR’S LINK BENGT BLENDULF FASTENER FRICTION – FRIEND OR FOE? from page 94 FIGURE 4 The impact on the tightening torque is enormous if we are basing it on the first example (Fig 3) and switching to the second (Fig 4). This can be done by accident or by purpose. If we want to reach a preload of nominal mean 22.7 kN and our k-factor is 0.31 the torque value would be 70.4 Nm according to figure 3. If the fastener is according to our example in figure 4, the required torque value would be only 38.6 Nm to reach the 22.7 kN. If we by mistake put the higher torque value on the fastener with coating the preload would be 120 kN or more than twice the tensile strength of the fastener. In that case the friction coefficient is not a friend but a foe. The fastener would fail long before we reached the projected torque value. I would suggest that you get the Torque Book for Fasteners from IFI if you are dealing with tightening of fasteners. It is not a perfect book, but will give you some control of the tightening issues. Our goal must be to get the best possible preload on the joint without killing the fastener. Let me also give you a few words about torsional stresses in the fastener. It would make an article by itself so let me just give you couple of hints about torsion. When you have a cylindrical body of any kind and you twist it, its capacity of carrying load through its axis is limited by the twisting. The limiting factor here is the friction between the internal and external threads in the fastener. If we can limit the coefficient of friction in the threads we will also limit the torsional effect. So, a good lubrication of the fastener is always a positive and “friendly” thing to do, just make sure that it is properly understood by the people holding the torque wrench. BENGT BLENDULF GUY AVELLON WHAT FASTENER DISTRIBUTORS NEED TO KNOW ABOUT SUBSTITUTIONS from page 96 Because of RoHS, the gold colored cadmium multiple bolt connection are torqued to the same value, plated nuts were discontinued. These nuts produced the higher strength fasteners will produce less clamp a consistent and low torque-tension relationship. The load than the others. Not the same clamp load, but less. replacement silver colored nuts are zinc with a wax Arguments have ensued with mechanical engineers and coating, which requires a greater torque for assembly. metallurgists about all steels (fasteners) have the same Therefore, using a cadmium torque on a zinc coated nut slope on a tensile graph. This is true, but not during produced lower clamping loads. assembly where instead of all tension, there is torsion. Bolts: Aside from the thread length differences Simply stated; it takes more force to stretch a higher and some head dimensional differences, many ASTM grade than a lower grade. Therefore, if you stretch the bolts may be substituted among themselves or with SAE higher grade fastener to the same amount or distance as bolts. However, regardless of the grade, make sure all of the lower grade, the higher grade has not been stretched the fasteners in a multiple assembly joint are the same the same distance and will consequently not produce grade and torqued the same. the same clamp force but instead, a lower clamp force than the lower bolt grade. The difference is torque, not This is by far the most important item to remember: tension. If a multiple bolt connection mixes different strength Always ask to be sure. A phone call is less costly grades in the assembly, even if all the fasteners in a than a law suit. GUY AVELLON


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