When to use washer or not??
When to use washer or not??
(OP)
Can any body shed some light on the existence of a standard or otherwise that governs the use of washers on studbolts? ie. When can you get away with not using them? Can apply to any application but specifically offshore oil and gas. Thanks!





RE: When to use washer or not??
The only time I would use a washer on a bolted connection is when the hole is slotted or drilled oversize. Oversize being more than a 1/16" larger than the bolt diameter.
RE: When to use washer or not??
A washer can provide multiple functions, the two most important ones being: 1) proper bearing area to avoid excessive surface pressure; and 2) hard, smooth, and constant-material-composition surface for good preload control. You must analyze the joint design (geometry, materials, and applied forces/moments) and calculate if the washer is necessary.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: When to use washer or not??
Another is to reduce the portion of the external load carried by the bolt, which is especially critical (because of fatigue) in the case of dynamic loads. It does this by increasing the relative stiffness of the clamped materials (k = AE/L, where A is the effective pressure area, which can be increased by using a hardened washer). In a bolted joint, the preload compresses the clamped members and stretches the bolt. An external tensile load will tend to separate the members, which would tend to stretch the bolt further (increasing the bolt load), but at the same time will also tend to relieve the compression of the clamped members (decreasing the bolt load). In reality, the joint will find some happy medium of compression/extension. The stiffer the members, the smaller the fraction of the external load the bolt will "see".
Shigley and Norton both cover this topic. I prefer Norton, especially for his coverage of calculating the joint stiffness factor. And boltscience.com has a great wealth of information on bolted joints.
Rob Campbell
RE: When to use washer or not??
RE: When to use washer or not??
RE: When to use washer or not??
We do use a hardened and ground flat washer under some H-11 SHCS being tightened against some PH S/S
RE: When to use washer or not??
RE: When to use washer or not??
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If the bolt is suffiently preloaded, it is not subject to fatigue. The cyclic load reduces the contact stress between what is being bolted together. Generally (and there are exceptions), the bolt stress does not change until the length of the bolt changes, which means the contact surfaces separate. So you have to overcome the bolt preload before bolts experience cylic loads.
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That simply isn't true. Maybe the key to the midunderstanging is the mistaken belief that the bolt does not change length until separation. The bolt, which is under tension in the first place, does indeed lengthen. The clamped members, which are in compression, relax by the same amount, so there is no separation as long as there is some of the initial compression still in the members.
The portion of an external load, P, carried by the bolt is:
Pb = kb*P / (kb+km)
Where kb and km are the stiffness of the bolt and the clamped members, respectively. Therefore, increasing the stiffness of the clamped members relative to the bolt decreases the portion of the external load carried by the bolt. But it does not eliminate it.
One of the figures commonly used to illustrate this concept is a block of wood wedged between a force pull gauge (essentially, a compression spring). In this case, the stiffness ratio is virtually infinity - try compressing a block of wood between your fingers. So pulling down on the block does not increase the load on the spring until the preload is exceeded and the pieces actually separate. The stiffness ratio in a bolted joint is much less than infinity.
Rob Campbell
RE: When to use washer or not??
Thread292-10824
RE: When to use washer or not??
It depends on the design - geometry, materials, and forces. The washer can provide increased bearing area and higher/more consistent preload, which may be necessary. However, the design may not require extra bearing area or more consistent preload, therefore it won't need washers.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: When to use washer or not??
Flat washers are a bad habit to get into, always try to fix the bearing surface where the nut lands. As stated previously we don't use washers on any flange bolting. I seen too many problems with flanges and leaks caused by the use of washers to correct a crippled flange. This has come in the form of cupped washers, washers as bad, proud metal, as the problem they are trying to fix, thinned, worn out washers, stacked washers, and so on.
If a washer is used on a flange due to a problem, real or otherwise, this flange is corrected the next outage.
Having said all that, if you deem a washer is needed make sure you get the proper one for the job.