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Are there any applications in which a Nyloc nut is preferred to a proper cap nut? Nyloc nutsenter image description here

As best I can tell: a Nyloc nut is just a specialized cap nut that will fail under lower torque and temperatures than a metal cap nut.

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Not all cap nuts are self locking, some are, although they can be a bit weird looking.

Nylocs are generally the first choice for general use as they provide good resistance to loosening due to vibration or flexing of the joint and will stay in position on a thread even when not under tension (unlike spring washers). Unlike locking adhesives they can be removed and adjusted without losing their locking properties and can, to some extent be reused.

There are certainly many alternative types of lock-nut which may be preferred for specific applications for example structural applications may use washers which bite into the joint, there are a whole variety of thread sealing adhesives, spring and star washers have applications and you can also get nuts with asymmetric threads which exert a radial load on the thread. all of these have their various advantages but nylocs are pretty ubiquitous for general use whre you just want something chap simple and reliable.

So typically they would be used for general engineering assemblies which aren't economical to analyse in exhaustive detail and need to be made to a price. Obviously for high temperature applications nylocs aren't suitable but that is usually reasonably obvious.

Cap nuts are generally used to prevent having an exposed thread protruding on the nut side of the joint this generally looks neater, reduces the risk of snagging and can help prevent the nut from seizing due to corrosion.

The major downside of cap nuts is that the bolt or stud needs to be within a few mm of the exact length required and there are a number of applications where they cannot be used eg if you need a nut some way along the length of a stud or need a reasonable range of adjustment. Also if you get the bolt length wrong you can end up with an apparently tight nut which isn't exerting the expected compression on the joint.

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  • $\begingroup$ So nylocs have meaningful "locking" characteristics even when they're driven with the nylon fully onto the threads – i.e., to an arbitrary position on a threaded rod? I can't find any specs on that usage, and I'm having a hard time imagining an application in which we say, "I want a nut right here, and I don't want it to move, even though it has no tension against anything but the threaded rod itself." (Because if it is bearing against something, and we want it to stay there, then I thought we always count on thread tension, perhaps supplemented by a locking washer or surface.) $\endgroup$ – feetwet Aug 25 '17 at 17:56
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    $\begingroup$ @feetwet A locking washer would require you to get two separate things and then put them together and have the right amount of two things. Sometimes it's easier (a lot easier) to have an all-in-one solution, both in terms of supply and labour. $\endgroup$ – JMac Aug 25 '17 at 20:45
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    $\begingroup$ @feetwet A feature of a nyloc nut is that even it the event it were to become somewhat loose (say the assembly it is holding degrades) it will still tend to stay in place on the fastener, while a plain nut or nut/ lockwasher will tend to vibrate off and get lost if not under some tension. $\endgroup$ – agentp Aug 25 '17 at 20:51
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A Nyloc nut is a form of locknut. The purpose is to prevent unintended loosening.

A cap nut is not necessarily a locknut. You can get cap nuts that have some form of locknut on the threads; but in general this is not a standard feature of all cap nuts.

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  • $\begingroup$ Doesn't the torque applied to either nut when turning it onto the threads determine the resistance to loosening? I thought the "lock" on threaded fasteners is primarily the product of tension and surface area of threads engaged (and it can be enhanced by applying various compounds to the threads). $\endgroup$ – feetwet Aug 25 '17 at 14:10
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    $\begingroup$ @feetwet The "locking" comes from elastic deformations in the threads (for these types of locknuts). They are specifically designed to deform more, so that the nylon goes into the threads better. The increased friction keeps the threads locked better. $\endgroup$ – JMac Aug 25 '17 at 14:38
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    $\begingroup$ @feetwet in static conditions, you would be essentially correct, but many, if not most, bolted joints are NOT completely static and will naturally tend to loosen. The nylon in the locknuts provides an elastic element that grips the threads and resists loosening due to vibration $\endgroup$ – DLS3141 Aug 25 '17 at 14:45
  • $\begingroup$ Plain cap nuts are mostly used for appearance and to prevent contact with bolt threads that would otherwise extend beyond the nut. They have no locking features. $\endgroup$ – DLS3141 Aug 25 '17 at 14:49
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    $\begingroup$ @feetwet "Doesn't the torque ... when turning it onto the threads determine the resistance to loosening?" No. You are ignoring plasticity, creep, thermal expansion, etc, etc. Note, there are theories of friction that are more advanced (and accurate!) than the elementary Coulomb friction model which consider "friction" to be "plasticity on a microscopic scale." $\endgroup$ – alephzero Aug 25 '17 at 20:35

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