Nut / Bolt fastener loosening - standard test methods 1

[ella0509]

Hi,

Just wondering if anyone knows of other suitable bolt / nut loosening tests or verification in common usage for a 'static' bolt joint without vibration or shock loads. I have read and witnessed a test on the Junker machine and heard of the NAS test methods, but I'm not really sure they fit our requirements as they seem to be a fairly violent test.

The application is for a clamp (similar to a heavy duty hose clamp - tensioned by bring cross bars together with bolt / nut) where the hose will suffer from slow dynamic oscillatory motion. I need to prove that the fasteners will not loosen over the lifetime of the application from this relatively slow dynamic motion and that any locking methods applied do actually perform as required.

I anticipate the fastener loosening would come mainly from creep and stress relaxation of any of the components making the initial 'bolt-up' fastening load reduce. We can cover this reduced load in our analysis, so I know what the worst case load would be (i.e. after creep etc) - but just not sure of what existing test method to apply. I suppose if it performs better on a Junker test before and after applying the locking method then this may be suitable (i.e. a relative performance comparison) - just concerned that it would not 'pass' this test method at all to give the lifetime confidence.

Thanks in advance.

 

[3DDave]

You need a means to ensure the preload is not lost - a spring element in the clamp that is a lower-rate than the hose material is required. A bolt and strap are typically too stiff to ensure preload.

After that any number of methods can be used - lockwire and locking compound are the most popular.

The key item is that preventing the fastener from turning does not prevent the joint from loosening. That's the job of a suitable elastic element with enough travel and low enough rate to accomodate whatever defects arise.

 

[desertfox]

Hi

Why not just use a pair of Nordlock washers and it won't come loose

http://www.nord-lock.com/products/wedge-locking/washers/introduction/

 

[tbuelna]

As 3DDave notes, there is a difference between a threaded fastener installation losing preload, and having the relative position of the mating components change or "back-off". A bolt/nut installation can lose preload without any relative movement between them if the clamped substrates creep or relax. You might be able to maintain a consistent preload over time in your rubber hose clamp fastener by using a compression spring under the nut.

The tendency for plastics and elastomers to creep or take a set under sustained loading is a primary reason these materials are not normally used as part of a preloaded joint. For reasons of reliability, many threaded fastener installation require two independent methods of locking. One is usually the prevailing friction in the threads due to axial preload, and the other is often prevailing friction in the threads due to radial interference produced by a locking element. If severe vibration is a concern, then some form of positive locking device would be used, such as a castellated nut/cotter pin or lockwire.

 

[ella0509]

Thanks all.

To further explain the problem I'll simplify the description slightly.

[ul]
[li]Assume we have a rubber band with one end rigidly anchored to the ground.[/li]
[li]The other end of the rubber band is rigidly attached to one end of a threaded stud.[/li]
[li]The other end of the threaded stud passes through a hole in a mounting plate - coaxial to the ground anchor point and fixed at a certain height above the ground.[/li]
[li]A bolt tensioner device then pulls the threaded stud vertically (reacting off the mounting plate) at a pre-set tension.[/li]
[li]This stretches the rubber band and imposes a load within the system.[/li]
[li]A nut is then hand spun onto the stud firmly against the plate to maintain the tension.[/li]
[li]The tensioning device is removed.[/li]
[li]Over time the rubber band and the fastener set will relax/creep - we have good data for these and with reasonable FOS can be happy with knowing that our imposed load at the end of service life is still in spec - assuming the nut stays where it was when installed.[/li]
[/ul]

We have a good handle on the applicable nut locking methods available. What I want to be able to do is demonstrate to the client that the nut locking method (whatever is chosen) will be sufficient to prevent the unwinding of the nut during transport, deployment and service. There will certainly be some relative transverse motion between the mounting plate / stud / nut and therefore it will undoubtedly begin to unwind and this would then reduce the system load.

This is my struggle - what tests would be suitable for this demonstration - is the Junker purely a 'violent' shock/vibration test like I have seen or can the frequencies and amplitudes, relative motions be adjusted to what our application will see in service? Would we set the test up just on the stud/nut components (imposed load on the stud) or would it need to include the rubber band with the load applied to the stud through this band?