Locking tabs

[MintJulep]

So, we know that the general consensus of the forum here is that split-lock washers don't work.

How do we feel about locking tabs? You know, the relativley soft metal plate that you bend a tab up against the flat of the nut, and another tab down against something else.

 

[Heckler]

I'm speaking from an aerospace background. Positive locking devices such as tangless locking keysert or locking HELI-COILs. Anything that has a tang or tab can be broken off becoming FOD which can damage a turbine engine. Most of the system level fasteners are safety wired.

 

[Screwman]

The critical thing in your staement is: "the relativley soft metal plate". Lock tabs are OK in light load applications where there won't be embedment of the bearing surface due to contact stress. However, it seems that these devices are always used in high load applications with either high mean or peak stress levels that result in embedment. Once that occurs, you have lost your preload and either loosening or fatigue failure is likely to occur.
If you know the in-service load environment you should design the preload and joint design such that they will survive without locking devices. If you don't know the service environment, I have had better luck using one of the thread adhesives that are available. They will prevent relative motion of the threads and will not encourage embedment.

Good Luck,
Dick

 

[unclesyd]

We still use "cat ear washers" on the rotating members of the compressor in a HNO3 plant. They are used on all three components, steam turbine, air compressor, and expander operating at 12,500 RPM . This machine was designed by a group from P&W. I don't know of any problems with the locking tabs.

We also use locking tabs on some SS fasteners on very large Centrifuges. The machines are all SS and are subject to vibrations problems.

Here is a interesting variation of the locking tab that I picked up on in another forum. I didn’t record the posters name.

http://www.stage8.com/howitworks.htm#

 

[MintJulep]

I was hoping to generate a more spirited discussion on this topic.

Screwman correctly picked up on my main concern. Any ideas how it might be possible to evaluate how much crushing/flow of the soft material happens during the initial tightening, and how much might continue to occur over time?

 

[MikeHalloran]

I don't know if they still do; GM used to use 'cat ear' washers under connecting rod nuts. They were _not_ dead soft, but were ductile enough to reuse a couple of times.

More recently, I bought a van with fancy wheel covers retained by bolts threaded into dead soft cross- straps, the ends of which were retained by two wheel nuts. The first time a wheel nut fell off, I threw away the wheel covers. A wheel nut's conical seat would make things difficult, but for a regular nut, it should be possible to analyze how hard the tab has to be.

I'm still a big fan of Loctite where temperature permits.






Mike Halloran
Pembroke Pines, FL, USA

 

[arto]

they use sheet metal lock tabs "all the time" on steam turbine bolting -
bend up one corner on the bolt flat, bend long part down over an edge, or use a long one for 2 bolts, bend up a corner on each.