Bushings in tension fittings (i.e. bathtub, channel) 3

[terning]

I have been researching tension fittings in Bruhn, Michael Nui, and technical publications. These offer insight when designing a new part to be installed.

My problem is that I need to fix a damaged hole using a press fit bushing to get rid of the damage in the hole. Can anybody point me in a direction to find info on how to find the max diam. the bushing can be before it will no longer react and tension load? Obviously if the bushing is as large as the washer or bolt head, which ever is bearing on the fitting, nearly no load will be reacted.

If anyone can offer me some insight it would be greatly appreciated. I look forward to hearing from you.

 

[jetmaker]

terning,

I do not have an answer for you on the maximum diameter of the bushing, but I do have a suggestion.

For tension joints that I repair, I assume that the bolt load is carried into the tension fitting in a conical distribution. The angle of the cone is arbitrary, but the smaller the angle, the more conservative the answer.

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As the bushing diameter increases, the cone's volume will begin to decrease. I try and restore the cone's volume by adding a thick washer or nested filler between the bolt head and the tension fitting, making the OD of the cone larger. Only count the additional volume in the base material and not that added by the washer.

I realise this is difficult to follow without proper diagrams, but hopefully you have the idea. If you have any further questions let me know.

Regards,
jetmaker

 

[Sparweb]

How about a flanged bushing? A simply cylindrical bushing will pull through, as you pointed out, but a flanged bushing, made of steel, like a NAS77 could suit the application. If you need harder steel or a more compatible material with the lug, you could always make something in-house. Putting a taper on the OD might help, too, for slightly different reasons than what Jetmaker had in mind.

STF

 

[edbgtr]

Terning
The initial design curves and formulae provided for the design of the new part, secure your ability to determine the extent of the bushing you can use. In the bathtub and tension clip (Brit. cleat) the bending in the end-pad defines, or drives (mostly), the tension load capability of the fitting.
The intended bushing does nothing for the out of plane bending strength of the end pads, and the net section through the tension bolt hole "drives" the bending capability of the end-pad. Conservatively, the BT end-pad, can be idealized as a beam constrained by the two parallel side-walls of the fitting.
Inserting a filler (Brit. throat-plate) allows you to partially "unload" the original end-pad and thereby alleviate the effect of the bigger bushed hole. The throat-plate and end-pad work as two separate "beams" in parallel, with the common boundary condition of the deflection at the bolt hole ensuring that the redundant beam system can be solved. It is possible to do this manually (classical compatible beam analysis) or you can use FEM. However, the manual method gives you an analytical tool to determine the throat-plate thickness variable and hence a solution for a variable bushing thickness. Please note Niu's comments about snugly fitting fillers in order for them to work most effectively. This comment becomes obvious when setting up the above compatibility analysis.
Hope this helps you solve your bushing problem.
Ed.