PEM fasteners for Shock Loads??

[miner00]

Has anyone had any good or bad experience using PEM's in situations where they might be exposed to shock loads after installation? I have used them in the electronics industry with no problems but I have heard rumors that they don't stand up well to impact loading.

IMO, a PEM that has been torqued properly is going to act exactly like a loose fastener once a screw is in place...the self clinching force will be insignificant compared to the clamping force of the screw securing it...

Any thoughts?

miner

 

[alexit]

Depending on the PEM style, your opinion above sounds reasonable.

We have had in several cases (due to high quality packaging delivery service) six PEM standoffs snapped simutaneously (all attachement points for a sub-component.) These break at the last (first?) thread, before the screw engagement range. In our case, the products packaging were redesigned, and no users have ever commented on broken PEMs.

 

[CoryPad]

miner,

You are correct - if you have sufficient preload, the clinching force is immaterial.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[griffengm]

Miner00,
Just my 2 cents.
One of our suppliers used "clinch" style fasteners in sheet metal assembly we purchased. We had a number of failures with these and finally went to another vendor.
We think there were two potential failure modes. My belief is tht they apply to PEM as well.
The first was failure of the sheet metal as evidenced by cracks radiating from the fastener. Either when the nut was installed or during our assembly process a stress was created and then magnified by shipping and operating forces.
The second mode was snapped bolts. These were, I believe, the result of impact loads magnified by a cantilever created where the id thread of the nut ended some distance from the plane of the sheet metal. Most of these occurred during shipment. The nut thread was recessed several turns into the nut in an apparent effort to prevent damage at installation. Since the change to a nut with threads into or at the sheet metal, I have not heard of a single failure in nearly 10000 units (~30000 fasteners).
We do use some pressed nuts as well but have avoided their use whereever substantial shear loads might be encountered in shipping/handling.
Hope this helps.

Griffy

 

[drawoh]

griffengm,

I wonder if this is not a sheet metal design issue.

You cannot place significant loads on the middle of a piece of sheet metal. You design it so that your loads are located next to one or more gussets, or you design your load to distribute itself over a large area of the sheet metal, or you double the sheet metal panel around the point the load attaches. If a PEM fastener is constrained from bending by the rigidity of the sheet metal panel, I predict failure.

I cannot imaging using PEM standoffs for anything heavier than a printed circuit board.

I am aware of shops using hammers to install PEM fasteners, in spite of the instructions not to do to. You ought to be picky about who fabricates your sheet metal.

I use PEM fasteners a lot, mostly for aircraft mounted hardware, and I do not have problems.

JHG

 

[griffengm]

drawoh,
Our original vendor for this part no longer supplies it. When we asked then to look at failures and issues they replied that other customers were using this design and not having problems so it must be something we were doing to cause it. So we solved it - don't buy their part.
Different design from someone who worked with us and the problem went away.
You are correct about it being a design issue. Their design placed pivot points some distance from the surface of the sheet plane to create a lever. They had been providing this part to us for years and then decided to change the design of this critical area. That's when the problems started.
These were not standoffs. We use standoffs as you suggest they be used and have no problem. We also use PEM style inserts in other applications with little problem. If I get my way we will use still more but this one instance caused us to look well at new apps.

Griffy