When and How to Use Jam Nuts in HALT Fixturing

Monday, March 09 2020

All-thread rod is a common component of fixturing used in HALT. The simplest HALT fixture for a product in a rectangular case is to use aluminum rails above and below the product with nuts, and all-thread rod used to clamp the bottom rail to the threaded holes in the table and the top rail to the product. However, this simple fixture can often come apart in vibration testing, even at low vibration levels, due to the unique nature of Repetitive Shock (RS) vibration.

If the product under test can deform or flex slightly under the clamping force of the top rail, as is common with a product in a sheet metal or plastic case, then the sharp shocks from the RS system can cause the product to compress for an instant, leaving the nuts that are clamping the top of the product with little force keeping them in place. With hundreds of these sharp shocks hitting the product per second a nut can easily loosen up and start ‘walking’ up the rod, and the fixturing will shortly fail.

Using two nuts in a jam nut configuration on the all-thread rod at the top rail can solve this problem, but it needs to be done correctly to be effective. Here’s the best sequence to follow:

• Put all top rail fixturing in place and hand tighten a single nut (with washer underneath) on each all-thread rod.
• Tighten the nuts down in a “star” sequence, being sure to avoid tightening one nut significantly more than the others as the rails are clamped down.
• · When all single nuts are sufficiently tightened, run the second, ‘jam’, nut down each rod. Snug the second nut onto the first nut so that it is tight against the first nut but has not moved the nut or increased the clamping force of the rail against the product. Then, holding the first nut with a wrench, tighten the second nut tightly against it. Repeat for all rods.

It’s not hard to do it right, and once you’ve done it right the fixture will stay together nicely, and you’ll avoid the irritation of having to stop a test to fix the fixturing!

And remember – if it’s not broken, you’re not done yet!

Neill Doertenbach

Customer Solutions Engineer

ESPEC NA