How to prevent bulkhead mount SMA connectors from rotating under torque?

[Scott_Guthridge]

I have a one-port device in a small aluminum box with a female bulkhead mount SMA jack coming out of the side. I put lock washers and nuts on both sides and tightened to 0.9 N-m (8 inch-lbs). When I attach an external connector and torque it, it's fine. But when I try to remove the external connector, my bulkhead connector twists. [It doesn't go into a PCB.] I'm thinking super-glue on the inside. Does anyone have a suggestion for how to secure it really well?

 

[dave3825]

So your bulkhead is torqued to 0.9 N-m (8 inch-lbs) in the chassis. When you connect the external connector to that, how much torque are you applying?

 

[prcguy]

When I was working in aerospace and broadcast I used a torque wrench on all SMA and most N connectors. However, I find for amateur type use I torque them at a much higher level due to what the OP is complaining about. Not sure what the max torque you can apply is and that would depend on if the SMA barrel and nut are brass or stainless, but with experience you can manually torque and carefully feel things as it tightens. I have purposely overtightened SMA connectors up to the point of almost failing then did S11/S21 testing and never found any VSWR or loss problems.

If its a permanent setup you can put a dab of epoxy on the threads when assembling then a dab on the flats of the nut but only on material that will never bend or wear in the future, otherwise you may not be able to take it apart to tighten again. I might consider epoxying a connector on a .090 to .125" thick metal panel but not on a .040" thick soft Chinese aluminum or plastic box. If the SMA chassis connector is in a punched hole with flats to keep it from turning and those flats have worn due to turning you can also put a dab of epoxy around the flats to give some material back to avoid turning. Or stipple the area under the nut around the flats with an automatic center punch to force the metal panel back into the flats on the chassis connector.

 

[Scott_Guthridge]

Some more information: here is the connector:



And its datasheet. Despite being brass, the datasheet specifies a mating torque of 7-10 inch-lbs, and a mounting torque of 15 inch lbs. Probably my main problem is that I torqued the mounting nut to only 8 in-lbs, not having a 15 in-lbs wrench. But the connector was still rotating after using only 4.4 inch lbs (0.5 N m) mating torque.

Here's what I did to that connector. I carefully sawed off the pins and filed the surfaces flat. I then surface mounted two 100 ohm RF resistors in parallel between center pad and shield. I cut a short length of brass tubing that has an I.D. almost exactly that of the base of the connector, and cut a brass disk from a sheet. Using solder paste and hot air, I soldered these onto the connector to make a shield. It looks like this:





I likewise made one with no resistors (open) and another with just the brass plate soldered directly onto the back of the SMA connector. As you can guess by now, these are meant to be VNA calibration standards with consistent delays and losses. The aluminum boxes are only for mechanical support so that I don't have to use two wrenches when attaching and detaching cables.

It's bad when these start rotating in the chassis, though, because then I can't remove the cable and have to take the top off the box and hold the back with pliers, risking damage to my nice little shield..

I will try some glue on the inner nut and try to tighten it harder.

 

[ArloG]

Me? A dab of epoxy would work. But how about soldering one of these babies to the connector with low temp. solder?
And then drill the box for a #4-40 screw and nut. I think that would work well.

 

[davidgcet]

do your washers have teeth facing BOTH directions? it looks to me like they are flat on one side and tooted toward the nut. if that is the case then flip one so the teeth bite the box. if you do use epoxy then rough up the box surface around the hole so the epoxy gets some bite as well.

 

[consys]

Locktite blue on the threads, removable if required. Or your wife’s nail polish for a choice of colors. Sounds sarcastic, not the intent. My go to for guitars & amps that get banged around. And when fashion changes you can get the leftovers for free.