SMA / SMC Tightening torque

According to the service manual for my spectrum analyzer: "when replacing semi-rigid pipe connections it is imperative that the following torque settings are used: SMA: 99 - 106 N-cm; SMC: 42 - 49 N-cm"

1. Is it really that imperative?
2. Does anyone know where I can buy suitable torque wrenches in the UK?

Thanks, Andrew.

I haven't used SMC much so the following applies to SMA.

You could probably hook a spring balance (or just a spring with a known extension at the required tension) to the other end of the spanner.

If the connectors are too loose, they may fall off or give unreliable contact after exposure to vibration. If you do them up too tight you can damage the connectors (I have read that the pins can be compressed too much amongst other things). If you do them up just tight enough that you think they won't come loose of their own accord, and no tighter, then you should be fine. The 7% or so tolerance that they specify in your manual is probably unnecessary for normal purposes. When using a network analyser, in my experience I have never been able to see any electrical difference between finger-tight (i.e. done up with my fingers and no spanner) and torque-wrench tight. The advantage of using the torque wrench is that I can then do them up tighter so that I have more confidence that the cables won't come loose later. If you'll be around to re-tighten the connectors then finger tight would probably do.

Chris

I once tightened an SMA with a TDR attached (at the other end of a cable).

There was a little spike of higher impedance that became narrower as I tightened the nut. It disappeared (roughly) when the correct torque was reached.

Regards, Allan

I vaguely remember seeing connectors with a lockwire hole in the shell, in case you have a mission-critical, high-vibration application (like a fighter airplane).

Cheers! Rich

RS Components sell the torque wrenches (certainly for SMA, I'd guess SMC as well). By default I' d say therefore that Farnell do too - they both have US distributors if you're over there. Mind you those distributors are hideously expensive (we get them from Nera for less!) but it should give you a manufacturer's name.

Cheers.

Ken

I can't find any on either's web site. Can you post the order code? Thanks.

I've found a few US companies on the web e.g.

Thanks to all the posters for your responses.

I've gone and tightened the damn things up by hand now - but I would still like to buy the proper tool for future use.