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SMA / SMC Tightening torque

A

Andrew Holme

Jan 1, 1970
0
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.
 
R

Rich Grise

Jan 1, 1970
0
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?

I'm not sure what you mean by "that" imperative, but I do know that
torque values are specified to keep ham-fisted operators from OVER-
tightening things, which could distort the dielectric, and worst case,
strip the threads. When I had occasion to use those type of connectors
in the military (USAF), we generally hand-tightened them, and used a
1/4" open-end wrench with just a little more than a feather-touch.
Less force at the end of the wrench than it takes to push a telephone
keypad button. We got, for all practical purposes, 100% reliability.

Get a little bitty torque wrench and a "crow foot", if you're really
nervous about it or have a PHB breathing down your neck. :)

As to where to get one, I know nothing about the UK other than that
Brit teeth are an endless source of humour - do you have anything like a
"Yellow Pages?"

Good Luck!
Rich
 
C

Chris Jones

Jan 1, 1970
0
Andrew said:
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
 
K

Ken Taylor

Jan 1, 1970
0
Andrew Holme said:
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.
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
 
Andrew said:
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?


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
 
A

Andrew Holme

Jan 1, 1970
0
Ken said:
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 -

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.
http://www.smelectronics.us/torquewrenchnncsmc.htm but this is 0.35 Nm for
SMC when I thought it had to be .42 - .49

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.
 
R

Rich Grise

Jan 1, 1970
0
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.

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
 
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