[TowerTalk] Reliability (or not) of SMA connectors

[Avery Davis]

Please bear in mind that the actual torque value depends on the type of 
connector and the application, and that over torque can make problems, 
too.  For SMA, brass connectors should be torqued between 3-5 in-lbs, 
while stainless SMA connectors should get 7-10 in-lbs.  Also, the usage 
is a consideration.  For lab/engineering (or shack) use where the 
connector is likely to be disconnected, a lower value should be used.  
For a final, permanent, installation, a higher value should be used.  I 
had an instance of an SMA torqued to 8 in-lbs on an assembly with 
replaceable screw-in receptacles, and attempting to remove the plug 
resulted in the receptacle coming off the assembly.  Since this was for 
lab/engineering use, we replaced our 8 in-lbs torque wrench with a 4 
in-lbs wrench and had no further problems.
Another problem I have seen a few times with stainless connectors is 
galling, which can ruin a connector. For engineering use, I prefer brass 
(sometimes gold plated) connectors for this reason.  I have thought 
about applying a stainless anti-galling compound to stainless SMAs, but 
I think a more common problem I have seen is cross-theading or other 
damage due to forcing the coupling nut with the wrench when the 
connectors are misaligned.  I prefer to carefully attach to full finger 
tight before using the wrench.  I assume that if the coupling nut 
becomes difficult to turn with my fingers before the connector is fully 
engaged, the connector is likely misaligned or damaged.
Here is a link for further reading:
https://www.microwaves101.com/encyclopedias/connector-torque

73,
Avery, WB4RTP