I have noticed that while playing with a right angle SMA connector that the resulting breaking torque varies which different matting techniques given the same setting on the torque wrench. There were two scenarios that I was interested in.
1. ) I finger tighten the right angle SMA connector on my RF cable. The mating connector is fixed to the table. Then, using an 8 inch pound torque wrench I tighten the connector WHILE preventing the connector body to rotate. The only part rotating is the coupling nut. I use my other hand to prevent the connector body to rotate. This keeps the cable in the same spot.
2.) I finger tighten the right angle SMA connector on my RF cable. The mating connector is fixed to the table. Then, using an 8 inch pound torque wrench I tighten the connector WHILE NOT preventing the connector body to rotate. The connector body rotates with the coupling nut and the cable ends up a number of degrees rotated from where it initially was.
After measuring the breaking torque on these mates on a relatively large sample set (each scenarios was performed 25 times), scenarios #1 results in about half as less breaking torque which is bad. ~3in-lbs versus ~6in-lbs in scenario #2. Breaking torque is measured with a dial torque wrench.
My question is: What is the correct way to tighten the connector? Is there a preferred method? Does anyone know or have insight to the phenomenon that causes less breaking torque?
My current theory is that there is extra running torque between the coupling nut and connector body when you hold the cable in place. This extra torque then detracts from the resultant pre-load making the breaking torque lower. I haven't been able to find anything on this. Would be helpful if anyone has thought about this before. I can make up a graphic to explain better if needed.
Note that 8 IN-LB are only for steel SMA connectors and 3.5mm connectors. Brass SMA connectors are 5 IN-LB. Which are you using; maybe it is damaging it? I have never measured the breaking torque, but your #1 technique is the proper one to use; not letting the body rotate. This is really critical for the HP style 3.5 mm connectors that have the fingers within the female center socket, as they can be torn out.
The other variable, even with the right torque wrench, is where you hold it.
I recall a greybeard once telling me in no uncertain terms that you only ever hold the torque wrench at the end - I've seen some hold it right up near the opening, and if you do that you can apply far more torque than the rated level.
First, I have been making sma connections for a long time. In the lab, I tend to tighten them finger tight unless I am making a phase measurement and then I use a plain old little open end wrench to slightly tighten. If I can't loosen it with my fingers, it is tight enough.
I don't think the obsession with breaking torque (Ha-ha, microwave parody waiting to happen) is insightful. If it is critical in practice, then stake the connection with epoxy (you don't need a lot).
The breaking torque is affected by surface finish and cleanliness. A connection made and broken many times will have less breaking torque for the same making torque. If I use a calibrated wrench I click it a few times (I hate break away wrenches).
It is wise to visually inspect both sides before making connections to look for dirt, damage, even metal particles on the Teflon surface (assuming there is teflon). The metal particles will lower the power handling of the connection and can degrade VSWR. (I worked on space products and we had to gage the connectors, everytime! Pins and dielectric and female holding force.)
If you see any thing, clean with a DRY q-tip and maybe GENTLE (dry) air clean.
Last Edit: 1 month 2 weeks ago by Desert Sage. Reason: Add more