Click here to go to our main connector page
Click here to go to our page on torque wrenches
New for March 2023: skip down the page to learn about thin and thick-wall SMA connectors, thanks to Rick.
The first information on this page was contributed by Ruth, a true expert in the subject of coax connectors! Further info was later contributed by Stephen T. Morley.
This question was sent in: what is the correct torque for different connectors, and in what documents give this information?
The MIL specs for connectors usually specify a "recommended mating torque" in the beginning of each slash sheet. Many connector manufacturers specify that value in the general specification pages for each connector series and I have not seen many inconsistencies among the various manufacturers. The only place you have to watch out is when there are brass versions of traditionally stainless steel products as in the SMA and SSMA styles. Due to the nature of the design if you torque a brass SMA to the specified torque of 7-10 in-lbs and the male housing is of the thin wall variety you can get a roll over condition. Most manufacturers will specify the brass connectors at a lower torque value... or only supply a thick wall male. Here are some of the more popular interfaces and the mating torque that I have always specified.
SMA (stainless steel) 7-10 in-lbs.
SMA (brass) 3-5 in-lbs.
SSMA (stainless steel) 7-8 in-lbs.
SMM (stainless steel) 2-3 in-lbs.
SMC (brass) 3-5 in-lbs.
Type N 12-15 in-lbs.
TK 12-15 in-lbs.
TNC 12-15 in-lbs.
SC 12-15 in-lbs.
This info came from Marcus on the more expensive, air-dielectric connector series, thanks!
For 3.5mm, 2.92mm, and 1.85mm recommended torque is 8 in-lbs.
To connect 3.5 or 2.92 to SMA, 5 in-lbs.
For 1 mm connectors, use 4 in.-lbs.
By the way, to convert from inch-pounds to Newton-meters, multiply inch-pounds by 0.113.
It is important to note that some interfaces like the Type N and TNC which can have a slotted or solid male housing must be watched carefully. The slotted housing is specified to a lower frequency and is much more forgiving than a solid housing. If a technician were to tighten a slotted type connector with his "calibrated" fingers it would most likely be OK and not particularly affect the performance. A solid housing however requires a closer watch on the torque therefore a torque wrench is highly recommended.
This info was sent to us by Mr. Morley after we posted the page, for the most part it agrees with Ruth's numbers:
Here are torque recommendations for coax connector coupling nuts compiled by Stephen Morley:
| Connector type |
Material |
Recommended torque
|
| American |
Metric |
| 7/16 |
Brass |
221 - 265 in-lbs.
(18.4 to 22.1 ft-lbs.) |
25 - 30 Nm |
| Type N |
Stainless steel |
12 - 15 in-lbs. |
1.4 - 1.7 Nm |
| Type N |
Brass |
6.2 - 9.7 in-lbs. |
0.7 - 1.1 Nm |
| TNC |
Stainless steel |
12 - 15 in-lbs. |
1.4 - 1.7 Nm |
| TNC |
Brass |
4.1 - 6.1 in-lbs. |
0.46 - 0.69 Nm |
| SMA |
Stainless steel |
7 - 10 in-lbs. |
0.8 - 1.13 Nm |
| SMA |
Brass |
3 - 5 in-lbs. |
0.34 - 0.57 Nm |
| SSMA |
Stainless steel |
3 - 5 in-lbs. |
0.34 - 0.57 Nm |
| OSMM |
Stainless steel |
2.0 in-lbs. |
0.23 Nm |
| SMC |
Brass |
1.9 - 3.1 in-lbs. |
0.21 - 0.35 Nm |
Here are torque recommendations for coaxial connector jamb nuts compiled by Stephen Morley:
| Connector type |
Hex nut size
(inches) |
Material |
Recommended torque
|
| American |
Metric |
| 7/16 |
Varies with manufacturer |
Brass |
310 - 354 in-lbs.
(25.8 - 29.5 ft-lbs.) |
35 - 40 Nm |
| Type N |
0.75 or 0.812 |
Stainless steel |
40 - 45 in-lbs. |
4.5 - 5.0 Nm |
| Type N |
0.75 or 0.812 |
Brass |
35 - 40 in-lbs. |
4.0 - 4.5 Nm |
| TNC |
0.625 |
Stainless steel & brass |
35 - 40 in-lbs. |
4.0 - 4.5 Nm |
| TNC |
0.562 |
Stainless steel & brass |
22 - 25 in-lbs. |
2.5 - 2.8 Nm |
| SMA |
0.437, 0.375 or 0.312 |
Stainless steel |
12 - 15 in-lbs. |
1.35 - 1.7 Nm |
| SMA |
0.312 Hex |
Brass |
8 - 10 in-lbs. |
0.9 - 1.1 Nm |
| SSMA |
0.250 |
Stainless steel |
8 - 10 in-lbs. |
0.9 - 1.1 Nm |
| SMB and SMC |
0.250 |
Brass |
3 - 5 in-lbs. |
0.34 - 0.57 Nm |
New for March 2023: From Rick. There are different cable configurations that can affect the amount of torque you apply to an SMA connector. Check out the images below. On the left is a thin-wall cable, on the right is thick-wall.
In Rick's own words:
The outer diameter of the reference plane has to sit within the 0.1810 Min on Female connector as specified per MIL-STD-348 Figure 310.1 and 310.2. But the inner diameter of the reference plane is a function of the coax cable design and what coax is being terminated.
When the body of the SMA is brass and a thin wall is used – usually the mating torque is lowered to 3-5 in-lb.
We tend to use stainless steel bodies and IF a thin wall is used, 7-10 in-lbs. is standard, but a thick wall is preferred – in my opinion.
Here's a bonus! Stephen Morley sent in this King's English/American English translation sheet for connectors, check it out!