Originally posted Nov 22, 2013
Straightforward advice and a trick question
Well, it’s more of a tricky question than a trick question. The answer is an interesting and useful fact and we’ll get to it in just a moment. First, allow me to be the thousandth person to remind you that connector torque can be important and helpful at high frequencies, and explain a few of the reasons.
Engineering is inherently challenging at microwave and millimeter frequencies. Many things that can be ignored at lower frequencies really begin to matter. Accuracy and dynamic range are already declining as frequency increases, and you don’t want to give up anything you don’t have to in terms of accuracy, repeatability or connection loss. Because the measurement solutions you buy—including analyzers, signal generators and associated equipment—are more expensive, you want to extract all available performance and preserve all the margin you can.
All these factors lead us to the use of torque wrenches. I don’t know about you, but my fingertips just aren’t strong enough or repeatable enough when it’s time to hook up an important measurement or make final system connections. I’ve noticed that I can’t unscrew a properly-torqued connector nut with my fingers, and that suggests appropriate tightening requires a wrench. Applying consistent torque points me specifically to a torque wrench, and avoiding damage means paying attention to its limiting function.
But what does proper torque really do for these connections? One way to summarize is to say that it places a consistent stress (in the mechanical engineering sense) on the nut and the rest of the connector structure. This has three important benefits:
- Consistent mechanical alignment and axial positioning of connector elements ensures the best connector performance (e.g., return loss) and repeatability.
- Sufficient strain reduces the possibility that the connection will be loosened by vibration, thermal changes or external mechanical action such as bending or twisting.
- Proper torqueing avoids connector damage from deformation due to excessive tightening.
The first and third benefits are often difficult to discern. Suboptimal connector performance can be a hard problem to isolate and connector distortion or damage from excessive tightening may be invisible to the eye.
The second benefit is something we’re all familiar with. After using our fingers and thinking we’ve got a connector tight enough, we later discover that it’s loose. Maybe the connector was bumped, or maybe something more subtle happened: Thermal cycles or cable motion reduced the strain from fingertip-tightened “too low” to “zero”—and the nut is unlikely to re-tighten itself from there!
Proper connector torque is simple for the vast majority of microwave and millimeter connections we use because there are only two torque values needed to cover SMA through 1.85 mm (70 GHz). SMA connectors should be torqued to 56 Newton-cm or 5 inch-pounds and all the rest—3.5, 2.92, 2.4 and 1.85 mm—should be tightened to 90 N-cm or 8 in-lbs.
And now to the tricky question mentioned above: What if you’re mating an SMA connector with one of the other compatible precision connectors, 3.5 or 2.92 mm? Should you use the SMA torque value or the precision-connector value? The answer is to follow the value for the male connector. Thus, if the SMA connector is male gender, then you should use the SMA torque value.
The power of wrenches must be used carefully because you can’t feel forces through them the way you do with your fingers. The potential for over-straining something is also greater when multiple or longer adapters are used or when devices are connected directly without cables that relieve bending stress. The diagram below describes one example: “wrench-lift stress.”
Using the appropriate wrenches to tighten or loosen connectors is good practice but they can cause wrench-lift stress and bend connectors if used the wrong way. Maintaining a small angle between the two wrenches during assembly and disassembly will avoid the problem.
The second wrench is typically a simple open-ended one, but it also has an important job to do. By preventing rotation of the connector mating surfaces while they are in contact, it reduces a major source of connector wear and damage.
Of course, connector torque and rotation aren’t the only sources of problems such as connector damage and poor performance. Other examples are discussed in the posts Loose Nut Danger and Male Front-Panel Connectors on Millimeter-Frequency Instruments: Why?
You can find a summary of torque values for different connectors, along with wrench sizes and part numbers, at http://www.keysight.com/upload/cmc_upload/All/EPSG086190a.htm.
Lastly, for more detail on torque and other coaxial connection issues and practices see the classic application note Principles of Microwave Connector Care (AN-326), Agilent literature number 5954-1566.