Bolt and nut will not loosen after tightening on device

[jenwilsonglia]

Describe the bug
When there is a problem with the bolt threading off of the nut, it becomes very difficult to loosen the nut and causes the nut to spin in the nut trap. This results in difficulty getting the device apart, and we have found that the plastic or the bolt needs to be severed in order to disassemble the unit.

To Reproduce
Steps to reproduce the behavior:

1. Test the device by assembling a rod in a rod clamp, a Schantz pin in a pin clamp, a bolt through both clamps starting with the pin clamp with a washer at the head of the bolt, and the nut in the nut trap of the rod clamp.
2. Tighten the bolt into the nut very tightly so that the rod and pin do not move in the clamps.
3. Then disassemble the unit by loosening the bolt from the nut. See if the nut starts to spin in the nut trap.
4. If this happens, the only way to get the unit to disassemble now is to either break the plastic around the nut or sever the bolt.

Expected behavior
The bolt should loosen from the nut, with the nut trap holding the nut in place while the bolt is loosened. The bolt should come free of the nut so that the unit can be disassembled.

Photos

Additional context
At first, we thought it might be an issue with the nut trap not having the proper tolerance and being just a little too big for the nut. However, we tested tightening and loosening the bolt and nut from parts that had good tolerance in the nut trap, and parts that didn't, and we were able to loosen the nut from the bolt without any spinning of the nut in the nut trap.

Failure Mode
The failure is in the threading of the bolt and nut when it is being loosened. When the threading fails, it is almost impossible to unscrew the bolt from the nut.

Effect
When this happens, the plastic or the bolt needs to be severed in order to disassemble the unit. This can create difficulty in taking the device off of the patient.

Cause
Not sure exactly what the cause is creating mis-threading of the bolt and nut when loosening. We will measure the distance of the threads to see if there is a difference in the bolts. We will also see if it may be an issue caused by using the nut more than once, or if it is caused by excessive pressure when tightening the bolt too much. These are all possibilities.

Mitigation / Control
Until we determine exactly what is causing the mis-threading, we do not know how to control this issue. More tests are necessary.

[apopanda]

Maybe the length of the bolt is the issue and it being forced to "bend" (or the nut to shift) with the exfix? We've got 2mm of clearance on each plastic part, when it is being tightened, the nut stays with the nut trap, the head of the bolt stays with the other side, the through-holes are cylindrical. This might be enough to put strain on the thread, since the nut is forced to turn at an angle in relation to the bolt. See my quick paint-fu below for an exaggerated view on it (I might need to get out my drawing tablet and Concepts next ^^)

[tareko]

This is a great first effort at using the structured process! A few small notes:

1. Failure mode may be better described as "what function is not working". In this case, the failure mode is that the bolt bolt cannot be removed from the nut after tightening.
2. The effect is one step before what you cite. It's not that "device has to be broken", but rather that the device itself now can't be removed because of the functional failure (failure mode) that the bolt cannot be removed from the nut. A potential solution to this is "break the device", rather than a cause of it.
3. In terms of causes, you identified several potential causes. Go ahead and list them along with your confidence, or even your theories. You're floating a good deal of theories, and that's a good starting point.
4. For mitigation, you may be overthinking this. For example, the simplest mitigation is "don't use the device". Another one might be "don't overtighten". These may not be practical mitigation techniques, but there they are.

It'll take time to get this way of thinking down, and perhaps we can engage people with expertise on it to help us understand it further. Like everything, it's a skill that has to be learned and mastered. I'll use the next few comments to expand on the issue further.

[tareko]

In terms of causes, a few theories have been floated here and on mattermost, so I'll summarize them:

1. That there is a pitch mismatch between the nut and bolt that is exacerbated when the system is under load and in tension (bolt is tight)
2. That the nut trap is too loose, which reduces the contact area with the nut and allows the issue to happen
3. That the bolt bends under load, which also alters the characteristics of the mating of the nut/bolt to make it impossible to remove. (Adam's theory above)
4. That thread deformation is happening from excessive tightening during testing (theory from Jen)

How Thread Deformation Occurs
Exceeding the Elastic Limit: When a bolt is tightened, it stretches within its elastic range, meaning it will return to its original length when the force is removed. Over-tightening pushes the bolt and the mating threads (in the nut or a tapped hole) beyond their yield point into the plastic deformation range.
Permanent Deformation: In the plastic range, the material permanently deforms or stretches. The threads can become worn, flattened, or stretched out, losing their ability to grip properly."

Are there others I'm missing?

[tareko]

Coming back to our FMEA, there are two functions that are failing here:

1. Use of the nut trap to reduce wrench requirements.
2. Ability to remove the nut

If we think of these to failures as separate failure modes, we can develop a new mitigation strategy that works immediately, which is to allow both the nut trap and a wrench to operate. That is to say, allow half of the nut to be outside the nut trap.

By doing this, the failure of the nut trap to act as a wrench / stabilizer stops being a catastrophic failure. The disadvantage of this is that it reduces the grip of the nut trap and increases the possibility of failure.

[jenwilsonglia]

Two failing functions, and my analysis after extensive tests:

1. Use of the nut trap to reduce wrench requirements:

Even without the nut trap, the wrench cannot loosen the nut off the bolt if the threads have been deformed after over tightening. So, even if half of the nut is outside of the nut trap (which is difficult to control sometimes), this will not result in loosening of the nut from the bolt.

2. Ability to remove the nut:

Different tolerances of the nut trap have been tested, and we've found that, even when there is a slight variance in tolerance causing the nut to easily fall out of the nut trap, if the bolt and nut are not over tightened, then the nut can be loosened from the bolt without any deformation of the nut trap. Therefore, the problem arises from over tightening of the nut and bolt which causes a deformation in the threads and an inability to loosen the nut, whether it is in the nut trap or not.

[jenwilsonglia]

Test findings:

When a nut and bolt are used for testing parts, they must be unused at first and tested by themselves before and after every test with parts in order to make sure that the nut can still turn down the threads of the bolt.

After several tests, even when not over tightened, the threads on the bolt will deform and the nut will fail to turn down the threads of the bolt. At this point, the bolt is unusable, but the nut can be tested on a new bolt and, if it turns down the threads of this new bolt, the nut is still ok to use.

If the nut and bolt are over tightened in order for either the rod or pin to not move in the clamps, then the nut and bolt can probably be loosened if they haven't been used before, although they will most likely deform the nut trap making the rod clamp unusable.

If the nut and bolt have been used before over tightening, there is a danger that the nut will not loosen from the bolt, even when released from the nut trap. At this point the bolt, nut and rod clamp are unusable. The pin clamp can be used again if it can be released from the nut and bolt.