Turnbuckles and loss of preload

Turnbuckles have a right hand thread and a left hand thread.



That implies that if I pull from both ends, the turnbuckle will resist the tension because the inner body would need to twist both ways simultaneosly to become unscrewed. The same applies to the compression case.

However, can they become loose against vibration? From what I just wrote about them it'd seem like that's not possible but it feels very weird to me. As long as the threaded rods are not allowed to turn, the turnbuckle should not be able to do it either so the preload/position will remain the same.

I know turnbuckles are sometimes locked in place using jam nuts or lock wire but I don't know if that's in cases where the rods could turn independently or if there is a mechanism that could cause a loss of preload even in the scenario I described.
I assume, as a safety measure, it's always convenient to use something like a jam nut but I would like to confirm if it is really necessary in the case where the threaded rods are not allowed to turn.

Juanda said:

That implies that if I pull from both ends, the turnbuckle will resist the tension because the inner body would need to twist both ways simultaneosly to become unscrewed. The same applies to the compression case.

I don't think that's right. If you twist the TB one way, it pulls both screws in, and visa-versa. So if it weren't for friction of the screw surfaces in the TB, tension would unwind the TB and the two ends would separate, no?

You're totally right. My description was incorrect. Additional locking measures are necessary to prevent the loss of preload.
I'm now wondering if it'd possible to make them with both threads in the same direction to avoid that loss of preload. The adjustment of length would not be as convenient but the joint should keep the same length at all times so the preload is sustained.
The middle body could move until it hits one end but as long as enough thread is still engaged in both sides when the collision happens it'd work I believe.

Juanda said:

I assume, as a safety measure, it's always convenient to use something like a jam nut but I would like to confirm if it is really necessary in the case where the threaded rods are not allowed to turn.

The turnbuckle must be locked somehow. Otherwise, some small boy will come along and adjust it while trying to find out how it works.

Since there is a left-hand and a right-hand thread, a jam nut, or lock nut, will be placed on the right-hand threaded side.

You could replace a turnbuckle with a threaded rod and two or more nuts, but it would be less convenient.

Baluncore said:

The turnbuckle must be locked somehow. Otherwise, some small boy will come along and adjust it while trying to find out how it works.

Since there is a left-hand and a right-hand thread, a jam nut, or lock nut, will be placed on the right-hand threaded side.

You could replace a turnbuckle with a threaded rod and two or more nuts, but it would be less convenient.

For sure. My interest though is more related to the mechanism of the turnbuckle and not the turnbuckle itself.
As usual in this forum, I'm talking about something I cannot give too many details about and give you the full picture although I'd love to. I will try to describe it as accurately as possible without crossing any line.

Image a structure partially held by articulated rods. Now these rods are adjustable in a similar way to a turnbuckle. I don't know if the word turnbuckle would still describe the rod but it's the best way I knew how to describe it. This structure will be tested against vibration so a locking mechanism like a jam nut, locking wire, adhesives, and/or others will be used. But I wanted to confirm how these turnbuckles could come loose in the first place.
I have already confirmed I was imagining the turnbuckle mechanism wrong so this thread already taught me something very useful. I would say the last detail to iron out is:
Is it worth it to make both threads in the same direction to prevent the loss of preload? The adjustment process would be way more inconvenient (even impossible depending on the space available) but it might be safer (the reasoning for this is at #3).

By the way, in case you're worrying, the structure does not put any lives at risk in case of failure or anything like that.

Juanda said:

Is it worth it to make both threads in the same direction to prevent the loss of preload?

Yes, but then you could not adjust it and the device would be redundant.

Turnbuckles are used to apply tension, or change the length of a rod.
Turnbuckles come loose due to vibration.

The opposite of a turnbuckle is probably a "coupler nut", "hex coupler nut" or a "joiner nut". It can be used to join two screw threads with a small but variable gap.

A fine length-adjustment device that uses two same-hand threads is a "differential screw".
https://en.wikipedia.org/wiki/Differential_screw

All right I got a much clearer idea now. Thank you all for the input!

The types shown in the pictures are used for supporting heavy exhaust piping of internal combustion engines and for belt tensioners.
Note the counter-nuts and the flexible elements for control of vibration.

In addition to possible vibration of the attached cables, the daily alternating heating and cooling of the turnbuckle and attached cabling can loosen the turnbuckle.