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Differential Turnbuckle

For very precise adjustments
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A "turnbuckle" is a widely available item (link) that generally has the following properties:
1. It consists of a middle part and two end parts.
2. Each end part screws into the middle part.
3. One end part has an ordinary "right hand" screw thread, while the other end part has a "left hand" screw thread.

Now consider an ordinary hardware-nut located in the middle of a long threaded rod. If you rotate the nut, it might be described as screwing its way toward one end of the rod, while unscrewing away from the other end of the rod.

An "initial configuration" of a turnbuckle might have both end parts only half-way screwed into the middle part. Now suppose the end parts are prevented from rotating, while the middle part alone is rotated, rather like that nut in the previous paragraph.

In this case, though, if the middle part screws its way toward one end part (part A), then because the other end of the middle part doesn't have the same kind of threading pattern as where part A is connected, end part B will also be moving closer to the middle!

And of course if we reverse the rotation of the middle part, the screwing action will cause both end parts to move away from the middle part.

I've described the ordinary operation of a turnbuckle specifically because it is relevant to this Idea. And there is one more important piece of background information that needs to be presented. See the link about "coarse thread" and "fine thread".

Over here in the USA, the non-metric bolts and nuts that are commonly available are typically offered in either of two screw-thread patterns --NOT "right hand" and "left hand", but "coarse" and "fine". (While left-hand threads are available in both coarse and fine forms, all are much less common than right-hand threads.)

While metric bolts and nuts are also available here in the USA, I've never seen any equivalent to coarse threads vs fine threads. So, any non-USA HalfBakers need to know about that before I can continue with this Idea.

Basically, a Differential Turnbuckle has a coarse thread on one end of the middle part, and a fine thread on the other end of the middle part. And both threads can be the common right-hand type.

What happens now, when the middle part is rotated and the end parts are kept from rotating, is that the middle will always approach one end part and recede from the other end part --but they will do it at different rates, because one end is rotating "coarsely" while the other end is rotating "finely".

The net effect is that the overall length of the turnbuckle changes, much like a typical turnbuckle. The difference is that the rate at which that overall length changes is MUCH less than a typical turnbuckle experiences. Which means that a Differential Turnbuckle is what you want to use when you want to make very precise length-adjustments.

Vernon, Jan 01 2012

About turnbuckles http://en.wikipedia.org/wiki/Turnbuckle
As mentioned in the main text [Vernon, Jan 01 2012]

About coarse and fine threads http://www.portland...n/thread-pitch.html
As mentioned in the main text [Vernon, Jan 01 2012]

Metric Threads http://www.engineer...-threads-d_777.html
[MechE, Jan 02 2012]

[link]






       A promising start to a new year. Critique will have to wait until after breakfast.
Alterother, Jan 01 2012
  

       Nice idea. It would also be a simplish way to improvise a turnbuckle, if the right combination of fine- and coarse-threaded bolts (or threaded rod) were available.
MaxwellBuchanan, Jan 01 2012
  

       //fine- and coarse-threaded bolts (or threaded rod) // [MB] That sounds like a fine idea by itself! At some fairly minor loss of strength, a rod could be threaded by (say) both 24 and 32 tpi, and function with either type of nut; similarly, "generic" #10 nuts would accept either pitch bolt.
csea, Jan 01 2012
  

       I'm not sure whether you can cut two different threads on the same bolt and have both of them work.
MaxwellBuchanan, Jan 01 2012
  

       Not with threads guaged as similarly as SAE 'coarse' and 'fine'. In order for two different guages to be 'cross- grained' in such a way that either or both of the corresponding fasteners' threads would make sufficient contact with the lands of the bolt's partial threads, one guage would have to be more akin to the rifling of a gun barrel in its twist ratio.   

       Er... it's easier to visualize than to actually explain...
Alterother, Jan 02 2012
  

       Some metric threads have 2 possible pitches. I know that 10mm (that's equivalent to about 25/64" plus a bit, for the UK oldies, or foreigners) has 1.25 or 1.5mm pitch (that's...Oh forget it...It's small, OK?), depending on which tap was picked up by accident.
Ling, Jan 02 2012
  

       'cuz in the idea they're both threaded the same direction... so if you have a 1.0mm thread on one side and a 0.9mm thread on the other it ends up 0.1mm per turn.   

       (this is too good not to be baked)
FlyingToaster, Jan 02 2012
  

       [21 Quest], the title word "Differential" refers to the Difference between a coarse thread and a fine thread. You can't get a turnbuckle shortening/lengthening action without it, if both threads have the same handed-ness (which is explicitly specified in the main text).   

       [FlyingToaster], when I first thought of this Idea several years ago, I found out it had been patented. But since it's not widely known, that's why I finally decided to post it here.
Vernon, Jan 02 2012
  

       Metric threads do have fine and coarse pitch. They're used even more rarely than English, but they do turn up every once in a while, mostly on things like stops or positioning elements where a finer adjustment is desirable.   

       And they are a bit of a pain, simply because unless you remember the normal numbers off the top of your head, it's easy to not realize the component you just spec'd isn't standard.
MechE, Jan 02 2012
  
      
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