Bolt movement question 2

[MechEngWannabe]

Hello, my wife has a medical device to lengthen her leg. It's called an external fixator. There is a bolt which is 1cm in diameter and threaded through a fixed part. I counted 10 threads per cm on the bolt. Using an Allen key, she is supposed to turn the bolt 4 quarter turns (i.e. one full rotation). My question is, what is the distance the bolt will move following this full rotation?

 

[MechEngWannabe]

Here is a pic for clarity

 

[dhengr]

Wannabe:
Think about this problem a little bit... If there are 10 threads per cm of bolt length, that’s called the thread pitch, wouldn’t it take ten full turns of the bolt to extend it 1cm. So, how much does one turn, (4)(quarter turns), extend it? Where did you get your engineering education and what kind of engineer are you? Did you happen to notice, when you logged in, that Eng-Tips is a forum for engineering professionals, not a forum for students or DIY’ers. As an aside, you may want to turn that bolt the other way, to shorten her leg. Remember, the longer her leg, the harder she can kick you in the butt, or elsewhere.

 

[MikeHalloran]

I can't resolve it from that photograph, but a single-start thread, the usual kind, will move 1mm in 1 turn with 1mm thread pitch.





Mike Halloran
Pembroke Pines, FL, USA

 

[MechEngWannabe]

Thank you and sorry for such an intellectually un-challenging question. Every ounce of my education and general common sense told me this was the case, that one full turn would progress the bolt 1mm, but I had to be absolutely sure. The distraction is supposed to progress at 1mm per day but the surgeon told her to make 4 quarter turns, 4 times a day. That's 4mm per day! Again, I am sorry if I offended anyone with this question. I wanted to get answers from engineers and not the local hardware store rep.

 

[MintJulep]

1/4 turn four times a day.

Four quarter turns per day.

Who would say "4/4 turns " when they could say "A full turn"?

Your wife misheard.

Never accept spoken instructions from a doctor. Make them type it.

 

[IRstuff]

Sounds like the case where the doctor ordered the patient to, "take one pill 3 times a day," and the patient couldn't figure out how he was supposed to regurgitate the pill so that he could take it again.

Rationally, 4-mm per day is a huge amount, and therefore, unlikely. 1-mm per day sounds more plausible, particularly if spread out as 4 separate 1/4-mm chunks, which is backed up by:

http://www.limblengtheningdoc.org/limb_lengthening_intro.html

TTFN
faq731-376

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[racookpe1978]

Let us hope your wife gets better.

Yes - I agree that the strain on your wife's leg should be very slow and very steady. The doctor (probably!!! ) does NOT want her to pull the leg too hard, nor too fast. So, CALL THE DOCTOR BACK - and confirm that you will turn the screw 1/4 turn very 6 hours. ASK the doctor if you can do this at 7:00 - when you wake up - and at 12:00 hours, and at 17:00 hours, and at 22:00 hours (before you go to sleep); or if you must set an alarm clock to make every turn exactly at 6:00 hour intervals.

I think he will give you some flexibility in the time. BUT CONFIRM THIS WITH THE DOCTOR!