# Power Screws in Machinery's Handbook 30th Edition (small size) -- Torque Equations



## hbogunb (Aug 13, 2019)

I'm working a power screw problem (Problem # 11 in the Mechanical Components section of Eng Pro Guides technical study guide) and having trouble finding the below formulas in my references as noted in the solutions: 




I checked MERM, Shigley, and Machinery's Handbook, and found similar equations but not the exact ones and it's making me nervous. 

The reason for the title of this post is because I suspect it is somewhere in Machinery's Handbook. I tried searching the index for each key term: Screws, Power, Threads, ACME, etc. and came up empty. I'm aware that there is a larger version; but I'm not sure if the smaller sized version might lack some of the material that the larger one has? 

Anyone that is more familiar and could shed some light on this please help. 

Thanks!


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## Audi Driver P.E. (Aug 19, 2019)

Well, I have the large sized Machinery's Handbook (28th ed) and it's not in there... not that I expected it to be different, they have all the same content, just smaller print in the small version. The test makers at NCEES like Shigleys and tend to design around what's included in there, so I would lean that route. I haven't done your calc with the various versions of the equation. Are the results different by any significant margin?  Shigley's (in my old version) does a pretty good treatment of the Acme thread in particular, and I trust it.  If the difference result in more than a couple percent, the test makers will specify which equation to use by some hint or will design answers that give enough wiggle room for use of any of the equation versions available.


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## Ramnares P.E. (Aug 19, 2019)

@justin-hawaii maybe you can clear up this question?


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## Ramnares P.E. (Aug 19, 2019)

Neither the Machinery HDBK 30th Edition nor Shigley 10th Edition has that identical formula.  I second what @Audi driver, P.E. recommended regarding using the existing Shigley formula and see what the % difference is.


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## Audi Driver P.E. (Aug 20, 2019)

I also looked at the MERM and I see that the formula there says that the relationships are different for ACME threads, so at least that difference is accounted for.


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## Numbers25 PE (Sep 4, 2019)

I was working this problem last week and sort of stumbled into the same issue. Looking back at my notes, it seems the pro guides equation is a strangely simplified version of equation 8-5 in shigleys (10th Ed). Substitute your lead term for pi*dm*tan lambda and and working through a few steps of simplification lead me to the equation found in the pro guides.

I'm assuming some more simplification would lead to the form found as Eq 59.2 in the MERM, but I didn't take the time to actually work it.

 This was just my thoughts and the conclusion I came to while working the problem. Like you, I like to cross reference all my sources when I'm unsure of where something came from.


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## ashu04 (Oct 12, 2019)

Hi, it is simple modified version of Shigley's equation 8-5( 10 the edition) I am attaching the conversion and  the answer is same and it is derived from the same equation.


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