What is the min reasonable pitch for an ACME power screw?

[enginuityep]

We have a design situation in which we're trying to get the highest possible axial force out of a 1.5" power screw. I'm assuming an acme thread is the best solution, and the standard pitch is 4, but my machine design book says other pitches are also commonly use. Does anyone know what the finest reasonable pitch is?

Thanks!

 

[drawoh]

enginuityep,

Please define "reasonable".

The last time I specified an ACME thread, I selected a pitch for which threaded rod and a tap was available. My definition of "reasonable" was resolved by a Google search. Yours could differ.

Probably, you have to do the calculations and find out what the bottlenecks are for your design. Will the screw buckle? Will the threads strip? Can the fabricator cut your weird custom thread?

JHG

 

[zekeman]

I don't think you can get much more force out of this screw system without going to a larger diameter. The force is primarily determined by the root diameter of the screw which isn't going to increase that much with a decrease in pitch.

 

[Cockroach]

I agree with the above discussion, it depends on your problem and boundary conditions imposed by the system.

One thing you can think about, after one revolution of the lead screw, how much would you want the nut to advance? This will give you your required pitch.

I've used several Acme profiles on power screws. The most common are six (6) and eight (8) threads per inch. Make the tolerance 2G for box-by-pin.

For the most part, four (4) TPI is probably too coarse for your application.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[DesignerMike]

Look on

http://www.nookind.com

for their "standard" offerings. They offer a 1.5-10 ... You might want to consider a ball screw depending on the application. You can generate more axial force with less torque (due to reduced friction).

Their web-site is pretty informative, and you can download 3D models of your selection.

Good luck

 

[enginuityep]

I just wanted to thank each of you as contributors to this thread--I've started using google a little and it's amazing how quickly information can be found on it.

Our limitations on screw size aren't very stringent and the environment is an outdoor--dirty environment. In the past we've been using a standard 60-degree cut thread profile as a power screw, and the load resistance coupled with the thread friction has been galding the threads to the point of locking up the system. My question about the thread pitch was regarding the mechanical advantage--has anyone's experience shown that increasing the pitch (from standard) really increases the mechanical advantage--or are there some frictional characteristics that come into play which nullify this effect. I've heard that there's not an appreciable mechanical advantage difference between course and fine pitch on 60-degree thread profiles.

Thanks again for each of your thoughts!