Column design values for jack screws

[HankVRSI]

I use machine screw jacks in my business to support and control mechanical equipment. Often the screw must be extended quite a distance in order to obtain the required range of travel. I have always used the root diameter to calculate the area and r for the kl/r value to use in the compression formula. Recently I ran across a calculation done several years ago that used the nominal diameter for calculating the r value and the area in order to obtain the critical load on the jack screw. I was taken aback and was wondering if anyone had any comments or justifications for using the nominal diameter on a long screw?

 

[desertfox]

Hi HankVRSI

Looking in my "machine design theory & practice" book it says that power screws can be designed on a tensile area which is larger than that given by the root diameter of the thread, this larger diameter is the average of the root and pitch diameters. The justification for this larger area is based on practical tests which have demonstrated that a threaded tensile section (specimen) is stronger than a plain specimen whose diameter is the root diameter of the threaded specimen. It also goes onto say however that when the unsupported length of the screw is equal to or greater than 8 times the root diameter the screw must be treated as a column. There is no reference in my book using the nominal diameter of the screw as the basis for tensile area and the book does state that if the designer wishes to be more conservative then the root diameter of the thread should be used.


hope this helps

regards

desertfox

 

[Lutfi]

I use screw jack as well. I try to avoid the ball screw jacks because they will back drive you on. If you use an ACME thread screw jack, it will never back on you. They are machined so that the thread angles will create friction that will lock the screws from backing up. Therefore, the controlling factor in these types is the column capacity of the screw.

To avoid the rigorous calculations, I use manufacturers charts that will give you safe load in tons for nay given screw extension. One of my favorite screw jack manufactures is Nook Industries. They make Action-Jac products. In Nooks ACME screw design guide, they use the root diameter in their computations of the compression load. See

http://www.nookind.com/acme/AcmeGlossary.cfm#ScrewColumn

;

From practical and theoretical standpoint, I tend to agree with their approach, which is same as yours. I would not feel comfortable using the nominal diameter without factoring down some where in my formula.

Regards,

PS. Nook is on line these days. Here is the web site

http://www.nookind.com

NOOK, please send me a check for the plug. LOL

 

[HankVRSI]

Thanks fox for the reply.

The AISC manual, which is the steel Bible for structural engineers, gives an area for both the root diameter and a tensile stress area of bolts. I feel sure that is the same area you are referring to. I believe it is basically the area you would get if you cut a section straight through the bolt. The area is slightly larger than when calculated by the root diameter and I would be, and have been, comfortable using the diameter calculated from that area if I needed to in order to justify an existing design. However the nominal diameter for a 2 inch bolt is about 14% larger than the root diameter. Since the allowable force is based on the fourth power of the diameter, that would be a 67% increase in the allowable. I’m very uncomfortable using that number without seeing some kind of logical justification.

 

[HankVRSI]

Thank you Lutfi

We use a lot of Joyce jacks and they also use the root diameter to calculate compressive loads.

Thanks for the link. It was interesting.

 

[Lutfi]

Joyce makes good screw jacks as well. I use them to articulate working platforms. They work great. I use a revolution counter (such as dynabar) on the motor that counts the revolutions. The PLC translate revaluations to inches of screw extension. I still use limit switches for redundancy. Man isn't technology great.

Good Luck

 

[HankVRSI]

Yes it is great. We use limit switches a lot also.

Considering the response I got here, I'm glad that design was never used. But I did want some verification that I was not being overly conservative.

Thanks again for the help.

 

[3dboy]

since column buckling is based on the radius of gyration which is sqrt(I/A), which drops out to R/2 for a circle, I cannot imagine there is a great difference between the root and nominal strengths. Whatever the case you can probably average the radii of gyration and come up with a safe answer.

 

[HankVRSI]

Euler buckling is based on l/r squared and area squared. Hence the results is based on the ratio of the diameters to the forth power. Therefore a small difference in diameter can mean a large difference in the allowable load. Nominal diameters can be around 25% larger than the root which means a 244% difference in the allowable load. Using an average for this would result in a 60% difference which is close to the factor of safety.

It finally dawned on me that I could run a FE analysis. For the particular case I was looking at, the allowable load increased 33% which surprised me.