General rule of thumb for maximum jackscrew extension?

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John Garner

Titanium

Joined

Sep 1, 2004

Location

south SF Bay area, California

• Nov 5, 2021

• #1

Is there a generally-recognized "rule of thumb" guideline covering the permissible extension of a compression-loaded jackscrew from its base, as a function of the jackscrew diameter? Similarly, is there a generally-recognized guideline covering the permissible overall length of a not-mechanically-attached "bottle jack" as a function of its base diameter?

Thanks in advance for your replies.

 

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MaxPrairie

Hot Rolled

Joined

Jul 9, 2015

• Nov 5, 2021

• #2

I would say most guys would base it off experience and throwing enough parts to recognize when a setup is rigid or not. If you don’t feel comfortable with you setup try something else to make it better. Put a 1-2-3 or 2-4-6 block in there to decrease the gap. Not sure what your actual application is here I guess.

 

E

Erich

Hot Rolled

Joined

Apr 4, 2017

Location

Poway, CA (from Location field)

• Nov 5, 2021

• #3

Are lives at risk?

 

EPAIII

Diamond

Joined

Nov 23, 2003

Location

Beaumont, TX, USA

• Nov 5, 2021

• #4

If you are going to overload almost any jack screw that you are using, your life IS AT RISK.

OK, lets qualify that a bit. Almost any jack screw over a #12 size.

Erich said:

Are lives at risk?

Click to expand...

 

R

Ralph_P

Stainless

Joined

Jan 14, 2003

Location

E. TN USA

• Nov 5, 2021

• #5

The female (nut) threads are only a certain length. All I've used had a stop on the male thread that bottomed against the nut.

 

EmGo

Diamond

Joined

Apr 14, 2018

Location

in the middle of the edge of the center

• Nov 5, 2021

• #6

John Garner said:

Is there a generally-recognized "rule of thumb" guideline covering the permissible extension of a compression-loaded jackscrew from its base, as a function of the jackscrew diameter?

Click to expand...

Basic rule for bolts is at least twice the diameter as thread length, so that'd be my "at least, must-have" rule of thumb if there's any load. Seems like other considerations would depend on circumstances.

 

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magneticanomaly

Titanium

Joined

Mar 22, 2007

Location

On Elk Mountain, West Virginia, USA

• Nov 5, 2021

• #7

Yes, we need to know if this is a machine setup question where rigidity is the goal, or a lifting or supporting application where only failure must be avoided.

For the first, my WAG is that any support should be less than 3x as tall as it is wide.....less than 2x is better.

For columns in engineering applications, 10:1 is a threshhhold often used to begin de-rating the unit compressive loading to avoid buckling....look up "slenderness ratio".. But that is if the load is well-centered by design. Size of base is less important than flatness , perpendicularity, and firmness of what is under it.

 

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Bill D

Diamond

Joined

Apr 1, 2004

Location

Modesto, CA USA

• Nov 6, 2021

• #8

It depends. A good design uses other means to locate the load and stop it from falling to the side etc. AS I recall when i asked about a sliding column inside a column, like a gantry crane leg. I was told at least 1/10 inside the lower leg. Of course in this example the lower larger column had some diagonal bracing so only the extended portion had to carry any off axis loads.
It is interesting to look at a telescopic tv antenna mast with guy wires every ten feet and realize how they get erected.
Bill D

 

plastikdreams

Diamond

Joined

May 31, 2011

Location

upstate nj

• Nov 6, 2021

• #9

Lift an inch, crib an inch...

 

J

John Garner

Titanium

Joined

Sep 1, 2004

Location

south SF Bay area, California

• Nov 7, 2021

• #10

Thank you, all. I appreciate the time and consideration you put into your responses.

A friend and former coworker had asked me to help him develop a written guideline for inexperienced techs. I'll pass your comments, in addition to my own, along to him.

A special response to Ralph P:

There are many older "bell / house" jacks still in use that don't have a mechanical extension stop, but instead have a window cast into the jack body. The jack operator is supposed to know to watch the window, and stop raising the jack, when the end of the jackscrew comes into view. There are also some jacks that have neither a mechanical stop nor a window; the Starrett 190 and 191 jacks -- and their copies -- fall into the no-stop, no-window category, as do a large number of shop-made jacks.

John

 

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Illinoyance

Stainless

Joined

Aug 24, 2015

• Nov 7, 2021

• #11

For jacks that do not have a stop or an observation window establish a max height for each jack based on the screw engaging the full length of female thread. Mark each jack with the maximum permitted extension.