Leveling feet ?

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earlgo
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Joined: Sat Jan 29, 2011 11:38 am
Location: NE Ohio

Re: Leveling feet ?

Post by earlgo » Tue Jan 22, 2019 10:32 am

from the previous calculation:
All together: P allowable = (40000* .0013)/( 2.4* .2) = 104.7 # This would be easy to bend if conditions were not perfect.
Adjusting for the new short projection:
All together: P allowable = (40000* .0013)/(1.3* .2) = 200# This would be a better situation.
John Hasler is right about the stress concentration factor but I wouldn't guess whether the threads were rolled or die cut. Rolled threads tend to have a smooth radius in the thread "crotch" while cut threads might not. If there were a sharp corner in the crotch then the stress concentration factor would be in the neighborhood of 1.5, so the actual load might be 100/1.5 = 133#
But this is too much information. :) (Engineering is an exact science based on assumptions.)
--earlgo
Before you do anything, you must do something else first. - Washington's principle.

jcfx
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Joined: Sun Aug 09, 2009 1:24 pm
Location: NY

Re: Leveling feet ?

Post by jcfx » Wed Jan 23, 2019 11:39 pm

John Hasler wrote:
Mon Jan 21, 2019 9:02 pm
Why would you use acme thread?
Back yard mechanics observations, acme threaded shafts are on scissor jacks that you'd
lift a car to change a car tire with, they also have a more robust thread compared to
regular threads. The leveling feet with acme studs are rated higher than regular threaded studs.
But they cost a lot more when you consider that if you DIY and buy acme threaded leveling feet,
you would have to invest in a acme tap for the whatever size acme thread.

jcfx
Posts: 460
Joined: Sun Aug 09, 2009 1:24 pm
Location: NY

Re: Leveling feet ?

Post by jcfx » Wed Jan 23, 2019 11:45 pm

Earlgo, the formula you posted is invaluable, and it's never too much info, I have to work it out
on my own and that's going to be difficult since I failed algebra twice ! :oops:

I'm curious as where the safety factor number comes from ?

John Hasler
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Location: Elmwood, Wisconsin

Re: Leveling feet ?

Post by John Hasler » Thu Jan 24, 2019 12:07 am

Earlgo has shown that the 1/2-13 UN threads are far more than strong enough. Acme threads are used for jacks because they are easier to turn but that doesn't matter here. The limiting factor is the maximum side load. It is determined by the minimum diameter regardless of thread type.

whateg0
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Joined: Sat Mar 28, 2009 3:54 pm
Location: Wichita, KS

Re: Leveling feet ?

Post by whateg0 » Thu Jan 24, 2019 9:55 am

If you think you want to go to a stronger thread, I would think a larger 60 degree thread would be far less expensive than an ACME. ACME threads are more efficient at converting a rotating force (torque) to a linear force. That is why they are used for things like car jacks. If strength along was the reason for it, surely the manufacturers would just us a slightly larger 60 degree thread.

Maybe you stated this already, but are you planning on releveling the table often? Or is it expected to be once in a blue moon sort of thing. I would guess since you have casters installed, that you are going to move it frequently. Though more complex, some sort of a retractable mechanism could effectively shorten the stickout while reducing the amount of time you spend screwing the feet in and out. Maybe something like this?

Image

Dave

earlgo
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Location: NE Ohio

Re: Leveling feet ?

Post by earlgo » Thu Jan 24, 2019 11:05 am

jcfx wrote:
Wed Jan 23, 2019 11:45 pm
I'm curious as where the safety factor number comes from ?
In this case it is the stress allowable divided by the actual stress, or the load allowable divided by the actual load.
Think of it as ice on the pond. It will hold 200# before breaking and if you weigh 100#, the SF is 2. If you weighed 400# the SF is .5 Any thing less than 1 is a fail, but no designer ever designs for less than a 2 or 3 safety factor in normal circumstances. If it is a life threatening situation, like overhead cranes or highway bridges, then the SF is governed by the local codes or government rules and could be much higher. Impact loads affect the allowable stress/loads, so that is a whole different set of rules. For static loads, SF of 3-5 is a safe rule of thumb.
If you are into stuff like this then find a copy of the American Institute of Steel Construction's book "Steel Construction Manual, Allowable Stress Design", and/or Roark's "Formulas for Stress and Strain". There is a lot if information in these references. :)
--earlgo (Engineering is an exact science based on assumptions.)
Before you do anything, you must do something else first. - Washington's principle.

jcfx
Posts: 460
Joined: Sun Aug 09, 2009 1:24 pm
Location: NY

Re: Leveling feet ?

Post by jcfx » Tue Jan 29, 2019 9:30 pm

whateg0 wrote:
Thu Jan 24, 2019 9:55 am

Maybe you stated this already, but are you planning on releveling the table often? Or is it expected to be once in a blue moon sort of thing. I would guess since you have casters installed, that you are going to move it frequently. Though more complex, some sort of a retractable mechanism could effectively shorten the stickout while reducing the amount of time you spend screwing the feet in and out. Maybe something like this?

Dave
Dave,
I'm not planning on leveling it or moving often, the wheels are there to help move it to it's final place once the mill is lifted
on to the stand, I have a very small shop so all of the lifting happens in a hallway, the wheels also future proof
moving it since i'm a sub tenant in a larger shop.
Jim

shorttrackhack
Posts: 42
Joined: Sun May 15, 2011 4:38 pm
Location: SoCal

Re: Leveling feet ?

Post by shorttrackhack » Thu Jan 31, 2019 3:04 pm

Have you ever considered shimming the casters?

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