Cantilever beam deflection

[slow-poke]

I'm fabricating extendable feet for my mill stand. (Max weight say 1000lbs, so only about 250lbs per corner). Not sure about the horizontal forces as the mill trys to lean side to side? I'm trying to use what I have at hand for material (steel).

I have some solid 1+3/4 x 3/4" that I could use for the feet that will slide up and down in a rectangular tube. When fully extended the solid bar will extend beyond the end of the tube by 6". Minimum length in the tube would be say 4-5".

Do you think these will be strong enough to minimize deflection?

I tried an online calculator and if I entered everything correctly results showed next to nothing < 0.001" which seems to good to be true.

 

[mickri]

I am not an engineer. I have done a lot of study on cantilever beams related to free standing masts on sailboats. The formulas for cantilever beams are not complex. Basically a cantilever beam has one end fixed in place and the other end is free to move as loads (wrong term??) are applied to the beam. The loads can be evenly distributed or be point loads anywhere along the beam or a single load at some point on the beam. A flag pole is a classic example of a cantilever beam.

I am trying to visualize what you are describing because it doesn't sound like a cantilever beam. A picture or drawing would help.

 

[slow-poke]

I am not an engineer. I have done a lot of study on cantilever beams related to free standing masts on sailboats. The formulas for cantilever beams are not complex. Basically a cantilever beam has one end fixed in place and the other end is free to move as loads (wrong term??) are applied to the beam. The loads can be evenly distributed or be point loads anywhere along the beam or a single load at some point on the beam. A flag pole is a classic example of a cantilever beam.

I am trying to visualize what you are describing because it doesn't sound like a cantilever beam. A picture or drawing would help.

When I think of a cantilever first image is of a balcony only supprted at one wall. Your flagpole is a better example of this situation, except upside down and assume the end on the ground is the unsupported end ie. free to deflect because it's not bolted to the floor and is free to deflect other than the friction due to the ~250lbs, the supported end is the upper half because it's inside the fixed rectangular tube.

Image is of WIP... the feet can move up and down and bolt at desired height. The stack of wood under the cabinet will be removed when I have four viable legs, I only have two so far.

 

[mickri]

That is not a cantilever beam. It is a column in compression with one fixed end (the upper end) and one pined end (the bottom end). The fasteners which hold the leg in place are in shear and take all of the down load. You can look up the shear strength of bolts to determine what size bolts you need. Be sure to add in safety factor. There are formulas to determine the size of the column to keep it from buckling.

 

[Winegrower]

It’s fine.

 

[slow-poke]

That is not a cantilever beam. It is a column in compression with one fixed end (the upper end) and one pined end (the bottom end). The fasteners which hold the leg in place are in shear and take all of the down load. You can look up the shear strength of bolts to determine what size bolts you need. Be sure to add in safety factor. There are formulas to determine the size of the column to keep it from buckling.

Well I used 2 x 5/16 grade 8 bolts, should be enough safety margin

 

[Dabbler]

If you used 1/8" wall tubing. and your telescopic leg were 1/8" wall tubing, the failure mode would be your 5/16" bolt. One 5/16" grd 8 bolt with rolled threads shears at the threaded portion at just over 4000 lbs.

My biggest mill is only 4000 lbs, and you (presumably) have 4 legs.

I hope this helps.

 

[Flyinfool]

Strength wise there is plenty there. The issue will be that in order for the legs to telescope means that there is clearance between the ID and OD. This means slop that will allow your mill to wiggle around on the legs. tightening the bolts will only clamp the inner tube if you are actually bending the outer tube. But this will also only lock it in one direction.

What might work better is to weld a nut on the inside of the inner tube on 90° faces this will clamp it solid in both directions.

 

[matthewsx]

Why not just figure out what height you want it at and weld the legs?

Pretty sure your plan will work out as designed though.

John

 

[slow-poke]

Why not just figure out what height you want it at and weld the legs?

Pretty sure your plan will work out as designed though.

John

My mill is located in a basement area with a low ceiling. With the table at ideal work height, I can no longer lift the lid off the pulley / V-belt enclosure, when lowered I can. I have not needed to get access to the pulley area for a couple of years but at some point I will.

The extendable foot is solid not tube, so plenty of threads. Slip fit is not sloppy, <0.01"