Question about steel strength??

jodebg · Mar 19, 2015

Working on plans to build a set of 3PH forks.

For the tines, the plans call for 2 x 2x 1/4" tubular steel- 43" long.
They have been load tested to over 2000lb lift capacity in this design.

I was considering using 3" x 1.5" x 3/16" inplace of the 2x2 called for in the plans.
This seems to be a better dimension for tines.

The steel guys did not know if the 3 x 1.5" was available in 1/4" thickness?

My question is how strong is the 3 x 1.5" tubular steel compared to the 2 x 2 x 14" sotck?

ishiboo · Mar 19, 2015

jodebg said:
Working on plans to build a set of 3PH forks.

For the tines, the plans call for 2 x 2x 1/4" tubular steel- 43" long.
They have been load tested to over 2000lb lift capacity in this design.

I was considering using 3" x 1.5" x 3/16" inplace of the 2x2 called for in the plans.
This seems to be a better dimension for tines.

The steel guys did not know if the 3 x 1.5" was available in 1/4" thickness?

My question is how strong is the 3 x 1.5" tubular steel compared to the 2 x 2 x 14" sotck?

Structurally, the extra 1" width is basically just extra weight.

The 25% loss in height, however, coupled with a 1/16" loss in wall thickness, will be a pretty big structural loss.

If you can, I would go with 3x2x.25" or the original 2x2. Otherwise your plan will be okay, you'll just have weaker forks.

By the time you buy that tube steel though, you could probably find forks alone on Craigslist for the same price... which will be solid steel, tapered, and quite strong.

LD1 · Mar 19, 2015

No go on the 3x1.5 (assuming you plan on laying it down so they are only 1.5" high). Just aint gonna be strong enough.

I even have my doubts about the 2x2 @ 43" long. but if thats what the plans call for, I wouldnt go any less.

Your proposed 3x1.5 is only about 1/2 the strength. Even if you could get it in 1/4", it still wouldnt be anywhere near as strong as the 2x2x1/4. Actually, 2x2x1/8 wall tubing would be stronger than your 1.5x3.

For forks though, I like 2x4x1/4 tubing. Nice wide fork. And quite a bit stronger than 2x2x1/4 tube

s219 · Mar 19, 2015

jodebg said:
Working on plans to build a set of 3PH forks.

For the tines, the plans call for 2 x 2x 1/4" tubular steel- 43" long.
They have been load tested to over 2000lb lift capacity in this design.

I was considering using 3" x 1.5" x 3/16" inplace of the 2x2 called for in the plans.
This seems to be a better dimension for tines.

The steel guys did not know if the 3 x 1.5" was available in 1/4" thickness?

My question is how strong is the 3 x 1.5" tubular steel compared to the 2 x 2 x 14" sotck?

You'd be losing both height and thickness with the 3 x 1.5 x 0.1875, so they won't be as stiff as the 2 x 2 x 0.25". Offhand, you lose about 25% due to thickness (all else equal) and 44% due to height (all else equal). Combined, you'd have to compute the area moment of each cross section, but it will probably be in the 50% range. So yes, quite a bit less strong by the back of my envelope.

LD1 · Mar 19, 2015

IF you want to know specifics, use a moment of inertia calculator for rectangle tubing. Plug in the numbers and see what you get. What you get is directly related to strength.

2x2x1/4 tubing is ~.91
3x1.5x3/16 is ~.53

the 2x4x1/4 I mentioned is ~1.68

So, 2x2x1/4 is "almost" twice as strong as the 3x1.5. And 2x4x1/4 is almost twice as strong again

jodebg · Mar 19, 2015

Thanks for all the great information.

I did not expect that the loss of strenght would be that great.

With the limited lift capacity of my 30hp Kubota B7800, I am attempting to
eliminate weight wherever possible. Around 760 lbs. on the FEL and 1650 on the 3PH.

I also like the profile of the 3 x 1.5" steel for the forks.

What if I milled some quality/straight grain lumber and inserted the lumber tightly into
the open cavity in each tine. Straight grain lumber can be good and stiff and add some
addtional strength to the steel? Bad idea?

LD1 · Mar 19, 2015

jodebg said:
What if I milled some quality/straight grain lumber and inserted the lumber tightly into
the open cavity in each tine. Straight grain lumber can be good and stiff and add some
addtional strength to the steel? Bad idea?

Sounds like more effort than its worth.

Id go with what is proven.

jodebg · Mar 19, 2015

What happened tho this...

".........there is only one way to find out."
"Ok, hold my beer and watch this..

Sodo · Mar 19, 2015

jodebg said:
What if I milled some quality/straight grain lumber and inserted the lumber tightly into
the open cavity in each tine. Straight grain lumber can be good and stiff and add some
addtional strength to the steel? Bad idea?

Wood will add very very little because it will do all its initial deflection barely even adding to the strength. Plus it can have no fixation at its ends. For the same weight it would be much stronger to add steel. Add flat bar at the root of the tine (top and bottom). Maybe weld the top one entirely before welding the bottom to pre-tension it in the upward direction (speaking of wood,,,, its like a woodie

).

If you are intent on minimum weight (and enjoy craftsmanship or making your projects take longer) step the thicknesses of your fork tine material. The root being 1/4" thick then 25% out, switch to 3/16", then 1/8" for the final half. I'd do cuts/welds on 45deg. But you will need full confidence in your welds (not to mention thick forum-skin)! Heh heh,,,,,,, Sorry guys dunno why this came to me.

If you like 3x1.5 x 3/16" then adding another 3" x 3/16" plate top and bottom at the root of the fork tine should be stronger than 2x2x1/4. I'd holesaw a few 3/4" holes in the plate center every 4-6 inches and weld up the spots.

Its my opinion that a B7800 will have very much diffuculty bending a 2 x 2 x 1/4". IMHO you can make the 3x1.5 work if you beef up the first 25% or 33%.