Simple Machines Revisited + more - making 2.5 ton lift

  • Thread starter nrobidoux
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  • #1
nrobidoux
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I always seem to get into "big" projects these days and need to whip out some math that has a lot of cobwebs. I'm not too worried about injuring myself... but I don't want to inadvertently have something break over a vehicle bed / trailer. I think most of the system components are good... I don't know if the system is good itself.

I was hoping for a check.

My goal is to construct a lift to raise ~2.5 ton reels of cable off a flatbed/pickup, the vehicle drive out and then I can drop the load. Reposition lift. Repeat several times and then lift the first of several onto a stand where I can then rotate the 2.5 ton mass... Fun stuff! :)

I was thinking initially 2 of https://www.walmart.com/ip/Hiltex-Rope-Hoist-Block-Tackle-2-Ton-Pulley-Wheel-Poly-Puller-Wild-Game-Deer-Hanger/106200975?wmlspartner=wlpa&selectedSellerId=2117&adid=22222222227047528458&wmlspartner=wmtlabs&wl0=&wl1=g&wl2=c&wl3=123154016438&wl4=pla-65687234442&wl5=21147&wl6=&wl7=&wl8=&wl9=pla&wl10=113148308&wl11=online&wl12=106200975&wl13=&veh=sem#read-moremm poly pro 2-Ton lifting blocks... giving me 14:1... but that's 320+ lbs (probably 4500 lbs not 5000 just being careful... and could be less if there isn't 40,000' on it). Too much for me, perhaps a 4-point lift with 4 of them. 28:1 is about 160 lbs of pull. Above the SWL for 6mm poly pro of 93 pounds using a safety factor of 12 but far underneath the 1100+ lb minimum break strength.

Each block setup has a hook which I was going to attach to a lifting eye. Each has a WLL of 2,700 lbs with a 3/8-16 thread. I was going to drill through the lifting frame and thread the lifting eye to a 7" 3/8-16 threaded stud.

This is where my math kinda leaves me the stud has a cross-sectional area of about 0.44" and tensile strength of 125,000 psi. But what about the threads? Am I going to pull this right out of frame? The other side was just going to be a nut? I found this: At = 0.7854( D - 0.9743/n)^2

D = diameter (.375)
n = turns per inch (16)
At = cross-sectional area of thread

Isn't this missing something like ... the nut is only 23/64" high with probably 75% of that with thread? Can I double-stack nuts? Giving each of the 2 layers the same torque?

Then there is the framing. This probably would be way easier with I-beams... but that seems expensive... and heavy. Memories of my dad with his homemade engine lift come to mind... but that was an engine not 4,500 lbs. This was done with 3 layers of 2x4s with the ends staggered sandwiched together with hefty carriage bolts.

I found a span calculator... IDK if it's what I need or not. But the span between "joists" is "touching" not 12" (the smallest I found). Was thinking about using 4-layers... and 2x6s but I really can't find the math anywhere (or I did and have no idea what numbers to use). I was going to puzzle piece the corners so 2 of the layers would go all the way over the vertical and the other two like 25%. Three carriage bolts. 2 - outer part of corner; 1 inner;

For the lifting area... Was thinking having metal plates over/under the framing. Dbl-ended studs through each corner, bolted with lock washers... the two lifting eyes side-by-side about the center of the plates. Idea was to help distribute the pressure.

Bottom of the vertical members would be a small A-frame as well. Top corners would have couple of 2x4s to triangle them. Was considering connecting the two bottoms together along the floor with a 2" x 4" x 12' cut to appropriate length. Edges of the boards to ceiling on top members and side walls to vertical members, bottom board would be flat on ground.

Final dimensions (maybe no staggered ends :D) -- 3' (W) (Bottom A-frame) x 8'+ (H) x 7' / 7.5' (L)

Open to ideas but that's where my initial thoughts are. Thanks.
 

Answers and Replies

  • #2
jrmichler
Mentor
1,978
2,525
Some thoughts:
Forget a 4 point lift. There's no way to balance the load, and you will end up supporting the load at 2 points.
Google torque tension chart to find the maximum tensile strength of bolts. Be careful, bolts come in many different grades.
Bolts loaded in shear are calculated completely differently.
A properly designed wood beam will weigh about the same as a properly sized steel I-beam.
A wood beam made from what's readily available will be heavier than a steel beam.
Cheap block and tackles (also cheap come a longs) are overloaded at the claimed allowable load. That's why quality ones cost so much more.
The actual pull on a block and tackle will be about double the theoretical pull due to friction.
Look at hand chain hoists. McMaster-Carr is a good source: www.mcmaster.com. They look expensive at first, but will do the job.
McMaster-Carr is also a good source for eye bolts and lifting eyes, complete with real load ratings. And their prices are reasonable.
McMaster-Carr is THE wish book for people building things.

We need a detailed sketch of your proposed structure.
 
  • #3
anorlunda
Staff Emeritus
Insights Author
11,209
8,614
Asking strangers on the Internet for safety advice is not wise. We can't know 100% of your situation, only a what you tell us. Your post makes no reference to codes and standards that may apply. Also, this is a public forum so it is not just your safety at risk, but the safety of anyone now or in the future who reads this thread.

Sorry, but PF rules don't allow discussion of dangerous topics. Thread closed.
 
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