Homemade tripod tree stump/root puller design

[MIG]

TL;DR Summary

Homemade tripod tree stump/root puller design question.

WHAT: I'm designing a simple tree stump puller. It consists of a tripod made of square (or round, if it's easier) steel tubing with a winch at the top. The cable from the winch drops down, attaches to the stump, then pulls the stump upward.

DIMENSIONS: The top of the tripod, where the winch sits, will be 5 feet high. The tripod legs will be evenly spaced from each other (120 degrees apart). Assume each tripod leg will be 30 degrees from a vertical line. I could change the vertical angle if 30 degrees is not optimal. The tripod legs will be tied together at the bottom, bit the tripod feet are not attached to any fixed point on the ground.

SAFETY: While the winch is engaged, no humans will be near. So, I don't think I need a huge safety factor. Once the stump breaks free, they typically weigh less than 100 pounds. So, the tripod is only under significant stress while it is pulling the stump out of the ground (and I'm standing 20 feet away).

QUESTION: If the winch is rated at 12,000 pounds pulling force, what size square steel tubing do I need for the tripod legs? I was thinking of 2-inches square tubing with 0.25 thick steel. Is that enough?

QUESTION: I might also use a "snatch block" with the winch cable. This loops the cable back upwards and doubles the pulling load to 24,000 pounds. What then?

 

[Baluncore]

Welcome to PF.
I guess you are pulling small trees or bushes. The tied tripod feet will be standing on the roots in the soil, so what will give?

Have you measured the force needed to lift a tree stump? If the soil is dry sand it should be easy. If the soil is damp clay it will be very difficult because you will need to let the air in under the roots, which are stuck to the Earth by atmospheric pressure.

 

[Lnewqban]

Have you engineered a solution for the head of the tripod, connection to legs and to winch?
Note abundant surfaces of contact ground-tripod.

 

[MIG]

Welcome to PF.
I guess you are pulling small trees or bushes. The tied tripod feet will be standing on the roots in the soil, so what will give?

Have you measured the force needed to lift a tree stump? If the soil is dry sand it should be easy. If the soil is damp clay it will be very difficult because you will need to let the air in under the roots, which are stuck to the Earth by atmospheric pressure.

I will mostly be pulling honeysuckle stumps less than 4-5 inches in size (that's the local invasive nuisance plant). I have over 200 of them. I might try it on stumps up to 6 inches in diameter. I don't know that it will useful beyond that.

The tripod will be on the soil, but the tripod feet will be 4 feet apart. So they usually won't interfere with the side roots in the soil.

I don't know what the typical lifting force will be. However, I have previously used a smaller 3,000 pound winch with success and it simply pulled the roots from the side. I'm guessing 12,000 pounds will be plenty (that's what the winch is rated for). I think I should assume the downward force on the tripod will be 12,000 pounds, and maybe 24,000 pounds if I decide to double up the cables.

 

[MIG]

Have you engineered a solution for the head of the tripod, connection to legs and to winch?

The head of the tripod will be a triangle-shaped 1/2-inch steel plate. The winch will be bolted on top of the plate with a slot cut in the plate for the cable. I'll weld three six-inch stubs to the bottom of top plate, and then the tripod legs will fit over the stubs.

If the tripod legs aren't strong enough and they buckle, it's hopefully no big deal. I'll be standing 20 feet away. I can simply try stronger tubing. But I'm curious, from an engineering standpoint, what the correct leg dimensions should be assuming a downwards force of 12,000 pounds, and then also for 24,000 pounds.

 

[Lnewqban]

Even if you go up to 24,000 lbf, if the legs are properly hinged, in such a way that most moments contribuiting to buckling are minimized, each leg would stand around 8,000 lbf / cos 30°.

Please, see:
https://www.engineeringtoolbox.com/load-steel-pipe-columns-d_1348.html

https://www.engineeringtoolbox.com/euler-column-formula-d_1813.html

https://www.engineeringtoolbox.com/square-hollow-structural-sections-hss-d_1478.html

 

[MIG]

If the legs are properly hinged, in such a way that most moments contribuiting to buckling are minimized ...

What does "moments contribuiting to buckling are minimized" mean? I don't know what a "moment" means in this context.

I had planned to firmly attach the legs to the top plate. However, I did not give that part much thought. Would it be better, or worse, to have a more flexible joint at the top of the tripod? Maybe the leg would attach with a pin and be free to pivot to some extent?

 

[berkeman]

What does "moments contribuiting to buckling are minimized" mean?

I'm no ME expert, but it seems like you would want to angle the footings in the soil so that they would absorb both vertical and horizontal forces to keep the forces on the downtubes in strict compression as much as possible.

See the back footing that @Lnewqban posted -- see how it is tilted so it is almost perpendicular to its downtube compared to the other 2 footings? I would think you would want to attach perpendicular footings to each of the 3 downtubes, and pre-dig the site a bit to set those footings into the soil at the proper angles.

BTW, thank for posting about safety issues and how you are handling them. We take project safety seriously here at PF.

 

[MIG]

Please, see:
https://www.engineeringtoolbox.com/load-steel-pipe-columns-d_1348.html

https://www.engineeringtoolbox.com/euler-column-formula-d_1813.html

https://www.engineeringtoolbox.com/square-hollow-structural-sections-hss-d_1478.html

I used Euler's Column Formula, but I don't think I did it correctly:

F = n PI^2 E I / L^2​

Here is each component:

• n = 4: Because both ends of the column (the tripod legs) are fixed. Maybe it should be 2 because only the top of the leg is fixed?
• PI^2: that's just PI squared: about 10.
• E: Modulus of elasticity. For steel, that would be about 30,000,000 PSI. I got that from this webpage: https://en.wikipedia.org/wiki/Young's_modulus
• I: Moment of inertia: From the webpage you posted, this is 0.745 for 2-inch square tubing with 1/4 inch thick walls.
• L : 70 inches. This is the length of a tripod leg. So,

F = 4 * 10 * 30,000,000 * 0.745 / 4900
F = 182,448 pounds
This seems high. Did I do this correctly? Maybe I mixed incompatible units?

BTW, thank you very much for the help so far. I have my general intuition on how thick the tripod legs need to be, but it's good to know how to formally calculate it.

 

[MIG]

I would think you would want to attach perpendicular footings to each of the 3 downtubes, and pre-dig the site a bit to set those footings into the soil at the proper angles.

BTW, thank for posting about safety issues and how you are handling them. We take project safety seriously here at PF.

Your comment about the perpendicular footings makes sense to me. But I have more than 200 stumps to pull. I'd rather not have to pre-dig tripod holes 200 times. Maybe if I can just use thicker steel, that would be worth it to me.

I definitely agree about the safety aspect. I'm glad you take that seriously.

 

[anorlunda]

I'm not an M.E. But I remember the tripod hoists we used for rescue at the Fire Department. They had solid legs rather than hollow. I think the issue is loss of strength if the leg gets dented.

This search shows many pictures of existing tripod stump pullers. Some of them may give you ideas.

 

[berkeman]

Your comment about the perpendicular footings makes sense to me. But I have more than 200 stumps to pull. I'd rather not have to pre-dig tripod holes 200 times. Maybe if I can just use thicker steel, that would be worth it to me.

I don't think it's that much effort. Just use a small pick-axe and a camp-size shovel. A couple of strikes per divot should be enough.

 

[MIG]

I don't think it's that much effort. Just use a small pick-axe and a camp-size shovel. A couple of strikes per divot should be enough.

Okay, I'll do that on the bigger ones. Thanks for your help.

 

[MIG]

View attachment 275498

What's scary about that picture is he's standing two feet away from a cable under ten tons of tension (assuming a ten-ton bottle jack) that's simply wrapped around the top of a stump. If the chain slips off the stump, or the cable somehow breaks free, who knows what ten tons of tension will do?

 

[Lnewqban]

What does "moments contribuiting to buckling are minimized" mean? I don't know what a "moment" means in this context.

I had planned to firmly attach the legs to the top plate. However, I did not give that part much thought. Would it be better, or worse, to have a more flexible joint at the top of the tripod? Maybe the leg would attach with a pin and be free to pivot to some extent?

A column can fail more easily by buckling when some force is applied perpendicularly to it, in addition to the compression load.
Same would happen if, instead of that force, a torque or moment acting on the same plane of the column tries to bend it.

After seeing your calculations, I believe that the type of connection of the legs to the head does not matter much, because you have a good safety margin for buckling.

Perhaps the articulate connection that we see in most pictures has the practical function of adjusting the separation of the legs on-site, according to irregularities of the terrain, roots, etc.
Note that a pin connection requires chains or cables by the lower end of the legs, in order to limit the spreading.

 

[Lnewqban]

I used Euler's Column Formula, but I don't think I did it correctly:

F = n PI^2 E I / L^2​

Here is each component:

• n = 4: Because both ends of the column (the tripod legs) are fixed. Maybe it should be 2 because only the top of the leg is fixed?
• PI^2: that's just PI squared: about 10.
• E: Modulus of elasticity. For steel, that would be about 30,000,000 PSI. I got that from this webpage: https://en.wikipedia.org/wiki/Young's_modulus
• I: Moment of inertia: From the webpage you posted, this is 0.745 for 2-inch square tubing with 1/4 inch thick walls.
• L : 70 inches. This is the length of a tripod leg. So,

F = 4 * 10 * 30,000,000 * 0.745 / 4900
F = 182,448 pounds
This seems high. Did I do this correctly? Maybe I mixed incompatible units?

BTW, thank you very much for the help so far. I have my general intuition on how thick the tripod legs need to be, but it's good to know how to formally calculate it.

You are welcome.
Your calculation did not mix incompatible units, you did it correctly.
For mine, I used n=1, for both ends pivoted.
Also, I calculated $I=0.55 inch^4$ for the 2x0.25 square tube that you mentioned in post #1.
My result was that one leg loaded with 9237 lbf maximum, could resist 32126 lbf without buckling, for a safety factor of around 3.5.

 

[mudrat]

Came across this post and had to show my tripod. Has been successfully loaded to 2,000#+.

Originally designed for pulling posts. Originally used come-a-long, switched to 1500# 12V electric winch as I got older...

Note the winch mounted on a strap of steel so it can hang on the eye bolt, or to chain wrapped around a tree to pull sideways. An eye bolt on the end of the strap acts as a cable guide to keep the winch from trying to twist sideways.

Legs are usually 7' 2x4, switched out to fit the job.

 

[NatureFriend]

I actually have experience dealing with Honeysuckle. You did not mention whether the plant was alive or dead. That is a big difference in the force required to remove (my physics answer). I would also suggest you are " p...ing in the wind" if you are trying to eradicate it if you are pulling live bushes. You are only dividing the root system into multiple new plants that will resprout. My approach would be to spray it with glyphosate (aka round up) to kill and then remove if you aren't fond of the esthetics (my biology answer). Here's a excerpt from O.S.U. bulletin on the subject. Good luck.

Controlling Non-Native Invasive Plants in Ohio Forests: Bush ...​

The Ohio State University
https://ohioline.osu.edu › factsheet
Aug 16, 2010 — Mechanical control alone is usually not a completely effective method of controlling medium to large bush honeysuckle shrubs.
Herbicide: Example Brand Names
2,4-D + triclopyr: Crossbow
Glyphosate: Roundup, Accord, and other herbi...
Triclopyr: Garlon 3A, Tahoe 3A