Torque Required to Drill in Soil

Good point. I'm wondering why this type of tool is chosen, and how much confidence their is in the soil tests. 'Soil' is a bit vague. There's a project I 'd rather forget in which we kept hitting boulders of rock (periglacial detritus). Tha cost was astronomic.

I'm posting anon because it still gives me bad dreams.

Piling depth gives a reasonable idea, but you need to be more specific. Overspec this - you'll be very sorry if you skimped for want of a few horses.

Hammer that thing at a depth foot or two, then get a torque wrench and rotate it(Torque might increase with depth). Take note of the reading add some margin twice or thrice the reading, then proceed with the rig to drill.

I am building a portable drill rig, and need to spec a motor for this. I will be drilling a 10" hole to a depth of 12'. Once the hole has been drilled, I will remove the auger and insert the Bell Tool, or Underreamer, to create the bell.

As I scale the tool from the picture, I believe the cutting edge to be approx 14.25", and extend 11" from the 8" diameter lower housing.

What I know about the soil is that it measures 1,500 psi. I will take samples again at 12' ... this number may change.

Char-Lynn motors have ratings that include Torque, Flow and Pressure. The flow and pressure are a function of the pump, and for the most part constant. I will be choosing a motor based on the torque specs.

I would like to know how much torque is required to turn this tool so I can choose the appropriate motor.

PW, Manufacturers push everything to their distributors, and distributors are Order Takers, "Do you have a Part number?" I have been waiting for 10+ days to hear from an Area Rep.

I can't get a clear answer from the tool supplier, either ... translation, they don't know how to calculate it either.

In calculating, it looks like you need a variable for the decreasing length as the distance gets closer to the axis as it travels up the cutting edge.

I also need to be able to incorporate PSI into the force measurement.

Can someone tell me how to calculate the torque requirements for this tool?

Thanks

It looks like a Big Beaver portable drill system. They have figured out the torque and it is in their specs. From littlebeaver.com

Soils are variable and some soils can be heavy, conglomerate, or loose. Sometimes even cemented. Unless you are building something to use many times, my option would be to hirer a cable tool rig. They could install large diameter casing to great depths. The casing could be bailed out and cemented in place. it may be the only option at low cost.

Kevin and TCM, thanks. Little Beaver offers 3+ sizes of Eaton Char-Lynn motors appropriate for this Power Mast system, and I have not gotten a straight answer every time I called. They are also fast and loose with the calculations. As I look at Eaton's Performance Charts, I don't see where they get their ratings for Torque or Speed. I don't want a motor that tops out at 88 rpm if I can get one that is faster with sufficient torque.

With the flow and pressure of the hydraulic pump I am considering, it looks like an 18 cu"/rev motor will produce 429 ft/lbs. The more expensive 24 will do the same at a slower speed, unless I provide more pump and horsepower. I was hoping to find that I would not need a 20 gpm, 3000 psi skid loader or tractor to provide sufficient torque. TMC, you comments have drilled holes in my previous confidence ... but thank you.

Unfortunately drilling big holes in the ground usually takes a a lot of power unless you have a lot of time to spare. There's no real way around it.

Personally when I do post holes I prefer to idle the tractor along and drill slow. I would rather drill a straight hole slowly whereas my dad prefers to open it up and drill crooked holes fast!

As far as drilling goes 60 RPM on a large auger looks surprisingly fast and generally has no problems moving the earth out of the hole unless it's real fine dry sand.

I am no expert on drilling, first of all, but it appears you already have what you want for the first part of the drilling - the straight sided vertical shaft. It seems you are concerned about extra horsepower requirement to handle the bell tool?

Does that bell tool spin out from the 8" figure at the top of the bell, and gradually extend outward by hydraulic action until it is laid out flat? If that is the case, you know when the bell is completed, then you should not really need that much more torque/pressure/rpm, since only a small radial cut is being made by the leading edge of the tool, though there may be some additional torque due to the bottom edge of the bell tool rubbing on a flat portion of the hole bottom.

I bet there are more variables here than most folks want to calculate, so why not just bump up the power by 50% to 150% of the power requirement for straight sided drilling and be done with it. At least the equipment should be rugged enough at that point to last? The additional torque requirement is not much with the first part of the shaft already drilled, since you should have pretty fine control over how fast the bell tool plays out to end of travel.

Why not contact a local well driller, they already have rotary rigs that can drill these holes for you faster than you can figure out what you need.

They have equipment that can drill through solid rock, on hand, and they know how to use it.

The well drilling rig above is of a "cable tool" design of which I drilled a lot of wells with years ago. This will not work for your application or drill bit.

The type of rig you need is a Rotary Well Drilling Rig. They can even 'mud off' the holes to prevent collapse in case of loose soil.

Give a driller a call and tell what you are trying to do...

The 1500 psi figure is almost surely the compressive strength of the soil. The shear strength, which is what matters, is likely to be considerably less.

Again, Thanks for the responses. I will be drilling piers next to the foundation at 7°, or 83° depending on your orientation, belling the bottom 30" and finishing with a piling cap. I have looked at several portable drill rigs, and they exceed my budget. So I am building the drilling rig, hydraulic power unit, and probably the bell tool itself ... and will enjoy it! The largest piece of equipment that would fit would be a mini skid steer or mini excavator.

The conflict is between speed and torque. I will be drilling in clay using a carbide tipped blade which requires speed. When I switch to the bell tool, I need torque. So for me, I am trying to size the motor by torque, and get as much speed as possible. It looks like the torque curve plateaus around the 18 cu"/rev motor, which was not offered by the drill rig manufacturer. I can spend more money on a larger motor without getting more speed or torque.

James, you are correct about the hole being drilled first. The belling tool has a 10" collar at top and will drop down the shaft. It is a mechanical system where the force of the drill rod pushes the cutters out. Then when the drill rod is pulled up, the cutters retract and scoop the dirt into the 8" housing. I am in Arlington, so you are welcome to come help and see how it works for yourself. ;-) And, you are correct about the radial cut and the leading edge of the tool. I want to make provisions for the extra 13" of cutting edge down there.

Adreasler, I remember Dad saying, "Well, this ain't gettin' it done." I guess it is best to look before you leap, and then get leaping.

And Tornado, I agree about this including shear strength, but I don't have that information.

Again, thanks to you who gave thoughtful answers.

Pitch of your drill handles shear strength of the soil (high pitch you need a high torque basically, if you have low pitch, low torque will also be required). You may reflect on the drill bit and get the idea.

Or over design a little bit your motor to drive your bit, assume you'll hit the hardest and toughest stone down under--then, you'll have a safe and working rig using that in your design.

Good luck

"The conflict is between speed and torque. I will be drilling in clay using a carbide tipped blade which requires speed. When I switch to the bell tool, I need torque. So for me, I am trying to size the motor by torque, and get as much speed as possible."

I should have spotted this before, as has happened many times before, the wrong question was being asked.

You don't need a MOTOR, what you need is a multi-speed GEARBOX! Also known as a transmission. You get a high-speed motor for the carbide tip, using the high gear on the gearbox to deliver 1:1 RPM transfer, and when you switch to the bell, you shift the transmission into low gear to get the torque you need.

For a fixed power output, speed and torque are inversely proportional⁽¹⁾, if you gear the speed down to half, you double the torque, cut speed to a tenth, and you get ten times the torque. This is basic physics for rotating shafts. When I was learning about electric motors to get my degree, I was taught how to determine the torque/speed graph. You marked the motor's maximum 'no load' RPM on the vertical axis, marked the motor's 'stall torque' on the horizontal axis, and drew a straight line between the points. That's it. Now I'm sure the curves for mechanical engines are a little more complex, since they may have varying internal frictions due to piston seals and such, but the base concept is the same.

Notes:

1. Not always a perfect [ slope = -1 ] line, and there are other energy losses due to the mechanical system, but it's a good starting point, and for most cases (where you just need to know the minimum horsepower motor/engine that will work) it's 'correct enough;' you'll be rounding the answer up to get the standard sized motor/engine you'll need.

Hi,

I guess you have completed your drilling rig by now.

I tend to offer my experience being a driller and a fabricator. Drilling a hole requires torque to turn the drilling bit and a axial force to determine the rate of penetration. Considering drilling in a sedimentary formations, where rotary with mud system is used then if the axial force applied on the drilling is constant (being enough to add weight to the drilling strings and not buckle or break it) then the variables such depth of drilling, diameter of drilling string and diameter of hole determines the measure of torque required.

Fabrication of a drilling rig ,as you say, the torque of the motor depends if there would be a gear with speed reduction ratio to provide more torque. Using 60mm diameter drilling pipe and drilling 200mm diameter hole to 90m depth in sand formations with hardpan or weathered lateritic formation, torque of 1000ft lb (1430Nm) should be adequate but some small rig manufacturers use lesser torque about a thirdth of this value.

But in your case of 12"(300mm) to 12'(3.66m)depth, a lesser torque of about 100ftlb might be adequate.

Thanks Grego. Other projects have forced me to push this to the back, so this is still in the research phase.

Speed reduction would add cost, and probably unusable torque. I see these units in skid steer and tractor applications. I assume that the extra weight of the machines would keep the unit from spinning. Excessive torque might turn this small drill rig might turn into a carnival ride. I plan on using a hydraulic gear motor to turn the bit.

I will be drilling in red clay.

BTW, as a fabricator, do you have any suggestions about the Bell Tool? I have designed something in CAD, but it is untested. Any suggestions you have would be appreciated.

Hi Texski, I have a suggestion for expanding the hole at the bottom but lam not sure if the hole formed will be bell bottom or something similar.

A cylinder with 10mm tolerance on its sides with the diameter of the bored hole should be used. Three vertical slots on its sides at angle interval of 120 degree. Three triangular plates are joined together at edges with a locking mechanisms and put inside the cylinder.when the construct is lowered to depth where the bell bottom begins, the drilling commences and the plates project through the slot to cut the soil formation and it goes down it tend to form a Bell, l believe. Well it is an idea but working on it might result on a detailed result. What do you think?