Torsional strength in a hollow steel body with milled slots
Torsional strength in a hollow steel body with milled slots
(OP)
Hi everyone,
I am trying to figure out how to determine the torsionsal strength of a hollow body/tube with slots milled in it. The tube is a downhole drilling device and the slots contains expandable reamer elements that stick out sideways into the borehole wall (underreamer). When drilling, a torsional force is acting on the body, twisting the body so the reamer elements get jammed (they pop out and in on hydraulic pressure). I have read with interest the comments by electricpete, zekeman and others, but have not quite found somethindg directly applicable.
A typical tool is between 4" and 16" OD, 2" to 4" ID and there are three slots milled in each tool. The slots are 1/4 to 1/3 of the circumference and are from 1 to 2 feet long. The steel is normally alloy, yield 135 ksi.
Thanks and regards
I am trying to figure out how to determine the torsionsal strength of a hollow body/tube with slots milled in it. The tube is a downhole drilling device and the slots contains expandable reamer elements that stick out sideways into the borehole wall (underreamer). When drilling, a torsional force is acting on the body, twisting the body so the reamer elements get jammed (they pop out and in on hydraulic pressure). I have read with interest the comments by electricpete, zekeman and others, but have not quite found somethindg directly applicable.
A typical tool is between 4" and 16" OD, 2" to 4" ID and there are three slots milled in each tool. The slots are 1/4 to 1/3 of the circumference and are from 1 to 2 feet long. The steel is normally alloy, yield 135 ksi.
Thanks and regards





RE: Torsional strength in a hollow steel body with milled slots
RE: Torsional strength in a hollow steel body with milled slots
I thought it might be possible to calculate it as one big tube, but remove the proportionate amount that represents the slots for the moment of inertia. But, then I get a linear curve and I think the curve should be more logarithmic. If the three beams are long enough, they will twist very quickly.
I have enclosed a picture that shows a similar tool, with three slots and the cutters inside.
Thanks
RE: Torsional strength in a hollow steel body with milled slots
Sure the result will be equivalent to three beams in bending with clamped ends, but seeing this as a torsion problem seems simpler to me.
And of course buckling considerations could govern upon the strength of the device.
prex
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RE: Torsional strength in a hollow steel body with milled slots
Cheers
Greg Locock
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RE: Torsional strength in a hollow steel body with milled slots
RE: Torsional strength in a hollow steel body with milled slots
gwolf
RE: Torsional strength in a hollow steel body with milled slots
RE: Torsional strength in a hollow steel body with milled slots
Yes, underreaming has been industry practice for years. Today, underreaming is going through a renaissance as it is used more and other purposes than before. That results in requirements that are way beyond what was seen in the past. We are trying to construct underreamers that can drill past 5.000 feet in a single run.
The main design problem is cutter blocks and lifespan and the only solution is longer cutterblocks with higher density PDC cutters. The next problem is that the pockets to fit the cutterblocks become longer and jamming due to torsional twist is the result. In many cases, the torsional twist is so high that the material yields. If the cutterblocks are jammed in expanded position, you cant get out of the hole and the customer will flip.
Anyway, a number of your comments has put me on the track again, so thanks all of you!