Loads from dragging a heavy mining cable 1

[metman]

The attached Word.doc lays out my idea of a method to determine load or force on a mechanical structure.

In the attached doc, the Known items 1-6 were given to me except for my addition of0.17 x 45⁰ = 7.65⁰ = ϴ)

We need to reposition the Side Arm and add additional support. A Preliminary redesign has been proposed by our Engineering team and they have asked me to analyze the structure.

I am still working on the force vectors acting on the structure but so far that is not a question.

Could someone please critique my method and tell me if you concur or disagree with this analysis?


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[desertfox]

Hi metman

What about the force required to accelerate the cable from rest to the required speed.
Other than that I think your on the right track but I would prefer to see a drawing or sketch of this arm if possible.

desertfox

 

[dhengr]

Metman:
What on earth are you trying to do? You really must define your problem much more completely, including some sketches, so someone other than you can see and understand what you are doing, if you want any meaningful discussion here. You can see it, the rest of us can’t, and we haven’t had the discussions with the rest of your engineering team, that you have about what you have and are actually trying to doing.

Isn’t a 17% slope, 17' vert. in 100' horiz., or 9.65°? What are you lifting 11.2', and pulling at the same time? My picture is a cable catenary, maybe 4 parts of cable, 100' +/- ? long, supported at your arm which is 11.2' above the ground on a truck, with the other end and the remainder of the cable length on the ground. Your description of your problem certainly leaves me guessing at what you are doing. Hydraulic booms are not usually designed to take significant lateral loads like you might be applying. What does your prelim. redesign look like?

 

[metman]

Isn't a 17% slope, 17' vert. in 100' horiz., or 9.65°

Yes you are correct. I thought I understood this but do not and need to. So it is the (arctan 17/100) = 9.65 degrees. Also you are correct on all accounts. I will provide more info later. I was brought into the melee at the end of Friday for about 5 minutes because our engineering mgr is on vacation and there was a field problem.

Only this afernoon did I get a 3D sketch etc. and just now a 3D model.

I appreciate that you took the trouble to even look at such a truncated description and the time to comment.

Desertfox: Good point but maybe the dynamic friction is enough lower than static to offset force of accelleration.

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[miningman]

Metman, my first reaction to your post was "This is another homework problem" but reviewing your profile this obviously isnt the case. However, I suspect that your engineering colleagues are re-inventing the wheel and the original suggestion that Arctan 0.17 is 7.65 deg suggests they might not fully understand what they are doing.....and the idea that 100% grade is 90 degrees might support my suspicions.

What you appear to be trying to do is done every month at some of the hard rock operations I have worked at., and the electricians would go absolutely ballistic at the idea of dragging hi voltage cables over rock. Four loops for 1100 feet is about right, I can do loops every 4-500 feet when I have to. There are specific machines available from the equipment manufactures designed for this... right now I am about to hang in excess of two miles of 13.8 kV cable... that 10 times the length you are dealing with and I suspect its a lot more than 11 lbs/ foot.


For a small job like yours , I wouldnt think twice about just using " miner muscle " being supervised by competant underground electrical personnel. Hope this helps you move the internal discussions in a more appropriate direction.

 

[dik]

I'll check with the electrical dudes on Monday and see what they do... current project we have 4 single conductors approx 4" dia, ea. and we're putting them down for several thousand feet... we'll see how they 'stretch' them... I think they come off a continuous spool and are just laid down, but I'll see how they are pulled where required. Some areas they are bedded in sand and in some areas supported by cable tray...

Dik

 

[desertfox]

Hi metman

I doubt the difference between the coefficients of friction would account for the acceleration force, read the references at the base of your calculation ie:-
"Also the friction actually decreases above a certain velocity"
Therefore whatever that certain velocity is the cable as to attain that velocity first so it as to be accelerated from rest.

desertfox

 

[dik]

For the 138 kV cable we are placing, they use single axle ‘dollies’ at each 10’ or so and attach a nylon rope to the end using a wire mesh puller (like Chinese finger trap). The dolly wheels are approx 12” dia. The cables are rated for 10,000 lb force (different cables have different load ratings) and the nylon rope is pulled by an electric ‘drum’ winch. A couple of turns are wrapped around the drum to restrict the tension on the rope going to the conductors. Using this method, two lengths of 2200’ of single conductor will be placed (total length is 4400’). There are four large conductors (one spare) in the trench plus half a dozen smaller ones.

After one of the large conductors is placed, the conductor is moved off the dollies, ready for the next conductor to be pulled. 250 approx of these dollies are used for this project.

If they are moving cables through conduits, they generally use a water based lubricant.

They don’t move the spools as I thought.

Dik

 

[dik]

Metman:

FYI

 

[metman]

...the original suggestion that Arctan 0.17 is 7.65 deg...

This was not my colleagues’ idea, it was my own misunderstanding. I thought as follows: 17 rise vs. 100 run = 17/100 = .17 so then 0.17 x 100% = 17%

Nobody told me nor did I think that (arctan O.17) = 7.65⁰

Because 45⁰ is 100% I thought it was a multiplication of “percentage in decimal form” times 45⁰ as follows:

1. Maximum slope: 17% ( 0.17 x 45⁰ = 7.65⁰ = ϴ )

dhengr corrected me as follows:

“ Isn't a 17% slope, 17' vert. in 100' horiz., or 9.65°?”

I verified this, i.e., (17% = 9.65⁰) with a chart in Caterpillar Performance Hdbk which shows %Grade vs. Degrees which I discovered by accident (explained further below).

dhengr: This is a huge help because I need a thorough understanding of Rimpull – off subject in this thread. I was going to make my own chart of %Grade vs. Degrees because during my study of chapter 6, Mobile Equipment Power Requirements, in 7th Ed of Peurifoy’s book, Construction Planning, Equipment, and Methods, I was having difficulty following his discussion on p145 where he says, “For slopes less than 10%, the effect of grade…” and then below that he says for angles less than 10⁰, sin α ≈ tan α (the small angle assumption). BTW I understand the small angle assumption. It was the confusion between % and angle that threw me off balance. By chance, while searching The CAT Performance Hdbk for Altitude Deration chart, I found the %Grade vs. Degrees chart so I did not have to make my own INCORRECT chart with 90 degrees at 100% grade. BTW, I challenged this at the time and said, "I thought it was 100% at 45 degrees."

If you’all are not confused by my rambling, you should be! But for me the clarity is crystal clear – well almost. Now if someone would be so kind as to instruct me how to attach multiple files in a single posting, I will provide some pics and explanation to clarify “what on earth we are trying to do.” Just this morning I learned how to scan these doc’s.

I asked Tecumseh Group several weeks ago how to attach multiple files but they did not answer me.


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[ScottyUK]

Easiest way to create multiple links in your post to stuff you've uploaded to engineering.com is as follows:

Upload your file.
Cut (CTRL-X) the attachment path from the text box in which it automatically appears.
Paste (CTRL-V) it into the main body of your post.
Upload the next attachment and repeat the steps above until you have all the upload links in the main body of the post.

You can leave the link to the final upload in the text box if you want - whatever is in the box gets appended automatically.


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[chicopee]

Your problem is not that difficult to solve. I did it in short period of time in general terms. Your problem is similar to conveyor type problems whereby it is loaded by a continous source such as sand from a hopper. I would suggest that you get a ME and Chem E handbook and study the topics under material handling specifically dealing with conveyors. Then you can determine the rim pull, whether the cable can sustain the pull without excessive stretching or breaking, and the horsepower needed to pull this cable.
Reading your post and attachment, I should remark that:1) if the cable has an horizontal velocity of a magnitude, this velocity should not change when the cable is pulled uphill,unless the cable become severly stretched 2) for the startup stage, I would tack on an additional 15% to overcome inertia of the cable 3) For coefficient of friction on dirt, I would use .5 4) your analysis sheet has too much extraneous details for the problem

 

[GregLocock]

Idle question - would there be a market for a physics based simulation for this sort of problem - up to and including 3d models of the surface in question, and your funny little trucks? Is elasticity of the cable an important variable -ie are you interested in dynamic cases where the cable snags and releases?



Cheers

Greg Locock


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[metman]

4) your analysis sheet has too much extraneous details for the problem
5) don't blame the PE, for whatever error he may have made, in public, blame yourself for not noticing it in time.

chicopee
4) The extraneous details were there for the non-engineers on staff to help them understand my method of analysis because our engineering mgr was gone.

are you interested in dynamic cases where the cable snags and releases?

Yes but actually we were more interested in cable tensile strength which we now have from an engineer at the mine along with all of the other specs for the cable. It appears to me that its strength is marginal for what they are trying to do and we have a solution to the problem but the mine will need to approve it.

Now that ScottyUK (Electrical) has so kindly shown me how to upload multiple files I will do so but later -- in a week or so just to satisfy everyone’s curiosity. I need to get busy on my main project now that my boss is back handling the mine cable problem along with some section modulus info I provided for a totally stout and different mechanism for pulling the cable instead of incorrectly side loading a boom.

There was a misunderstanding between the requirement for a side boom arm on this cable-reel-truck and what we, the supplier of the truck, were shown as an example of its intended use. I will explain as best I can when I attach pictures later because I was only peripherally involved and not privy to a lot of detail like being there at the mine site.


Thank you all for some interesting and helpful comments.

Cheers



Design for RELIABILITY, manufacturability, and maintainability

 

[metman]

Attached is a photo of a truck with a boom lifting an electric mine cable. This is not one of our trucks. This is what was represented to our personnel as the intended application. The mine folks were lifting and pulling the cable parallel with the boom. This was to move about 100 feet of cable at a time perpendicular to the axis of the cable. We added an arm on the side of a cable reel truck for this purpose.

Attached are two photographs of an example of a cable reel truck that was available for me to take a photograph last week. This is not the truck in question. One truck that was being designed two months ago when I came on board and has subsequently been delivered had a cable reel to handle 1,500 feet of 4-3/4 diameter electric cable – NOT pictured here. So there is some expertise here on this subject but there was also some misunderstanding. I am working on a totally different product line.

The attached sketch is companion for the analysis sheet originally attached in this thread. A coefficient of friction = 0.5 for dragging the cable in dirt is probably closer to reality as someone here suggested but for structural integrity I needed worst case so I used coeff = 1.

Because there would obviously be problems side loading a boom arm, the first solution proposed was to add steel restraining cables attached near the end of the arm and to the truck bed at the other ends. I have free body diagrams with resultant cable load but it is now moot because a more practical and substantial solution was devised by our field service guy and VP – while I watched them sketch in appreciation of their innovation -- involving an insulated hook and some proprietary details.

Thank you once again for your input. I hope to return to this thread in the future to gain more understanding on this subject if needed.

http://files.engineering.com/getfil...4775-83d5-e009f0e6d071&file=Cable_Lifting.pdf

http://files.engineering.com/getfil...2e54-4c64-a5ea-a8603f2a838a&file=IMAG0120.JPG

http://files.engineering.com/getfil...4a92-4f2a-90e3-288e70a01e49&file=IMAG0121.JPG

http://files.engineering.com/getfil...206f02fef&file=Elect_mine_cable_drag_load.pdf

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[GregLocock]

Thanks for that. These are the cables I'm used to handling - about 10 tonnes breaking load.

http://sydney.edu.au/warrencentre/bulletin/NO46/ed46prd4.htm

Cheers

Greg Locock


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[chicopee]

"Because there would obviously be problems side loading a boom arm" one safety rule with booms is that they are not designed for side loading. Read the manufacturer's instructions regarding this particular subject.