drawbar design 1

[Frankendave]

Hi all,
I'm after some indication that I'm heading in the right direction. I need to design a drawbar to be used underground to tow broken down trucks out of the hole, its been a few years since I've done anything like this and I'm stuck out bush with no books or notes to lean on.

So far I've worked out the force parallel to the incline (1:6) due to the mass of the truck.

Then I've basically done a dog bone design (with hole at each end for pin coupling) and used the basic stress = nominal force / area < ultimate tensile stress with a safety factor of 1.5.

I also did a quick check on the shear stress.

Am I anywhere near close or have I simplifed it too much?
Would friction increase my force needed to start towing?

Any help would be greatly appreciated

 

[GregLocock]

You should use a factor safety of at least 4, in fact I'd use about 10. Your model is far too crude to start being clever with small factors of safety.



Cheers

Greg Locock

 

[FredGarvin]

When in doubt, make it stout. You're on the right track, but Greg has a very good point.

The other thing you haven't mentioned is the jerk involved. Impact forces like that will be much higher than the static applied forces you have already looked at.

 

[zekeman]

Dont forget the external load e.g. stuck in mud plus the impact load when your pulling truck impacts the rod on making contact. Both are very complicated.

 

[Frankendave]

Thanks all,
I'll try the safety factor of 4, I'm trying to keep it as light as possible so two blokes can lift it. I think the jerk factor would be negligable as there is no give in the system. Just thinking about that and wondering if it will work without that jerk???
Thanks for the help.

 

[JStephen]

You can also check shear on the pin and bearing between the pin and the bar.

Depending on what you are pulling with- it might be easier to design for the maximum pull that your tow vehicle can do, rather than trying to figure out the force to move a stuck vehicle.

The safety factor required would depend somewhat on the consequences of a failure. If the only consequence is a matter of inconvenience, it would make sense to use a lower factor. If a towed vehicle rolls back and wrecks, use a much higher factor.

I once saw an over-the-road dump truck that had been pulled with a chain & bulldozer (while stuck). The chain broke right near the hook, and the hook came back through the windshield and broke the back glass in the cab. I don't think you'd have that problem with your towbar, but something to keep in mind.

 

[Tmoose]

If they're broken down AND stuck in the mud a stretchy rope might be the preferred connection, to allow a running start and thus greater extracting power. Yep, if something breaks it will get exciting.

http://www.masterpull.com/results.cfm?catid=1148

Whenever I tow something there is a significant jerk in the sytem.

 

[aviat]

If you don´t have a towing rope you can use two chains with an old tire in the middle to take the shock.

 

[patprimmer]

I find it hard to imagine the floor of an underground mine is so smooth so as to be able to calculate the grade that accurately. There will be bumps and pot holes that cause considerable variations, and if hit with any speed, inertia might increase the instantaneous load to somewhat higher than the force exerted by the power of the tow vehicle.

Is it simply a broken down truck on a steel rail , or is it something running on road base with pneumatic tyres.

One other factor when working underground is fire risk. Could a breakage result in a crash that might create a fire or explosion risk.

To reduce the load on the only 2 available men, can you mount one end on the tow vehicle, with a winch to a higher point on the tow vehicle, so it is easier to lift.

Maybe mount it on the tow vehicle with the free end to high. Do this above ground where there are more facilities. Support it with a crude friction type block and tackle, so it can be easily controlled when lowering with only one man on the rope. A friction type block and tackle can be made by running a rope through eye bolts.

Try it above ground first. Practise it several times. Do it where breakages will not result in injury.

Just my 0.02 cents worth. Sorry if it's a lecture, but I am imagining a situation where someone can get hurt. Underground mining is dangerous enough as it is.

Regards
pat pprimmer@acay.com.au
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