Crazy talk! Using a rope prusik on steel cable?

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Crazy talk! Using a rope prusik on steel cable?

Postby OpenTrackRacer » Aug 20, 2013 6:32 pm

We're going to be working on a project very soon which involves the use of a very old mining hoist. The hoist car is supported by a steel cable about 1" to 2" in diameter. I was thinking about how to add additional safety systems and the thought crossed my mind to make a prusik out of 11mm Pit Rope to grab the cable should the hoist motor/brake fail.

:yikes:

Does anyone think this would actually work? I'd guess the weight of the hoist car and three people to be somewhere around 1,100 pounds. Of course if we're far down the mine shaft, the weight of the cable could add several thousand pounds. The idea would be for the prusik to engage before things got moving too fast. This would be a last ditch do something or everyone in the car is dead safety so as long as it could work and didn't add more danger I'd be willing to try.

Yes, I always come up with crazy stuff.

Thoughts?

Mike
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby caverdan » Aug 20, 2013 6:40 pm

IMHO...... rope will not work to stop it. You would be better off trying to jam something steel between the pully and cable........like a 6'rock bar. :yikes:

If there is question about this rig......why don't you use SRT? :shrug:
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby GroundquestMSA » Aug 20, 2013 6:58 pm

I have no idea really, but it seems that one problem may be the diameter of the proposed prusik. Even on a massive cable like that, is pit rope going to be too big to grab? Prusiks are fun to play with. I used prusiks to climb a metal light pole while waiting for friends in a parking lot after a baseball game. I don't think that rope & cable are inherently incompatible, but this rope with this cable and these loads... I'll wait to hear what the experts have to say.
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby snoboy » Aug 20, 2013 7:26 pm

Thoughts? Yup...

Hard to say if it would work. I don't know if there would be enough friction to stop the system before speed built up too much. When I look at prusiks locked onto a rope, it always seems the main line is bent, not sure how much that adds to the grip. I have used 11mm prusiks on 1.5inch cable as anchor points, but they had other backups, and also a plate clamped below that would have prevented them slipping very far. We never did any pull test on that configuration.

I can't see any disadvantage off the top of my head. Make sure the prusik tender has gloves, and no chance of getting sucked in.

I wouldn't use Pit rope - too stiff, find something soft like Sterling. I would also use tandem prusiks, maybe even three.

Alternate idea - can you run independent safety lines parallel to the hoist and have passengers use something like a Petzl AS'AP? This would only work if there was low chance of the riders being entangled in the cart in case of a failure. That's more or less what workers do when working on some types of hanging scaffolds (swing stages).

Edit to add:
I have used 8mm prusiks on cable that was probably 1/2" (12.7mm) and we pulled pretty hard on it with no slippage.
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby johnlhickman » Aug 20, 2013 10:34 pm

The easy answer is that I have used 1" webbing to pull 3/8" aircraft cable at a ropes course so I know it would hold. Of course, we had a static load and were not using it to catch a load. You may run into several issues related to wear on the prusik-style hitch as the cable passes through your interface and the shock load on the system since you are using cable and not rope. Safety can be good, but you don't want it to imped the operation of the primary system.

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Re: Crazy talk! Using a rope prusik on steel cable?

Postby NZcaver » Aug 21, 2013 2:47 am

I would not rely on Prusik hitches gripping a very old - presumably rusty and burred - large diameter steel winch cable if it starts going out of control with several thousand pounds of force on it. Assuming the cable and hoist and anchor system has integrity, I would think a good mechanical clamp brake should suffice. Or... find another way.
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby paul » Aug 21, 2013 6:24 am

I would have though that ensuring the hoist's safety brake was in full working order would be better than relying on a Prusik?
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby Carl Amundson » Aug 21, 2013 9:23 am

Here is an Italian website that shows ascending knots used on steel cables. They use slings not rope for this application.
It even shows a climbing system that is made out of slings that can be used to ascend steel cables.
http://translate.google.com/translate?hl=en&sl=it&tl=en&u=http%3A%2F%2Fwww.eneafiorentini.it%2Ficorsroc%2Fnodroc3.html

This site is wealth of information on ascending knots and all their many variations.
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby rjack » Aug 21, 2013 10:24 am

I've never seen a hoist motor or brake on the actual trolley. Around here the motor and brakes are generally fixed installations which move the cable itself (spooling a smaller cable onto a drum). 2" is enormous. Seems silly to ask but how do the motor and brakes grip the cable in the first place? I am imagining your prussik is going to have alot of wear from that old rusty cable very quick, and especially if you are starting to accelerate.
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby David Grimes » Aug 21, 2013 7:55 pm

I use prusiks on steel cable everyday in my side job (ziplining), we use them for emergency brakes and when we have to perform rescues. We use a small diameter rope on a 1/2" galvanized steel cable but they hold remarkably well, even under a load that opened a petzl trac plus pully like a soup can. With that, I can say that it likely would not work at all for your purpose, a fast moving person on a zipline will move a prusik a good ways before coming to a stop, with the kind of weight your talking about it likely would not even slow you down.
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby OpenTrackRacer » Aug 22, 2013 6:03 pm

Thanks for validating the insanity of my idea. Everyone's comments pretty much echoed the voices in my head. Oh wait, that sounded bad...

:tonguecheek:

We'll be rappelling into the mine and have full ascending gear to get back out. However, there is a hoist and it works and hasn't killed anybody yet. We'd love to ride the hoist back up rather than climbing. Due to the age of the system and modern modifications, we're worried about a runaway condition. That's part of the reason we'd only ride the hoist up, not down. Even then, some method of stopping a runaway would be nice.

Any cable grab would be at the surface either below the main sheave or between the sheave and the hoist drum. My initial research has turned up small grabs but nothing so far that looks like it could stop and hold several thousand pounds (and nothing for that large of a diameter cable).

I wonder if a prusik made out of the proper material could even hold the weight? Theoretically, if the prusik was below the sheave and had a minder above it, it would start to grab the moment the cable reversed and started to descend (just like progress capture on a haul system).

The voices in my head are back...

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Re: Crazy talk! Using a rope prusik on steel cable?

Postby rjack » Aug 22, 2013 6:24 pm

Why not ride the hoist, but have a seperate static line along which you slide an ascender as the hoist lifts you? If the car were to suddenly reverse and fall back down into the shaft, you'd end up hanging on your ascender. Maybe not an ideal situation, but do-able.
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby OpenTrackRacer » Aug 22, 2013 7:22 pm

The hoist car has a roof and there would be three people inside on each trip. Also, having a rope in the shaft that close to the moving car and cable would probably add danger rather than reduce it. I would not want to ride the hoist (even without a roof) and be tethered to anything external. If you get snagged, the hoist is barely going to slow down as it rips the offending limb off.

rjack wrote:Why not ride the hoist, but have a seperate static line along which you slide an ascender as the hoist lifts you? If the car were to suddenly reverse and fall back down into the shaft, you'd end up hanging on your ascender. Maybe not an ideal situation, but do-able.
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Re: Crazy talk! Using a rope prussic on steel cable?

Postby caverdan » Aug 23, 2013 11:22 am

I always ask myself......what's the worst that could happen? :yikes: :shrug: :yikes:

Why not weight down the bucket and try to stop it? The only way to truly know if your idea is plausible is through experimentation. If you try webbing......I'd go with a 2" trucker strap. :big grin:

(I can see an episode of Myth Busters on the horizon.)
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Re: Crazy talk! Using a rope prusik on steel cable?

Postby rjack » Aug 28, 2013 1:18 pm

OpenTrackRacer wrote:The hoist car has a roof and there would be three people inside on each trip. Also, having a rope in the shaft that close to the moving car and cable would probably add danger rather than reduce it. I would not want to ride the hoist (even without a roof) and be tethered to anything external. If you get snagged, the hoist is barely going to slow down as it rips the offending limb off.


Good point.

You could send the hoist down by itself and rappel on your own though. Only riding it up gives you 1/2 the hoist exposure time.
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