Cable car disaster in Italy 3

[LittleInch]

Not a huge amount of detail at the moment but a dreadful tragedy and difficult to understand how this could happen. The factor of safety on a cable car cable must be huge as it is so vital.

https://www.theguardian.com/world/2021/may/23/at-least-eight-dead-in-italian-cable-car-accident

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[LittleInch]

Well I'm not sure I really wanted to see that and at least glad they fuzzed out the faces.

But there sure was a huge force on the tow cable in the reverse direction such that the cabin was violently flung backwards. I guess several hundred metres of cable might supply that much force as it sags between the towers, but it seemed more than that for some reason.

Horrible horrible incident.

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[IRstuff]

Is the towing motor uphill or downhill? I'm thinking uphill, and that the towline snapped because it got jammed on the downhill side and the tow motor kept trying to pull the uphill side upward, but the cable wouldn't move, so it stretched until it snapped. This accounts for the extreme acceleration after the breakage, such that the car was turned 45 degrees, until the tension on the cable reduced enough for the car to catch up and level out. The angle would indicate that the downhill acceleration was on the order of 1g for almost a second and a half.

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[LittleInch]

I think for a two car system, the motor is often at the bottom. Hence the car coming down helps pull the car going up if that car is heavier or controls the car coming down if that one is heavier.

This helps in preventing you having to send a big power cable all the way to the top and easier to maintain the motor.

Any big cable car like that that I've been on as you get off at the top it slowly moves up the ramp as the weight in the car reduces and the line stretch is reduced a bit.

Still doesn't explain quite why there appears to be so much force pulling the car backwards, but you have a long weight of cable it is pulling up the hill I suppose which rests on the pulleys. So there would be a significant force, but not one which it hadn't seen for its 50 years of operation.

I've not seen any information on the cable released, but as the main people seemingly responsible are in prison (or at least one is) awaiting trial, this might not be seen until then.

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[LionelHutz]

I think for a two car system, the motor is often at the bottom. Hence the car coming down helps pull the car going up if that car is heavier or controls the car coming down if that one is heavier.

That makes a lot of sense. It appears the uphill cable broke on the way back down the hill, not in the station. Based on when the broken end reached the upper station, the break was maybe somewhere between the station and the first tower down. The upper pulley just being an idler would explain why the cable was able to reverse direction and pass backwards through the upper station easily.

Originally the cable went from one car up and around a drive pulley (or pulleys) at the top and then down to the other car.

The cable that seemed to cause the initial downwards acceleration was always going down, period. I don't know why you're finding this so hard to understand when it can clearly be seen in the video. I also don't need a grade school education on how the upper cable balances the weight of the cars.

 

[IRstuff]

It took 2/3rd of a second for the car to be tilted about 45 degrees; that is not a pure gravity load. The only thing in the system that can apply a load that quickly is an extremely high tension on the tow cable. It takes nearly two seconds before the car is upright again, at which time it should only be under gravitational acceleration.

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[LionelHutz]

No, it wasn't just gravity. Isn't the travel distance about 800m and going uphill? The amount of pull from holding up that 800m of tow cable shouldn't be ignored. I'd take a guess that the tow cable length from the bottom to top could easily weight 15,000 to 20,000lbs.

 

[waross]

A cable that long must grow and shrink a considerable amount with temperatures.

One system that I have seen has a very large diameter drive wheel with two grooves. The cable will go around one groove and then around a similar diameter, sliding idler wheel a few diameters away. The idler is slightly tilted so that one edge lines up with the bottom groove on the drive wheel and the other edge lines up with the second groove.
The serves the dual purpose of increasing the contact area and thus the friction on the drive wheel and the idler wheel maintains a constant tension on the cable.
The idler may be held against the tension of the cable by a weight and multi-part block arrangement.
If the drive is at the bottom, then the upper cable is needed to let one car pull the other car upwards as the first car is pulled downwards.


Bill
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