Ideas sought for arresting the fall of a large mass

[RobertCasey]

Hi
We are developing a machine that involves a mass of the order of 300-400 tonnes suspended on cables. The mass will be lowered at a constant acceleration of about 0.5g to a maximum velocity of 12m/s. We need to have a secondary system that is able to trim the velocity of the mass should it exceed the maximum velocity, or to arrest its fall should the acceleration exceed the design value (i.e. if one of the suspension cables fail).

Does anyone have any ideas about how this could be done? We are thinking about secondary ropes that reel from drums at the top of the shaft and which are monitored and braked if neccessary, or maybe using caliper brakes just like a lift. Is there anything else that might be suitable, or has this situation been solved somewhere else already?

Any help is greatly appreciated.

 

[ornerynorsk]

You could check out the design of safety brakes on elevator systems. I hear that the Taipei 101 project is using ceramic-shoed brake pads, with velocity in that same range you're looking at. Good luck!

 

[JamesSmith]

There is a system employed in some elevator systems that involves some sort of clutch... if the elevator falls, and a part of the device begins to spin too fast, drag of some sort is created.
I do not know what exactly the device is, or exactly how it works, but it is something you can checkl into.

Crude idea - if this involves a true potential emergency, one crude back-up might be a parachute.. probably a dumb idea, but it never hurts to think WAY outside the box just to get a new direction.

 

[nbucska]

what is the max. distance ? <nbucska@pcperipherals.com>

 

[prex]

If your mass is not guided by some sort of rails or guides, then I guess that only a secondary system of cables could be used, as you suggest: however this would be difficult to design for a sufficient reliability.
If a guiding system is present, then you could consider the elevator parachutes suggested by others, or a kind of shock absorber (if the falling height is not excessive) or an eddy current brake with permanent magnets: this is used in the free fall towers in amusement parks and its main characteristic is the very high reliability, as it requires no power nor mechanical actuation to perform its action. prex

http://www.xcalcs.com

Online tools for structural design

 

[RobertCasey]

I have done a few more calculations and I think we can get the mass down to about 280Tonnes. The peak speed will be just under 10m/s and the total stroke about 13m. This means it will drop the entire distance in about 2.7s.

I think that in the event of the mass accelerating outside the design range (either because of a suspension rope failure or as part of normal operation) the mass does not need to be brought to a halt, but it certainly must be prevented from accelerating any further so that it becomes a controlled descent. Caliper brakes do seem to be a obvious choice (which could be triggered by an external monitoring system) and provided that they can generate a total reaction equal to the mass times 1g then it should prevent the mass from accelerating further.

However, this machine will operate every 80 seconds and there is a possibility of the weight accelerating outside the design range every time. I would be worried about heat build up/premature wear that a normal caliper brake would have to endure and so I am wondering if there is any other system/material that might be able to generate such high reactions quickly and repeatedly without problems.

 

[Hush]

This might come across as a stupid idea because of the mass and stroke involved but how about some form of air or hydraulic damper? A piston or series of pistons with sized orifices.

 

[GregLocock]

Is the acceleration important? If you really can't accept more than 0.5 g then you need a counterbalance of half the mass, or some fancy leveraged smaller inertia.

To limit the velocity reliably I'd look at Hush's idea.

Probably we'd all be a bit less vague if we had some idea of what you were trying to do. Cheers

Greg Locock

 

[cncprogramming]

I would apply secondary independent hydraulic energy absorbing sistem (washers with precise calibrated holes)
This sistem must be mounted on separated bases and joined
to heavy weight(Cables) with two degree of fredom.

 

[RobertCasey]

The mass would form part of an amusement device - a giant catapult - with the rope from the mass passing through a pulley system to the accelerate another part of the device. At the end of the stroke the mass needs to be decelerated and stopped relatively quickly. We have looked at eddy current brakes (too many needed) and now we are concentrating on an air piston arrangement and hydraulic buffers. I think that trying to 'trim' the acceleration of the mass during the stroke is not really practical so we will deal with the excess speed that will be generated at the vehicle in another way.

 

[Shortstub]

Most the suggestions, thus far, involve vertical constraints. The following might sound strange, but how about a &quot;TOOBS&quot; or Try-an-Out-Of-the-Box-Solution {always wanted to say that}? Best analogy I can give is that of falling tree or falling dominos (or is it domini) in a chain, but in reverse.

Think of a column, of suitable mass, free to rotate about its base and somehow connected to the falling mass. I'll leave the somehow part to you. Now you have the kinetic energy of a rotating mass, controlling the rate of decent of a falling one.

BTW, if you aren't aware, O-O-B is the latest buzz word in &quot;Problem Solving!&quot; I added the 'T' and 'S' bits!

 

[chicopee]

Dynamic braking is a possible solution if you decide on an arrangement of a cable and drum system. Modern elevator technology uses dynamic braking to slow descents and ascents before reaching designated landings. Brakes then are energyzed to hold elevator cabs in place. Elevators have flyball governors that will engage emergency brakes and may not be practical if the braking system is used over and over.

 

[Shortstub]

RobertCasey,

I meant to say a column pivoted, not rotated, at its base, like a felled tree, only it can be initially positioned anywhere in its 360 deg range of motion, i.e., +Y, -Y +X, or -X, and 0,0 is the pivot point!

Now, go out and amuse!

 

[stageleft]

I don't if this will be of help but I've done something similar on a much smaller scale (1 tonne out of balance on a 7 tonne counterweighted vertical door). I assume you have rope drum driven by a brakemotor and gearbox arrangement to lift this beast. On the &quot;down&quot; cycle, try back driving the suitably efficent/inefficent gearbox and sticking a hydraulic gearpump on the back of the drive motor. Use a one way valve so hydraulic circuit is only effective during lowering cycle. In &quot;down&quot; cycle pump fluid thru a circuit of course and fine control needle valves (switch between course and fine control mechanically, say travelling nut on lead screw chain driven off drum shaft to actuate a change over valve). In my case this was the primary system of decent control and had to work in power fail conditions (fire safety curtain), so may be you need a governed system before your system kicks in. Depending on your duty cycle you may also need a heat exchanger (or very large oil reservoir) and watch for pump cavitation.

 

[sancat]

Hi,
After two weeks, I would like to add an idea,
What about using a motor and a regenerative variable frecuency drive to absorbe the energy in the system. In that way, you can give back to the electrical circuit the energy to be disipated, and you can control the amount of torque to be used and thus the tension on the steel rope. I have not done the numbers on the economical meaning of the electricity, but it sounds like an interesting idea. Another posibility is to use the energy to move a big blower, since the blower will have a cubic law against speed, it will achieve equilibrium with your falling system at some point (with some calculations, not so fast, I expect)
sancat

 

[FourJawChuck]

If you made the guide rectangular and the guide hole for the mass to match you could use two spring loaded channel beams made so that upon emergency application the spring pressure would force the two beams apart and through friction stop your load, high phosphorus cast iron shoes are used on rail systems against steel rail with good service life. You could use any method of collapsing the two rails together to allow the load freedom of movement.

Think of the spreading opposing rails used on other amusement park rides such as roller coasters used to retard car movemement as an example.

 

[RobertCasey]

Thanks for all your suggestions - some very interesting ideas. The problem with such a huge mass was that all the ropes/drums/sheaves/governers/motors etc that are needed are huge and are all heavily contributing to the inertia of the system. The retarding forces needed to trim the acceleration/speed of the system were so large that we were worried about heat build up, shock loading and wear on any component that would be used so frequently. Therefore we have managed to reduce the mass down to a &quot;reasonable&quot; 250T and have abandoned trying to control the speed - that will be done directly to the train after the launch. If the cables break then we have managed to find some powerful caliper brakes that will at least prevent the mass from accelerating away (the high phosphorous brakes suggested by FourJawChuck were interesting), and we have spoken to a company who say that they can supply hydraulic buffers to catch the weight at the end of every stroke. So, I think we have just about got to a solution. Of course if anyone else can add anything, I am still open to suggestions.

 

[Shortstub]

Robert,

Is there a relation between your project and the new thrill ride &quot;the Sky Swatter&quot; in Utah, mentioned in the April issue of &quot;Popular Mechanics?&quot;

 

[RobertCasey]

No. That ride was designed and built in the US by S&S. We are based in Europe.

 

[Speedy]

This is just 'Food For Thought' but have a look at how they raised the Kursk Russian submarine. They employed hydraulic pistons under the pulley wheels, this avoided shock loading due to the wave motion on the ship etc. Seems to be over elaborate for your application but a neat bit of lifting all the same.

Cheers Speedy

&quot;Tell a man there are 300 billion stars in the universe and he'll believe you. Tell him a bench has wet paint on it and he'll have to touch to be sure.&quot;