Mirror ball drop

[Deathride]

Ok the mirror ball motor is going to be mounted to some plywood 3/4 the rope were using will be Professional climbing rope the ball needs to be dropped quickly and then taken off stage the mirror ball is 50 cm diameter the rope will have one knot for the pulley and a fail safe which will be tied to a scaff bar which will be linked to the whole set so :D I would want to know a way how to minimise the jolt from the sudden stop any suggestions will be grateful ( I have had doubts about this from the start but someone ( the director) has said it'll be fine

[Darkfold]

It will be safe to do as long as there is noone on stage and every part of the system is rated at double the shock load generated by instant stop of the mirror ball.

 

Climbing rope: This is only rated for a single shock stop, after which it must be replaced.

3/4 Ply... God alone knows what this will hold without testing it first, as it all comes down to the glue.

Mirror Ball: Most balls are not rated for repeated shock stops. The expensive ones will be, but not a good idea.

 

Better way: Pulley system running from ball to side of stage (multiple pulleys required)

Experienced flyman has the rope at all times and controls a quick descend. This allows you to stop the ball reliably and will give a controlled stop over a longer time period than shock stop at the end of the rope. Longer time = Less sudden force applied and so the effect can be repeated more frequently (read as more than once, but not necessarily on the same night)

 

Ball fixing and recovery: Send on a crew member who you trust disguised as a twirly(actor), get them to do it.

 

Big problem: watch Austin Powers: Gold Member. On the submarine they drop in a model of the Earth and a Meteorite. Watch it, cringe, and realise why we don't want you to do an uncontrolled drop with a heavy ball.

[Andrew C]

It will be safe to do as long as there is noone on stage and every part of the system is rated at double the shock load generated by instant stop of the mirror ball.

 

Mirror Ball: Most balls are not rated for repeated shock stops. The expensive ones will be, but not a good idea.

 

Out of interest, where do you get the "double" figure from?

 

I'll bet that no mirror balls are rated for anything, let alone a shock loading!

[Darkfold]

Double comes mainly from "It happens"

 

Rating at double the shock load (as in twice the instant decelleration force required) will give quite a lot of leeway and ensure that the ball (and everything it is attatched to) is well within the tolerance of the system.

 

Aye, I doubt it too on the rated mirror ball front, but you never know, some manufacturer may have come up with it...

 

Also would like to reemphasise that Climbing rope MUST be replaced after being subjected to a shock load.

[Bryson]

We take you now to a quick message from the management....

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[j_b]

Out of interest, exactly how far are you planning for this ball to fall?

 

What do you anticipate the velocity of the ball to be the instant before the rope snags? What is the mass of the ball? As Energy = 1/2 x mass x velocity² it could be interesting to work out how much energy is involved here...

 

 

Seriously, your username suggests what one outcome of this exercise might be, and please please, use some basic punctuation! I find it extremely difficult to understand what you are saying - initially, upon first reading your post, I thought that you had said that 3/4 of the rope would be professional climbing rope (whatever that is).

[paulears]

As for actuallydropping the mirror ball. I can't find one source of mirror balls that has a suspenson point rated for a drop. The other thing is that tiles fall off these things so easily when you just touch them, let alone drop them, that this is rather a difficult thing to assess, as to safety, and as a result, shouldn't be done.

 

My physics classes (which admittedly are somewhat in the long distant past) seem to indicate that the force resulting from a drop and sudden stop will be more than your double figure. Your physics classes are more recent than mine - what maths did you use?

[Darkfold]

Aye, they are more recent.

 

Sorry if not clear,

 

(FinalVel)squared = 0 + (2*9.81*distance falling through in metres)

 

squareroot(answer from above) = deceleration required for [edit]: one second stop

 

Force= mass of ball*deceleration

 

Rate the rig to stop [edit] many times this shock force (whats the UK requirement?)

 

^^ this rating is the one I was talking about using double of for the reason given above. [edit] Have been corrected on this. Look below a ways.

 

If you cant follow through the maths, dont bother trying it. Also, you are correct, the mirrors will probably detatch (had forgotten that one...) and so you are likely to have a bald mirror ball :P :)

I would not want to do this given the option.

 

Edit: original equation used was

(v^2)=(u^2)+(2*a*s)

with a of 9.81

u of zero for a stationary start

^ indicating power of.

[paulears]

Surely in a drop like this, it is the total energy involved that is important? This, if I remember it correctly, means you have to add the kinetic to the potential energy which then becomes mechanical energy. As I said, though, my brain's a bit old now - so I may have misremembered. Whatever.........

 

The more maths we apply, the more answers come up, making the comment "it's safe" become "it's probably safe".

[V.A.]

Darkfold - Sorry to disagree with you here, and you may well be correct (mechanics never was my good module in maths), but you seem to be saying that a velocity is a deceleration.

 

I think you'll also find that Force=mass of ball * acceleration; and in this case acceleration will take a negative value. Force is a vector quantity (it has direction and magnitude), so you can't just go ignoring negative terms!!!

 

You are also assuming that there is no air resistance - Newtonian Mechanics is not really applicable as anything other than a rough guide when you're in the "real world".

[zonino]

the vector can be in whatever direction you define, if you define towards the stage as forwards, the acceleration will have a postive value, if you define up from the stage as forwards it will have negative....

 

air resistance? is this really going to have that large a factor in this situation? over 4 metres?

 

I've had the ring on a mirror ball snap on me before, fortunately nobody was on the dance floor at the time... we now replace them at regular intervals, in your case I'd do this every night

[V.A.]

The vector can indeed be in a direction of your choosing, but the formula is still for acceleration in that direction, not decelleration.

 

Dependant, of course, on the size of the mirrorball, air resistance could be significant.

[Darkfold]

Edit: am checking facts....

 

Have a look at Maccalder's post a while back in this thread, we agree so far...

 

 

Edit again: *DOH* It would be true for a 1 second stop. (accelerate at equal to velocity, but in oppostie direction for one second gives 0 velocity).

 

For 0.01 second stop, factor of 100 increase in deceleration required.

And apologies for getting the rated load capacity requirement wrong, I am not a rigger, just a maths student (ALevel Mechanics) :)

 

Talk to a competent rigger before trying to drop a load as UK regulations are strict for good reasons, in particular, show them exactly what you plan to do. And read the disclaimer at the top of the page/forum.

[bruce]

We can call the final velocity equal to the deceleration, simply because we are stopping it instantly.

 

That, my good sir, is nonsense. Dimensionally wrong as well....

 

Bruce.

[Jivemaster]

You must get the maths right!

 

You can buy "Bungy cord from B&Q (and others I quess)

 

You must define the stages of movement, then you can use bungy cord to allow a controlled deceleration.

 

First the PLAN

Then the Maths

Revise the Plan

 

Then try one drop with NO-ONE even Near,

 

Then you can decide if it is safe.

 

Reduce the moving mass drastically Dont try to drop bits of wood, A "piece of ply " from 4 metres would KILL

 

Do ensure that the ball has a through bolt with a large washer opposite the ring then the loads are spread rather than concentrated on a vunerable thread into the foam.