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Dolly Accident...Caught on Video!


Karl Lee

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wow, that is dramatic! Might the problem have been that the grip at the very back of the dolly could not stay back far enough on the swing around because of the subway tracks? The farther back he can stay the more counter weight he provides. So at the beginning of the shot he has all the counter weight room he needs, but as the dolly moves on the track and he swings around, he can only go back a limited amount so he has less counter weight to offer.

 

That is one complicated math problem to figure out on the set. Was that a rehearsal or an actual take? I am curious if there are prior takes to what we are seeing. If there are prior takes and the dolly did not fall over, than that might mean the tolerance on the back end was so tight that simply anticipating the move of the dolly just a moment too soon versus being a counter weight a moment longer was all it took to cause the dolly to tip over.

 

Plus I noticed the track was elevated with wedges and that could have had an affect as well on a swing around.

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I see two triangles, first; camera to grip to point on floor at tracks. The second triangle; camera to point on floor at tracks and then straight up to top of the crane (perpendicular to the floor's point). I believe (imho) that the math of the second triangle is to blame for the fall. As the distance between camera and the point on the arm of the crane expands out the force between the line from camera to track's point increases. I believe the solution would be more weight at the base of the crane to justify the extension of the crane's arm(with weight of camera operator)

 

While it's true extending out the grip might help, his position is more of a fluid up and down of the arm not stability of a crane.

 

I also agree that even an empty test run might (should) shown this instability where grips(human strength) might of caught the weight instability before catastrophic failure.

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p.s. the math should always be done on the most extended out point of the move and that's why I see this as a two-demension problem not accounting for the swing. the beginning is just in a more stable position and I don't believe momentum had any factor in this example.

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Clearly not enough weight in the back, as I was watching it I thought, hmmmm, not many weights on the back of that thing. When I've seen that type of crane used there are typically a lot more weights. As the crane extends out and the load gets further from the centre, physics takes over, and the rig goes over.

 

I always make it a policy to personally inspect any rig that will go over the top of an actor. If a camera fell off the crane and landed on the actor, well I can't image what devastation that would cause. So once the rigging is done, I'll ask the ACs to walk me through the entire set up. I want to see the safety chain going from the camera direct to the crane structure. There's no such thing as being too careful in these cases.

 

R,

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p.s. the math should always be done on the most extended out point of the move and that's why I see this as a two-demension problem not accounting for the swing. the beginning is just in a more stable position and I don't believe momentum had any factor in this example.

 

That's what is so tricky about this. If full extension happens before the grip can be completely behind the rig, then the counter force provided by the grip is much less. But then this brings up another question, what percentage of counterweight is the grip at the back responsible for versus using more counterweights as Richard Boddington suggested. Are there actual standards?

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Although it's a little bit armchair CSI...

 

The unit is obviously heavy on the operator end of the arm, that combined with:

 

- the rotation of the unit had just aligned the arm with the direction of motion

- the continual rise of the centre column

- a bit of dodgy track, maybe a joint or wedge fail - (the 'pop' at 28 secs... - within 1 sec the unit stops on the tracks and over it goes ?)

 

Did someone mention motion control - or at least operator on remote wheels and focus?

 

And as David mentions, lower it's CoM - and/or extend it's natural pivots (the dimension of the base) - >edit:as Bruce suggests

Edited by Chris Millar
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That was a needless accident. Due to overconfidence.

The crane providor should give data tables for the hard ballast needed for any load (operator and camera kit) to be safe at any position. The fulcrum or pivot point will be the centre of the dolly wheel unit nearest the camera.

If the grip hates reading or is really bad at high school math he can do a dry run using a stand in mass for the camera and operator loads.

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I haven't seen one of these things used in Toronto in a long time now, everyone uses remote heads, no need to have a human operator out on a crane.

 

R,

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I haven't seen one of these things used in Toronto in a long time now, everyone uses remote heads, no need to have a human operator out on a crane.

 

R,

 

Interesting point. I never thought about remote heads from a safety standpoint until now.

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The fulcrum or pivot point will be the centre of the dolly wheel unit nearest the camera

Neat! another physics discussion :)

 

I would say that if the wheel locks up the pivot point actually runs along the circumference of the wheel.

 

If the wheel is free to turn then again it's a moving target, close to the wheel axle as you point out, but not perfectly centre (conservation of momentum, friction etc...)

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I'd love to see the movie, but... facebook. Is it anywhere else?

 

I'm using a raher slow Windows XP pc ( with IE ) Phil, and have to wait a while for the video to load ( ... I even get a 'do you want to run this script' prompt - click yes ) and it eventually plays.

 

I'm not on facebook and it's openly accessable.

 

John S ^_^

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what percentage of counterweight is the grip at the back responsible for versus using more counterweights as Richard Boddington suggested. Are there actual standards?

The grip shouldn't be providing any counterweight. The crane should be balanced with weights, not with people. The Grip is there to guide it, nothing more.

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The grip shouldn't be providing any counterweight. The crane should be balanced with weights, not with people. The Grip is there to guide it, nothing more.

 

How is it possible to have the weight counterbalanced so that in both the high or low position the grip is not needed? I agree the grip should not have to put too much force to keep it from moving up or down too quickly, but a grip is back further than the the counter weights and a person weighing 150 pounds or more is still a significant amount of counterforce back there.

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You balance the weight on the front end due to the camera, operator, etc. with weights at the back end to get as close as practical to equal counterbalance but obviously you need a grip to manipulate the arm and safety it, but he isn't putting real weight on his end with his body - ideally he could let go and the arm just gently floats slightly up or down.

 

The problem here was that the rig was overall too heavy for the base, the center of mass was too high to be safe -- as you swing the arm around, there are forces involved other than just up or down. In theory, perhaps a perfect balance could be achieved with the center of mass being higher up the base, but to be safe, you can't rely on perfection, nothing with a human being at one end of the arm can be that stable, even extending his legs could create some instability during a swing.

 

A long time ago on a low-budget affair, I was on a dolly during a 360+ degree tracking shot, a very fast move around and around a tree -- the director suddenly asked the grip to boom up on the dolly arm and the whole dolly just hopped off of the track due to centrifugal force when the center of mass went up.

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Don't know how these things are 'supposed' to work, but definitely not enough counter balance and/or support to counter the torque exerted by the guy+camera.

 

While a remote head would have eliminated the operator's potential injuries... the same principles apply and the camera could have toppled into actors at the final position, if there were any, if sufficient consideration for the change in effective 'lever arm' along the path of movement...

 

Personally I hated engineering statics and dynamics... but apparently it does come in useful at times... even if only dimly remembered...

Edited by John E Clark
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even extending his legs could create some instability during a swing.

Yip, and it's what is what he did when he got the feeling things weren't going to plan and unfortunately in the exact direction to exacerbate the situation.

 

While a remote head would have eliminated the operator's potential injuries... the same principles apply and the camera could have toppled into actors at the final position, if there were any, if sufficient consideration for the change in effective 'lever arm' along the path of movement...

Very true a remote head wont solve that. (a competent grip should)

 

Still though, it's obviously preferred in terms of safety. A remote head will have a lot less weight, and wont move around so much ... Also all those fun issues that come with the operator mounting and dismounting the jib.

 

It looks awkward also, so maybe even the arguments of being hands on vs. wheels fail to make the case valid?

Edited by Chris Millar
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