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Dynamic Balance


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#1 Jerry Holway

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Posted 12 January 2006 - 11:54 AM

Here's an alternate and possibly faster, more simple way to find dynamic balance:

Start with the camera c.g. directly over the centerpost!

Balance roughly with the battery.

You will not be in dynamic balance, but you know that you must move the battery in and the camera back to get into dynamic balance.

(read the dynamic balance primer, available on Tiffen's or my website, if you don't know why and want to...)

Move the battery in by increments (maybe start in 1/2" increments or so), and rebalance with the camera and test spin.

Repeat until the sled pans flat.

This should take very little time, and with experience with your rig, work even faster.

Hope this helps.

Jerry
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#2 Alec Jarnagin SOC

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Posted 14 January 2006 - 01:05 PM

This method works VERY well. It is the way I've always done it as I find it much easier on the brain. 1/2" increments may be too much depending on the type of set-up you use. I use Jerry Hill's battery base with a TB-6 (on an XCS sliding bracket) which keeps the monitor low in comparison to the batteries. Since I have a worm gear for the batteries, I'm just giving it a few turns on top and bottom and then a test spin.
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#3 Eric Fletcher S.O.C.

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Posted 15 January 2006 - 01:45 AM

I use a different method that works very well for me.

Balance the rig statically as normal.
Set your drop time.
Now tilt your rig 90 Degrees like when you do for a drop test. Pan the rig 90 degrees so the battery rack and monitor are horizontal. Now do a drop test.
If the rig rolls to Monitor side move the batteries out or the monitor in. If the rig rolls to the Battery rack side move the monitor out or batteries in. Retrim for headroom and do another Panned drop test.
When you get it to drop flat, spin it... It should be dynamic

I've been doing it this way for several years and it works great.
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#4 Jerry Holway

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Posted 15 January 2006 - 01:33 PM

Alas, Eric, all you are doing is getting really, really good static side to side balancing. Many years ago I proposed this same solution to myself, thought I had "the answer" and then realized sadly why it was not so...

I only wish Alec had told me of his method earlier....

Jerry
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#5 Alec Jarnagin SOC

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Posted 15 January 2006 - 02:29 PM

"I only wish Alec had told me of his method earlier...."

Jerry,

It never occurred to me that you hadn't been doing this forever and a half. I think I started doing this after an article YOU wrote (maybe GB). I remember thinking the gist of it all was to keep the CG of the camera just behind the center post to compensate for the monitor resting above the batteries. It seemed a logical way to approach it all to simply mount the camera with its CG above the post (or just behind) and use this as a starting point. Another approach is to leave your sled configured ever so slightly front heavy and then throw the camera on and after you have done a static balance (which now requires the camera to have been moved to behind the post), you'll be close. Like you said, once you get to know your sled, it is easier. When I switched from a 1.5" post to a 2", I found that I was off a bit because the larger post gives the illusion that the weight is still over the post when in fact the camera's CG has been moved back (you really have to think about where the center of the gimbal is versus the post).
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#6 Charles Papert

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Posted 15 January 2006 - 03:27 PM

What I would really love is an empirical explanation/solution for the "gremlins"--particularly the one where the rig is all lined up properly, no component rotated out of square, and yet during the spin it precesses to the side rather than fore and aft.

I just hate that.
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#7 Eric Fletcher S.O.C.

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Posted 15 January 2006 - 03:39 PM

Alas, Eric, all you are doing is getting really, really good static side to side balancing. Many years ago I proposed this same solution to myself, thought I had "the answer" and then realized sadly why it was not so...

I only wish Alec had told me of his method earlier....

Jerry



When I do it my way the rig spins flat and that by definition IS dynamic balance.
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#8 Jerry Holway

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Posted 16 January 2006 - 11:38 AM

What I would really love is an empirical explanation/solution for the "gremlins"--particularly the one where the rig is all lined up properly, no component rotated out of square, and yet during the spin it precesses to the side rather than fore and aft.

I just hate that.


Chas-

Me too.
Chas, I know you know all this stuff, but here are some ideas for anyone with this sort of problems:

It may look all square and nice, but each part that is not attached to the camera may not have its c.g. in its physical center. Remove components and test, or balance with a practice cage, carefully centering it over the post side to side, and balancing by wagging the battery or monitor slightly side to side if your rig permits this. Otherwise add small weights to one side or the other of the monitor.

You may be slightly off balance fore/aft or side to side. Try balancing top to bottom lightly (long drop time) to minimize these effects.

Inexepnsive, cheaply made, or damaged equipment - especially in the gimbal ? may also exhibit these problems, as would anything that could move.

Eric, you method seems to work fine for you and your rig, I can't (and shouldn't!) argue that, so continue to use it. However, your method will not work for all rigs in all configurations. Changing the height of the monitor will not result in a change in your sideways drop test, but it will greatly effect dynamic balance.

The whole discussion of dynamic balance is way too long for this forum, but the methods we propose will work for any rig.

Again, I suggest others reading this thread take the time to read the dynamic balance primer.

Jerry
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#9 RonBaldwin

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Posted 16 January 2006 - 02:40 PM

let's have a "Balance Off". First to spin balance flat wins...uh...more beer! Maybe at Dalt's after Cine-Gear this coming June? We have Steadicam masters, but do we have Drunken Steadicam Masters?

Ron B
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#10 Charles Papert

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Posted 16 January 2006 - 05:26 PM

Ah, but imagine how ugly it could get--one man's flat spin may be another's "not good enough". Magnifying glasses and calipers would be produced out of back pockets; laptops displaying 3D models of the spin pattern would litter the bartop and eventually, fisticuffs and buggery in the parking lot. In other words, the usual Steadicam Guild get-together.
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#11 PeterAbraham

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Posted 21 January 2006 - 04:02 PM

Fisticuffs and buggery?? Dammit to hell.

I gotta move to L.A. :D

Yeah, a bad bearing in one of the two races might allow for static pleasures and dynamic nightmares. Also ( and I hesitate to say this because I don't mean offense to anyone more experienced ), panning the rig perfectly flatly to begin with is a somewhat humbling experience for newer Ops. You can balance as carefully as possible, and blow it in the pan.

If one is positive that one is exerting perfectly even rotational forces on the centerpost as one is spinning, then all other things are brought to bear. If one is unsure, then one may get a false negative, as they say.

Peter Abraham
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#12 Oliver Roetz

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Posted 10 March 2006 - 05:51 AM

Hi everyone,
again, it's about dynamic balance. I searched the forum but could not find any helpful information.
The procedure of achieving DB is absolutely clear and has not been a problem for me so far.
But now I have to do some TV work with a triax cable and zoom remote. Fast pans are to be expected. Since I can't spin the wired rig for obvious reasons, I was thinking of balancing the rig statically and dynamically without being wired at all(no triax, no zoom, no tally light). Then I would connect all necessary cables and rebalance to be as close to DB as possible.

Am I right? Or would you guys do it in a different way? Any help is highly appreciated!


Keep flying safe, Oliver
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#13 jay kilroy

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Posted 10 March 2006 - 08:19 AM

Oliver,

You're absolutely right. I work with triax all the time and that is exactly how I balance. Do you have to attach the triax right to the camera or do you have a nice piece of thin flexible cable you can use? I have the Tiffen triax set up with my rig. The triax comes into my adaptor on my vest and then from my vest to the camera I have a great lightweight, very thin and flexible cable I use. Shoot me an email and I can send you pictures of my set up.

jay kilroy
jkilroy@steadicam-ops.com
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#14 Marc_Abernathy

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Posted 11 March 2006 - 02:14 PM

dr. Kilroy,

id be interested in those photos also.

DM2 (at) marctronixx (dot) com
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#15 Oliver Roetz

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Posted 15 March 2006 - 02:59 AM

Jay,
thanks again for your mail. It turned out to be a heavy set up but everything worked very well. Drop time was about 4 seconds and the rig was pretty easy to handle. The images are not really the best but maybe you just have a look.

Keep flying safe, Oliver


Attached File  Setup1.jpg   117.85KB   795 downloads
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