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Formula for Inertia Calculations in various axes?

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#1 Chris Poynton

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Posted 14 December 2012 - 01:07 AM

The handbook section on antlers throws around a lot of inertia calculations in terms of "pounds per square inch" (lbs/in2) for various rigs, and antler setups.(p.254-5 in first edition)

e.g. The 6ft super antlers (approx 8 inches above the camera) increased roll inertia by nearly 7,000 lb/in2

Could someone clarify what distances you need to measure (heights or radii?) and the formula for calculating inertia in multiple axes?

I am guessing this is a complicated spreadsheet requiring lots of rig inputs, and it may have been generated somewhere in the deep dim past. I attempted to map some of the variables in the attached photo but it obviously becomes "Dynamic Balance formula on steroids"

And a supplementary question: ... the inertia measurements look enormous and seem very abstract. Do they actually relate in a meaningful ratio to the measure of actual pressure being applied by the fingers on the post?

Such actual numbers are a academic when doing mods in the field, where it all comes down to "feel" and "rules of thumb", but this an aspect of steadicam theory that has not had a lot of coverage, so I am just putting the question out there.

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#2 Jerry Holway

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Posted 14 December 2012 - 09:29 AM

And to Chris, who asked a reasonable question:

Different people learn in different ways. The numbers are there to give you an idea how one of the most basic aspects of a Steadicam's design works, and a way of comparing effects - say of doubling a weight at a certain distance vs. doubling the distance. It's just about understanding the tool you have in your hands.

I could have just said, more weight at a greater distance adds a lot more inertia, but someone else would ask "how much" or "why," and I saw it as my job to explain things a bit more fully.

Distances are measured from the axis of rotation (X, Y, and Z through the gimbal center). The pan axis, for instance, is the axis running through the center post, and the distances are measured horizontally to the weight. The height of the weight above the gimbal is not a factor in the inertia for pan - but it is for tilt and also for roll.

Calculating inertia should not be academic when designing a Steadicam system, nor in making dynamic balance calculations, or stress and materials analysis, etc., all of which require some math. Some of that math might be of interest to some operators, to others it isn't. If you do mods to your rig, you should have a basic understanding of how it works, and be able to do some of the calculations yourself.

Yes, you do feel it at the fingertip level, and you see it in your operating.

Very straightforward! I hope this helps.

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#3 Ian Thomson

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Posted 14 December 2012 - 08:34 PM

one of the things that I greatly appreciated during the lake arrowhead workshop (april 2012) was the focus on "less-book" "more-practical" in regards to balancing and operating.

don't get me wrong, I think that the book is fantastic (especially for someone who is in a smaller market with less operators to hang out and drink/discuss with), but the over-riding advice that I took away from the workshop was... "do"

try setting the rig out of dynamic balance, trim for headroom, fight the "incorrect" settings and see what it feels like. shorten the distance between monitor/batteries, feel how that works... spread out the mass and see how that changes things...

of course using "the math" may not be high on the list for some operators - and others may benefit greatly. but an understanding of how the math works is of a great help to me when shifting stuff around on the rig to increase/decrease the feeling of pan speed etc...

loving the discussion that comes from questions - I'm still learning, and will always be :)

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