Truss Wobble with Unbalanced Movers

[Seano]

If feasible, might this help things?

 

It would help things enormously.

Actually, if the distance between trusses were short enough you could achieve similar effects with a couple of scaff tubes between trusses.

 

The downside is practical: you lose the ability to fly the trusses independently of each other - this may mean you need a bigger motor controller.

More importantly, for example, if there's a bit of set in the way of the upstage truss on the load-out, you can't get on with derigging the downstage truss while you wait. (Thereby wasting valuable <insert vice of choice here> time..)

 

But you don't need a box truss for extra stability - any kind of 2d layout (ie: anything other than a straight line) will have something of the same effect. You could have a 'T' or an 'L' for example.

 

Sean

x

[Cormac]

Hiya,

I find putting in an extra motor pickup near a midpoint on a truss helps to reduce the problem your talking about.

My example would be on a standard 40' truss rather than using two pickups use a third,50' truss use four rather than three,

 

Seems to help,

 

Cj

[Bix]

Matt: Thanks for the comments, unfortunately screens are invariably set mounted.

 

Seano: Will try my best to get some funky shapes going! Not always possible in all venues but seems the most practical answer.

 

Cormac: The truss in question is actually normally on 5-6 motors!

 

Thanks guys!

[Tomo]

The cause of the wobble is (as stated by Sean) rotation of the truss along an axis parallel to it's length.

- We'll call this axis X.

 

This significant rotation happens because the truss has a very small moment of inertia about the X axis.

To reduce this, we need to increase that moment of inertia somehow.

 

Moments of inertia depend on two things - the bits of mass (eg your lights, the truss) involved, and the distance of those bits from the axis.

Note that you can have a very large moment of inertia while still keeping the centre of mass on the axis - stick a light the same distance either side of the axis!

 

There are two ways to solve this:

1) Move the axis further away from the mass particles.

2) Add mass particles further away from the axis.

 

When hanging Apex-Up, the classic wrap where you go around the top chord as well as the bottom chords creates two possible axes of rotation - the top chord itself, and the shackle.

 

Obviously, the top chord is very close to the mass (your lights)

 

By avoiding the top chord in your wrap, you move the axis to the shackle - and if you use a long basket, this can be quite a long way from the mass.

 

You could also bolt something perpendicular to the truss with weight on the ends.

 

There are lots of other ways to increase the moment of inertia, so you should be able to find something.

 

Alternatively, if you can pick up the truss from points that are not in a straight line (eg T shaped truss etc), you've essentially eliminated the axis entirely as the truss would have to slacken one or more chains to wobble.

 

It could still sway of course!