Truss Wobble with Unbalanced Movers

[Bix]

As most new MACs and other movers don't seem to be centre balanced, I'm suffering from a great deal of truss wobble on award jobs with big mad stings, especially with low trim heights. Unfortunately I can't bridle points to meet the motor so can't stabilise that, but most of the wobble/swing seems to occur below the motor anyway.

 

Mostly using litebeam/minibeam, with the lite variety suffering more due to reduced weight however mini seems to swing for a longer time.

 

I've had a few Vidiots asking me to chill out on the ballyhoos as their lovely projections move around the frame but the PM quickly counters that and wants the movement back.

 

Hanging the AV on separate trusses is sometimes an option but only in a few venues where points/budget allow.

 

Any ideas? Maybe to dampen the mover somehow?

 

Someone once piped up with "Leave the clamps loose", I just smiled politely.

[Simon Lyall]

From my meagre experience (so don't take this as stone) when I was suffering slight movement problems of a bar with two macs on it was worse when they were both 'twisting' together. By inverting the 'twist' so when one went left the other went right is dampened it. I haven't really ever got problems with up/down causing any movement. So if your ballyhooing maybe try having one go left when the other goes right may reduce it for you.

 

The other thing that comes to mind (however I have never tried this) is an arrangement of guy-ropes to steady things, but I can't really comment on the feasibility of this or recommend this due to not doing it myself!

 

 

 

Simon

[Ynot]

I'm afraid you've hit the problem that is trying to use movers on a non-rigid floor-mounted truss arrangement.

By dint of the basic physics of the equation, stands + truss + moving heads = trouble.

 

There have already been a couple of threads about this very subject, not least about whether or not truss mounting is in fact a potential topple-hazard.

[Simon Lewis]

By dint of the basic physics of the equation, stands + truss + moving heads = trouble.

 

Ynot,

 

The OP mentions motors.... and I read it as the suspended truss was wobbling....

 

Basicmonkey, can you decribe exactly how the truss is flown?

[gareth]

... floor-mounted ... stands ...

I think you've got hold of completely the wrong end of the stick, Tony. The OP doesn't say a thing about stands or ground support. In fact, he specifically mentions motors, points and bridles ...

[ceecrb1]

yeah I read it as hanging too...

 

I'm off to have a wee thinky about it!

[Ynot]

I think you've got hold of completely the wrong end of the stick, Tony. The OP doesn't say a thing about stands or ground support. In fact, he specifically mentions motors, points and bridles ...

Hmmm.... ©

OK - after a re-read, I think you may well be right.

Seems I'm on the wrong page here.

 

Now there's a first...............

(Not!!)

 

:(

My apologies to the OP.

[Seano]

When you say 'wobble' I'm thinking 'roll' - ie the truss is essentially rotating about its long axis, rather that actually swinging from side to side (which would involve the motor moving too).

 

Two things spring to mind:

a) How is the truss picked up? You could try picking it up with a 'deeper' basket, wrap or whatever. You lose height, but gain stability. If its picked up on hard fixings, that'll be your problem - try losing them and using a steel wrap or a spanset instead.

b) You could increase the inertia of the truss (in terms of 'roll'). An obvious way to do this might be to rig some scaff horizontally across the top chords, perpendicular to the truss. (Think of the way a tight-rope walker uses a pole.) It'll look odd, but you could possibly disguise the pole by giving it a job to do - dangle a teeny tiny mirror ball off each end, put a couple of birdies on it, a pixelline either side of the truss if you want to get fancy, whatever.

 

If the truss is swinging - trick string is prolly the way forward.

 

hth

Sean

x

[slipstream]

More weight on the truss will also damp the effect, a lightly loaded bar is much more likely to respond to the nodding bucket then one with some extra lamps on.

 

Sam

[Leggy]

I don't know which desk you are using, but if you can damp the path on the p+t channels that can help.

Leggy.

[DaveBeattie]

If you have any flying height spare on the truss I would suggest you might try using a couple of stiffeners. Rig a couple of vertical scaff poles next to the flying points on the truss (connecting to both top and bottom of the truss). The tops of the poles should have flying eyes on them and the winch chains should pass through these eyes. When flown this will move the centre of rotation of the truss up to these eyes and will effectively swap a large rotational movement for a small translational one (more accurately a rotational about a distant centre). The stiffeners should not take any weight. They are simply there to dampen the rotation.

 

I must say that I haven't done this on a truss before, but I have done it on a bar with a couple of PAR 64s and a couple of VL5s. I used three 5'x 1" x 1" steel stiffeners attached to the bar with U-bolts. This prevented the PARs from nodding in opposition to the VL5s.

 

HTH

 

Dave

[GregB]

Just starting with the basics (so not to offend) when I hang a job like this I use span-sets looped through the truss in such away as to leave a very short distance from the truss to hanging point (chain/motor..). The shorter this distance the less swing. it also helps if the truss is hung 'apex' pointing down not up (yes seen that) as this again provides better lateral stabillity.

The idea of rigging a bar perpendicular to the truss I feel would only help exentuate any swing. As said above only more waght in the center of the vertical axis will reduce swing.

[Bix]

Thanks for the replies guys...

 

Simon Lyall: Guys sound interesting but on the tight time constraints already in place I doubt that I'd manage to get them done. As regards P/T it seems to be both tilt getting things shaking more than anything for me. 550s and 700s seem to be very unbalanced, as do Alphas.

 

Ynot: I am indeed talking about hanging rather than ground support!

 

Simon Lewis: Truss is flown invariably from 1/4 or 1/2 ton pros/lodes, often trimmed to the blocks or sometimes with 1m trim.

 

Seano & GregB: Truss is always on fiber/flat spans or steels, always basketed as close to metal as poss, tried various ways on the minibeam but to no avail. Apex down isn't an option on the lite due to space constraints I'm afraid.

 

Slipstream: On my last awards tour I had 1/2 ton of PA, 12 movers and 18 channels of generics on the front truss. Not sure how much else I could fit on! See your point though.

 

DaveBeattie: That sounds like a great idea and has triggered the brain cells into action.

 

Another option I've been contemplating is to sub hang the AV on a short bar on 1m spans below the main truss off the top chords to dampen the roll effect. Might give this a go in the warehouse.

 

Cheers again guys, welcome any ideas...

[Seano]

I must say that I haven't done this on a truss before

I guessed not. It isn't practical for a number of reasons.

Also it isn't necessary - unlike a bar, a truss has some 'width'. So when you pick it up with a wrap (steel rope or roundsling), you are in effect making a 'bridle' between the frond and back chords, with its apex at (or just below) the hook of the motor. This makes a kind of triangular bracing with regard to the truss rolling, and therefore has the same effect as the 'stiffener' you describe, but much more easily. (Which is why it was my first suggestion earlier.)

 

Incidentally, a note on terminology: A motor, such as a CM Lodestar, is a hoist - not a winch.

 

 

Just starting with the basics (so not to offend) when I hang a job like this I use span-sets looped through the truss in such away as to leave a very short distance from the truss to hanging point (chain/motor..). The shorter this distance the less swing.

You're 180 degrees out of phase with reality there I'm afraid - the *longer* this distance the less prone the truss is to rolling about its axis.

 

Also, see above. Regarding box truss, or triangular truss flown apex down:

The truss wrap is in effect a bridle between the front and back top chords (or bottom chords, depending on how its picked up. Slinging a truss to minimise the height of the apex of this bridle above the truss is also maximising the included angle of the bridle, and in turn increasing the tension in the pick up sling along with the compressive force (horizontally) between truss chords. It isn't good practice to allow this angle to exceed about 90 degrees without thinking carefully about what you're doing. (That 'included' angle hits 90 degrees when the apex of the pickup is half the width of the truss above it - eg: when slinging a 12" truss, the truss wrap meets at a shackle 6" above the top chords.)

 

 

it also helps if the truss is hung 'apex' pointing down not up (yes seen that) as this again provides better lateral stabillity.

Triangular truss now, ok. As above - apex down - the pick up is a 'bridle' as mentioned above.

Apex up, in most cases the pick up is no longer a bridle in the same way, because the top chord is usually included in the wrap (or the truss may be picked up solely by the top chord). This means that the height of the motor above the truss is now immaterial - the point about which the truss/wrap rotates is still close to the top chord - hence increased 'wobbliness'.

 

You say "seen it done" as if hanging a triangular truss apex up in somehow weird behaviour. It isn't (neccessarily), there can be compelling reasons to choose either orientation - the most obvious reason to choose 'apex up' in a lighting truss is to make two chords available to rig lights on, rather than just the one.

 

The idea of rigging a bar perpendicular to the truss I feel would only help exentuate any swing.

Exacerbate? Yep, you're probably right here. In this case that would probably be a bad idea, and I probably shouldn't post when I've just come in from the pub.

 

Sean

x

 

Ey up - posts crossed...

Seano & GregB: Truss is always on fiber/flat spans or steels, always basketed as close to metal as poss, tried various ways on the minibeam but to no avail.

See above, I've been telling you the exact opposite to GregB's advice - try making that basket deeper - so more drop from the motor to the truss, not less. (Not always an option with limited headroom in hotel rooms and the like, I know.)

 

Apex down isn't an option on the lite due to space constraints I'm afraid.

Speculating here, but it may be worth experimenting with:

You could try picking it up with a wrap of bottom chords only (as opposed to the more conventional wrap around bottom chords and then crossing under top chord). Stability is an issue, so you'd need either a wrap fully around each bottom chord to prevent the truss from shifting within the wrap. Better yet, two separate slings, one on each of the bottom chords Bring this 'bridle' together a little above the top chord (but not touching it), and you should gain the stability of hanging 'apex down' without losing any significant height.

 

Slipstream: On my last awards tour I had 1/2 ton of PA...

With that kind of ballast available to hang under the truss, you shouldn't really have a problem. How is the PA rigged?

 

Sean

x

[Matt Riley]

A quick thought from someone who doesn't pretend to call himself a rigger. In this situation are you just hanging a truss at the front of the stage, or is there often a rear truss which you rig lights and screens off etc? If you've got two parallel trusses, and you've got a sufficiently capable motor controller may it be possible to join the back truss to the front truss using extra truss and corner pieces, making one big rectangular truss structure hung off 4 points. If feasible, might this help things?

 

Matt