which section shape for an allu boom arm?

[Scho]

need a light weight boom arm and was wondering which will section shape would be the stiffest?

box?
triangular?
hexagon?

for the rig design i've got in mind the box would be the easyist to work with, hex a close second BUT i'm willing to suffer with triangular section if it's a lot stiffer.

cheers,

rick.

[japhilip]

From experience with camera cranes, I'd say triangular gives the better stiffness to weight compromise, particularly if you're swinging the camera around.

If you've already got the materials though, and you know it's strong enough, I'd use that. I take it the material you have (in mind) is tubular not solid bar? If you haven't already bought the materials, going for a larger cross section (for a given wall thickness) will give you additional stiffness/strength for not a huge amount more weight.

[VxDuncan]

Depends on the loads and the directions. If it's a simple bending moment in one direction the "I" beam is the way to go. Personally, I'd consider using something like the Rexroth extrusion profile that's designed for test rigs etc. VERY still and easy to add things etc. Quite expensive though, but easy to get hold of via RS trade counter...
href='http://rswww.com/cgi-bin/bv/rswww/searchBrow... target='_blank'>http://rswww.com/cgi-bin/bv/rswww/... 
(I've used it a couple of time to make camera widgets)

[Scho]

HMM. great stuff guys, thanks for the info.

just to answer a few q's:

i don't have any material yet

i was thinking hollow-not solid tube

i'm not swinging the arm around too much but stiffness in all directions would be a puls i think.
the main problem i'm dealing with is vibration. i need to keep the camera as still (in relation to the car the arm is attached to as possible)

what wall thickness do you guys suggest? i'm thinking the arm will be 3m in lenght but supported for the first 1m or so. to carry about 2kg in kit at the end

thanks again,

rick.

Edited by Scho on Thursday 8th November 11:13

[japhilip]

Scho said:

HMM. great stuff guys, thanks for the info.

just to answer a few q's:

i don't have any material yet

i was thinking hollow-not solid tube

i'm not swinging the arm around too much but stiffness in all directions would be a puls i think.
the main problem i'm dealing with is vibration. i need to keep the camera as still (in relation to the car the arm is attached to as possible)

what wall thickness do you guys suggest? i'm thinking the arm will be 3m in lenght but supported for the first 1m or so. to carry about 2kg in kit at the end

thanks again,

rick.

Edited by Scho on Thursday 8th November 11:13

3m is quite a long arm. Can you give a little more detail?

On the other hand, 2kg isn't so much. It's been a while since I did any structural engineering design, so I'd err on the side of caution. I've used cranes up to 40ft long that have wall thickness in aluminium of around 3 to 4 mm, probably more like 3mm actually. However, they do have cable rigging to support the arm over its' length. Might be worth considering this. It might also reduce movement at the end of the arm.

If you can stop the arm vibrating in relation to the car, vibration wouldn't be too noticable. It's angular movement/vibration that is more noticable than linear movement. If that makes sense.

[poah]

the automotive rig system has a wire tether to stop movement

[Scho]

poah said:

the automotive rig system has a wire tether to stop movement

cheers...

am i right in thinking if i don't put a hinge on mine it will be more rigid OR does the triaglulation(real word?) of the two halfs of the boom braced by the wire make it stiffer?

if i put a hinge in the boom i might use another smaller diamiter section tube for a brace instead?

ta

rick.

[japhilip]

Scho said:

poah said:

the automotive rig system has a wire tether to stop movement

cheers...

am i right in thinking if i don't put a hinge on mine it will be more rigid OR does the triaglulation(real word?) of the two halfs of the boom braced by the wire make it stiffer?

if i put a hinge in the boom i might use another smaller diamiter section tube for a brace instead?

ta

rick.

Not having the hinge will make it stiffer. However, you could consider having a locking arrangement for the hinge, so you can tighten it with a spanner for instance.

A smaller diameter tube might be better for the brace, as it will brace in two directions, whilst wire will only brace in one (tension).

[GreenV8S]

Why isn't the camera mounted directly on the boom? With those extra pivots and arms it looks as if there is plenty of scope for it to twist/nod, which will affect the picture far more than a bit of wobbling up and down.

[japhilip]

GreenV8S said:

Why isn't the camera mounted directly on the boom? With those extra pivots and arms it looks as if there is plenty of scope for it to twist/nod, which will affect the picture far more than a bit of wobbling up and down.

That's a 'magic arm'. They hold 10kg's or more. Very rigid, when they're in good condition. We use them frequently with broadcast stuff.