Is this gonna hold?

[DrewL]

Heya,

I'm trying to build this ...Don't ask why (well it's part of a film camera crane )

(I know the sketch isn't perfect, it's a schematic draw I made with Word)

I'm not totally sure that this rig is going to be be safe, so I'd like to know what you think - in terms of loads and stress on the different parts.

The parts are:
A - "Lazy Suzan", the rotating base. Rated load 1000 lbs, so that's not what I'm worried about.
B - V-shaped metal bar (I think steel), 1/16" thick.
C - r-shaped metal bar (again, seems like steel), 1/8" thick.

I plan to use 1/4-20X stove bolts to connect A and B, and a 5/16" hex bolt between B and C.
Later on, C attaches with two 5/16" bolts to another part, which is not what worries me.

On top of the lazy suzan I intend mounting a 6-pound camera.

Any thoughts?
Thanks

 

[pna]

If I understand the diagram correctly, all the load will be going through the area where the bolt attaches B to C, but this piece of metal is only 1/16". Looks a bit suspect to me, but I can't calculate whether it would work or not. I would be happier if B was made of thicker material and if B and C had a greater area of overlap.

 

[masona]

Spot weld it will strengthen it.

 

[david and julie]

Bracket "C" requires a gusset plate welding to it and at least two bolts instead of one.

What does it fasten to?

 

[AdamW]

I can't figure this diagram out particularly well either, you haven't included much information on it. Where exactly does the "V-bar" attach? How far does it hang off the "lazy susan" before it meets the right-angle bracket?

I have assumed your lazy susan weighs 1.7kg (I can't deal with Imperial units for calculations!), that assumes it is 3mm thick, a solid disc of steel 30cm across. Your 6lb camera, well that weighs about 2.7kg. So, you have a mass of 4.4kg centred around 18cm from the corner of your angle bracket. That is a moment of 7.76Nm acting on that 1" wide bit of 1/8" steel. I am not a mechanical engineer so I couldn't say how much the bracket will bend, or if it will even yield/fail, But I suspect that this will not be rigid enough for your needs. The bracket would act like a spring, and make the camera oscillate up and down too easily, and it would not hold the camera particularly level.

As Dave says, it needs a gusset. 1/8", that's only 3mm or so. Not very thick, and will be thinner across the bend.

The problem with the single bolt between B and C is not the strength of the bolt. The bolt would be in tension, metal bolts are strong in tension. However, you need a second bolt there to stop it swivelling about if it loosens off slightly.

If you can find someone who uses a package such as "Pro/Engineer", they can draw this out for you and it will tell you if it will work or not. I have seen some very complex structures designed on ProEng, with a few clicks the engineer was able to then show that it would work in stainless steel but not in T5 aluminium.

 

[TexMex]

put a dogs head on it and mount the whole thing on the rear parcel shelf of your car. It'll look perfect.

If you're going to use it to capture video footage while the assembly is on the move, the results will give you motion sickness, (IMHO ).

 

[david and julie]

The problem with the single bolt between B and C is not the strength of the bolt. The bolt would be in tension, metal bolts are strong in tension. However, you need a second bolt there to stop it swivelling about if it loosens off slightly.

Apparently my necks always on the swivel when I go to the pub! well thats what Julie says anyway.

 

[DrewL]

Woah! No responses for days...and then suddenly bang!

So first of all, thanks for the replies, everyone. I don't know why I'd want to put a dog's head on a 12" lazy suzan, but'd definately give it a try.

Seriously, I built the thing, and TexMex might be right - what comes before anything risks yielding is undesired elasticity - because of that thin B bar.
Problem is, I don't have access to welding equipment. This whole venture was a use-what-I-can-get-at-Home-Depot kind of project.

So the easiest thing to do seems to be adding a second V-bar - thus doubling the thickness, which will make it less prone to shaking as well as to yielding. I'm not worried about the single bolt swivling.
OR, I might just take the whole darn thing off and attach a real tripod head to C and that's it

AdamW, a special thanks for the detailed calculations. The lazy susan weighs barely 400g, and the V-bar attaches at the optimal place (the center of the V joint), other than that everything seems correct.

 

[DrewL]

I got rid of the whole thing - obviously it was not a good design.
I ended up mounting a tripod head to C, which worked fine and gave me the results I wanted.

Thanks for the feedback!