Are C-clamps rated?

[pudgeo2]

I was recently talking with a sales rep, who informed me that C-Clamps and Side arms are not rated for overhead use. I can see where this comes from, but I find this hard to believe since C-Clamps have been the standard for ever.

[soundman]

Mega clamps are http://www.thelightsource.com/products/1/view

Malleable Pipe Clamps 500lbs working load <- taken from BMI supply catalog.

[derekleffew]

Your rep is correct. Cast iron C-Clamps are not rated, as cast iron, as opposed to forged, is inherently brittle and thus cannot be load rated. We've had this discussion before, particularly in this thread, regarding hanging SeaChangers from side-arms.

[jmabray]

Derek is absolutley correct. Cast Iron C-Clamps are not rated.

But other ones, like the mega clamp or the aluminium clamp from L&E are.

[cutlunch]

This maybe a silly question as we don't use these type of clamps in NZ as much. I understand the rating of the clamp but is there a rating on the bolt or this all together? Why I ask is apart from the teeth it seems the tightness of the bolt is what stops the light from falling off the bar if the centre of gravity does not run directly through the center of the clamp.
Here we use Hook clamps the same as Europe.
http://www.blue-room.org.uk/wiki/Hook_Clamps
These have the advantage in that the light will stay on the bar even if the bolt comes loose unless you have the light over the bar. But these are good in schools etc because the bolt normally has a handle not a bolt head. Also the clamp holds to the lights yoke using a bolt with a wing nut. So as long as someone hasn't come along with a spanner and overtightened the wing nut you don't need tools to hang these lights.
Getting back to my inital question, if a bolt is lost out of a C clamp and replaced with one from the hardware store does this derate the C clamp loading capibilty?
I think the last time I used a C clamp was on some old Colortran Fresnels.

[derekleffew]

cutlunch, note the LightSource link above. The specs say:

Safe Vertical Working Load 600 lb 273 kg

Emphasis on the word "Vertical." The C-Clamp bolt (as opposed to the yoke bolt) takes no weight and is only for holding the clamp in position on the pipe. The yoke bolt on the MegaClamp I have in front of me is a Grade 5.

Not sure why your hangers never caught on here in the US, except for PARbars and pre-Series 300 Vari*Lites. I've often heard them referred to as "G-clamps."

[Logos]

The Hook Clamp or G clamp was I believe invented by Strand in the 50's I think there is a wikipedia article.

[JD]

Cast C clamps have always given me concern ever since High School (many, many years ago) when I was tightening one and the top just broke off! Wasn't even that tight. Castings are funny things.

[derekleffew]

Originally Posted by Logos

The Hook Clamp or G clamp was I believe invented by Strand in the 50's I think there is a wikipedia article.

Didn't find anything in wikipedia, but stumbled upon the Strand archive, cited here: 1959

• Strand launch the Hook Clamp and free British theatre from the tyranny of the traditional 2-bolt, 2 nut L Clamp.
• Strand Electric's office in Toronto, Canada opens.
• The Century Izenour 10 Scene preset is introduced.

[Les]

I'm confused. If C-Clamps and sidearms aren't rated for overhead use, where are you supposed to use them? I'm hard pressed to think of any hanging position in a theatre that isn't over someone's head. Is the type of clamp in the picture below what we're talking about?