C-clamp lifespan

[quarterfront]

Hey, so I've got about 150 units, more than half of which have been in service for 21 years. They all are equipped with cast steel "Altman" or "Colortran" C-clamps. They're in use year round, 8 shows a week every week except for about 9 weeks of changeover over each year. We do 6 changeovers a year and at least half of the units get re-hung each changeover.

What I'm getting at is that these clamps have been under tension for 21 years and flexed a lot of times.

I've only seen a C-clamp break once - I was young and strong and trying to do a weird focus and got overzealous tightening it down. Clamp busted at the top bend of the "C" and the unit ended up hanging by the safety cable.

That said, I find myself wondering, in a preventative maintenence state of mind, whether anybody has any experience with these reaching a point where the metal gets fatigued and clamps start breaking without any provocation. Last thing I want to see is a chunk of clamp hitting the deck in the middle of a show, actors and audience stop paying attention to the story and thinking about their lawyer's phone number instead....

Anything anybody can offer here?

[JD]

Well, bridges are under a lot of stress for many many years, and usually do not fail. When they do, it is often runaway corrosion. In an engine, the cylinder walls are under repeated impact torture. Same thing. I can't say that I know of a "life span" issue, but C clamps are now coming under scrutiny as they don't really have an official structure rating. I would use the opportunity to slowly upgrade to modern and officially rated clamps such as the mega-clamp.

[icewolf08]

I have not heard of C-claps having a useful lifespan limit. I wouldn't be surprised if 80% of the C-clamps in use at my theatre are at least as old as yours. I think the big thing is to just keep an eye on them, check them as part of your routine maintenance. If you see that the "spine" of the clamp is bent, or if any part of the clamps are mis-shaped, chipped, or cracked, then pull them from service and replace them.

[derekleffew]

I would say that the useful lifespan of a C-clamp is approximately equal to that of a STEVETERRY; perhaps longer if not abused or not used for other than intended purposes.

[cdub260]

I wouldn't worry too much over the life span of a c-clamp. About half of my c-clamps are 40+ years old and in excellent shape. Given that I'm at an outdoor venue, you would expect that rust would be an issue, but it hasn't been. As icewolf08 mentioned, just make inspecting your c-clamps a part of your routine maintenance and you should be fine. What you're looking for is signs of bending and stress fractures from over-tightening in the "C" part of the clamp, and any damage to the threads on the bolts. An additional step that I take with my c-clamps is to periodically hit the threads with Tri-Flow, a spray lubricant, this lubes up my bolts and gives them some minimal protection from the elements, but has the down side of making the threads more suseptable to collecting dust, as dust loves to stick to oil. Do not ever use WD-40. While it has its uses, lubrication is not one of them despite any claims they make to the contrary.

[ship]

First, falling on the audience shouldn’t be a factor if you properly safety cable the fixture - that’s what they are for and you always use them. Even at work bought a bunch of the improved Euro versions of them in three classes of load rated weight for use on gear, though I still buy in bulk factory manufactured safety cables of the normal 3/16" and 1/4" size both and or make by way of go/no-go gauge checked other than normal length safety cable for other gear. That’s the key. If hanging say a 10K Fresnel, it’s not a safety cable it’s at least 3/16" steel with a shackle or load rated chain.

Next, the clamps have been discussed over the years in depth on stagecraft forum and the way of the industry will some day go with the above Euro style load rated safety cables that are much safer in shock load but at yet won’t work with most gear given the thimbles that are very important. This much less you cannot buy the same screw gate snap hooks in the states yet. And yet I mean in shopping for them.

Also on the above past debate from stagecraft, or perhaps more recent one... was mostly about clamps. Different domestic ones and Euro ones plus the cheseborough types. Lots of options amongst them amongst the Mega Clamp that would tear up a truss beyond what a truss condom would work with in dealing with a normal, heavy or extra heavy duty type C-Clamp.

On the clamps, this once the concept of safety cable is mentioned.... once got into a long off-line discussion with Frank from stagecraft about the subject a few years ago. One might given his unique personality guess how the debate went. Still it was of interest and I certainly also use other types of clamp at work as with those more normal Altman type or Heavy duty ETC or Altman type or Extra heavy duty Altman or even Colortran type. This in addition to the various G, J and cheseborough clamps of various types. And mega and other clamps.

Whole bin full of half cheseboroughs of both Doughty style with hex nut bolt and normal flat head screw style plus a few others in the bin for repair constantly. Spend hundreds of dollars per year in just replacing the swing arm bolt assembly for each and any brand. This after the three minute rule in repairing the threads do a dinged up bolt. If it will take more than three minutes to repair the bolt by way of triangle or thread file, damage is substantial or more than a minor ding to the threads in the lower half of the swing arm thus structural, the swing arm assembly gets replaced. Major liability issue in figuring out what can be fixed, repaced or as Doughty sees it in not providing replacement swing arms... not safe once bad.

J or G clamps... seen just as many bent, threads dinged and lots of other in such clamps for me being a secondary choice. Lots of them on the market, in my opinion none are better though bent as opposed to cast is a valid point in strength or reliability due to fatigue. On the other hand, not as easy to use. You know that as it were in other terms, screw you set screw is useful if in good condition. Even of these clamps at one point bent up a few dozen of my own out of 1/4"x2" steel plate. Not easy but can be done and they still function properly. For the set screws, it was carriage bolts ground down to set screws and than as typical with grade 2 bolts used as a retaining mechanism for the clamp.. Plus the un-supported distance of between the bolt touching the pipe and clamp in distorting it =- some even factory knobs or screws of this type are not sufficient in size or grade. All about tension and torque on them also. Grade 5 is available as with proper aircraft grade set screws.

On the other hand as with any screw or clamp, take the 10" C-Wrench away from the tech people in thinking that the torque of 1/4 turn past finger tight for them is the same as that of 1/4 turn past finger tight for that of a 8" C-Wrench. “That’s nice.. You go sweep the floor.”

No matter what the clamp or tool, and I do have some wing nut T-handle tighteners I custom make even, over tension is a problem that any clamp will fail with.

On the clamp age... Got some C-Clamps that are in perfectly good condition that are at least 50 years old now, at least at home. I still use them. No signs of metal fatigue or bending and that’s what you are looking for. During the late 90's there was one run of C-Clamps that were made off shore without a name brand on them to watch out for on the other hand. Sort of have a varnish coating to them in look. Not real finish, sort of a brownish like gloss coat above the clamp casting. Destroy these clamps - they are unsafe in being lesser casting materials used and not a real C-Clamp one can most often see deformed or cracked.

Beyond this, never use other than a hardened bolt/set screw for the clamp. Such bolts get mushroomed and or deformed in bending and not holding. If you need a longer bolt, only use proper set screws if not at least grade five cup ones. Far too often I see C-Clamps coming back from shows with grade two hex screws in them with a mushroomed tip. Once that tip’s mushroomed, cut the bolt before attempting to remove it. This otherwise the flared tip will ding and screw up the threading in the clamp. If stuck bolt or bent bolt, cut it where its possible in removing it from the top or bottom, don’t attempt to remove it anyway or you will get a bad clamp.

Mostly, C-Clamps... different grades of them, and age is not persay a factor in safely using them. Yes metal fatigue is a good concept to study and valid in point but for the most part if properly used and mostly you can see this, a C-Clamp don’t have an age just as it don’t have a load rating. More a question of inspection and proper use. Ages old or brand new... just cut the bolt off of an ETC clamp tonight. Bolt was bent and it was having problems getting removed. Took a lot of re-tapping of the clamp to make it proper again and the clamp itself was inspected to verify its safety.

Overall. If in doubt, throw it out and or damage it so someone else won’t take it home without understanding why it went into the trash. Cut it in half or ding the heck out of it so it can no longer be used. At times without proper supervision, that even if more expensive is the only solution. I properly train and or supervise anyone working on clamps at work. Didn’t ask for that responsibility but it’s my job and supervision I am tasked with. Very important yet on the other hand age of the clamp is less a factor over proper use and inspection of it.

[gafftaper]

I'm not a metal expert but it seems like a C-clamp is either going to be good or bad. It seems highly unlikely that it will be hanging just fine and then suddenly fatigue and fail. True you are putting a lot of strain on and off that clamp for years, but it's designed to take that strain. If they are over tightened then I would expect them to either break on the spot or when loosened they would remain misshaped due to the metal being bent past it's fatigue point. It seems highly unlikely to me that a C-clamp would just suddenly break in the middle of a performance, days after the load was placed on the clamp. On the contrary it seems most likely that it's going to break as your are placing that strain on it one last time.

So all that summed up it seems that a good regular visual inspection of the c-clamps is in order, but as Derek said those clamps should be able to be used for many years without fear. Yeah if you've got clamps that are so old they don't have any sort of manufacturer's label on them you should probably replace them. But if they are 20 years old, say Altman on the side, and appear in good condition I wouldn't worry about them.

[cdub260]

Originally Posted by ship

On the other hand as with any screw or clamp, take the 10" C-Wrench away from the tech people in thinking that the torque of 1/4 turn past finger tight for them is the same as that of 1/4 turn past finger tight for that of a 8" C-Wrench. “That’s nice.. You go sweep the floor.”

But I need my 10 inch C-wrench. It's the only one I can twirl around on my pinkie finger.

[church]

While we are correct to be cautious we also need to be careful not to go crazy because we can quickly reach the point that it is not possible to do anything. I work in an industry where we design everything with tremendous safety margins, everything is analysed and then tested and the test data is correlated back to the analysis. We have full traceability of all materials down to knowing where and the date of the smelting with test certs for raw material we also only machine parts out of solid metal.

Great you say yes but the cost and schedules are horrible a simple 5-50 fastner costs $10. So if we took the same approach for C Clamps they would cost about $1000 each retail and a S4 would cost about $30,000 each. So before people get too carried away the information provided by Ship and overs is appropriate; Buy good quality clamps. If it shows any sign of deformation, damaged threads or cracking replace it. Visually inspect all equipment on a regular basis i.e. anually and document the inspection. Whenever you are adjusting something give it the once over if something doesn't look right fix it. No different to your car - you check the tires, lights, oil, coolant, glass etc. weekly and follow the manufacturers service schedule don't you?

Anyway here are some thoughts on clamp failures. A clamp is not going to simply fail because of the ticking of the clock. A clamp will fail because it has:

1) a manufacturing defect that reduces its mechanical properties below what it is required to have for its intended service application. This defect may be detected by X ray, high magnification inspection of the grain structure or non destructive testing to a derated load that is below the elastic limit of the material

2) been subjected to a shock load or vibration load beyond its design load plus safety margins for a time interval that excedds the fatigue life of the material used in the manufacture.

3) been subjected to a static load that exceeds the design load plus safety margin to a level that exceeds the elastic limit of the material resulting in fracture.

4) been made from the wrong material i.e. not the specified material. This is not usually detectable by casual investigation, 6061 Aluminium with a T5 temper is visually the same as the T6 temper and even 7075 aluminium but different material strengths but they can be detected with XRF Spectrometer. Not something you are likely to have around.

The elastic limit does not change with time. It changes with heat and depending on the material this is significantly beyond anything you would see in a theatre i.e. 200 degree centigrade and higher. This is why in a fire even the structural steel deforms and civil engineers require coatings over the steelwork to increase the temperatures at which this happens. However at these temperatures you and your audience are not present.

[quarterfront]

First, falling on the audience shouldn’t be a factor if you properly safety cable the fixture - that’s what they are for and you always use them.

Dude, I always safety everything. I'm talking about the piece of broken steel C-clamp that comes away from the clamp if the clamp busts falling and beaning somebody. Just 'cause the unit doesn't fall, that doesn't mean that that piece of clamp wouldn't really hurt if it hit somebody on the head.

once got into a long off-line discussion with Frank from stagecraft about the subject a few years ago. One might given his unique personality guess how the debate went.

Yeah, on and off I've tried subscribing to the stagecraft list but I just can't take it.

Overall, from the replies, it looks like so far nobody's ever seen the thing I'm wondering about happen, and that's pretty reassuring.