clamping force


When a company says that its clamp can extert X amount of force, what does that mean? Before the clamp will snap? That a user will not be able to tighten it any further?
Specifically I am interested in the specs on the "Quick-Grip One-Handed Bar Clamp / Spreader" shown here: http://www.coastaltool.com/cgi-bin/welcome.pl?ref=froogle+page=/clamps_vises/quick-grip/qg_bar_clamp.htm
It says that the clamp "Exerts up to 250 lbs of clamping pressure". I've used this clamp and tightened it as best I could by hand, squeezing the trigger. Assuming I have a "normal" strength grip, would that mean that at that point the clamp is exerting 250 pounds? What might the multiplier be? What if I used another clamp to squeeze the trigger of the first clamp? TIA. -- Igor
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wrote:

Average hand force to twist the handle generates that much linear force.
This is generally a combination of the helix angle on the screw thread, moderated by the bendiness of the frame.

Well they're lying.
And they're Irwin, so don't buy it anyway.

No, see my previous point about "lying".
These aren't usually bad clamps for lighter gluing, and you don't need 250lbs to clamp a glueup anyway, or else you get squeezeout. But leave it a few months until the pawl wears and you're lucky to get a few lbs out of this type of clamp. The best ones I've found are by Wolf (turquoise and black) and even those don't last very long.
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http://www.coastaltool.com/cgi-bin/welcome.pl?ref=froogle+page=/clamps_vises/quick-grip/qg_bar_clamp.htm
I have several style clamps, many of which easily exert much more clamping pressure with MUCH less effort on my part than my Quick Grip clamps.
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That simply means you are using the wrong clamp.
Quick Grip clamps are OK for a quick holding, but IMO, they are not well suited for serious work. I'd much rather use a Bessy K or F. Even the lightweight UniClamps are better but come in size up to 18".
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wrote in message

No, I learned early on that they are for better suited lite duty but I was more or less giving a warning to the OP. He was considering using another clamp to squeese the trigger on the Quick Grip clamp.

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wrote:

Actually, it was more of a hypothetical -- i.e., what sets the so-called 250# limit? Will the trigger just stop activating the clamp, regardless of pressure applied to the trigger? In furtherance of the "regardless" question, I thought, "What would happen if I used a 2nd clamp to pull the trigger?" (This reminds me of the photo I saw online somewhere (via the wrec) of the guys in a warehouse using a forklift on a larger forklift to lift some worker high enough to reach something.) -- Igor
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Pounds are used to measure force, not pressure. There is an important difference.
--

Larry Wasserman Baltimore, Maryland
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Indeed. A clamp which can deliver a force of 250 lbs can give a force of thousands of pounds per square inch if the contact area is very small, or almost no pressure at all if the contact area is large.
-j
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on 3/15/2005 3:12 PM J said the following:

Easily demonstrated by placing your hand on the floor and asking your lovely wife to step on it quite slowly. First barefooted and then while wearing her high heels.
There's more than one reason you don't see a lot of vinyl tile in business places. High heels will dimple it better than a ball peen hammer.
You'll be back at that lathe or table saw in four or five weeks! Guaranteed!<g>
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Lots of replies, but the real answer is up to the point of clutch slip or screw jamming.
The type referenced are used to hold things in place in lieu of a helper in my shop. Though primarily "extra hand" devices, they can also be used effectively as clamps, especially on properly prepared material.
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Thanks, all, for the replies. I brought up the question because of some _rough_ stress testing to failure. (Maybe I should have said "OT", since I raised the question not in the context of clamping wood?) I had set up a bench tester and used the quick-grip clamp to generate the force to stress a part between two components of the tester. You see, NASA has been bugging me and I was behind schedule on a part for the next Shuttle launch ... But seriously, ladies and germs. I had not been able to cause failure using my >200# weight at the end of a lever (much more force than the clamp could apply because it was near he base of the lever), but I was hoping the clamp could give me some sort of rough _perceived_ metric -- such as, "as shown in this photo, part did not fail when 250# of force was applied." Yes, I could try to do the math, based on the length of the lever and my own weight, etc., but I thought the clamp looked better in a photo than would me hanging on the end of the lever. Looks like maybe I should get a pulley, a bucket, and some sand.
Also, with the discussions here about tool "ratings", such as hp, I thought the clamp rating numbers deserved an airing. Again, thanks. -- Igor
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