Clamping force

[Grahamshed]

I keep seeing clamps expressed as 2000 ( or whatever ) 'N'. Does anyone know what the 'N' stands for and how it relates to pounds per square inch ?

 

[Phil Pascoe]

Newtons?

 

[Grahamshed]

Newtons?

That was my guess but how many apples is that ?

 

[Cheshirechappie]

I think 'N' is probably 'Newtons', defined as the force required to accelerate a mass of 1kg by the rate of 1 metre per second per second. More helpfully, 1kgf (kilogramme force) = 9.81N (Newtons).

There isn't a direct relationship with pounds per square inch (which is a measure of pressure or stress, not force). However, there is a direct relationship with force measured in imperial units; 1N = 0.225lbf. So 1 pound-force is about 4 Newtons, give or take a bit, and 1Newton is about a quarter of a pound-force, again, give or take a bit.

 

[Gav1]

Newtons measure a force and psi measures pressure, so you cannot make a direct comparison.

2000N is equivalent of 200 kilograms.

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[Grahamshed]

Thanks guys. In terms of things like F clamps I would have thought 'pressure' and 'force' were much the same thing. ( obviously not !! )

For clamping pieces while glueing what sort of 'N' is required ?

 

[Gav1]

Force is mass times acceleration, so if we work backwards knowing 2000N, and acceleration is gravity (10 is close enough for us woodworkers) 2000 divided by 10 gives us a mass of 200kg (450lbs in old money).

So the maximum force the clamp can exert is the smae as putting a 200kg lump of something on to what you're clamping.

As for how much force to apply, as much as is needed to hood the joint securely without damaging the work?! :?:

I think I've explained that right, happy to be corrected!



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[Phil Pascoe]

Just use as much force as is neccessary to tighten the joint - there's no point squeezing all the glue out , or creating stresses that needn't be there.

 

[Cheshirechappie]

Just use as much force as is neccessary to tighten the joint - there's no point squeezing all the glue out , or creating stresses that needn't be there.

+1 to that.

The quoted clamping force for any given clamp is a bit meaningless, really. Without being able to measure the exact torque applied to the clampscrew, you won't know what exact force the clamp is exerting. If you get a couple of feet of tube over the clamp handle, put your foot against it and heave, you'll get enough force to permanently distort the clamp.

You don't really need to know the force being exerted in figures, pounds or newtons. The force you need is enough to close the joint and hold it closed while the glue cures. Tight, but not silly.

I suspect the clampforce figures quoted in manufacturers' literature is a bit like the old 'CD' figures (coefficient of drag) they used to quote for aerodynamic drag of cars. Utterly meaningless in the real world, but it could be used to sell cars.

 

[RogerS]

Just use as much force as is neccessary to tighten the joint - there's no point squeezing all the glue out , or creating stresses that needn't be there.

+1 to that.

The quoted clamping force for any given clamp is a bit meaningless, really. Without being able to measure the exact torque applied to the clampscrew, you won't know what exact force the clamp is exerting. If you get a couple of feet of tube over the clamp handle, put your foot against it and heave, you'll get enough force to permanently distort the clamp.

You don't really need to know the force being exerted in figures, pounds or newtons. The force you need is enough to close the joint and hold it closed while the glue cures. Tight, but not silly.

I suspect the clampforce figures quoted in manufacturers' literature is a bit like the old 'CD' figures (coefficient of drag) they used to quote for aerodynamic drag of cars. Utterly meaningless in the real world, but it could be used to sell cars.

+1.

If you need to apply that much force to tighten your joints then they are too tight to begin with!

 

[Grahamshed]

Its not that I need much force to close my joints

I have just brought a couple of the Bessey single handed EZS clamps and been comparing them with a couple of same size Irwins I have. ( by the very scientific methoud of clamping a piece of wood to my bench ) With the Irwins I can 'maneuver' the wood in the clamp no matter how much force I exert. With the Bessey it locks solid.

The Bessey is obviously much better built and stronger but 4 times the price. I will be getting some more anyway because I like them but was just wondering if you actually need all that force.

 

[Louise-Paisley]

the quoted force is the max force the clamp can exert before the clamp fails, or exceeds it design specification should I say. In much the same way you can probably spin a router bit faster than its spec but doind so gives a very real risk of failure.

The pressure exerted by the clamp (at max force) is the force/ area of contact. Without a block to spread the load the clamping face of an f clamp is likely to be only about 1 square inches, therefore the pressure exerted by the clamp alone at max force is 200*2.2 PSI or about 440PSI. If you spread the load with a good thick block of wood which is say 4 x 4" then you are spreading the force over 16 square inches so the pressure would be 27.5 psi.

In both cases it is more than enough to clamp something up to glue!

 

[dickm]

As an aide-memoire, a Newton is reckoned to be about the force exerted by a typical apple resting on the surface. (Honest!)