Measuring Chuck Clamping Force

[Nerdlinger]

Hi Everyone,

Parts are moving in my chuck. We refurbished the serrated jaws to no avail, so we started looking into the clamping force. If we run the calcs on hydraulic pressure, actuator surface area, and the resulting force on the draw bar our chuck should be producing 20,000 lbs of "total clamping force." It is a four jaw chuck, so 5,000 lbs for each jaw?

We have a small hydraulic force gage and when we put it between two jaws I would expect to see 10,000 lbs, but are getting a little less than half of that. (see attached pic.) I figger we are either getting about half of the clamping force we should be OR I am thinking about this wrong and for some reason we should be expecting to only get about 5,000 lbs with a force measuring gage.

Do you think we should expect to get about 10,000 lbs or 5,000 lbs when measuring this way? I think it should be the former but I am often wrong.

Thank you for your help!

 

[DouglasJRizzo]

You may want to email the chuck manufacturer. There may be factors you're not taking into consideration.

 

[Orange Vise]

The number of jaws doesn't affect the mechanical advantage. It's just drawtube force divided by diametric jaw travel (combined travel of two opposing jaws in your case).

What's the make and model of that chuck, and what's your PSI reading?

 

[Nerdlinger]

You may want to email the chuck manufacturer. There may be factors you're not taking into consideration.

Yeah, I’ve been working with them but they’re in Germany so communication is a bit slow. At one point they said that 5,000 lbs from each jaw should be displayed as 5,000 lbs on a force gage but to me it should read (5,000 lbs/jaw) x (2 jaws) = 10,000 lbs. I’m sure they know what they’re talking about but I almost can’t believe it.

 

[Nerdlinger]

The number of jaws doesn't affect the mechanical advantage. It's just drawtube force divided by diametric jaw travel (combined travel of two opposing jaws in your case).

What's the make and model of that chuck, and what's your PSI reading?

That’s true…I wonder if I can measure the travel of the drawbar and compare that to the travel of the jaws.

The chuck is an HWR Inoflex VK026. Right on the chuck it says 50KN max draw force and 130KN max clamp force. The pressure gages say 3.2MPa and the area of the hydraulic actuator is 103cm^2.

 

[Red James]

You cannot add the 4 jaws together to get total force. If you weigh 200 pounds, step on a scale that reads 200 pounds, you will not get 400 pounds of force.

 

[GENERALDISARRAY]

Parts are moving because its a 4 jaw. Like a 4 legged table on the patio at my favorite local brewery.

 

[maguilera]

A hydraulic force gage usually can only measure force in one direction, therefore your results make sense
Mind you that this clamping force you´re measuring will decrease as the spindle speed increases, due to centrifugal force.

 

[yardbird]

I hafta ask, you ain't close to the bottom of the cylinder stroke are ya?

Brent

 

[Nerdlinger]

That's a good question, but we DID already look into that. We set up the jaws so minimal travel is required to grip the part...the idea being that there is still stroke left over to keep from bottoming out.

 

[Nerdlinger]

You cannot add the 4 jaws together to get total force. If you weigh 200 pounds, step on a scale that reads 200 pounds, you will not get 400 pounds of force.

I'm just not entirely certain of the nomenclature regarding the hydraulic power chuck jaws. I believe what they call "total force" is the sum of individual clamping forces produced by each jaw. That's what the manufacturer said, at least, but I wasn't sure if it was an industry standard, more or less, that someone here could help clarify. I've never actually tried measuring chuck clamping force in pounds before with a force gage so I'm not sure how to compare what I am getting to what the mfg states.

 

[BobM3]

I agree with Maguilera. I've not used a check force measuring tool but if it installs with one face against one jaw and the opposite face against the opposite jaw with no other reaction holding the transducer then I'd think a 5000# force from one jaw would read 5000# (not 10,000#). But if the maker of the gauge wanted to he/she could put a factor of two on the readout so a 5000# load would read as 10,000#. You need to check with either this gauge maker or another that could tell you the standard.

Load cells will report the load on the face of one side of the cell. They don't include the reaction force on the other side.

 

[CalG]

Opposing jaw clamping forces do not sum.

Think if only one jaw were fitted, opposing an infinite resistance. The proverbial "brick wall". Still only 5k.

NOT 5K plus a brick wall.

 

[Billy_C]

I'm curious about the clamping force as well. Going by the hydraulic pressure never made sense to me because the cylinder design could be different on different machines. Different piston surface area with the same pressure will result in different force, right? Different chuck designs seem like they could make things different as well. Maybe I'm just not familiar enough with the internal drawbar mechanics.

 

[beege]

Yeah, I agree with the brick wall thing. I remember that old brain teaser that had 2 cars going 60mph at each other. Impact on each car were as if each had hit a brick wall at 60, instead of the "first thought" of 120. 5000 pounds should be the total squeeze on a part.

And another thought.... if you had six jaws instead of 2, would you expect 30k of clamping force? Where would that even come from?

 

[ARB]

Have you pulled the chuck apart and serviced it?

 

[Scruffy887]

Most of these chucks have a sweet spot for max clamping. Not at max stroke, not at min stroke. Kit has this sweet spot defined with tic marks.

 

[Nerdlinger]

Yeah, I agree with the brick wall thing. I remember that old brain teaser that had 2 cars going 60mph at each other. Impact on each car were as if each had hit a brick wall at 60, instead of the "first thought" of 120. 5000 pounds should be the total squeeze on a part.

And another thought.... if you had six jaws instead of 2, would you expect 30k of clamping force? Where would that even come from?

Yeah, this is EXACTLY where my spidey-sense was telling me I may be thinking of it wrong. I was thinking each jaw would simply clamp by the ratio one could calculate from the info on the chuck face (total clamping force = 2.61 X draw bar force) divided by the number of jaws. But then if I remove jaws 1 and 3 to only measure the clamping force between jaws 2 and 4 it should double, right?

It is a somewhat-sophisticated chuck, in my opinion, with four self-centering jaws. So maybe there is something in the mechanics, too, that I am misunderstanding. I will report back what I find out with the manufacturer, but they DID already say once that I should expect to get 1/4 of the theoretical "total clamping force" when measuring the way I am between two jaws. I imagine they are correct, but it just seemed so unintuitive I had to ask here.

 

[Nerdlinger]

Have you pulled the chuck apart and serviced it?

Yeah, I was certain that was it because the manual says, "Problem - loss of clamping force Solution - clean chuck." It was full of gunk so there was no way the grease could get to where it had to. Alas, no change after a good cleaning.

 

[Nerdlinger]

I'm curious about the clamping force as well. Going by the hydraulic pressure never made sense to me because the cylinder design could be different on different machines. Different piston surface area with the same pressure will result in different force, right? Different chuck designs seem like they could make things different as well. Maybe I'm just not familiar enough with the internal drawbar mechanics.

Yeah, the drawbar cylinder has the surface area marked on it, so I multiplied the hydraulic PRESSURE by the cylinder SURFACE AREA to get the drawbar FORCE. From there I used the info on the face of the chuck to determine what the clamping force should be. My problem is I'm not entirely certain on how to go about measuring that. It looks like I forgot the mythbusters where they crashed two cars into each other.