Author Topic: How to test the gripping force of a chuck?  (Read 6578 times)

Offline awemawson

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How to test the gripping force of a chuck?
« on: September 11, 2014, 09:46:59 AM »
I've been installing a hydraulic chuck on the Traub CNC lathe, and it occurred to me that to have some means of testing gripping force would be mighty handy.

I've only operated it on much reduced hydraulic pressure at the moment during testing, but even at 5 bar as opposed to it's intended 22 bar I cannot move a bar tapping axially with a copper hammer - but that's a bit of a crude test!

Problem is there are too many variables. The jaws move a maximum of 3 mm, and being bored jaws are seated on serrations and bolted on, so themselves are movable.

Only method that I can come up with at the moment is to turn up a slug of lead, or other very malleable metal, grip it with the chuck, release and examine the (presumable) deformation. Then do the same with a manual chuck of similar size and compare them.

I suppose some sort of deformable tube filled with hydraulic oil and equipped with a suitable pressure gauge is another possibility?

Any suggestions welcome.
« Last Edit: September 11, 2014, 10:46:29 AM by awemawson »
Andrew Mawson
East Sussex

Offline awemawson

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Re: How to test the gripping force of a chuck?
« Reply #1 on: September 11, 2014, 11:15:56 AM »
 :coffee: Google has found me this clever little gizmo:

http://www.schmachtl.cz/web_get_img_data?aID=533906

I'd imagine there are three pistons the the jaws bear on with lip seals communicating with a central chamber that the gauge attaches to.

Now can some mathematically versed member tell me how to relate the pressure read on the gauge, to the force on the three pistons. Piston area obviously influences it.

Working in Imperial units if (say) we have a pressure of 100 lbs/sq in reading on the gauge, each piston contributes 1/3rd so 33.333.. lbs sq  in  per piston, so (to make calculation easy) if we have a piston area of 0.333.. sq in is that a force of 100 lbs on each jaw.

Or is it more complicated than that?

It's a bit like the 'Dead Weight' testers you can get to calibrate gauges I suppose, but I've never used one or seen how that they are set up  :scratch:
Andrew Mawson
East Sussex

Offline Lew_Merrick_PE

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Re: How to test the gripping force of a chuck?
« Reply #2 on: September 11, 2014, 11:27:26 AM »
Andrew -- If your distribution pressure is (say) 100 psi, then each piston is pressurized to 100 psi.  The piston area (i.e. cylinder bore) X the pressure (100 psi by your example) is applied to each piston.  Thus, if the piston is °.500 inch, then the piston area is (.25▓*pi =) .1963 in▓ and, for a 3-jaw chuck, this would give you a total clamping force of (3 * 100 lbs/in▓ * .1963 in▓ =) 58.9 lbs.

Offline awemawson

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Re: How to test the gripping force of a chuck?
« Reply #3 on: September 11, 2014, 11:35:16 AM »
Thanks Lew for your reply.

It doesn't look to be beyond the capabilities of a Madmodder to make such a device  :ddb: I suppose the issue is to size the pistons so that a standard gauge reads sensible figures. I'd imagine lip seals are available in a multitude of sizes and it's be a case of choosing one that works out sensibly mathematically and making cylinders and pistons to suit.

Bleeding all the air from it might be fun.
Andrew Mawson
East Sussex

Offline Lew_Merrick_PE

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Re: How to test the gripping force of a chuck?
« Reply #4 on: September 11, 2014, 11:26:00 PM »
Andrew,

Dual o-ring seals should be sufficient for most hydraulic pumps (3 ksi or lower pressure).  I often design piston parts for pyrotechnic actuators that see something in excess of 60 ksi for short periods of time that allows me to design o-ring seals (30% compression) that use a single o-ring.  In essence, you want a "gap" between the piston & cylinder of .004/.002 inches.  I have (and am still "debugging") a spreadsheet for designing both static and dynamic o-ring seal glands from (SAE, NAS, and MS) standard o-ring sizes and materials.  If you need a hand in such an effort, drop me a line.

Basically, for a male gland piston/bore seal I need: Minimum ID of Bore, % of compression desired (which comes from SAE tables for desired pressure loads), and the o-ring material you would like to use.  Everything else is (almost) automagic.  This is a back burner project that moves ahead by fits and starts (and stops).

Offline Meldonmech

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Re: How to test the gripping force of a chuck?
« Reply #5 on: September 12, 2014, 03:41:10 AM »

     Andrew I do like the little gizmo you have discovered, do you intend making one?

                                                      Cheers  David


Offline awemawson

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Re: How to test the gripping force of a chuck?
« Reply #6 on: September 12, 2014, 05:09:41 AM »
David,

I'm not yet committed, but it's on the list of possible little projects for winter  :thumbup:
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: How to test the gripping force of a chuck?
« Reply #7 on: September 12, 2014, 06:46:19 AM »
Do you know Poldi hardness measurement system? Anyway the big idea is that you strike a plunger that presses a specimen of known strengt and a hard ball againsta unknown specimen. Indentations (of the ball) are measured from both specimens and hardness is deducted from the table. Anyway, this is pretty simple method and it produces results that are not 1:1 brinnel etc. hardness.

http://www.measureshop.biz/images/products/ultra/Ultra_tisk-193-i2.jpg

Now, we are not measuring Brinnell hadness here, but only a force. Just incidently same method:
http://home.iitk.ac.in/~kamalkk/Image10.gif


Could this idea work in sort of reverse engineering manner here? If a bush that has holes squarely against the jaws. Holes accomondate one ball bearing "ball" each. In the centre there is a round rod. When jaws are closed, an indentation is made. Now. If you have a shop press, or something close to it (I would not be surpriced, if someone had a deadweight calibrator) and you would use same size ball bearing to press indetations on the same rod. When you get close to same size indentation, you have pretty close the same force than the jaw?

Right or wrong?

I'm a little handicapped mechanically (I'm electrical engineer we did not read that much ironbashing). So, if I'm completelty off I don't mind corrected.

Pekka

Offline awemawson

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Re: How to test the gripping force of a chuck?
« Reply #8 on: September 12, 2014, 06:54:18 AM »
Pekka,

That sounds a nice simple thing to make and use, thanks for the suggestion I may well try it out.

The  '3 hydraulic piston' one has the advantage of giving a reading that can be watched as the jaw pressure is adjusted.

I can see a series of mini projects being spawned here  :ddb:
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: How to test the gripping force of a chuck?
« Reply #9 on: September 12, 2014, 07:20:08 AM »
Happy if you find that idea usefull.

That meter surely looks spiffy and it's a instant reading.

My line of though was that you probably have a meter or set value for the pressure setting. Now, if you make a table of the pressure vs. force (indentation) you can set it later easy and if something does not check you can allways fall back known rod/ball/indetion size to check system up.

I saw once a measurement station for load cells, they had a really nice system to minimize frictions and enviromental effect. one thing that I found genious was that they were rotating the cylinder piston - all friction was sliding friction, static friction would be very much bigger loss.

Pekka

Offline Jonny

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Re: How to test the gripping force of a chuck?
« Reply #10 on: September 12, 2014, 04:48:27 PM »
Beat me to it Pekka.

A simpler idea though never tried it can be to make up a self contained container thats held in the jaws that will collapse. Link this to a pressure guage just make sure you take a reading from a designated point  ie 0 psi or 34 psi or whatever.
As cylinder compresses so will the internal pressure, sure I am missing something?
To pressurise could use oil or air, its low pressure so no harm should things go pear shaped.

Standard nitrile O rings will take in excess of 10000psi and operate for years at 300 bar in a static arrangement. On moving pistons have had several Viton pieces last 8 1/2 years running 120bar at about 140 cycles per week with just one o ring 1mm cross section. I prefer to expand up on inner bore and compress o/d with minimal width guaranteed to work from zero psi. Some specs rely on pressure against the o ring to create a seal either on I/d and face they always pack up and distort.
Unless its a 1mm cross section o ring I usually go for a compression on o/d around 0.3mm for sub 1 1/4" bores, still nice and easy to screw 1.5 to 2.4 cross section together by hand.

Offline hermetic

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Re: How to test the gripping force of a chuck?
« Reply #11 on: September 14, 2014, 07:22:13 AM »
Do you want to know the pressure exerted, or the grip? a test of the actual gripping capacity could be done by gripping the plain part of a bolt in the chuck and using a torque wrench on the hexagon. you could then work out the surface area of the chuck jaws, and extrapolate mathematically from there (don't ask how, I leave that to the wife, she has a maths degree:-) Or have I missed the point?

Offline awemawson

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Re: How to test the gripping force of a chuck?
« Reply #12 on: September 14, 2014, 08:18:06 AM »
Basically I want to check that the way I've fitted it allows it to apply the force specified by the manufacturer (Schunk).  I need to adjust the operating hydraulic pressure to get the right amount of grip without exceeding the maximum pull on the chuck (22kN)

Researching (googling) this I've come across all sorts of interesting things. For instance I found one site with a video demonstrating how the grip reduces HUGELY as the speed increases due to the centrifugal force on the jaws. Sadly I now cannot find the link !
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: How to test the gripping force of a chuck?
« Reply #13 on: September 14, 2014, 09:47:30 AM »
.....

Researching (googling) this I've come across all sorts of interesting things. For instance I found one site with a video demonstrating how the grip reduces HUGELY as the speed increases due to the centrifugal force on the jaws. Sadly I now cannot find the link !

Anything like this?

     


Pekka
« Last Edit: September 14, 2014, 12:52:46 PM by dsquire »

Offline hermetic

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Re: How to test the gripping force of a chuck?
« Reply #14 on: September 14, 2014, 10:06:51 AM »
Right Andrew, see what you mean, not straightforward at all, what is needed is a link between rpm and pressure so that as rpm increases, pressure rises Subject to the max safe working pressure, I am skating on thin ice at the very edge of the abyss here, so I will leave it to "them as knows". I will watch and hopefully learn.
Phil

Offline awemawson

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Re: How to test the gripping force of a chuck?
« Reply #15 on: September 14, 2014, 11:10:58 AM »
Pekka, although I've seen that Youtube video, the one I'm thinking of isn't youtube but a manufacturers demo one. It shows a device measuring the force and remotely monitoring it by wireless transmission on a hand held display. As the operator increases the spindle rpm the display shows the gripping force almost halving at top speed given a constant operating pressure :bugeye:

Phil, my machine hasn't got the facility to change pressure under program or panel control - it's a 19 mm spanner on the regulator  :ddb: So really I'm only wanting to check it statically. I have to say that the little 3 cylinder hydraulic device looks quite easy to make, and even accepting that it won't be calibrated it will be a very useful comparative device. Apparently as the operating ramps in the chuck degrade the grip reduces rapidly, though to be honest it probably is a 'non problem' with my low usage.
Andrew Mawson
East Sussex

Offline Lew_Merrick_PE

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Re: How to test the gripping force of a chuck?
« Reply #16 on: September 14, 2014, 12:41:22 PM »
OK, the chuck jaws have a given mass (measured in either kg or lbm) and a specific center of mass (also called center of gravity).  If you have these to values, the acceleration they undergo is given as a = v▓/r where r is the distance in m or inches from the center of the spindle to the center of massv is the rpm of the spindle * r * 2 * pi / 60 and will end up in either m/sec or in/sec.  Thus, your acceleration ends up as: a = 4 * pi▓ * rpm▓ * r / 3600.

1 lbm X 1 g = 1 lbf.  Thus, if you are using lbm and inches, your force per chuck jaw will be: F = ma = (M lbm *  4 * pi▓ * rpm▓ * r / 3600) / 386.09 -- where 386.09 is 1 g in inches/sec▓.

The caveat here is that though a kg is supposed to be a measure of mass, the unit kgf, which is 1 kg X 1 g has become quite common in recent years and, if your measure was in kgf, then the acceleration needs to be divided by 9.807 m/sec▓ to turn it into a measure of gravities.  I work with a number of European-based companies that regularly supply "mass" values in kgf -- and often do not distinguish between mass and force which leads to major confusion (especially as the "proper" measure of force in the metric system is that worthless measure the Newton -- which is why the kgf has come into usage)!