Computing force from arbor press

crossthread · Jan 17, 2019

I need to control the force applied by my arbor press fairly accurately. Too much and it over presses. Not enough and it doesn't do the job. I replaced the handle of my press with a "click" style torque wrench but I am not smart enough to compute the mechanical advantage of the rack and pinion. The torque wrench will allow me to be consistent but I need to be able to document how much force I am applying. If I had a strain gauge (which I don't) or a very small set of scales calibrated in hundreds of pounds (which I don't have either) I could measure it directly. Can anyone tell me a slick way of converting foot pounds of rotational force to vertical force from a rack and pinion? Thanks.

Rob F. · Jan 17, 2019

The easy way is to use a bending beam torque wrench, or a dial type and see what it is reading when you are at the pressure you want.

Cole2534 · Jan 17, 2019

Use the pinion's pitch radius. If that's an inch, one ft-lb on the pinion shaft should be 12lbf on the ram.

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fciron · Jan 17, 2019

It's just a lever with both forces applied on the same side of there fulcrum. Your torque wrench gives you the effort force for an imaginary 12" lever (if it's in foot/lbs.). As pointed out above, the length of the applied force lever is the pitch radius of the pinion.

Here are some formulae for levers Levers

crossthread · Jan 17, 2019

Thanks Rob. Yes that would be the most direct way of being repeatable but I need to know what the actual pressure is at the anvil so I can document it. This project is for doing intaglio pressing into gold and silver. Trial and error is very costly and once the die is raised it is very difficult to register again. I have a rough idea of how much pressure to apply for different alloys but I am either too stupid or too old to compute it properly using this arrangement. I think after I turned 70 my brain went to oatmeal or something. Hell I've got to take half a viagra to keep from pissing on my shoes.

Rob F. · Jan 17, 2019

You can get a load cell and put it on the end of the ram, then you just read the numbers on the display. Or get a flat one and put it under your workpiece.

crossthread · Jan 17, 2019

Ahhh the pitch radius. Thanks guys. I may look into a strain gauge Rob. I was hoping I could just figure this out knowing the force applied to the pinion in foot pounds. Sounds like I can. Thanks again.

gbent · Jan 17, 2019

Put a bathroom scale under the ram.

Cole2534 · Jan 17, 2019

crossthread said:
Ahhh the pitch radius. Thanks guys. I may look into a strain gauge Rob. I was hoping I could just figure this out knowing the force applied to the pinion in foot pounds. Sounds like I can. Thanks again.

For sure!

Force= 1/pitch radius (in feet) times the torque (in ft-lb)

Example- pinion pitch radius of .625", delivered torque of 19 ft-lb
1/.0521 (.625/12) * 19 = 19.19 * 19 = 364.6lbf

You can use inches too, just change both units to reflect it.

adama · Jan 17, 2019

Lot lot easier to do this with hydraulics and a pressure gauge. other option used in a lot of die forming work is to simply compress a die spring x amount with the press simply bottoming out against a fixed stop, the force from said die spring then doing the work - limiting the applied force to the work to a very repeatable amount.

Bill Simmons · Jan 17, 2019

The above posters have given good advice. Having not seen the setup, I will add that the torque wrench socket end must be centered on the pinion and not attached with some sort of adapter that adds length to the torque wrench or additional calculations will be required.

I hope when you say "strain gauge" that you mean a load cell. A strain gauge could be used but the math and equipment required to use one would be overwhelming. A strain gauge or multiple strain gauges are often components of load cells.

Miguels244 · Jan 17, 2019

adama said:
Lot lot easier to do this with hydraulics and a pressure gauge. other option used in a lot of die forming work is to simply compress a die spring x amount with the press simply bottoming out against a fixed stop, the force from said die spring then doing the work - limiting the applied force to the work to a very repeatable amount.

I second the die spring approach.
Very accurate and repeatable.
I’d start here.
MISUMI USA: Industrial Configurable Components Supply

TDegenhart · Jan 17, 2019

adama said:
Lot lot easier to do this with hydraulics and a pressure gauge. other option used in a lot of die forming work is to simply compress a die spring x amount with the press simply bottoming out against a fixed stop, the force from said die spring then doing the work - limiting the applied force to the work to a very repeatable amount.

Great idea but, the spring needs to be calibrated and the thickness of work has to been held constant.

Tom

Miguels244 · Jan 17, 2019

If you can’t measure the pinion diameter there’s another way to determine the ratio.
Using a handle of known length and an indicator determine the ratio by direct measurement.

AA+ · Jan 17, 2019

Consider measuring the distance the hand moves against the distance that the press moves. The distance ratio is the same as the force ratio.

Best wishes --- Allen

DDoug · Jan 17, 2019

Bill Simmons said:
The above posters have given good advice. Having not seen the setup, I will add that the torque wrench socket end must be centered on the pinion and not attached with some sort of adapter that adds length to the torque wrench or additional calculations will be required.

I hope when you say "strain gauge" that you mean a load cell. A strain gauge could be used but the math and equipment required to use one would be overwhelming. A strain gauge or multiple strain gauges are often components of load cells.

You must be a retired college perfessor...... :crazy:

Bill Simmons · Jan 17, 2019

Digger,
Retired yes. But never a perfessor. But, when I was working I installed and setup a lot of strain gauges. There were so many issues that I would guess that only about half the time they provided accurate data. Used them in rocket engine testing. If you wanted thrust you used a load cell. If you just wanted to verify the timing and direction of forces they were great.

adama · Jan 17, 2019

TDegenhart said:
Great idea but, the spring needs to be calibrated and the thickness of work has to been held constant.

Tom

Yes but tourque wrenches, friction and pinion wear also makes that far from accurate too, if you need better than about 20% hydraulics or pneumatic is sweat, lots of industrial applications use a humble-spring, there pretty dang constant and so long as the spring is long enough and you only need a fraction of its stroke the errors from part thickness affecting force become low single digit % errors. Equally the stop the press bottoms on can easily be shimmed - made adjustable.

All depends what level of repetition the OP needs. Presumably from the description its just to cap the upper limit of force to a bellow assy damaging level, hence why something as simple as a spring setup can work well. If you need to apply say 458,5lbs of force, then a torque wrench ain't going to get you there either sorry, if you want sub single digit % variance on force, you have to measure it directly and feed it back into the loop.

Rob F. · Jan 17, 2019

digger doug said:
You must be a retired college perfessor......

Did I hear we are in the midst of a rocket scientist? :Ithankyou:

TDegenhart · Jan 17, 2019

adama said:
Yes but tourque wrenches, friction and pinion wear also makes that far from accurate too, if you need better than about 20% hydraulics or pneumatic is sweat, lots of industrial applications use a humble-spring, there pretty dang constant and so long as the spring is long enough and you only need a fraction of its stroke the errors from part thickness affecting force become low single digit % errors. Equally the stop the press bottoms on can easily be shimmed - made adjustable.

All depends what level of repetition the OP needs. Presumably from the description its just to cap the upper limit of force to a bellow assy damaging level, hence why something as simple as a spring setup can work well. If you need to apply say 458,5lbs of force, then a torque wrench ain't going to get you there either sorry, if you want sub single digit % variance on force, you have to measure it directly and feed it back into the loop.

I think we are saying the same thing, but let me summarize. There is a spring attached to the end of the ram. The spring works directly on the part, perhaps through a die set to control movement. The ram goes against a hard stop so that with a constant thickness of work, the force is always the same.

The spring, if it is a die spring, needs to be calibrated as die springs are only linear after they are pre-loaded, and they have a very high spring constant (k). A low k spring will accommodate thickness variation but require longer travel to achieve a high force.

If I had my druthers I would use a pneumatic or hydraulic cylinder as the force can be controlled very closely and the thickness variation is unimportant. k for a cylinder is constant.

Tom

Computing force from arbor press

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