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outback
10-17-2012, 04:41 PM
I need to measure a few pounds of mechanical force. Ya, I know I can compress an air cylinder and measure the air pressure.

Years ago, when I worked a day job I, I used a device in an arbor press, that would send to a digital display the amount of force (in pounds) that was applied. I cannot remember what the device was called. It was a small cylinder about 3/4" dia and 2" long with two wires comming out of it.

The same device is also used in electronic weighing scales.

I would like to either find one or make one for a project I have going. Nothing fancy either. Depending on how these things work, possibly I could use a digital volt meter for a readout.

What are these things called? Jim

Guido
10-17-2012, 04:55 PM
http://www.omega.com/pressure/psc.html

--G

oldtiffie
10-17-2012, 05:00 PM
Stress gauge? Strain gauge pehaps.

http://en.wikipedia.org/wiki/Strain_gauge

Mcgyver
10-17-2012, 05:26 PM
what you had was a load cell....but for a few pounds how about the cheap small electronic scales that use a strain gauge. just put it at the receiving end of the force; unlike a balance they work in any direction

if you want the strain part you can pull one out of a cheap harbour freight digital scale. As I understand it they measure down to a very tiny amount of voltage (nanovolt?) and use like a 24 bit AD converter (ie change in voltage from strain gauge is over its range is very small)......so I'd suspect measuring with a standard volt meter wouldn't work.

can you describe the set up?

rohart
10-17-2012, 05:40 PM
The simplest strain gauge is a thin layer of resistive metal that snakes back and forth sandwiched between layers of plastic. You stick it onto a load 'beam', something that bends when you press one end of it. The strain gauge is either stretched or compressed very slightly as your load beam bends with the force. This lengthens or shortens those lengths of resistive metal, and the total resistance changes. You need some fairly sensitive, but possibly quite simple, electronics to measure the changes.

This kind of strain gauge can be picked up new from electronics stores for very little, and you can make up a simple bridge circuit to monitor what's going on for a similar pittance. You could get the whole thing going on a bread board for ten dollars. You'd need to do some reading, though.

strokersix
10-17-2012, 06:14 PM
A 1 square inch area piston in an oil filled cylinder with a pressure gage works really good. Scale as needed. I have one I made for checking engine valve spring pressure (force at installed height). I've used it for other purposes too.

Bob Fisher
10-17-2012, 06:36 PM
What is the magnitude of the force are you trying to measure and what sort of resolution do you require? As others have mentioned, you need a load cell, but there are other ways to skin this cat. You could use a fish weighing scale or a tool once used to measure distributor point force. It all depends on how accurate you want to be. Bob.

Peter N
10-17-2012, 06:36 PM
You need a force & deflection gauge. Chatillon is probably one of the better known brands, and I used one of these years ago for measuring Bulb (Lamp) insertion force vs deflection in the contact pads of rear lamp assemblies for Cars.

http://www.chatillon.com/products/gauges/index.aspx

Haaving said that, I made a bench tet rig for a customer a few years ago incorporating a load cell & meter from Mecmesin, then outputting data to a PC.
Very simple to put together, Mecmesin link here and testrig piccie below. http://www.mecmesin.com/force-testing-products/force-instruments/universal-indicator-and-loadcells

http://www.btinternet.com/~p.neill/TestRig_1.jpg

lbhsbz
10-17-2012, 06:55 PM
I made one for a valve spring checker. I used a harbor freight short ram jack, measured the diameter of the piston, calc'd the surface area, then submerged it in a bucket of hydraulic oil to reasemble and replace the fitting with a pressure guage while submerged (make sure no air got in the system). Apply force, read gauge, do a bit of math (I made a chart) and you've got your force. Ideally, I'd have like to have a cylinder with exactly one square inch of surface area, but couldn't find one...the HF short ram was about 1.17" IIRC.

aostling
10-17-2012, 07:07 PM
I need to measure a few pounds of mechanical force.

To what accuracy?

outback
10-17-2012, 07:22 PM
"Load Cell", that is it. I have not heard that term in 10 years.

I'm building a cycle tester for a customer where I need to pull on a piece of electrical wire with a force of 4 pounds. I'll be using air cylinders to do the pulling. I'm aware I can calculate the area of the cylinder piston and regulate air pressure for a theoretical pulling force.

But, I would like to have a way to check the actual pulling force. I would like to have an air cylinder push against a load cell then adjust a regulator to a specific pushing force.

A cheap digital scale has some possibilities or something like that. I would like to make something if possible. Thanks, Jim

Mcgyver
10-17-2012, 08:00 PM
compression is easier....but here's an idea.

you make something sort of resembly the shape of a C clamp - maybe out of a piece of AL channel. digital scale gets fixed to the inside of the none moving leg - you make the AL extrusion is much longer than a C clamp to suit the scale. Outside of that leg as a eye and cable goes to the item being pulled. The other end, instead of a screw clamp, has a plunger and spring. Plunger presses down on scale, say it reads 10 pounds - force exerted by the spring. you pull on the plunger (or whatever you attach to the plunger) until the scale reads 8 lbs and there's 4 pounds of force on the cable to the test piece. You'd have to support in on something of the cable runs other than vertical.

A friend who has expensive ones says these cheapos are 0.1% accurate by his comparison testing....quality and calibration of scale is up to you and the customer

Bill736
10-17-2012, 08:16 PM
I suppose the most simple way is to attach a 4 lb. weight to the wire, using pulleys and a string. A spring scale , like a fish weight scale, is next on the simple list. (Accuracy is not so good with a fish scale, but a good fish scale adds 3 lb. to the real weight anyway !) A mechanical balance scale, like an old fashioned laboratory scale, is also simple, and pretty accurate. Whatever you finally use, be sure you can easily check the accuracy, and recalibrate if necessary, with a simple weight.

mechanicalmagic
10-17-2012, 08:46 PM
Digital Fish Scale, HF:
http://www.harborfreight.com/catalogsearch/result?q=fish%20scale
DJ

camdigger
10-17-2012, 10:45 PM
Load cells systems usually are easier to build if the sensor is in compression. The easiest way I've seen to accomplish this is to have the load cell sandwiched between the contact point of two links. When the links are pulled apart, the load cell itself is in compression.

outback
10-17-2012, 11:59 PM
The digital fish scale should do the job just fine and the price is right.
Thanks Dave

Jim