Simple DIY Force Gauge

by lachies_restorations

5.4K2714

Intro: Simple DIY Force Gauge

I’ve always wanted to know the clamping force of my vises in my workshop, but force gauges can range anywhere from $600-$1000 (aud). So I decided to make a cost effective version for around $90 (aud). I’ve put on a video for all the visual learners out there too.

STEP 1: Remove Hydraulic Coupling

So firstly we need to remove the hydraulic coupling as we won’t be needing it. Unscrew the cap which will reveal the 17mm hex head. Using a spanner or socket unscrew it and make sure it’s kept upright as the ram will be primed with oil, if you spill any it will cause air to enter the system which will then need to be bled out before the next step.

STEP 2: Fill Up With Oil to Extend Piston

Using an oil can filled with hydraulic oil form a tight seal in the filler hole and pump in the hydraulic oil slowly until the piston is fully extended as in picture 2. Be sure to pump slow so as to not allow air to be forced through the system. Fill the oil to the bottom of the threaded hole.

STEP 3: Sealing the Thread

I used Teflon tape (plumbers tape) to wrap around the thread, 3 or 4 times around should be plenty. Be sure to wrap the tape with the thread so when you are screwing the gauge in it doesn’t pull off the tape.

Note: Preferably you should use an appropriate sealant but Teflon tape is all I had and will do the job.

Also a Note: Before you screw in the gauge, lightly push the piston in until the oil reaches the top of the filler hole. This will make sure there is no air in the system but also allow sufficient movement so as the gauge is screwed in it will push the piston out before giving a preloaded false reading on the gauge.

STEP 4: Screw in the Gauge

Using a 14mm spanner, screw in the gauge. Be sure to go slow and be careful that you don’t go too far and have the gauge bottom out. Remember it’s only a brass thread so take your time. In this case I screwed it in 4 turns, it was snug but still left sufficient room at the bottom and wasn’t so tight that I risked snapping the brass thread. Turn the gauge to were you want it to face, in my case I have it facing the piston side.

STEP 5: Almost Done But Not Quite

Now it’s time for some maths… I know how much you all live maths. Our gauge reads in PSI or Pounds per Square Inch, the problem we have is our piston isn’t 1 square inch so the gauge will read differently to the actual pressure applied. If you paid $600 for a fancy one it would’ve been machined to exactly 1 square inch and no calculation would be needed… but we’re saving money here remember. So we need to find the area of the piston that is in contact with the oil (known as the Cylinder Effective Area) this can usually be found on the spec sheet of your hydraulic ram. In my case for this 20t ram the area of the piston is 4.43 inch².


Now for the math…


Whatever force is applied to the piston will read 4.43 times less on the gauge (Pascals Law). Therefore whatever reading you get on the gauge needs to be multiplied by 4.43 (area of the piston) to get the actual reading.

e.g The gauge reads 1000 PSI, we would simply multiply 1000 x 4.43 to give us the actual reading of… 4430 lbs… easier than you thought hey?

STEP 6: Using It

So here I have clamped the force gauge in my 100mm Dawn Offset vise, I’ve clamped it down as hard as I could by hand (no cheater bar, I’m not an animal). It’s reading at 790 PSI. So multiplying that by my magic number 4.43 gives this vices a clamping force of 3500 lbs (equal to around 1.59 tonne).

STEP 7: In Conclusion

Well you now have yourself a handy little tool. Not a daily user for sure, but hey, it was cheap.

KEY POINTS TO REMEMBER:

You MUST know the area of the piston that is in contact with the oil. If you can’t find a spec sheet then before filling it with oil remove the piston and measure it. To find the area use the formula πr² or alternatively an online calculator if you’re unsure.


Once you have the piston area make sure you multiply the gauge reading by that number to give you the actual reading in pounds.


Hope this helps someone, if you have any questions or feel I’ve left something out let me know below and I’ll try sort it out. Happy building.

STEP 8: Example of Its Use


Here’s a video I made using the gauge to measure various size vises. Just an example of the many ways it could be used.

14 Comments

datoo786 2 years ago

As the piston compresses the fluid to give a reading on the gauge, what brings the piston back to datum position?

datoo786 2 years ago

Considering the system pressure, are we saying that the pressure gauge needle return spring has enough force to push the main piston back to datum? If that is the case, a video would help because the video stopped after the main piston was pushed in.

Dwargh 2 years ago

Didn't know there's a need to measure pressure in a vice! Nice!

Metric system FTW! :)

rjkorn 2 years ago

I have a similar 20T ram from harborfreight. It has too short of a travel to really be useful to me as a frame straightener tool. I was going to do the same thing but never got around to it. I will have to try it. Should be easy to use since it has a 50mm piston which equates to almost exactly 3:1 conversion (50mm dia piston = 3.04 sq in surface area)

ChipsWoodShop 2 years ago

I love this idea. Not a big deal, but once you multiply by your piston area in square inches, the resulting clamp load is in pounds, not PSI.

chimewizard 2 years ago

"no cheater bar, I’m not an animal" made me guffaw :-D This is excellent, I have no immediate use for one but - heh! - when did that ever stop anyone making a thing? :-) When introducing the pressure gauge to the ram body, is there likely to be an air pocket already in the gauge? Can it be bled out (if there is one)? Great project, thanks for sharing.

lachies_restorations 2 years ago

I’m actually not sure… this gauge had a ball bearing on the bottom to prevent air getting in until pressure is applied. But it was a cheapie from China so it is possible there could be a tiny amount of air in the gauge (there shouldn’t be). Don’t know how you’d remedy it though… I’m very trusting haha.

Karmudjun 2 years ago

Just curious - the air bubble in the gauge face - is it compressing along with the gauge compression mechanism, or is it remaining the exact same size and the oil pathway from the piston to the gauge compression mechanism is separate from the gauge face oil? There is error introduced if the air bubble is much more compressible than the oil filling the gauge face - I don't know how one would account for the air compressibility when calculating the force.

Nice build though. It would be a headache to open the gauge and copy the psi dial with the multiplier as a new ring of psi numbers. That would be some extremely fine etching on a scale to accomplish the multiplier alongside the original scales.

LucianoB16 2 years ago

The oil in the gauge dial is dampening oil to keep the needle from fluctuating too rapidly when pressure changes. Usually a light mineral oil. The bubble shown has no effect on the calibration of the dial. It can usually be topped up by removing a small rubber plug at the top of the gauge.

Karmudjun 2 years ago

Thanks for the info - if it were part of the pressure reading system - that would be a wrench in the accuracy of the system - very nice build!

Norm1958 2 years ago

I built a similar device for measuring valve spring strength using a 1 1/8" shaft for a piston which conveniently has a surface area of .994" which is so close to 1" the pressure gauge reads directly. No math, yeah!

datoo786 2 years ago

I am just guessing, how about lorry or truck wheel cylinder, which are commonly obtainable and to save more money?