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.

PRODUCTS:

1x Short Body Ram (I used 20t but 5t or 10t would be fine too)

1x Pressure Gauge

1x Hydraulic Oil

TOOLS:

1x 14mm Spanner

1x 17mm Spanner

1x Oil Can

1x Teflon Tape (Preferably oil resistant sealant)

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.

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.

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.

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.

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?

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).

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.

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.