Twenty Dollar Twenty Minute Load Cell

[7HC]

I hesitate call this a project because there's so little machining involved (drill two holes and tap a thread), but I figured it might be of interest to anyone else who needs to measure the tongue weight of their trailer, which is important when moving that mill, grinder, lathe, or in my case a 10K lb boat.

The pic makes the title pretty self explanatory, a $12 Wal-Mart 2 ton bottle jack, a $4 gauge (eBay), and $4 in brass fittings and jack oil, then twenty minutes to complete it.

The procedure is to first disassemble the jack by unscrewing the cap complete with piston while keeping the base clamped in a vise.
Discard the oil along with all the carp floating in it, then clean everything up taking care not to damage the piston seal or any of the 'O' rings.

Next, drill an 1/8" hole horizontally into the base from outside, followed by a similar one vertically to intersect it inside after unscrewing the cylinder bore.
Open the first 3/4" of the horizontal hole to 21/64" and tap it 1/8"-27 NPT which will probably be the size of fitting you step down to.

Finally, measure the piston head diameter before cleanup, reassembly, and filling with fresh oil.

Now all that needs to be done is to calculate the area of the piston head (pi * r squared) in order to be able to apply a correction factor to the psi reading from the gauge (mine was 0.7), and not only do you have a tongue gauge, but you also have a dedicated trailer jack as well!

To broaden the scope slightly, the same technique can be used to adapt a press, or floor or pallet jack to display their loading too.



M

 

[4GSR]

Is that 90 deg ell rated for 2000 psi? I don't think so.... Have you proof tested the jack to 2000 psi?

I would suggest using hydraulic fittings good for 3000 psi.

 

[jumps4]

that was a good idea
steve

 

[7HC]

Is that 90 deg ell rated for 2000 psi? I don't think so.... Have you proof tested the jack to 2000 psi?

I would suggest using hydraulic fittings good for 3000 psi.

Your point is well made, and no I don't know the pressure rating of the ell.
The load I'm measuring, the tongue weight of my trailer, is ideally 1000lbs which is a reading of approximately 1500psi on the gauge.
I operate it from the right side, pumping until it supports the tongue which raises the ram about an inch, then I step back to see the reading.
It hasn't let go yet but if it does, being a casting it'll probably crack, causing an instant loss of pressure and a nasty mess.

I worry more about hydraulic hose pinhole leaks which can inject oil into the skin causing serious injury.

M

 

[4GSR]

7HC,

The idea and thought behind the load cell is a great idea.

I'm just safety conscious about things like this. I've seen 90 deg ell malleable steel fittings about this size blow with about 2,000 psi across them, just because we were too lazy to go buy the correct fittings!

Just one note about oil and when it penetrates your skin. It take less than one drop of oil to get in your blood stream and stop your heart from beating! Be careful around pressurized oil of any kind!

 

[Tony Wells]

I'll be with Ken on this. I've worked around high pressure (something about us oilfield guys, eh Ken?) and you can't be too careful. And fittings rated properly for this aren't that expensive. Just a note of caution and safety. On some applications you might get away with this, but someone else using a setup like this for 20 tons instead of 2 would be in trouble, I'm afraid.

 

[7HC]

I'll be with Ken on this. I've worked around high pressure (something about us oilfield guys, eh Ken?) and you can't be too careful. And fittings rated properly for this aren't that expensive. Just a note of caution and safety. On some applications you might get away with this, but someone else using a setup like this for 20 tons instead of 2 would be in trouble, I'm afraid.

I agree, and my orginal post should be taken as an example to explain the principle, and anyone modifying a jack or hydraulic cylinder in that way would be well advised to use fittings that could take pressure in excess of that which could be generated by the device.


M

 

[Tony Wells]

I think it's a great idea, and all of our hydraulic presses should have a way to monitor pressure. I have a 20 ton press that I really would like to implement this on. I just need it to move up on the "to do" list. That list is way to long just for the shop, paying jobs aside, and in addition to the things around the house that need taking care of.

I wish you had taken a few pics with the jack apart. I'm not altogether clear on where to create the passageways. I'm sure I could figure it out, but it would be nice to see how someone else did it.

 

[7HC]

I think it's a great idea, and all of our hydraulic presses should have a way to monitor pressure. I have a 20 ton press that I really would like to implement this on. I just need it to move up on the "to do" list. That list is way to long just for the shop, paying jobs aside, and in addition to the things around the house that need taking care of.

I wish you had taken a few pics with the jack apart. I'm not altogether clear on where to create the passageways. I'm sure I could figure it out, but it would be nice to see how someone else did it.

Most jacks are made the same way and it's not very complicated, so I can probably describe it satisfactorily.

You disassemble it by first draining the oil (usually a rubber plug on the side), then unscrewing the collar at the top which usually has flats on it.
The collar does two things, it allows the piston to pass up through it when the jack is operated, and it holds the reservoir in place, which is the thin outer shell of the jack.

Once the collar is removed, complete with the piston, the reservoir can be lifted off the cast base which reveals the tube that's screwed into the center of the base, that tube being the housing that the piston rides in.

Unscrew the tube because that's where you need to drill down into the base, i.e. below the piston. Then, on the opposite side to the 'pump', drill through the side of the casting to meet up with the first hole.
I used an 1/8" bit for both holes, then opened up the first part of the horizontal one to accomodate a 1/8" NPT tap (21/64" if I remember correctly).

That's it really. If you're lucky you might find that the area of the piston is 1 sq/in, in which case you can read the load in pounds directly from the gauge, if not you need to calculate the conversion you'll have to apply to the reading.

Let me know if that's as clear as mud and I'll try to clarify.


M

 

[Tony Wells]

Thanks for the details. Should be simple enough. This is an older Sears jack, and there is a leak somewhere on it that makes a mess, so I think a rebuild kit, or a seal kit anyway, and some time and I will do this to it. Like I said though, the "to do" list is pretty long.

Being a 20 ton jack, I doubt the piston is as small as 1 square inch, as the ram is about 2-2 1/4" or so in diameter. Simple enough math though.