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I'm just copy and pasting this...found elsewhere. Thought I'de pass it on. If you see something wrong, point it out.
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My homemade jack scale.
You will need to locate a hydraulic bottle jack that has a service port. This will be a screw out plug on the bottom of the jack. Unfortunately, jacks at Wal-Mart and Pep Boys (made in China) no longer seem to have the service port. So, you may need to search out yard sales or an older friends garage to find a bottle jack with the port. Remove the plug from this port and install a pressure gauge - I selected a liquid filled gauge that will measure up to 3,000#. To convert this to weight, measure the diameter of the jack cylinder (mine was 1.125 inches) and apply a little math to get the area of the cylinder (3.14 x radius squared). In my case 1.125 diameter /2=0.5625 radius x 0.5625 radius = 0.3164 x 3.14159 (pie) = 0.994 (my jack factor). Multiply the area by the pressure and you get weight.

The above information was originally found on an RV Camper forum and written by “Professor95”.
I changed the numbers to match my jack dimensions.
If you use a jack with a 1-1/8” cylinder you can use the direct read from the gauge and be accurate within 6 pounds per thousand. Gauge reading =1000# = 994# actual.


Gauge at Northern Tool was 12 bucks and the adapter fitting was about $6.59.
If you have a rubber plug and spring behind the service port metal screw remove the rubber and leave the spring. If the spring is so small in diameter that it slides into the new fitting you should cut a small hole in the rubber disk and use it as a spring stop. I think the spring holds pressure on a metal ball valve.

The front of my Baja weighs 706#







Dia Factor

1.125 0.994018711
1.5 1.767144375
2 3.14159
2.5 4.908734375
 

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don't bottle jacks use a bigger bore than their rod diameter like most cylinders...like say a 2 inch bore and a 1.5" rod...if they do that would throw everything off
 

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Meh, just go out to your Work truck and take 4 load cells and an indicator and build a scale.



Oh yeah, Guess it helps if you are a Scale tech:flipoff2:
 

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Not any more, now we have nist traceable pressure producing testers.
The power plants where I worked had NIST traceable cylinders with precise bore diameters and NIST traceable precise masses that were placed on the cylinders and used to calibrate pressure gauges from inches of mercury up to multiple thousands of PSI. AFAIK, this is still the only method accepted by ASTM for gauge calibration, ie-displacing a known volume of oil with a known mass pressing against a known surface area.
 

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Have to take a look at my jack, but if there is no service port I'd think that you could drill and tap the body to install a pressure gauge?
 

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don't bottle jacks use a bigger bore than their rod diameter like most cylinders...like say a 2 inch bore and a 1.5" rod...if they do that would throw everything off
you are correct, you need to know the actual piston (+seal) diameter to accurately determine the area to use.
 

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you are correct, you need to know the actual piston (+seal) diameter to accurately determine the area to use.
I'de think even if you had no clue, you could build it, then weigh a couple things you already know the weight of for reference and figure it out from there.
 

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I'de think even if you had no clue, you could build it, then weigh a couple things you already know the weight of for reference and figure it out from there.
true, but I just wanted to clarify/confirm the comment i quoted.
 

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I wondered about that as well, so I emailed U.S. Jack and they were very helpful...

Here's what he said...



Braxton,

A 5-ton jack with a gauge port is what we refer to as a 5-ton hi-range, simply because it has a higher service range than a standard 5-ton jack, the part number is a D-51012. The ram diameter for the jack is 1.375", when a gauge is calibrated specifically to read in tons it has to be set up to read 5-tons on a 1-3/8" diameter ram. The actual bore diameter of the ram cylinder in which the ram slides is 1.506" I.D., then you have the polyurethane cup mounted to the end of the ram which makes the seal against the side walls of the ram cylinder and the O.D. of the cup is actually flared out to a dimension that is slightly larger than the I.D. of the ram cylinder to insure a good seal.

I have attached a drawing of the ram cup, the one used in this particular jack is the 2-58350-3. The drawing gives you a pretty good visual as to why they call these "U" cups, as the hydraulic oil fills the "U" it pushes the lip against the I.D. of the ram cylinder. All of this technical data is fine, but you will still need a gauge calibrated for 5-tons on a 1-3/8" dia. ram. If you do not have a source for getting a special gauge, let me know, we have a source in Tucson, AZ................. but the gauges are not cheap by any means. Hope this info helps you out Braxton.

Bob Conrad - Gen. Mgr.
U.S. Jack

----- Original Message -----
From: Braxton brantley
To: [email protected]
Sent: Tuesday, June 24, 2008 5:03 PM
Subject: bore size

Hey, I have one of your 5 ton jacks with the gauge port and was wondering, is the bore size of the ram the same as the diameter of the rod or is it larger? Just wondering so I can use the pressure gauge to measure weight as well, thanks.


Here's the pic of the seal.



And after looking at some numbers, it looks like difference in rod/ram size can make a decent change in the weight figures...around 200lbs at 7oo psi with this larger 5ton jack.
Sorry for the lack of sleep wording.
 
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