I wanted to test pull out strength of bolts with my shop press. I needed something to give an indication of force. I bought the cheapest gauge on ebay. It is a Harbor Freight press so they will get along well. It is a pretty decent gauge!
I started to disassemble the jack. This took a LOT of force.
There was dirt and loose burrs inside the unit. These are clearly made at a price point. Machining finish, deburring, and cleaning are not of concern. Most likely this will collect and sit in the reservoir for the life of the jack.
The cylinder was about 1/4" thick DOM. I got it off with a pipe wrench, but it really fought me. A chain wrench would have been a better choice. Chain wrenches, however, cost more than 1/2 what I paid for the bottle jack. I am showing you how I did it, not necessarily the right way!
You could see the plug marks under the paint for the internal porting. I needed a 1/4 NPT tap and my own port into the cylinder bore. My ports needed to avoid any existing ports. Since I was dealing with very large forces, it would also be nice if I did not crack the base!
I laid it out in CAD. I went with an 1/8" shallow hole in the cylinder base. The 1/4" NPT holes has a 3/16" port which meets up with the 1/8" hole. There is plenty of meat where nothing is going to break. The gauge piping is all schedule 80 or better.
Harbor Freight gave me new seals in a rebuild kit when I bought the jack so I installed them. The piston is a hard plastic cup seal forced up against the cylinder wall by an o-ring. My guess is it does not seal air very well. I would think this helps bleed air out of the normally upright cylinder. The bleed procedure never states to flip the jack upside down. Air rises up towards the piston and this is it's only way out.
I cleaned everything up. I removed many burrs I made and existing burrs. The gouges from the pipe wrench are on the outside and won't hurt anything.
The numbers:
Radius = 56mm/25.4/2 = 1.102in
Area = 3.14 * 1.102in^2 = 3.82in^2
Theoretical
Force = 20T * 2000lbs/T = 40,000lbs
PSI = 40,000lbs/3.82in^2 = 10,471lbs/in^2
Actual (pressure relief trips between 7-8 kpsi)
Force = 8000lbs/in^2 * 3.82in^2 = 30,560
30,560lbs/2000lbs/T = 15.28TA 20 Ton bottle jack will not produce 20 Ton of force. There is a built in safety factor. It is just as well. I am not 100% sure the press would hold 20 Tons.
Thanks for looking! There are more pics, CAD, and CAM here...
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