Basic Question about hydraulic system pressures
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  1. #1
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    Default Basic Question about hydraulic system pressures

    The answer to this question will be painfully obvious to most folks here but I'm a tad on the slow side!

    I have a hydraulic shop press with a air/hydraulic pump and a 1 1/2" ram. Mounted between ram and pump is a pressure gauge. If I press an object and see 1000 psi on the gauge, then the force at the end of the ram is 1000 X 1.767, or 1767 pounds of force! Is this correct?

    If that is indeed correct, is there simple way to have a gauge read actual ram force as opposed to applied PSI. Maybe a 'corrected overlay' glued to the front of the gauge!


    Stuart

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    Your math is correct. It might be possible to recalibrate the gauge to read higher, but I doubt there is enough adjustment available. There is a product called "Presstype" that had rub on letters. The type is on a plastic sheet you position over the surface and rub to transfer it. In the 60s and 70s when I was making specialized equipment, I would frequently sand off words like "milliamperes" and replace it with "percent scrap" or some such thing.

    Here is a Google. It is still sold under several different names. I got mine at art or office supply stores. If you are sufficiently adventurous, you can add another scale inside of the original or remove the original and make a new one in its place.

    Google

    Later I learned to silk screen and made a lot of locomotive speedometer dials for the Mexican market with both MPH and kilometers per hour, but you are not likely to go to that much effort for one dial.

    Bill

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    Enerpac and others have gauges that are calibrated in lbs or tons for a given size ram. Often the gauge will have multiple scales to accommodate different size cylinders. For instance, I have a 30 ton / 10k PSI hollow cylinder. I use the scale that has 30 ton as the maximum value.

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    The human eye is pretty good at interpolating between lines . . . figure Force for three or four gage points, magic marker, and done.

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    The truck axle and transmission lab I worked in circa 1968 had a Dake 75 ton press. We needed a more accurate gage to measure ram force. I ordered a high accuracy gage with a stock PSI dial. The instrument lab took the glass off and used the dead weight calibrator to verify that the pointer was indicating the correct pressure. Then I applied Letraset dry transfer lines, numbers and letters to create a custom dial that read ram force in pounds in addition to pressure in PSI. The result could pass for OEM.

    Letraset - Letraset Transfers

    A paper chart hanging next to the press would work almost as well, but I had fun reworking the dial. In later times we would use an electronic pressure transducer and custom computer to read out force and pressure in either English or metric units and record the data.

    Larry

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    Question: does the actual tonnage figure actually mean anything? Or do you just need a way to repeatedly obtain the same force?

    I have a series of 4 different PSI range gauges I use on my 75 ton press. They are plumbed into the system using hydraulic quick couplers, so that they can be swapped out in a few seconds. This permits the press operator to select a gauge for the task at hand. If you are doing light duty work, a gauge reading 0 to 10,000 PSI isn't sensitive enough to be useful, but a gauge reading 0 to 100 will be very sensitive. When I have a bunch of bars to straighten, I find by trial and error at what pressure the part actually yields at, and then it is quite easy to run the next one right up to that pressure and have it yield. Differences in yields strengths between different batches of material is even observable, but I always know when I am getting close to the magic point at which I can get a permanent bend to occur. Watching the gauges, is just watching a number, the actual tonnage required doesn't even enter my mind.

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    Quote Originally Posted by L Vanice View Post
    The truck axle and transmission lab I worked in circa 1968 had a Dake 75 ton press. We needed a more accurate gage to measure ram force. I ordered a high accuracy gage with a stock PSI dial. The instrument lab took the glass off and used the dead weight calibrator to verify that the pointer was indicating the correct pressure. Then I applied Letraset dry transfer lines, numbers and letters to create a custom dial that read ram force in pounds in addition to pressure in PSI. The result could pass for OEM.

    Letraset - Letraset Transfers

    A paper chart hanging next to the press would work almost as well, but I had fun reworking the dial. In later times we would use an electronic pressure transducer and custom computer to read out force and pressure in either English or metric units and record the data.

    Larry
    A wide range of fonts is available. In 1971 I needed a nameplate for my 1941 Army Indian. I was able to match the several fonts with large Presstype, made one about 18" high and photographed it down to 3" for the artwork to expose Kodak Photoresist coated metal which I then etched. On something like Larry's gauge dial it is almost impossible to tell it from a printed original.

    Bill

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    Stuart,
    Is the piston also 1.5" in diameter? My 50 ton Dake press has a 2" ram and a 5" piston and cylinder diameter, that makes it direct reading i.e., the gauge reads in psi which equals ten times the number shown in tons.
    For example if I press to 900 psi I get 9 tons of force, quite easy to read the gauge, very well thought out by Dake.

    I have added an electrical pump and it maxes out at 1700 psi (auto hoist unit) which means I can only go to 17 tons- a built in safety device! We added a hydraulic bypass valve and now we can set the pressure to bypass at the desired yield point, it works very well. Hand pumping is used for going above the 17 ton limit.
    I do find the gauge a bit clunky for small bends so Hu Flung's idea of adding a range of gauges is a very good idea, I might just make that happen.

    Michael

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    Make sure you attach a sign to your press saying "Install proper gauge for intended pressure range usage before operation". I got a couple now that got 'sprung' It is kind of like leaving the chuck wrench in the chuck, so, now I always take and install the high pressure gauge as soon as I'm done with my low pressure application.

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    Quote Originally Posted by M. Moore View Post
    Stuart,
    Is the piston also 1.5" in diameter? My 50 ton Dake press has a 2" ram and a 5" piston and cylinder diameter, that makes it direct reading i.e., the gauge reads in psi which equals ten times the number shown in tons.
    For example if I press to 900 psi I get 9 tons of force, quite easy to read the gauge, very well thought out by Dake.

    I have added an electrical pump and it maxes out at 1700 psi (auto hoist unit) which means I can only go to 17 tons- a built in safety device! We added a hydraulic bypass valve and now we can set the pressure to bypass at the desired yield point, it works very well. Hand pumping is used for going above the 17 ton limit.
    I do find the gauge a bit clunky for small bends so Hu Flung's idea of adding a range of gauges is a very good idea, I might just make that happen.

    Michael
    Michael,

    My press is a dinker..only 10 ton. It turns out the cylinder bore is 1 11/16" so I did what was suggested above and made a small chart to hang on the press that has gauge pressure conversions to ram force. It's simple...but that's because I'm simple. It seems to work.

    Stuart

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    Stuart,
    It sounds like you have it figured out, simple is best and your chart will work well.

    The ram housing on the Dake press is a very large storage tank for the hydraulic fluid, it is impossible see or guess the size of the bore and I was surprised that it was 5". It makes total sense when you run the numbers for a 50 ton press.

    Also so be safe with your press, ten tons is still a lot of stored energy.

    Michael


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