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  • Another calculation problem?

    I need to size the hydraulic cylinder to lift the table on my sheep shearing machine.

    I need to know how much force will be needed to lift the arm at the yellow circle with the ? in the middle. There will be two of these lift arms. One on each side of the table.

    Location: The Black Forest in Germany

    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  • #2
    We need to know the exact angle between the cylinder axis and the arm at the start and end points to be sure, but assuming the angle is about 30 degrees, the force required is about 1950kg. The force required will increase as the load is raised, and you need to add some to overcome friction in the system.
    Any products mentioned in my posts have been endorsed by their manufacturer.

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    • #3
      With the arm lowered the angle is 15 degrees. With the arm raised it is 82 degrees. Is the 1950kg. for each arm?
      Location: The Black Forest in Germany

      How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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      • #4
        Some observations, not meant as criticism, as there may be space or other constraints that I am not aware of:

        The cylinder would be so much more effective if it could be arranged to push the load rather than pulling. The rod diameter takes away from the area of the piston.

        The location and angle that the cylinder is pulling places it at the least mechanical advantage when the load is greatest. As the framework approaches vertical, less force is needed, but the cylinder is approaching more of a right angle where it can assert more force.
        Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
        ~Practitioner of the Electromechanical Arts~

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        • #5
          The figure was a total for both arms, but that's MUCH more of a swing than I envisioned, so I'd like to see a diagram with the arm in the raised position and in the lowered positioned.

          Weston makes two good points that you should try to incorporate into your design.
          Any products mentioned in my posts have been endorsed by their manufacturer.

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          • #6
            I assume the pivot point of the contraption that's lifted is that circle just above the right end of the blue line.

            Measure the horizontal distance from the pivot to a vertical line through the location of the 150 kg load. Multiply this distance by 150.
            This gives the moment about the pivot in kg-mm.

            Measure the shortest distance from the centerline of the piston rod to the pivot. The force required times this distance is your lifting moment, and has to be greater than the other moment by whatever margin you consider appropriate.

            So, if that second distance is L, then

            Force required = k X 150 X 1300 / L,

            where k is your design factor, which you have to select. Obviously, k should be greater than 1. I wouldn't make it less than 2.5, but that's nothing more than a wild guess, because I don't know enough about your machine and possible hazards.

            The distance from pivot to rod will increase as the load goes up, and the force required will decrease, as Weston says. If you arrange the pistons to push, the opposite may apply.
            Last edited by cameron; 07-16-2014, 08:57 AM.

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            • #7
              Use moments, clockwise, then anticlockwise about the centre, just force times distance clockwise and anticlockwise, easy, ish
              Handy thing to know
              Mark

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              • #8
                Thank you for the help. The hydraulic cylinders are on order. I understand Weston what you are saying and I agree but it needs to be above the floor line and not block other areas of the machine when the table is in the down position.
                Location: The Black Forest in Germany

                How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                • #9
                  Originally posted by winchman View Post
                  the force required is about 1950kg.
                  1950kg at what accelleration? (probably 9.8 m/s^2, right )

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                  • #10
                    Originally posted by Royldean View Post
                    1950kg at what accelleration? (probably 9.8 m/s^2, right )
                    Is acceleration a factor? I hope BF is not planning to use this to catapult the sheep into the next field after it's sheared?

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                    • #11
                      Originally posted by cameron View Post
                      Is acceleration a factor? I hope BF is not planning to use this to catapult the sheep into the next field after it's sheared?
                      That would sure save me a lot of time and money. I wouldn't need all those fence panels to get the sheep where I want them. Do you think it will be necessary to put one of those horns like they use for trucks when they back up? Maybe a loudspeaker by the pen where they land...........INCOMING!!!
                      Location: The Black Forest in Germany

                      How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                      • #12
                        I don't want to rain on the parade, but there seems to be a good amount of steel or some metal being lifted here. So I must ask, is the mass of all that metal included in 150 kg or just that inside the yellow box? Or is that just the weight of the sheep?

                        That being said, I have to totally agree with Weston Bye. It would be a much better design if the cylinder were mounted vertically, with the fixed point directly below the point of attachment on the arm and using it to push instead of pull. It would apply maximum force when the arm is in the lower position where it would need more and that force would decrease as the arm is raised and less force is needed. You would probably need to extend the point of attachment on the arm to allow for the length of the cylinder. Adjust as needed to get the swing angle you need.

                        Why don't you just train the sheep to climb on and off the table? I mean, are they retarded or something?
                        Paul A.
                        SE Texas

                        And if you look REAL close at an analog signal,
                        You will find that it has discrete steps.

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                        • #13
                          I have a lot of space considerations to deal with in this project. As it is I am making a few concessions as to what is below the plane that would be the floor of a trailer this machine gets mounted on.

                          I might also consider a setup like the one in this picture. The yellow frame is slid back in the picture to reveal how the cylinder forces work.

                          Location: The Black Forest in Germany

                          How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                          • #14
                            Much better.
                            Weston Bye - Author, The Mechatronist column, Digital Machinist magazine
                            ~Practitioner of the Electromechanical Arts~

                            Comment


                            • #15
                              Yes, I agree, that looks better.

                              It looks like you are starting the stroke with the arm on your pivot at about a 45 deg point back toward the cylinder. This reduces the force of the cylinder to about 70% as applied. I would try to make that arm point straight down at lowered position so the amount of force at the start, when you need the most, would be close to 100% of what the cylinder can deliver. From that point, you should be able to get your 67 (82-15) degrees of travel with the reduction of force just about equal to the reduction in the effective load. It all balances out quite nicely. This should make maximum use of your cylinder's capability in case some of the sheep are extra fat.
                              Paul A.
                              SE Texas

                              And if you look REAL close at an analog signal,
                              You will find that it has discrete steps.

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