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Hydraulic press design - FEA

  1. Feb 16, 2016 #1
    Hi guys,

    Working on a 50-ton hydraulic press design using a standard H-frame design, like this:

    bva-imp5506-hydraulics-press.jpg

    I'm working on the design of the vertical members. Similar to the photo above, the verticals have 1.25" diameter holes drilled for pins that support the horizontal bed. The problem: while determining the size of steel to use, I'm finding that the hole stresses require a much thicker steel than what I see being used in commercial machines.

    Specifically, in order to support the maximum 50-ton load, that equates to 25,000 lbf per vertical support. If I use two 1.25" pins per side (4 total) to support the horizontal bed, that works out to 12,500 lbf of force per hole.

    I've modeled a 1"x4" A36 flat bar at some length with 1.25" dia. holes space 5" apart. The base is fixed, there is a bearing force of 12,500 lbf per hole (so 25,000 lbf total), and a reaction force of 25,000 lbf along the top of the bar. Running SolidWorks Simulation, I get the following:

    img1.JPG img2.JPG

    The max stress is in the top hole, 27,600 psi. While this is OK, if I use, say, 1/2" plate, it yields. Commercial units I've seen use 1/2" or less. Many only use a single pin per side. I just don't get it. Am I doing something wrong here? Are the manufactures not taking into account the hole stresses? Are they not as important as I'm suspecting?

    Any feedback would be greatly appreciated. Thank you!
     
  2. jcsd
  3. Feb 16, 2016 #2

    Baluncore

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    Plastic design can greatly reduce the mass of a design since it takes a small amount of the material safely past the yield point. Maybe you should consider plastic deformation of the hole. Allowing some yield will limit the peak stress as the force is then spread over a greater area.

    I use induction hardened, high tensile hydraulic rod for the pins. Since pins must be slightly smaller than the holes there needs to be a pear shaped deformation of the holes to bear the full load on the surface of the pin.

    The bed is usually made from channel, so the pin is against, and deforms the flat flange. That is not good for the bed channel's web, or the pin. For heavier presses, the bed needs to have holes through thicker plate or collars.

    The presses that use 4 vertical bars tend to be a bit unstable as the misalignment of the head and bed can eject the tool or work piece. It is better to use a single channel for the vertical on either side. Select or fabricate channel with a heavy flange.

    Vertical hole separation of 5” gives you 5” steps. You might consider drilling less holes in the vertical columns. By drilling two holes, one above the other in the bed, say 3” apart, you can then use less holes, say 6” apart, in the vertical, to get 3” steps. My last press design used three holes in the bed to give a “vernier” choice of bed height and so better utilisation of the ram length without unstable packing.
     
  4. Feb 16, 2016 #3
    Excellent information! Thank you very much. I am working on this as a senior capstone project for mechanical engineering. We're taught so much to keep things below yield, not much has been discussed in my courses about designing in the plastic region. I discussed this with one of my professors today, who is a licensed PE, and he did agree. Although for the purpose of this project, they may not want us to produce a design that is yielding. I'm still working on clarification there.

    C-channel was looked at for the vertical members (and is preferred), but standard C-channel is tapered and the hole stresses required the change to use flat bar. With some allowable yielding, that would certainly open back up the possibility to use C-channel, or plate bent to such a shape. Obviously the commercial units work, so I don't doubt the design. We just need to produce a justifiable design.

    Use of a single pin per side would also be preferred, but again, with the hole stresses simulated, distributing the loads over two pins per side was needed to prevent yielding. I'll continue to dive into understanding plastic design better. If you know of any specific resources that may help, please let me know. Thank you!
     
  5. Feb 18, 2016 #4

    Baluncore

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    I think you misunderstood my suggestion. I suggest only one pin per side, but two possible holes, in the bed, one above the other. That halves the number of holes needed in the columns for the same bed position step size.
    If the holes in lighter columns were fitted with collars to spread the pin load, halving the number of collars would be a saving worth taking.

    For shear, the cross-section area of the pin rises with the square of the diameter, but the pin bearing area only rises linearly with diameter, and with column thickness. Have you solved for optimum pin diameter and therefore column thickness?
     
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