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    Discussion in 'The main mechanical design forum' started by Jaimech, Jan 27, 2019.

    1. Jaimech

      Jaimech New Member

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      Hi All,

      How to avoid a Stick slip or a chatter when a cantilever load falls by gravity on a double guide linear slides as shown in picture.The cantilever load is approximately .5 kg and its centre lies 40mm from axis of shaft dia(5mm). The linear travel is approximately 40mm. The load chatters even though I have provided linear bush bearing on one side of the shaft. How to get a smooth movement both up and down without any chatter.

      Thanks,
      Jay
       

      Attached Files:

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    3. MSHOfficial

      MSHOfficial Well-Known Member EngineeringClicks Expert

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      There are differences between the static and dynamic friction that is the cause of the vibration (jerking) when the object is transitioning between being stationary and moving on the surface.

      Stick-slip in seals can cause softening, swelling, binding, drag, wear, and even failure in a mechanism. Here are the key factors to look out for to avoid stick-slip in your mechanism:

      Keep surface roughness not too low or too high. The material properties like hardness, elasticity, friction coefficients and maybe chemical properties sometimes should be compatible. You can use lubrication for friction coefficients too high or if your mechanism allows it. The contact pressure seems low because the weight is not to high of the object so you can most probably increase the roughness by a notch. Check dissipation of heat, frictional losses shouldn’t be too high.
       
    4. s.weinberg

      s.weinberg Well-Known Member EngineeringClicks Expert

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      As much as you can, make the bearing length as long as possible on each rail.

      Also if possible, line up your two rails with the load, rather than having the line between the shafts perpendicular to your load. That should help quite a bit.
       
    5. MSHOfficial

      MSHOfficial Well-Known Member EngineeringClicks Expert

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      Yes we did what you are suggesting, we made the bearings thicker and as long as we could go. but the idea was to reduce the excess loading. Not to change the design of the object. However, since the design team couldn’t come up with anything else they had to change the design.

      But the second thing that you are suggesting. You want us to rotate the rails by 90 degrees ? that is what I understood. If that is the case its not possible. Those rails are not connected to anything on the sides they run down the length of the cylinder and form something like chair legs. So we have 8 legs to support the whole thing. I guess the bend in the image file made it look like the rod extends to in the radial direction. But that was my mistake. Thank you for your suggestions. Very much appreciated.
       
    6. s.weinberg

      s.weinberg Well-Known Member EngineeringClicks Expert

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      Not sure I understand correctly, but to clarify my suggestion, I'm not suggesting that you change the rails from vertical to horizontal.
      Rather, right now, you have the rails side by side, and the load between them and offset, forming something like a triangle. The issue is that the moment on each bushing is very large - the moment to balance the load is basically being made across each individual bushing, which isn't a big distance - and the distance between the rails isn't helping you that much.

      If you lined them up instead, so that you had one rail in the middle, a 2nd rail behind, and the load in front, it would be far better. Basically, a small amount of friction on the back bushing would create a large reaction moment around the front bushing to balance out the load.

      Assuming that's still not an option, you could look at tightening the fit between your bushings and rails, though that increases the risk of general binding. Or use a different lubricant.
       

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