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  • friction fitting with dry ice

    Discussion in 'The main mechanical design forum' started by chofer, Jan 22, 2013.

    1. chofer

      chofer Member

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      I need advice from an experienced person, I found some literature on friction fitting with dry ice but the first
      attempt failed miserably, the process that I used was, I measured the shaft, (1 1/4" precision 1044) it was 1.248" I subtracted .003 from that, and I got 1.245", I machined the hole into the base plate (44W) to the 1.245 measurement... With a caliper I got an .0025 interference, so, my bore was 1.2455 and my shaft was 1.248 , I thought it was perfect...

      Ok the dry Ice now.....

      I covered the shaft with dry ice, for 45 minutes, (the sales guy where i got the dry ice said 10 minutes would work) I heated the base plate (44W steel) with the torch till it was nice and hot... Fist I tired with just placing it by hand, then by hammer, then by sledge and then the hydronic press... Cooling the pin and heating the plate between each attempt. Anyway... I got it in with the press... And it had gouged into one side and the shaft was out of square...
      So I just removed the pin and now I need some advice, the plan is moving to an 1 3/8" precision shaft,

      Any help would be great please and thanks...
      C
       
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    3. Ramana Rao

      Ramana Rao Well-Known Member

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      I think you have to start from the basics.
      1. Co-efficient of thermal expansion of the shaft and the plate.
      2. The temperature of dry ice.
      From this calculate how much the shaft will shrink by dry ice, and similarly how much the hole will expand in the plate. As the shaft has to slide into the hole an additional clearance is required. If everything is fine, then ensure that the finish of the mating surfaces is good. A small entry taper of 3 degrees on the shaft, will help center it during insertion.
      Hope this helps.
       
    4. chofer

      chofer Member

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      I have no knowledge about how much the thermal expansion, my reason for asking around for advice, and... I can understand the logic behind a 3 degree taper, interesting... I guess that's something that can be done... Thanks!
       
    5. Erich

      Erich Well-Known Member

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      How large is your plate? Did you heat the entire plate uniformly or did you just heat up an area around the hole?

      I suspect you just heated the area around the hole. Doing this will make the hole get tighter NOT looser.

      The cold steel around the outside keeps its shape, the hot metal near the hole cant' expand so it gets forced into the hole.

      Put your plate in the oven at about 400 F for an hour.

      Dunk your shaft in dry ice.

      They should fall together.

      Steel expands at 0.0000063 inches/inch per degree F That is 6.3 E-06 in scientific notation.

      Assuming 70 F room temp a 400 degree oven will make your 1.375 hole expand by 0.0029 inches

      The dry ice should make the shaft shrink by 0.0016 inches for a grand total of 0.0045 inches.

      3 thousands is a substantial press fit for a solid shaft, what are you doing with it?
       
    6. telecomguy

      telecomguy Member

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      "I suspect you just heated the area around the hole. Doing this will make the hole get tighter NOT looser." I'll place a bet on this one. Good call Erich.
       
    7. chofer

      chofer Member

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      Yup that's exactly what happened!!! :) the base is 18" wide by 9 1/4" across and 1 1/4" thick,

      Seeing that I've never done this, I imagine a propane torch will be alright? I was using oxygen/acetylene torch,
      One question... About how long does the pin need to stay in dry ice?
       
    8. chofer

      chofer Member

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      I was planning on a .0025 interference...
       
    9. tomatlots

      tomatlots Member

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      Thats a LOT of interfefence. I would suggest .0005 to .001 inch. This can be done with the dry ice alone.
       
    10. mvalenti

      mvalenti Well-Known Member

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      ^yup, what they said!
       
    11. Rafael_Amen

      Rafael_Amen New Member

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      If the plate is too large to heat it all, you can try leaving it at room temperature and chilling the shaft with liquid nitrogen instead.
       

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