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  1. #16
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    662

    Default

    Loaded the nut and mounted on the screw, twice
    The complete process is simple and trouble free.
    The nut even balances on the transfer tube and spacer without need of a supporting stand.
    Easy to finger drop about 10 balls, then fish them around with the blunt end of tweezers until they either enter the return tube or go down the helix.

    First run was 108 x 1/4" balls.
    Used thread wires to estimate gap between balls at 2 mm.
    Feels 'loose' and the nut drops under gravity over full length of the screw.

    Second run was 54 x 3.65 mm on one circuit, and 47 x 3.65 mm plus 7 x 1/4" on the other circuit.
    The tread wires were marginally tighter to enter the ball gap, but still about 2 mm.
    Can definitely 'feel' that the nut is firmer, but still loose in my mind.
    The nut tries to drop down the screw, but is reluctant, especially near the ends, but it does rotate several turns before stopping.

    Note total ball counts:
    - I reported 103 originally, now 54+47+1(not used small one) = 102 (damnit),
    - originally 103 (or 102) as found, now 108 loaded by me.

    My judgement is that the 1/4" balls are too loose, but what size to purchase ?
    US price for a 6.37 mm ball is about USD 0.72, so 110 balls is a significant purchase, have to get it correct.
    Have no idea about the effect of a lubricant film on the fit, or the allowance for heat expansion.

    John
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  2. #17
    Join Date
    Sep 2009
    Location
    Newcastle
    Posts
    163

    Default

    Do the balls need to fill the ball path length? I don't think so.

  3. #18
    Join Date
    Apr 2009
    Location
    Kingswood
    Posts
    662

    Default

    I think the ball size is determined by the fit to the screw groove, it affects the pre-load, backlash and friction factors.
    The balls in the path length simply need 'working clearance', currently about 2 mm.

    20 microns for lubrication plus 20 microns for heat over 54 balls is 2 mm, the whole current working clearance.
    If I increase ball size by 5 microns I would probably have to drop the ball count to 53.

    Note that I have to get new balls, the old balls are faulty and the 1/4" too small.

    I currently think 6.37 mm balls are worth trying.
    These may be too tight, then I can try alternating 6.37 mm with 1/4".
    But, the risk is that the screw root diameter plus ball diamater is too great, then I would have to re-order balls at 6.365 mm.
    Count will be such as to leave at least 2 mm in the path length.

    John.

  4. #19
    Join Date
    Oct 2011
    Location
    Norwood-ish, Adelaide
    Age
    54
    Posts
    5,255

    Default

    Can you use a gauge pin to accurately size the groove? That would at least give you a maxiumum ball diameter. Not sure how loose is too loose but will some of that play go if the ends of the screw and the nut are constrained so that they can only move in one direction (that is mounted as they would be on a machine)?

    Michael

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