Gear profile shift question
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  1. #1
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    Default Gear profile shift question

    Guys I have a pair of gears 35T and 70T mod1.25 (approx. 20DP if it helps) that are on fixed centres. I want to change this to a 1:1 ratio which of course means 52.5 t average.

    My question is very simple:

    Is it better the cut a pair of 52t gears and shift the profiles up to suit or 53t gears and shift the profile down? They are drive gears in the work head of my gear hobber.

    Any constructive advice appreciated.
    Pete.

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    It's a horse apiece between the two.
    ( or... six of one; half dozen of the other )

    As I'm sure you're already aware, it amounts to minor Profile Shift, either way. If it were me, I'd likely just lean toward the plus side and make a pair of 52T gears. It would not hurt to mildly harden them to alleviate any fears of strength, just to give yourself peace of mind.

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    Thanks ZK

    Being very much an enthusiastic novice I've only ever cut a profile-shifted gear to a drawing (for a guy who managed to get a drawing off Harrison for an obsolete part). Is it right that I figure the OD for a 52.5 tooth gear then just touch off and cut the 52 teeth to normal depth, or is there some other consideration?

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    can't you work backwards from the centre distance to get an even number of teeth ?

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    Quote Originally Posted by Limy Sami View Post
    can't you work backwards from the centre distance to get an even number of teeth ?
    Probably Sami but I've found that if I cut 20dp gears instead of 1.25 mod it brings the centre distances even closer to ideal so that's what I'm going to do.

    1.25mod 52.5 tooth pair = 65.625mm centres
    20dp 52 tooth pair = 66.04mm centres

    Only 17 thou difference. I'll make my blanks 20 thou small then cut them 20dp

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    As ZK mentioned above- keep your shift factor positive- cut the 1.25 mod teeth with a positive profile shift. If you only have a 20 DP hob then. . . . Standard practice is to enlarge the blank diameter twice the profile shift amount and then cut to standard depth plus whatever backlash amount you need.

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    As you shift out, pressure angle goes up. Down, the opposite. So if you're looking for strength, out is better. Quietness, in.

    More or less. I don't think it matters for what you are doing, but that's theory, anyhow

    In practice, out is more common. You don't get any involute below the base circle so maybe that's why ...

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    Or... you could keep the 1.25 mod and add a small amount of helix to the teeth to get the 66.625 center distance. With profile shifting, the gear's constant velocity is lost, though probably won't be noticed.

    helix.png photo - David photos at pbase.com




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    Quote Originally Posted by David_M View Post
    With profile shifting, the gear's constant velocity is lost, though probably won't be noticed.
    Hunh ? How do you figure that ? All you are doing is running on a different portion of the involute.

    Involutes are friendly that way. They are forgiving of all kinds of stuff.

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    Emanuel, on further thought, I'm wrong. Now I can visualize how it works or at least I think I can.


    Gear meshing

    Involute gears have the invaluable ability of providing conjugate action when the gears' centre distance is varied either deliberately or involuntarily due to manufacturing and/or mounting errors.
    Last edited by David_M; 05-20-2019 at 06:38 PM.

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    If you don't care about being exactly 1:1 and don't mind prime numbers, you could cut a 52 and a 53 so that the teeth engage all other teeth as they go around to even out the wear.

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    Quote Originally Posted by David_M View Post
    Now I can visualize how it works or at least I think I can.
    The whole drop-tooth thing confused me for a long time too. "How can this work ?"

    Then a brainstorm. It's just running on a different section of the curve, dummy !

    Oh. Right.

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    Quote Originally Posted by BoxcarPete View Post
    If you don't care about being exactly 1:1 and don't mind prime numbers, you could cut a 52 and a 53 so that the teeth engage all other teeth as they go around to even out the wear.
    He needs an EXACT ratio of 1:1 - it's the hob spindle of a gear hobber so that ratio is important !


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