# Re: Bevl Gear Question!!

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Posted by Bill on May 26, 2000 at 18:22:31:

In Reply to: Bevl Gear Question!! posted by Tom D on May 22, 2000 at 13:56:52:

Tom,
Perhaps the best way to conceptualize a straight bevel gear is to imagine that the pitch changes as you move from heel (large end) to toe (small end). The circular pitch decreases to match the decreasing pitch diameter as you move along a tooth toward the small end. Involute tooth proportions (depth, tooth thickness, involute shape) all maintain their same proportions to the pitch calculated at any pitch diameter.

This is somewhat over simplified, disregarding tooth crowning, root angle modification, etc. But the idea here is that the imaginary "thin gear" shrinks in all dimensions as you go toward the toe.

I may be misleading you if you are visualizing a plane gear like a flat "slice" taken from a flat spur gear. A bevel's tooth point contact in tight mesh(conjugate action) travels along an involute curve, starting perpendicular to the bottom or root, and ending perpendicularly to the top land. Since the top land and root line are not parallel, the involute curve does NOT lie in a plane surface. Therefore the involute exists in an imaginary curved surface that intersects all tooth flanks perpendicularly.

If you want a thin gear equivalent to bevel gears in mesh, you have to imagine two very thin bowls with teeth cut at the rims. Also, each bowl's rim of teeth would be curved from top of tooth to bottom.

Bevel gears are NOT like a gear that has an oversize OD at the heel, and undersized OD at the toe. If you've ever tried to simulate a bevel gear by hobbing or shaping on a tilted axis, you'll quickly discover that the tooth form is completely unacceptable, usually with no top land at heel and much undercut at the toe. Such a gear does not mesh with a "true" bevel gear.

HTH,
Bill

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