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why choose one pressure angle versus another

why choose one pressure angle versus another

why choose one pressure angle versus another

I notice that many suppliers offer both spur and
worm gears with differing pressure angles.  I know
that the pressure angles or the gears must be compatible,
but my question is "why choose one PA versus another"?
For instance, if the a gear is available with the same
DP, face width, etc.  why choose 14.5 degree versus 20
or visa versa?

[I'm looking to learn more abour gears in order to
build my own motorized telescope platform.  Any web
sites with introductory level info would also be helpful.]

Jeff Anderson-Lee

RE: why choose one pressure angle versus another

14.5 is the "old" standard. It was developed from an even earlier "composite" standard which combined cycloidal and involute gear profiles. I believe this is still used in watches and clocks. A variant of composite is used for molded plastic gears.

20 degree gears are stronger, suffer less from undercutting for small numbers of teeth.
Supposedly they increase forces tending to push gears apart, but I have never seen a case where this is a problem.

Worms sometimes use even greater angles for more strength.

20 degree gears should be chosen in 99.9% of cases.

Some references: Guillet's Kinematics of Machines, Machinery's Handbook. Marks Standard Handbook for Mechanical Engineers.

RE: why choose one pressure angle versus another

The standard specifying 14.5 pressure angle gears was discontinued in the early 1970's. As there are thousands of inch pitch gears with this pressure angle in use, numerous stock gear suppliers still manufacture to the old standard.

20 degree gears are the standard throughout Europe and Asia and they have been the standard in the US with the publication of AGMA specfication 2001-B88.

The advantage of 14.5 degrees is a smoother mesh, resulting in lower noise. The disadvantages are less strength and a higher threshold for undercutting.

While 20 degrees is the current standard, there are many applications where a 25 degree pressure angle is used. These are applications where a strong gear is required but the number of teeth needs to be small and there are no restrictions on noise.

RE: why choose one pressure angle versus another

Historical note:  The 14.5 degree pressure angle was first chosen because its sine is very close to 0.25, which simplified calculations.

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