Helical Gear Form Questions 1

[rgeiss]

Has anyone heard of or seen the phenomena where the base circle diameter of an involute helical gear is larger than the root diameter??? I am running into this an can't determine if it is a problem or not.

See attached screenshot of the beginning of a model for a helical gear in PRO-E. Shown is a Base Circle Diameter (large blue diameter), the root diameter (small blue diameter), and the involute curves for the tooth profile. Notice how the involute curves are do not extend to the small diameter blue circle. That is because my calculations are setting the base circle diameter (where the involute tooth profile is to be derived from) larger than the root diameter.......????????

Ryan

 

[spigor]

It's common, the teeth will be undercut.

http://en.wikipedia.org/wiki/Gear#Tooth_profile

You can address it e.g. by increasing the number of teeth, introducing helical angle, increasing pressure angle or decreasing dedendum. Since finding a best design is somewhat complicated, gearing books typically contain graphs and tables which allow you to find the parameter set which is the good solution. Usually only a certain level of undercut is accepted for reasons discussed in those books.

 

[mraabe]

It is common for small number of teeth, that the base diameter is larger than the root diameter.

For a spur gear the base diameter is db=d*cos(alpha), the root diameter is df = d - 2*mn*hfP + 2*mn*x

With alpha=20° the base diameter is larger than the root diameter if z < 33 * (hfP - x)

For helical gears the limit is smaller depending on the helix angle.If the base diameter is larger than the root diameter it doesn't mean there is undercut since there still is a root radius. For spur gears the undercut limit is 17 teeth, but the base diameter is larger than the toot diameter for up to 40 teeth with x=0.

Markus