Page 166 - Gear Technology Solutions

Figure 9: Axial force Fz created by pressure angle

The question, if the ratio between involute and outer cone follows the same
recommendations as for spiral bevel gears has to be answered with “not exact-
ly”. Because the spiral angle is zero at midface, the arc of the cutter radius has
to be extended far outside of the heel, until a 90° angle between the cutter ra-
dius vector and the radial distance vector is achieved. This is the point where
the flank line has the same local curvature as an involute would. The farther
this point is away from the outer cone, the quicker the contact location will
move under load in order to ultimately reach the involute point. This shows that
the cutter diameter is the only parameter to control the virtual involute point in
face width direction [1]. For Zerol bevel gears it is sufficient if the involute to
outer cone ratio is between 1.5 and 2.5. In case of large Zerol gear (400 to
600mm outer diameter) the value of 1.5 is recommended. For small Zerol
gears of 50mm and below the ratio of involute to outer cone can be chosen up
to 2.5.

151