Page 285 - Gear Technology Solutions

Figure 10: Bevel worm pinion with large lead angle (left)
and low lead angle (right)

19.7 Bi-Directional Operation

If a speed increaser is used bi-directionally, then it has to transmit the same
torque on the drive side and on the coast side. The effect of coast side driving
like the afore mentioned reduction of backlash down to zero has to be reduced
(Figure 6 with reversed force directions). One solution is the use of straight
bevel gears. Straight bevel gears can be designed and manufactured today
with low motion error and acceptable contact ratio. Also grinding of modern
®
Coniflex Pro straight bevel gears is applied today for many industrial applica-
tions. If spiral bevel gears are used as bi-directional speed increasers, then a
reduced spiral angle and a large cutter diameter should be used. The recom-
mendations for speed reducers in textbooks and design manuals call for a 35°
spiral angle and a cutter diameter which puts the involute point at the outer di-
ameter. This is called the ratio of involute to outer cone. In case of a bidirec-
tional speed-increaser, the spiral angle should not exceed 20° and the cutter
diameter should be chosen close to the gear mean diameter. This means, if
the mean gear diameter is 190mm, then the cutter diameter should be 7.5”.
The involute to outer cone rule was established to achieve a favorable contact
movement under load affected deflections on the drive side. In case of a low
spiral angle of 20°, and driving on both the drive and coast side, the rules of
the involute to outer cone would result in a very small cutter diameter, which is
not recommended for speed increasing transmissions.

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