Teeth
Straight bevel gears have straight teeth that meet along their length at once. Spiral bevel gears have curved teeth that meet gradually.
Noise and Smoothness
Spiral gears are quieter and smoother, especially at speed. Straight bevel gears make more noise, which is acceptable for slow drives.
Load Capacity
Because more teeth share the load in a spiral set, it can carry more torque than a straight set of the same size.
Speed
Straight bevel gears are usually limited to lower pitch line speeds. Spiral bevel gears run well at high speed, particularly when ground.
Thrust
Spiral gears produce axial thrust whose direction depends on the hand of spiral and the direction of rotation, so the bearings must be chosen to carry it. Straight bevel gears also create thrust, but it is simpler.
Cost
Straight bevel gears are simpler and cheaper to make. Spiral gears cost more but save space for the same load.
How to Choose
Use straight teeth for slow, light, low-cost drives. Use spiral teeth for anything fast, heavily loaded or noise-sensitive. If you are replacing a gear, use the same type unless the housing and bearings have been checked. See bevel gears and spiral bevel gears.
