The basic cylindrical gear, with straight teeth cut parallel to the shaft axis and used between parallel shafts. Spur gears produce no axial thrust, are easy to make and inspect, and tolerate small center distance errors well. Because each tooth engages across its full face at once, load changes abruptly as teeth enter and leave mesh, so they are noisier than helical gears at high speed.
It's just a spur gear set, no thrust to worry about, so ordinary ball bearings on that shaft are fine.
Race and rally boxes often run straight-cut spurs on purpose: no thrust load, less friction, and the whine comes with the deal.
Also heard as
- straight-cut gear
- straight spur
Do not mix it up with
- Helical gear: Helical teeth are cut at an angle to the axis, giving smoother, quieter engagement but adding axial thrust.
Related terms
Helical gear
A cylindrical gear whose teeth wrap at an angle around the blank, engaging gradually for quiet, strong running.
Contact ratio
The average number of tooth pairs in contact at once, which largely sets how smoothly and quietly a pair runs.
Involute
The curve traced by the end of a taut string unwound from a circle, the shape of almost every gear tooth flank.
Whine
A steady, pitch-changing tonal noise from gears, rising with speed and caused by tooth meshing errors.