The spur gear is the simplest gear design used in standard transmissions. Spur gear teeth are cut straight, parallel to the gear shaft. During operation, enmeshed spur gears have only one tooth in full contact at any given moment with two and a half teeth in partial contact. Use of multimesh gearing can increase the partial contact zone to four teeth and thereby reduce peak loads applied to individual gear teeth. The straight-tooth design is an advantage of a spur gear. Straight teeth minimize the possibility of popping out of gear, a critical consideration during acceleration, deceleration, and reverse operation. For this reason, spur gears are often used in the reverse geartrain.
A disadvantage of spur gears is noise, specifically a clicking noise that occurs as teeth come into contact with each other. At higher turning speeds, the clicking noise becomes a constant whine. Helical gears are quieter and, because of this, are often used in preference to spur gears to eliminate gear whine.