Bevel gears, with straight teeth around the periphery of the cone, can mesh on intersecting axes, especially when the loser shaft and output shaft are perpendicular to each other. These include the most common straight bevel gears, spiral bevel gears, as well as beveled and hypoid gears.
Straight bevel gears are the simplest bevel gears. When they mesh, their teeth are momentary line contact. These gears are capable of transmitting the proper torque, but they are not as smooth as the spiral bevel gears because the straight bevel gears are in linear contact. Straight bevel gears can withstand medium loads.
The teeth of the arc bevel gear are bent and slanted. Due to the helix angle, there is a large overlap of the teeth so that at least two teeth are in contact at the same time during rotation. Compared to straight cylindrical gears, each spiral bevel gear is subjected to less load, but the rotational speed can reach 8 times that of straight cylindrical gears. As a result, spiral bevel gears have a high load carrying capacity.
Equal diameter right-angle bevel gears and bevel gears with equal number of teeth are used in transmissions where the input shaft is 90° to the output shaft. A hypoid gear is a type of bevel gear with arc teeth that is suitable for situations where the two shafts are perpendicular but not crossed. They are often used to connect drive shafts to the rear axle of automobiles, although they are often mistakenly referred to as helical gear units.
A helical gear is a gear in which the peripheral teeth are cut out at an angle to the axis, rather than parallel to the axis. As shown in Figure 1e, the teeth of the helical gear compensate for the axis at an angle so that it spirals around the shaft. Due to the sloping teeth, helical gears transmit motion more smoothly than spur cylindrical gears and can drive heavy loads because they are meshed at an acute angle instead of 90°. When helical gear shafts are parallel, they are called parallel helical gears;When they are at right angles, they are called helical gears. Herringbone gears and worm gears are named based on the geometry of helical gears.