Introduction

Bevel gears are used when the direction of the rotation of the shaft needs to be changed. The teeth on bevel gears can be straight, spiral or hypoid. In the case of the old Howard Clock we're restoring, the teeth are straight. They are cut on a cone instead of the cylinder blank and are used in pairs to transmit rotary motion and torque where the bevel gear shafts are at right angles to each other. They are usually mounted on shafts that are at right angles to each other though they can be designed to work at other angles as well.

The main advantage of this kind of gear is that it makes it possible to change the operating angle. The downsides include the fact that each wheel of the gear can only work with its complement and no other. The wheels must also be very precisely mounted to ensure that the motion is transmitted properly.

Bevel gears in the Howard Clock

In the Clock we're restoring, there are two sets of bevel gears that work in tandem to transmit the motion from the winding mechanism at the base to the clock faces at the top. The first set is connected to the hand setting mechanism and transforms horizontal rotary motion from the winding mechanism to vertical rotary motion. This is then transmitted up the main vertical shafts to the next set of bevel gears which in this case consists of one horizontal wheel mounted at the top of the vertical shaft transmitting motion to four other wheels corresponding to four possible faces of the clock. The bevel set here transmits the vertical rotary motion to horizontal rotation, which is transmitted through the universal joints to the clock hands making them rotate.

While the clock has only three faces, there are four wheels at the bevel gear set at the top and four universal joints. A possible reason for the extra face (and hence joint, and gear wheel) is to ensure that the gear is properly balanced out.

References:
http://www.fi.edu/time/Journey/Time/Escapements/geartypes.html
http://science.howstuffworks.com/gear4.htm
http://en.wikipedia.org/wiki/Bevel_gear
http://www.hzpt.com/tech/coniq.html