A sector gear is a flat spur type device that only has teeth along a short section of its circumference; the remaining portion of the gear is smooth. These gears are used to drive other elements or are driven themselves in mechanisms which only travel through a short arc and not a complete revolution. They are typically found in mechanisms which feature short, repetitive, or reciprocating movements such as clocks, actuators, and steering units. Sector gears may have only one short section of toothed surface or two opposing sections depending on the particular application. Depending on whether the output is radial or linear, this gear type typically drives another sector gear or a rack gear.
When short throw, gear transferred motion is required in a mechanism, it is unnecessary to use a full range gear to initiate the operation as a large part of it will never work. Examples of these short cycle, reciprocating operations include clock mechanisms, linear and radial actuators, and the steering units on certain vehicles. Often the range of motion in these applications may be close to a full 360° turn, but more often far less. The extent of the motion will, along with space constraints, transfer rates and torque requirements, dictate how many teeth are included in the sector gear, and what its total radius will be.
The elements driven by sector gears may be round, spur type gears, or flat rack type linear gears, or may even themselves be sector gears. Again, torque and transfer speed requirements play a role in determining the eventual design, size, and tooth pitch of these driven gears. The sector gear is also frequently the driven gear as is the case with low speed and torque sluice gates. A large semi-circular sector gear is attached to the gate and driven to lift or lower it by a smaller, full range spur gear. Where large amounts of input torque or higher operating speeds are required, the configuration would be reversed.
One of the benefits of sector gear designs is their ability to have dual outputs, often with different pitch values. These gears typically have two, or sometimes more, sets of toothed sectors along their edges which drive different parts of a reciprocating mechanism. Unlike a full range gear, the individual tooth sets may be cut at different pitches if speed differentials are needed. Sector gears may be made of a variety of materials including ferrous metals, brass, and high impact plastics.