Involute gear

Two involute gears, the left driving the right: Blue arrows show the contact forces between them. The force line (or Line of Action) runs along a tangent common to both base circles. (In this situation, there is no force, and no contact needed, along the opposite common tangent not shown.) The involutes here are traced out in converse fashion: points (of contact) move along the stationary force-vector "string" as if it were being unwound by the rotating circle.

The involute gear profile is the most commonly used system for gearing today. In an involute gear, the profiles of the teeth are involutes of a circle. (The involute of a circle is the spiraling curve traced by the end of an imaginary taut string unwinding itself from that stationary circle called the base circle.)

In involute gear design, all contact between two gears occurs in the same fixed, flat plane (the Plane of Action), even as their teeth mesh in and out. Further, the contacting surfaces are always perpendicular to the plane of contact, so the dominant contact forces (in a well designed system) are always in the plane. This way, the moment arms are kept constant. This is key to minimizing the torque/speed variations which produce vibration and noise in lower quality gears. Note that the involute profile only minimizes the teeth from rubbing each other every time they mesh, and the high localised stresses can be the dominant source of wear. It is not possible to design a gear tooth profile which rolls through the mesh without some rubbing.^{[citation needed]} Service life is often managed by using hard materials and direct lubrication. When friction wear is a critical issue, chain drives can help reduce maintenance requirements.

The involute profile can be generated using a hobbing machine with a rack form. Rack pressure angles of any degree are theoretically possible, however only 3 have been in common use; 14.5 degrees which was used many years ago but now is obsolete, 20 degrees which is the most common and 25 degrees which is generally only found in the USA.

Small pinions have addendum modifications to stop interference.

v • d • e Gears Gear systems Spur gear systems • Worm drive • Rack and pinion • Epicyclic (planetary) gearing • Sun and planet gear • Harmonic drive • Cycloidal drive • Non-circular gear Gear shapes Spur gear • Bevel gear • Spiral bevel gear • Helical gear Geartooth profiles Involute • Cycloid Gear mechanics Transmission • Differential • Gear coupling • Gear train • Bicycle gearing • Continuously variable transmission • Offset (gears) Examples In Bicycles: Cogset • Derailleur gears • Hub gear • Shaft-driven bicycle • Sprocket In Horology: Wheel train See also Ball screw • Leadscrew • Jackscrew • Belt drive • Chain drive • Gear manufacturing • Freewheel

External links

• Kinematic Models for Design Digital Library (KMODDL)
  Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes an e-book library of classic texts on mechanical design and engineering.

External links

• Involute gear at How Stuff Works - with animation