Harmonic Drive
Encyclopedia
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
system that can improve certain characteristics compared to traditional gearing systems (such as Helical Gears or Planetary Gears). It was invented in 1957 and is now produced by Harmonic Drive LLC. The advantages include: no backlash
Backlash (engineering)
In mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established...
, compactness and light weight, high gear ratios, reconfigurable ratios within a standard housing, good resolution and repeatability when repositioning inertial loads, high torque capability, and coaxial input and output shafts. High gear reduction ratios are possible in a small volume (a ratio of 100:1 is possible in the same space in which planetary gears
Epicyclic gearing
Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear...
typically only produce a 10:1 ratio).
Disadvantages include a tendency for 'wind-up' (a torsional spring rate) and potential degradation over time from mechanical shocks and environment.
They are typically used in industrial motion control
Motion control
Motion control is a sub-field of automation, in which the position or velocity of machines are controlled using some type of device such as a hydraulic pump, linear actuator, or an electric motor, generally a servo...
, robotics
Robotics
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...
and aerospace
Aerospace
Aerospace comprises the atmosphere of Earth and surrounding space. Typically the term is used to refer to the industry that researches, designs, manufactures, operates, and maintains vehicles moving through air and space...
, for gear reduction but may also be used to increase rotational speed, or for differential gearing.
History
The basic concept of Strain Wave Gearing (SWG) was introduced by C.W. MusserWalton Musser
Clarence Walton Musser inventor of the harmonic drive is an old hand at masterminding design breakthroughs. He is credited with over 250 major inventions and discoveries....
in his 1957 patent. It was first used successfully in 1964 by Hasegawa Gear Works, Ltd. and USM Co., Ltd. Later, Hasegawa Gear Works, Ltd. became Harmonic Drive Systems Inc. located in Japan and USM Co., Ltd. Harmonic Drive division became Harmonic Drive Technologies Inc.
The electrically-driven wheels of the Apollo Lunar Rover included harmonic drives in their gearing. Also, the winches used on Skylab to deploy the solar panels were powered using Harmonic Drive. Both of these system were developed by The Harmonic Drive Division of United Shoe Machinery Corp.
On January 1, 2006, Harmonic Drive Technologies/Nabtesco of Peabody, MA and HD Systems of Hauppauge, NY, merged to form a new joint venture, Harmonic Drive LLC. HD Systems, Inc. was a subsidiary company of Harmonic Drive System, Inc. Offices are maintained in both Peabody and Hauppauge.
Mechanics
The wave generator is made up of two separate parts: an elliptical disk called a wave generator plug and an outer ball bearing. The gear plug is inserted into the bearing, giving the bearing an elliptical shape as well.
The flex spline is like a shallow cup. The sides of the spline are very thin, but the bottom is thick and rigid. This results in significant flexibility of the walls at the open end due to the thin wall, but in the closed side being quite rigid and able to be tightly secured (to a shaft, for example). Teeth are positioned radially around the outside of the flex spline. The flex spline fits tightly over the wave generator, so that when the wave generator plug is rotated, the flex spline deforms to the shape of a rotating ellipse but does not rotate with the wave generator.
The circular spline is a rigid circular ring with teeth on the inside. The flex spline and wave generator are placed inside the circular spline, meshing the teeth of the flex spline and the circular spline. Because the flex spline has an elliptical shape, its teeth only actually mesh with the teeth of the circular spline in two regions on opposite sides of the flex spline, along the major axis of the ellipse.
Assume that the wave generator is the input rotation. As the wave generator plug rotates, the flex spline teeth which are meshed with those of the circular spline change. The major axis of the flex spline actually rotates with wave generator, so the points where the teeth mesh revolve around the center point at the same rate as the wave generator. The key to the design of the harmonic drive is that there are fewer teeth (for example two fewer) on the flex spline than there are on the circular spline. This means that for every full rotation of the wave generator, the flex spline would be required to rotate a slight amount (two teeth, for example) backward relative to the circular spline. Thus the rotation action of the wave generator results in a much slower rotation of the flex spline in the opposite direction.
For a Strain Wave Gearing mechanism, the gearing reduction ratio can be calculated from the number of teeth on each gear:
For example, if there are 202 teeth on the circular spline and 200 on the flex spline, the reduction ratio is (200 − 202)/200 = −0.01
Thus the flex spline spins at 1/100 the speed of the wave generator plug and in the opposite direction. This allows different reduction ratios to be set without changing the mechanism's shape, increasing its weight, or adding stages. The range of possible gear ratios is limited by tooth size limits for a given configuration.