A harmonic drive, also known as a strain wave gear, is a high precision gear system used to achieve large speed reduction ratios with very low backlash. It is widely used in robotics, aerospace and motion control systems where accurate positioning and compact design are essential.
The harmonic drive operates using a flexible spline, circular spline and wave generator to transmit motion. This unique mechanism allows multiple teeth to engage simultaneously, providing high torque capacity, smooth operation and excellent positional accuracy.
The harmonic gear allows high reduction ratios with concentric shafts and
with very low backlash and vibration. It is based on a very simple construction utilising
metals elasto-mechanical property.
Harmonic drive transmissions are noted for their ability to reduce backlash in a
motion control system. The principle of operation of harmonic gears is through the use of a thin-walled
flexible cup with external splines on its lip,placed inside a circular thick-walled
rigid ring machined with internal splines. The external flexible spline
has two fewer teeth than the internal circular spline. An elliptical
cam enclosed in an antifriction ball bearing assembly is mounted inside the flexible
cup and forces the flexible cup splines to push deeply into the rigid ring at two opposite points
while rotating. The two contact points rotate at a speed governed be the difference in
the number of teeth on the two splines This method basically preloads the teeth, which reduces backlash.
Harmonic drives have speed reductions single stage of 30:1 to 350:1. The lower ratios
are limited because of fatigue considerations on the flexispline member.
The harmonic drive is normally not suitable
for high power levels above 6kW ..
A typical large duty Harmonic drive will have dimensions of about 280mm dia x 150 mm long and will be able to
continuously transmit an output torque of 2270 Nm at an output speed of 20 RPM and
and input speed of 2000RPM (4.7 kW output)- This unit will be able to transfer higher
torques for short time durations e.g a momentary torque 9000Nm. The efficiency would be
about 70%. This unit would weigh about
42 kg. ..... A worm drive for a similar drive (1800 RPM imput speed ,70-1 reduction same
output torque )would weigh 327 kg and would be in a case about
750mm high x 360mm wide x 500mm wide..Surprisingly for this duty the efficiency of the
worm unit would be 77%
Wave Generator:
The wave generator is an oval-shaped cam with a thin ball
bearing placed around the outer circumference of the oval cam. The wave generator is mounted onto the motor shaft.
Flex Spline:
The flex spline is a thin, cup-shaped component made of elastic
metal, with teeth formed along the outer circumference of the cup's opening.
The gear's output shaft is attached to the bottom of the flex spline.
Circular Spline:
The circular spline is a rigid internal gear with teeth formed along its
inner circumference. These teeth are the same size as those of the flex spline, but the circular spline has two more teeth than the flex spline. The circular spline is attached to the gearbox along its outer circumference.
The wave generator is mounted on the drive shaft and as it is rotated it imparts a
continuously moving elliptical form or wave-like motion to the Flexspline. This causes
the meshing of the external teeth of the Flexspline with the internal teeth of the
Circular Spline at their two equidistant points of engagement to progress in a
continuous rolling fashion. It also allows for full tooth disengagement
at the two points opposite the minor axis of the Wave Generator.
Since the Flexspline has two less teeth than the Circular Spline and because
full teeth disengagement is made possible by the elliptical shape of the Wave
Generator, each complete revolution of the Wave Generator causes a two tooth
displacement of the Flexspline in relation to the Circular Spline.
This displacement is always in the opposite direction of the rotation of the
Wave Generator (see diagram)e.g if the Wave Generator is rotating in a CW
direction, the two-tooth-per-revolution displacement of the Flexspline will be
in a CCW direction and vice versa
In this way, a basic three element
harmonic drive component set is capable of functioning as a speed reducer.
Input from a main power source through the Wave Generator is at a high speed, but
the two-tooth-per-revolution displacement causes the Flexspline, which is the output
element, to rotate in the opposite direction of, and at a considerably slower
speed than, the Wave Generator. The reduction ratio which results can
be calculated by dividing the number of teeth on the Flexspline by two
(the difference between the number of teeth on the Circular Spline and
the Flexspline). If a fixed Circular Spline had 80 teeth and an
output Flexspline has 78 teeth, the ratio would be 80/(80-78)=40:1
Harmonic drives typically achieve single-stage reduction ratios between 30:1 and 350:1. The ratio is determined by the difference in the number of teeth between the circular spline and the flex spline.