Harmonic Drives

In the late 1950's, Harmonic Gear Drives were invented and since then, these have been using a very important mean of speed/motion controller. Gradually, a lot of developments and innovations have been taken place in these drives. Gradually, these gear drives have become the primary choice for those applications, in which repeatability and precise positioning accuracy are required.

Now a days, these drives are available in different combinations, like actuators in which encoder and motor are installed, fully housed gear heads, or in the form of custom configurations with specific materials. These Harmonic Gear Drives are available in customer specific designs for simple adaptability. Although, a lot of changes have been taken place in these gear drives since their inception, but one thing which is remain unchanged. This unchanged thing is the operational principle of these drives. These drives have high rotational ratios, high torque-to-weight ratio and zero backlash. These all characteristics of these drives are due to their operational principle.

Components: Harmonic Gear Drives
The entire process of operation of the Harmonic Gear Drive comprises three components. These three components are:

• Wave Generator
  The assembly of a steel disk and a bearing is called as wave generator. The wave generator is also called as wave generator plug. An elliptical shape has been there on the external surface of the wave generator plug, which is machined carefully to a precise specification. Around the bearing plug, a specially designed ball bearing is pressed. This causes the bearing to comply to the similar as the wave generator plug as far as the elliptical shape is concerned. Commonly, as an input member, the wave generator is used and normally it is affixed to a servo motor.
  
  
• Circular Spline
  It is a firm steel ring having circular shape and consists teeth on the inside diameter of it. Circular spline is fixed on the housing arrangement of this drive and it doesn't rotate. Teeth of both the circular spline and Flex spline are meshed with one another. The shape of this meshed arrangement is like an ellipse engraved concentrically within a circle. From mathematical point of view, an engraved ellipse will touch a circle at two distinct points. But, here, as there is a finite height of the gear teeth, so there are two distinct regions of the teeth engagement instead of two distinct points.
  
  
• Flex spline
  It is a thin-walled steel cup having teeth machine. Into its outer surface, its teeth are machined, which is near the open end of the cup. It is radially submissive and this is due to the long diameter of the cup. Although, it is remained torsionally stiff. At one end of the cup, there has a rigid boss that provide a jagged mounting surface. At the time of assembly, inside the flexspine, the generator is inserted. This insertion should be done in the same axial location according to the flexspline teeth. There is an elliptical gear pitch diameter found on the outer surface of the flexspline. Flex spline is the outer element of the harmonic gear drive mechanism.
  
  

Harmonic Gear Drives Operation
On the drive shaft, the web generator is set and the web generator continuously rotates on it. Consequently, it imparts wave like motion on the flexspline. This wave like motion induces the meshing process between the external teeth of the Flex spline and circular spline's internal teeth. This meshing process conducts at their two equidistant engagement points and this goes on in a continuous rolling manner.
After every complete revolution of the wave generator, two tooth displacement takes place in the flexspline in comparison to the circular spline. This two tooth displacement takes place because Flex spline has two less tooth than the circular spline and the elliptical shape of the wave generator. The directions of the two-teeth displacement and rotation of the wave generator are always opposite. If direction of the first one is clock-wise, then direction of the other will be anti-clock wise. Only the three components are capable in forming a complete harmonic gear drive. The high speed input (power) comes from the main power source through the wave generator. When, this power input reaches to the external element i.e. Flex spline, the speed of the Flex spline gets reduced. The ratio of reduction can be calculated by the following formula:

Harmonic Gear Drives Characteristics

• High Speed Reduction Ratio
  This gear drive has high coaxial reduction ratios. This ratio lies in between of 1:30 to 1:320 and this high range of ratio is derived by this drive by using only the three above mentioned components.
  
  
• Free of backlash (lost motion)
  The backlash value of this drives teeth meshing value is very negligible. This value is almost equal to zero.
  
  
• High precision
  Very high degree of positional and rotational accuracy can be obtained by using this type of drive. This is possible due to the mating process of the gear teeth. The special type of mating of the teeth ensures almost nil pitch errors and also neutralizes the accumulated pitch errors.
  
  
• Small numbers of components and ease of assembly:
  The high reduction ratio from this drive can be obtained by only using the three components and all these three components are arranged co-axially. Therefore, the assembling process of this harmonic gear drive is very simple.
  
  
• Small-sized and lightweight
  By using only three components, this drive gives high torque ratio as well as high speed reduction ratio. Therefore, this drive is preferred over other type of drives having the same magnitude of torque ratio and speed reduction ratio. This drive is almost 50% lighter than other kind of drives.
  
  
• High torque capacity
  Special steel is used to make flexspline with a high degree of resistance against fatigue. Unlike the other common type of driving force transmission devices, the teeth in the flexspline are meshed in such a way so that these can generate a high degree of torque. That is why, almost 30% of its teeth always take some load and these teeth make direct face to face contact.
  
  
• High efficiency
  At each mating region, each tooth face very little slide motion. This reduces the amount of motion loss due to the frictional force drastically. This helps the harmonic drive to maintain high level of efficiency.
  
  
• Quiet, vibration-free operation
  As the tooth doesn't make any rolling contact with one another and each tooth's circumferential speed is also low, the force provided by the teeth is well balanced.
  
  

Harmonic Gear Drive Applications

• Robotics
  These drives assist the robot manufacturers in manufacturing robots because these drive have almost zero backlash, low vibration, high positional accuracy and compact designs. Their light weight also make them useful in preparation of robots.
  
  
• Machine Tools
  This drive helps in maintaining accurate control on the motion for axis retention and for changing tool. These drives are used in MRI / Cat scan gantries, patient beds and rehabilitations equipment.
  
  
• Military Aerospace
  These drives are used in the rotating and tilting the antennas and arrays. Apart from these, these drives are also used in positioning aerials and telescopes along with driving lunar vehicle motions.