What is Strain Wave Gear a.k.a Harmonic Drive? A Perfect ...
In this tutorial, we will explore the concept of Strain Wave Gear, commonly known as Harmonic Drive. We will start by discussing its operational principles and subsequently design our own model to 3D print, which will allow us to visualize its functionality and enhance our understanding.
What is Strain Wave Gear?
The Strain Wave Gear represents a distinct mechanical gearing system engineered to deliver exceptionally high reduction ratios while maintaining a compact and lightweight design. In comparison to conventional gearing systems, like helical and planetary gears, it can achieve reduction ratios of up to 30 times in a similar footprint. Additionally, this system boasts zero-backlash characteristics, high torque capabilities, precision, and reliability, making it suitable for various applications, such as robotics, aerospace, medical machinery, milling equipment, and other manufacturing devices.
The invention of the Strain Wave Gear is attributed to C. Walton Musser. The term "Harmonic Drive" is widely recognized as a branded name for Strain Wave Gear, trademarked by the Harmonic Drive company.
How It Works
Let's delve into the operational mechanics of the harmonic drive. It comprises three primary components: a wave generator, a flex spline, and a circular spline.
The wave generator is elliptical in shape, featuring an elliptical hub along with a specially designed thin-walled bearing that conforms to the hub's elliptical contour. This component acts as the input for the gear set and is attached to the motor shaft.
As the wave generator rotates, it creates a wave motion.
The flex spline resembles a cylindrical cup crafted from a flexible yet torsionally resilient alloy steel material. The cup's sides are slender, while the base remains thick and sturdy.
This design allows the flexible opening of the cup while maintaining rigidity at the closed end, making it practical for output with an output flange affixed. Notably, the flex spline has external teeth around the cup's open edge.
Conversely, the circular spline is comprised of a solid ring with internal teeth. The circular spline incorporates two additional teeth compared to the flex spline, a fundamental aspect of the strain wave gear's design.
When the wave generator is introduced into the flex spline, the flex spline adapts its shape according to the wave generator's elliptical configuration. As the wave generator turns, it radially distorts the flex spline's open end. Both the wave generator and flex spline are situated inside the circular spline, aligning the teeth for meshing.
The unique elliptical form of the flex spline results in the meshing occurring solely in two distinct areas, positioned on opposite sides across the flex spline's major axis. Consequently, as the wave generator turns, the flex spline's interfacing teeth with the circular spline gradually shift position. This tooth count discrepancy leads to a small backward rotation of the flex spline relative to the wave generator for every 180-degree spin of the wave generator, facilitating the advance of the flex spline teeth by just a single tooth.
This mechanism continues such that a complete 360-degree rotation of the wave generator correlates to a two-tooth positional change for the flex spline. For instance, if the flex spline comprises 200 teeth, the wave generator must undergo 100 rotations to facilitate a single full rotation of the flex spline, yielding a 100:1 reduction ratio. Here, the circular spline will hold 202 teeth, adhering to its design that mandates a two-tooth excess compared to the flex spline.
The reduction ratio can be computed with the formula: ratio = (circular spline teeth - flex spline teeth) / flex spline teeth. Thus, with 200 teeth on the flex spline and 202 on the circular spline, the ratio results in 0.01—indicating a speed reduction factor of 100 times relative to the wave generator's rotation, with the negative sign denoting the opposing rotation direction.
We can arrive at various reduction ratios by modifying the tooth count or adjusting either the mechanism's diameter while maintaining the same tooth size or by altering the dimensions of the teeth while ensuring the gear set's overall size and weight stay the same.
Strain Wave Gear & Harmonic Drive 3D Model
With an understanding of the theory behind the Strain Wave Gear, we can now transition to practical application—designing and assembling a 3D model that can be printed.
I utilized Fusion 360 to create a model of the Strain Wave Gear. All designated parts are 3D printable; hence, we only need bolts, nuts, and some bearings for assembly. For the input, I opted for a NEMA 17 stepper motor.
The essential components—the circular spline, flex spline, and wave generator—were carefully designed to consider the limitations of 3D printing regarding accuracy and precision. I selected a module size of 1.25 and employed 72 teeth for the circular spline. Consequently, the flex spline must comprise two fewer teeth, totaling 70, resulting in a reduction ratio of 35:1 while maintaining a compact gear size.
Due to the difficulty in sourcing specialized thin-walled bearings, I substituted them with regular ball bearings arranged in an elliptical configuration. I ensured the ellipses' dimensions correspond with the flex spline's inner wall measurements, with the major axis radius being 1.25mm greater than that of the flex spline and the minor axis radius 1.25mm smaller.
The wave generator consists of two sections designed for easy attachment of the bearings, with one section featuring a shaft coupler to secure the NEMA 17 stepper motor.
Supporting structures surround these three fundamental components. The output side of the housing will accommodate two bearings with a 47mm outer diameter, securely fastened using bolts and nuts. The output flange is constructed from two sections joined with bolts and nuts to allow for easy attachment.
You can download this 3D model and explore it on Thangs.
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