The Advantage of Strain Wave Gear - KOFON Motion Group

Why strain wave gear?

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• Since power is transmitted through multiple tooth engagement, strain wave gear (harmonic reducer) offers high output torque capacity.

• Strain wave gears (harmonic reducer) are backlash-free. Positioning accuracy can be within one arc-min. Repeatability accuracy can be within 5 arc-sec.

• Strain wave gear (harmonic reducer) units are reversible and can be used for speed increase as well as speed reduction.

• With only three elements high single stage reduction ratios ranging from 50:1 to 320:1 can be achieved. 

• Strain wave gears (harmonic reducer) exhibit very high torsional stiffness over the whole torque range, as well as almost linear hysteresis behaviour.

• Strain wave gearing the teeth come in contact with an almost pure-radial motion, and have essentially zero sliding velocity, even at high input speeds. This results in minimal wear and long operating life.

• High efficiency be available up to 85 %.

• The strain wave gears (harmonic reducer) can provide the option of a central hollow shaft which can be used to pass cables, shafts through the centre of the gear.

Strain wave gears are a lesser-known gear reduction technology, meaning many engineers and designers may be missing out on their unique benefits.

Without understanding how strain wave gears work and their advantages, projects may suffer from using less optimal solutions that are bulkier, heavier, less precise, and less efficient.

This article provides a comprehensive overview of strain wave gears, explaining their operating principles, components, configurations, and key strengths and limitations. Armed with this knowledge, readers can make informed decisions on whether strain wave gears are the right choice for their applications.

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What is Strain Wave Gear

A strain wave gear, also known as a harmonic drive, is a unique type of mechanical gear system that uses the elastic deformation of a thin-walled component to transmit torque. This innovative design offers several advantages over traditional gear systems, including high gear reduction ratios, zero backlash, and compact size.

Strain wave gears were invented in by C.W. Musser, and have since found widespread application in robotics, aerospace, and precision machinery. The key distinguishing feature of a strain wave gear is the use of a flexible “flex spline” that conforms to an elliptical “wave generator.” As the wave generator rotates, it causes the flex spline to deform and engage with a rigid “circular spline,” resulting in a high-ratio gear reduction.

Components of Strain Wave Gears

1. Wave Generator: An elliptical cam with a thin ball bearing that is mounted on the high-speed input shaft. As the wave generator rotates, it imparts a continuously moving elliptical shape to the flex spline.
2. Flex Spline: A thin-walled, flexible cylindrical cup with external teeth that is slightly smaller in diameter than the circular spline. The flex spline is deformed by the wave generator into an elliptical shape, causing its teeth to engage with those of the circular spline at two opposite regions.
3. Circular Spline: A rigid ring with internal teeth that is mounted within the gear housing. It has a slightly larger diameter and typically two fewer teeth than the flex spline.

How Strain Wave Gears Work

As the wave generator rotates, the flex spline deforms to engage with the circular spline at two opposite regions across the major axis of the ellipse. The flex spline teeth which are in engagement with the circular spline teeth are under load and deflected like a cantilever beam. As different flex spline teeth engage the circular spline during each revolution, the mechanical power is transmitted from the wave generator to the flex spline and then to the circular spline.

Normally the wave generator is the input, and either the flex spline or circular spline can be fixed to produce output rotation. If the flex spline is fixed, the circular spline rotates in the same direction as the wave generator. If the circular spline is fixed, the flex spline rotates in the opposite direction of the wave generator. The difference in the number of teeth between the flex spline and circular spline determines the gear ratio. For each complete rotation of the wave generator, the flex spline moves by two teeth relative to the circular spline, resulting in high gear reduction ratios.

Types of Strain Wave Gears

Hat Type

In the hat type configuration, the flex spline features a hat-shaped flange extending from the toothed cylinder. This flange allows for mounting the flex spline to a housing or frame, providing support and preventing rotation. The hat type design is often used in applications where the flex spline needs to be fixed and the circular spline serves as the output.

Cup Type

Cup type strain wave gears feature a flex spline with a cup-shaped extension opposite the toothed cylinder. This cup interfaces with the housing, allowing the flex spline to rotate and serve as the output member. The circular spline remains fixed in this configuration. Cup type designs are commonly used when a rotating output is required.

Gear Reduction Ratio

One of the key advantages of strain wave gears is their high gear reduction ratios in a compact package. The gear reduction ratio is determined by the number of teeth on the flex spline and circular spline.

The reduction ratio can be calculated using the formula:

Ratio = (Nc – Nf) / Nf

Where:
Nc = Number of teeth on the circular spline
Nf = Number of teeth on the flex spline

Typical reduction ratios range from 30:1 to 320:1 in a single stage. This high reduction capability eliminates the need for multi-stage gear trains, resulting in a more compact and efficient design.

Advantages of Strain Wave Gears

1. Zero Backlash: The unique meshing mechanism of strain wave gears eliminates backlash, ensuring high positional accuracy and smooth motion transmission.
2. Compactness and Lightweight: The high reduction ratios achievable in a single stage allow for a compact and lightweight design compared to multi-stage gear trains.
3. High Torque Capacity: Strain wave gears can transmit high torques due to the large contact area between the flex spline and circular spline teeth.
4. High Efficiency: With efficiencies typically above 80%, strain wave gears minimize power losses and generate less heat than other gear systems.
5. Coaxial Input and Output Shafts: The concentric arrangement of input and output shafts simplifies machine design and saves space.
6. Reconfigurable Ratios within a Standard Housing: Different reduction ratios can be achieved by changing the tooth counts while maintaining the same housing dimensions.

Disadvantages of Strain Wave Gears