The machinery industry relies on various gear systems to achieve precise motion and power transmission. Strain wave gears and cycloidal gears are two such technologies, each with unique characteristics.
This article will delve into the operating principles of both strain wave and cycloidal gears, exploring their differences in backlash, torque capacity, and precision. We will compare their performance regarding shock resistance, size, efficiency, and cost. Furthermore, we will provide typical applications and discuss the torsional stiffness of each to aid your understanding.
What Is Strain Wave Gear
Strain wave gearing, also known as harmonic gearing, is a mechanical gear system that utilizes a flexible spline deformed by a rotating elliptical plug, called a wave generator, to engage with the internal gear teeth of a rigid circular spline. This unique method of power transmission offers distinct advantages, such as zero backlash, high gear ratios, and compact size.
The core components of a strain wave gear are the wave generator, the flexspline, and the circular spline. The wave generator is an elliptical disc with a ball bearing, which fits inside the flexspline and causes it to deform into an elliptical shape. The flexspline is a thin, flexible cylinder with external teeth that mesh with the circular spline. The circular spline is a rigid ring with internal teeth. The flexspline has slightly fewer teeth than the circular spline, which enables the gear to achieve high reduction ratios.
What Is Cycloidal Gear
A cycloidal gear is a type of gear system that uses a rotating eccentric cam or disc to drive a series of pins or rollers, producing a reduction in speed and an increase in torque. The central component of the cycloidal drive is a cycloidal disc that has a series of lobes. This disc is mounted eccentrically on an input shaft. As the input shaft rotates, the cycloidal disc wobbles, and this motion is used to drive output rollers, which are connected to the output shaft. The output rollers fit into holes on the cycloidal disc. These holes are slightly larger than the rollers, allowing for the necessary motion to drive the output shaft. The unique design of cycloidal gears results in high efficiency, low backlash, and the ability to withstand high shock loads.
Difference Between Strain Wave Gear and Cycloidal Gear
Operating Principle
Strain wave gears operate based on the elastic deformation of a flexspline. The wave generator causes the flexspline to deform into an elliptical shape, engaging with the circular spline at two points along the major axis. As the wave generator rotates, the engagement points move, and the difference in teeth between the flexspline and circular spline produces a reduction in output speed and increase in torque.
Cycloidal gears, in contrast, utilize a cycloidal disc mounted eccentrically on an input shaft. This disc wobbles as the input shaft rotates, driving output rollers within the disc that in turn rotate the output shaft. The motion is characterized by an eccentric movement, converting rotational input into reduced rotational output.
Backlash
Strain wave gears are known for having extremely low to zero backlash. This is because the gear teeth are always fully engaged in a region along the major axis. Some manufacturers claim their strain wave gears have absolute zero backlash.
Cycloidal gears, while not achieving zero backlash, generally offer very low backlash, though typically higher than strain wave gears.
Torque Capacity
Strain wave gears are capable of transmitting high levels of torque, particularly considering their compact size and weight. They offer a high torque-to-weight ratio making them suitable for applications requiring powerful torque outputs in small packages. Some precision strain wave gears can handle a rated torque ranging from 10 Nm to over 7000 Nm.
Cycloidal gears are also well known for their high torque capacity, often considered superior to strain wave gears for very high load applications. The robust design and ability of the cycloidal disc to distribute stress across multiple points allow them to handle heavy loads and shock forces more effectively.
Precision
Strain wave gears are known for their high precision and repeatability due to the constant engagement of the gear teeth and the absence of backlash.
Cycloidal gears also provide good precision, particularly with regards to positioning accuracy due to low backlash. However, their precision is sometimes considered slightly lower than that of strain wave gears because of their more complex geometry and pin-roller system.
Shock Resistance
Strain wave gears have good shock resistance due to the multiple teeth engaging simultaneously and the elastic nature of the flexspline. However, they are generally less suited to applications involving very high loads or significant shock forces when compared to cycloidal gears. Some strain wave gears have a shock resistance rating of 300m/s².
Cycloidal gears are known for their excellent shock resistance. The design, with its robust cycloidal disc and multiple points of contact, can withstand sudden or irregular forces. The pin-and-gear structure maintains continuous and distributed power-transmission contact, enabling some variations to endure momentary exposure to four times their rated torque.
Size and Weight
Strain wave gears are very compact and lightweight, owing to their design which allows for high gear reduction ratios in small form factors. The size and weight of the gear do not vary with the reduction ratio. This makes them suitable for applications where space is limited.
Cycloidal gears are generally more robust and may be larger and heavier compared to strain wave gears for similar torque capacity, but they still offer a compact footprint for their performance.
Efficiency
Strain wave gears are characterized by their high efficiency, often reaching up to 85% in nominal operation. This is primarily due to the smooth rolling motion of the flexspline, which minimizes wear and tear. The teeth come in contact with an almost pure-radial motion, and have essentially zero sliding velocity, even at high input speeds.
Cycloidal gears have good efficiency, with single-stage efficiency approaching 93% and double-stage efficiency around 86%. While their efficiency might be slightly lower than strain wave gears due to eccentric motion and multiple contact points, this is compensated by their exceptional durability and torque capacity.
Cost
Strain wave gears can be more expensive due to their complex manufacturing process and the precise components involved, particularly the flexspline and wave generator. The cost drivers include the high-precision machining of the components and the material used in the construction.
Cycloidal gears are generally more cost-effective to manufacture than strain wave gears, as their designs are less intricate and do not require the same level of precision.
Typical Applications
Strain wave gears are commonly used in robotics, aerospace, medical equipment, and high-precision industrial machinery. They are often found in robotic arms, where precise positioning is critical, in satellite mechanisms and in other applications where high reduction ratios, zero backlash, and compact size are essential.
Cycloidal gears are frequently employed in heavy industrial applications, such as conveyor systems, and in robotics where high torque and shock resistance are needed. They are also used in machine tools, automation systems, and other applications requiring reliable and robust power transmission.
Torsional Stiffness
Torsional stiffness is determined by measuring the torsional angle at the output when the input is blocked and torque is applied. Strain wave gears offer high torsional stiffness, especially in specific designs, which is important for maintaining positional accuracy under load. Some strain wave gears can have twice the torsional stiffness compared to other strain wave gears.
Cycloidal gears also have a good torsional stiffness due to their robust construction and the multiple contact points. The design of the cycloidal disc and the rollers provides a stable and rigid connection between input and output, which minimizes torsional deflection.
