Has the strainwave gear had its day?

Most robots, from industrial arms to space exploration to customer, are actuated by an electric motor coupled with a gear box on their joints. As the motor spins, the gear box acts as a speed reducer and torque multiplier. There are numerous types of gears, all suited to different applications – spur, worm, planetary, cycloidal and planetary gears all have their place.

A major issue facing robot manufacturers operating geared motors is backlash, otherwise known as the lost motion between the gear teeth, which is a design trait built into many speed reducers so that the gears mesh without binding and so that lubrication can be applied to lessen the chance of overheating or damaging the teeth. While typical industrial robots have an accuracy of around ±0.1mm – and many can be even more accurate – gear backlash can reduce this accuracy in robots and machine tools, which is why it is concerning to manufacturers.

For many robotics applications, strain wave gears, also known as Harmonic Drives after the company that trademarked the technology, are the first choice. This is because they boast zero backlash, which leads to a higher accuracy robot – important in precision manufacturing or medical applications – and are lightweight and compact. They also have high gear ratios when compared with planetary gears. They are made up of three major parts, a wave generator, a flex spline and a circular spline.

The strain wave gear is therefore in hot demand, but its high precision manufacturing process, combined with its popularity, means that there is a long lead time, creating a bottleneck for robot manufacturing. Also, high-precision machining of gears with minimal backlash isn’t cheap and when typical industrial robots usually contain around six gears and surgical robots need 32, the costs can quickly add up.

As robot technology evolves, the industry could greatly benefit from a scalable, flexible, accessible speed reduction technology. There is also an opportunity to create a gear with even higher strength, low noise, low vibration and the need for even less maintenance.

While a broken down gear is not a disaster as an automation parts supplier can source you a replacement, it can cause downtime, disrupt production and add unexpected costs. As well as robotics, the benefits will be felt in other fields too like automotive, aerospace, renewable energy and machinery.

What’s next for robot gears?

After half a century of incremental change in gearing technology, numerous start-ups are now coming forward with new approaches to the age-old speed reducer problem. IM Systems, a Dutch company, for example, has reinvented the gear so that it has no teeth. Its product, the Archimedes Gear relies on frictional contact to convert speed into torque using flexrollers made from hollow steel cylinders to compress and transmit the rotational power. While each point of contact isn’t as strong as interlocking mechanical teeth, up to 30 rollers can contribute to the traction.

Interestingly, the Archimedes Drive can reach impressive single-stage reduction ratios of up to 10,000:1, boasts high torque, high accuracy and zero backlash. Its design, using bearing-like components, means that it would not be subject to the same lead times as other solutions and would help to deal with the shortage of gears. The reduction in cost, too, could make them an enticing option for robot manufacturers who require a large number of gears. In addition, the company can design variations of its drives for use in any axis of the robot.

Another new business, Motus Labs, has also come up with a possible solution to many of the industry’s challenges — the Motus M-DRIVE. The technology was developed on the principles of kinesiology and kinematics, the study of human and non-human body movement, to simulate the motion of an athlete running.

It even includes hips, knees, legs and feet. This is a gearless drive design, which uses mating blocks instead of teeth and replaces them with low friction, aluminium and plastic mating surfaces. The new method maximises torque and minimises slippage and flexing. Other benefits include increased contact area and efficiency, lightweight materials and greater rigidity.

Sophie Hand is UK country manager at EU Automation