FEATURE Medical
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Diff erential compact drives for medical applications
From robotics through to medical infusion systems, differential compact drives can deliver reduction of over 300:1 in a small, two-stage design, explains Vivek Salve, Lead Mechanical Design Engineer for Gearboxes at Portescap
W
earable injection devices deliver vital drugs such as insulin to the patient at planned intervals, in
the comfort of their own home. A key requirement of a wearable injection system is small size, giving the patient comfort with optimum usability. As a mobile device, these systems must also be energy effi cient and, for round-the-clock use, quiet operation is important, too. The motion system that drives the device, including the motor and gearbox, also has to fulfi l these needs, including be highly compact and lightweight yet provide the required power. While the motion system has to be able to deal with varying liquid viscosities – hence the requirement for a capable torque range – the gearbox also has to be able to provide a suffi cient high speed reduction to ensure precise drug delivery. Applications with similar requirements include the control of collimators used to fi lter x-rays and gamma-rays, as well as the motion of antenna positioning systems. A key requirement common to these uses is a gearbox that delivers high reduction, combined with a compact length. To fi t these needs, diff erential compact drives have been developed. Also known as a compound planetary gear train or a compact planetary diff erential, this
Wearable injection devices deliver vital drugs such as insulin to the patient at planned intervals
Portescap R32 gearbox, perfect for medical designs
gearbox provides a mass to reduction ratio that is signifi cantly lower compared to a planetary gear design.
Conventional planetary gear technologies are the choice for many applications as a result of their power transmission effi ciency and relatively high power density. However, there are constraints, the most signifi cant of which is the relatively low ratios that can be achieved. Typically, single-stage planetary gearhead ratios range from 3:1 up to 10:1, and this upper limit can’t be exceeded because, above that range, pinion gears become too small for reliable or eff ective operation. Inversely, planetary designs can’t
reduce less than 3:1 as the similarly-sized pinion and sprocket below this ratio wouldn’t allow space for the planet gears. As a result, the best combination of pinion and planet-gear size, performance and life are ratios between 4:1 and 8:1. Higher ratios than 10:1 can be achieved by designing in additional one or more planetary stages, but this increases unit length, making it less suitable for applications requiring compact footprint. More stages also increase the number of moving parts, adding to the noise and weight of the system. For the same reason, friction also increases, reducing the overall design effi ciency, reliability and lifetime. Crucially, any additional stage also adds an extra cost burden.
In many epicyclic gear train applications, the ring gear is fi xed, while the star carrier is the output and the sun gear is the input to the planetary system.
34 March 2023 | Automation
Alternatively, the diff erential compact drive comprises a two-stage split-ring epicyclic gearing system. The input stage ring gear is fi xed, whilst the output stage ring gear is rotating, connected to and driving the output shaft. In this system, the input stage features planetary gearing, and the output here becomes the input to the second stage, whereas the ring gear of output stage is the fi nal output. The benefi t of this design is that the
reduction ratio and power output are much higher. At the same time, the diff erential compact drive achieves a much more compact footprint. The reduction can be higher than 300:1 in a two-stage compound planetary diff erential, a ratio that could only be achieved with a conventional planetary design by adding three or four stages.
Applications that depend on high limits
of torque can remain with a conventional planetary design. Similarly, diff erential compact drives aren’t the desired options at the far extremes of reduction, below 50:1 and above 400:1. Most industrial applications however, such as those in robotics, industrial tools and medical infusion systems, are within the gear reduction range. Engineers striving to achieve designs that are compact, highly- effi cient and of low noise should consider diff erential compact drives as a better gearbox option.
CONTACT:
Portescap
www.portescap.com
automationmagazine.co.uk
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