Planetary gearheads

As industry becomes more automated and robotised the need for high precision gearboxes in machinery and plant engineering grows. Ian Carr, managing director of Drive Lines in Bedford, explains how planetary gearheads have precision and power designed into them.

Machinery and systems such as robots, packaging machines, pick and place units, tool changers in machine tools, printing presses and welding systems all rely on precision motion. Likewise, medical equipment, navigational aids and astronomical telescopes also need cutting edge motion systems.

While at first glance some of these applications may seem to have little in common, in fact their power and motion systems are based on similar fundamental principles.

Looked at as a drive system, the main requirements are in fact very similar: high torque must be provided, whilst low backlash is essential. To achieve this, torsional stiffness is required, and high acceleration and braking are often demanded. More often than not, there is also the need for a compact design, or the need to fit within a demanding space envelope.

Such drive systems have a high design content. Many are truly bespoke, one-off designs, while others may be customised variations of a standard design. However, like all drives systems they are typically made up in large part from standard components, servomotors, gearheads, couplings etc.

Thus a precision drive system designer will select components, optimising one with the next with the ultimate goal of achieving each project’s performance specification. They will also be aware that each type of component has certain characteristics and advantages, so will know, for instance, when to specify a servo motor rather than a stepper or which type of coupling to use in given situations.

It is fair to say that precision drive system designers make regular use of planetary gearheads. In fact they are used almost exclusively as the major joints in both articulated arm and delta robots, are ideal for many bespoke machines and are a firm favourite for medical scanners. So let us look first at how they work, then at why they are good for precision drives.

Operating principle

Planetary gears are also called epicyclic gears or sun-and-planet gears, the latter being useful for helping visualise - their operating principle. Dismantle a planetary gear and at first it looks complicated, but you can soon identify a central sun gear, surrounded by, and meshing with, some planet gears. (Typically there are three or four planet gears, but it could be fewer or more.) Holding the sun and planet gears together are an outer ring gear with internal teeth that mesh with the planets. There is also a planet gear carrier, but this does not transmit power rather being a structural element that keeps the gears in place.

If the sun gear is turned, the meshing teeth cause the planet gears to turn, which then cause the ring gear to turn. It sounds complicated and looks hypnotic, but in fact is very simple. It is also notable that the ring gear turns in the same direction as the sun gear.

Significantly, while the sun gear rotates relatively quickly, the outer ring gear rotates more slowly. In fact the ratio of their two speeds is derived from the ratio of their two diameters – the planet gears act as idlers, their role being to transmit power between the sun and ring, in other words their diameter does not affect the overall speed ratio between input and output.

This configuration offers some very significant advantages over other gear layouts. For instance, all the parts fit inside the ring gear making it a compact unit. Because there are a number of planet gears, there are many teeth meshing with one another, each pair transmitting power and adding together to give a high torque capability in a compact unit. The multiple meshing of teeth also helps to significantly reduce backlash compared to a gear system where there is only one meshing pair. Also, because the power transmission is spread equally across each planet gear, there is an innate symmetry which helps prevent the gearhead from flexing under load.

Thus it can be seen that planetary gearheads have a lot to attract designers of precision drive mechanisms. They are compact, powerful, accurate and robust.

In order to make planetary gearheads even more attractive to potential users, manufacturers such as Premium Stephan make them in a range of formats, so that individual requirements can be met from a standard model or one that has been only slightly modified. Premium Stephan can supply their gearheads as sub-assemblies, enclosed gearboxes or fitted to a motor to form a unitary component. They are available in a range of sizes and can be supplied with either solid shafts (with shaft length to match the application) or hollow shafts, the latter allowing a feed-through of cables. Generally they are designed for use with standard mineral oils, although specials are available.

Now to look in more details at a particular planetary offering, let’s take the Melior Motion range, again from Premium Stephan. This has many design details that help ensure performance is as precise and reliable as possible. For instance, the gear teeth have a unique profile that guarantees backlash is less than 0.6 arcmin and will not change by more than 10% over the nominal operating lifetime of 20,000 hours. This reduces the costs associated with initial set up and subsequent servicing and replacement.

Further, the Melior Motion’s torque capacity is exceptional for such a small footprint, so energy efficiency is high. Thus heat generation is low, allowing standard lubricating oils to be used rather than expensive specialist ones. It also leads to low starting torque, inertia and braking loads, and therefore smooth and reliable operation.

Tilting and torsional stiffness are high, leading to precise positioning and high repeatability. In fact the Melior Motion was designed with robot applications very much in mind, hence its compact size, low maintenance, and high performance.

In conclusion we can say that planetary gearheads are enabling more and more potential users to think about adopting precision servo drive technologies for their machines and systems. First time users in particular may appreciate technical support from a company like Drive Lines, which not only supplies gearheads and associated equipment but also has a wealth of expertise and experience in specifying precision drives for a wide range of applications. Established over 30 years ago, Drive Lines has worked on applications as diverse as hot drape formers, golf buggies, lifting systems, laboratory equipment, cement mills drives and a door closure for a medieval cathedral.