Linear Actuators: what they’re and how to decide on them

A linear actuator is a self-supporting structural system capable of transforming a circular motion generated by a motor right into a linear motion along an axis. Helping to produce movements such as the pushing, pulling, elevating, lowering or inclination of a load.

The most typical use of actuators entails combining them with multi-axis Cartesian robot systems or using them as integral parts of machines.

The principle sectors:

industrial automation

servos and pick-and-place systems in production processes

assembly

packaging and palletisation

Indeed, just think of applications such as aircraft, laser or plasma chopping machines, the loading and unloading of machined pieces, feeding machining centres in a production line, or moving an industrial anthropomorphic robot along an additional external axis in order to increase its range of action.

All of those applications use one or more linear actuators. In accordance with the type of application and the performance that it should guarantee in terms of precision, load capacity and velocity, there are various types of actuators to select from, and it is typically the type of motion transmission that makes the difference.

There are three major types of motion transmission:

belt

rack and pinion

screw

How can you ensure that you choose the appropriate actuator? What variables does an industrial designer tackling a new application should take into consideration?

As is commonly the case when talking about linear motion solutions, the necessary thing is to consider the issue from the best viewpoint – namely the application and, above all, the outcomes and efficiency you’re expecting. As such, it is value starting by considering the dynamics, stroke size and precision required.

Let’s look at these in detail.

High Dynamics

In many areas of business design, akin to packaging, for instance, the demands made of the designer fairly often need to do with speed and reducing cycle times.

It is no shock, then, that high dynamics are commonly the starting level when defining a solution.

Belt drives are often the best solution when it involves high dynamics, considering that:

they permit for accelerations of as much as 50 m/s2 and speeds of up to 5 m/s on strokes of so long as 10-12m

an X-Y-Z portal with belt-driven axes is typically capable of handling loads ranging from extremely small to approximately 200kg

in accordance with the type of lubrication, these systems can offer notably long maintenance intervals, thus making certain continuity of production.

Wherever high dynamics are required on strokes longer than 10-12m, actuators with rack and pinion drives tend to be a superb solution, as they permit for accelerations of up to 10 m/s2 and speeds of up to 3.5 m/s on probably infinite strokes.

The selection of a different type of actuator would not guarantee the same results: a screw system, which is undoubtedly a lot more precise, would certainly be too gradual and wouldn’t be able to handle such lengthy strokes.

Lengthy Strokes

Systems created by assembling actuators in the typical X-Y-Z configurations of Cartesian robotics typically, in applications corresponding to pick-and-place and feeding machining centres along production lines, have very lengthy strokes, which may even reach dozens of metres in length.

Plus, in many cases, these lengthy strokes – which usually involve the Y axis – are tasked with dealing with considerably heavy loads, typically hundreds of kilos, as well as quite a few vertical Z axes which operate independently.

In these types of applications, the best choice for the Y axis is certainly an actuator with a rack and pinion drive, considering that:

thanks to the inflexibleity of the rack and pinion system, they’re capable of operating alongside probably unlimited strokes, all whilst sustaining their inflexibleity, precision and efficiency

actuators with induction-hardened steel racks with inclined teeth which slide alongside recirculating ball bearing rails or prismatic rails with bearings are capable of dealing with loads of over a thousandkg

the option of putting in a number of carriages, every with its own motor, permits for quite a few independent vertical Z axes.

A belt system is good for strokes of as much as 10-12m, whilst ball screw actuators are limited – within the case of lengthy strokes – by their critical speed.

Positioning Repeatability

If, then again, the designer is seeking maximum precision – like in applications such as the assembly of microcomponents or certain types of dealing with within the medical discipline, for example – then there is only one clear alternative: linear axes with ball screw drives.

Screw-driven linear actuators provide the very best efficiency from this viewpoint, with a degree of positioning repeatability as high as ±5 μ. This performance cannot be matched by either belt-pushed or screw-driven actuators, which each attain a most degree of positioning repeatability of ±0.05 mm.