Linear Actuators: what they are and how to choose them

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

The most common use of actuators involves combining them with multi-axis Cartesian robot systems or using them as integral components of machines.

The main sectors which employ the use of linear actuators are:

• industrial automation
• servos and pick-and-place systems in production processes
• assembly
• packaging and palletization

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

All of these applications use one or more linear actuators. According to the type of application and the performance that it must guarantee in terms of precision, load capacity and speed, there are various types of actuators to choose from, and it is typically the type of motion transmission that makes the difference.

There are three main types of motion transmission:

• belt
• rack and pinion
• screw

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

As is often the case when talking about linear motion solutions, the important thing is to consider the issue from the right viewpoint - namely the application and, above all, the results and performance you are expecting. As such, it is worth starting by considering the dynamics, stroke length and precision required.

Let’s look at these in detail.

High Dynamics

In many areas of industrial design, such as packaging, for example, the demands made of the designer very often have to do with speed and reducing cycle times.

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

Belt drives are often the ideal solution when it comes to high dynamics, considering that:

• they allow for accelerations of up to 50 m/s2 and speeds of up to 5 m/s on strokes of as 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
• according to the type of lubrication, these systems can offer particularly long maintenance intervals, thus ensuring continuity of production.

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

The choice of a different type of actuator would not guarantee the same results: a screw system, which is undoubtedly much more precise, would certainly be too slow and would not be able to handle such long strokes.

Long Strokes

Systems created by assembling actuators in the typical X-Y-Z configurations of Cartesian robotics often, in applications such as pick-and-place and feeding machining centers along production lines, have very long strokes, which can even reach dozens of meters in length.

Plus, in many cases, these long strokes - which usually involve the Y axis - are tasked with handling considerably heavy loads, often hundreds of kilos, as well as numerous vertical Z axes which operate independently.

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

• thanks to the rigidity of the rack and pinion system, they are capable of operating along potentially unlimited strokes, all whilst maintaining their rigidity, precision and efficiency
• actuators with induction-hardened steel racks with inclined teeth which slide along recirculating ball bearing rails or prismatic rails with bearings are capable of handling loads of over 1000kg
• the option of installing multiple carriages, each with its own motor, allows for numerous independent vertical Z axes.

A belt system is ideal for strokes of up to 10-12m, whilst ball screw actuators are limited - in the case of long strokes - by their critical speed.

Positioning Repeatability

If, on the other hand, the designer is seeking maximum precision - like in applications such as the assembly of micro components or certain types of handling in the medical field, for example - then there is only one clear choice: linear axes with ball screw drives.

Screw-driven linear actuators offer the best performance from this point of view, with a degree of positioning repeatability as high as ±5 μ. This performance cannot be matched by either belt-driven or screw-driven actuators, which both reach a maximum degree of positioning repeatability of ±0.05 mm.

If you would like to learn more about this topic and find out the nine essential features to consider when choosing linear actuators, read our in-depth report: