Why Planetary Roller Screws Deliver Maximum Performance in Robotics

You may not be familiar with roller screws, but these compact, highly efficient screw drives are an important technology in the production of everyday devices and products. In fact, the manufacture and assembly of your car likely involves more than 20 roller screws performing functions such as stamping, tube bending and riveting.

But roller screws are not limited to the automotive and heavy industries, such as injection molding and machining. On the contrary, roller screws can produce the smallest displacements (movements) of any screw technology while remaining extremely quiet and smooth in operation, which makes them ideal for a wide range of robotics applications.

What robots need: high efficiency, compact size

When it comes to precision screw technologies, there are three main options: lead screws, ball screws and roller screws. Lead screws are versatile in design and materials, but their efficiency and load capacity are relatively low. Ball screws offer good efficiency, but when an application requires a small screw diameter and a small lead, load capacity suffers because the load-bearing balls must also be very small. 

Roller screws, on the other hand, offer strong thrust force in a compact package, as well as superior efficiency, the best of any option. As an example, when comparing load capacity and efficiency for different screw technologies of the same diameter (all with a 1 mm lead), a roller screw has four times the load capacity of a ball screw and twice the efficiency of a lead screw. 

This combination of performance and design characteristics makes roller screws an ideal solution for many robotics and related applications. For example, robots that use linear actuation to connect joints (similar to how the biceps controls the movement of your forearm) benefit from the higher load capacity and lower weight of roller screw drives.

In robotic gripping applications, roller screws can provide the extremely high holding force needed for a secure grip, and can do so with minimal input torque. The key to this combination of high force and low input torque is using a screw with a very small lead.

Roller screws can also be used in robotic end-of-arm tools. In these applications, the roller screw's ability to generate high force in a small housing provides an advantage over other solutions. This is because keeping weight (mass) low is critical for components located at the end of a robot's arm. 

Heavy payloads at the end of a robot arm create significant inertia, which makes precise motion control difficult and, in some cases, requires larger motors and other drivetrain components. This superior power-to-weight ratio is a key reason why spot welding robots and automated riveters overwhelmingly use roller screws for actuation.

Trends in robotics

Modern computing, manufacturing methods and battery technologies have driven rapid progress in automation and electrification, enabling many new robotics and mobile applications that were impossible just a few years ago. One example is the development of active prostheses (prostheses powered by motors), which significantly improve the function and mobility of people with limb amputations.

Unlike large industrial robots, in which each joint uses its own motor and gearbox, many active prostheses use a screw-driven linear actuator, allowing smaller, lighter and more durable designs. While active prostheses can use various types of screws, roller screws have an important advantage over other technologies: an unmatched ratio of performance to size. 

For example, a roller screw only 3.5 mm in diameter, small enough to fit in the palm of a hand, can push loads of up to 227 kg and perform movements in the micron and submicron range to better mimic human joints and fingers.

Electromechanical exoskeletons are another rapidly developing robotics technology. According to forecasts, the exoskeleton market will grow by more than 46% from 2021 to 2026, with active exoskeletons accounting for a large share of this growth (compared with passive exoskeletons).

Although we would like to be able to program robots to perform any manual task that is too heavy or risky for humans, there are too many variables for this to be practical for many tasks, such as trimming branches from trees in residential yards. The solution is a human-controlled exoskeleton or an actively controlled robot. 

This combines human decision-making with lower risk, fewer injuries and increased load capacity. Like prostheses, exoskeletons face design constraints related to weight, size and complexity. Here, the extremely high ratio of force to size and weight makes roller screws an ideal solution.

And the smaller and more mobile a robotic application becomes, the more constraining attributes such as power consumption, size and weight become. Whether drones, autonomous underwater vehicles (AUVs) or space rovers, the requirements are the same: high efficiency, strength and reliability in the smallest possible footprint.