Small seals, big impact

Every gram counts in the drive to build better robots, with lighter designs helping increase the speed, acceleration, and precision of robotic movement while improving energy efficiency and workplace safety.

The operating time of automated mobile robots (AMRs) in e-commerce warehouses is as dependent on weight as it is on advances in battery technology. Collaborative robots (cobots) designed to operate alongside humans without barriers are safer when they are lighter. For high-precision tasks such as semiconductor fabrication, building implantable medical devices, or robots that conduct brain surgery, robots need to be smaller simply to be able to do their jobs.

"When the first industrial robots appeared in the late 1970s, the goal was simply to make them work reliably – that’s what everyone focused on," says David Kaley, Trelleborg Global Segment Manager for Industrial Automation. "Then they needed to make them faster, and they found out that faster comes from lighter: the less the moving mass, the faster robots can go."

A new range
The Stefa Mini portfolio of seals promises to enable a new leap forward in the decades-long development of robots. With cross sections as small as 3 millimeters, the seal range is designed to be used in joints, servo and direct-drive motors, gearboxes, and speed reducers of smaller, lighter robots.

The product comes in three designs: the Stefa Mini Radial Shaft Seal, the Stefa Mini Cassette Seal, and the Stefa Nano Radial Shaft Seal. While smaller seals alone do not make a substantial difference to overall weight, they allow for a smaller housing, which opens the way for significant weight reductions.

Made from Trelleborg's proprietary fluoroelastomer (FKM) compound, Stefa Mini seals reduce energy lost to friction, allowing for greater speed.

Typically, as the coefficient of friction reduces, the seals ability to seal reliably can also lower. Trelleborg has carefully calibrated the coefficient of friction while refining the profile of the radial shaft seal’s lip to optimize the pressure it applies to a shaft’s surface, ensuring leak-free operation. This means the seals keep grease and oil sealed safely in robotic joints, servo motors, and speed reducers while allowing high rotational speeds and acceleration. Tests conducted both internally by Trelleborg and by customers have proven that they are durable too.

"Designing a seal that runs well for 5,000 hours, maintains low friction, and doesn't leak is a big challenge," Kaley says. "That was the demand from the customers, and we've achieved these criteria during customers' tests."

Development of the Stefa Mini seals began when a customer requested an ultra-small seal for the gear motor on an articulated-arm robot. After coming up with the unique profile design for the Stefa Mini Radial Shaft Seal, Trelleborg designers decided to develop a cassette seal and nano version to create a new family of seals.

Where will they be used?
The primary use case for the seals remains articulated-arm robots, as lowering the housing weight makes the swinging mass of an articulated arm easier to manage. Smaller seals result in smaller arm housings. This is particularly the case for cobots with articulated arms, which have historically had to operate at lower speeds to avoid injuring people working alongside them.

"Collaborative robots have become significantly lighter – a robot arm weighing 75 kilograms now weighs 20 kilograms," Kaley explains.

Reducing the size of gears and motors is not the only way of reducing robot weight; lightweight materials can also replace stainless steel for the rotating shaft.

“It is difficult, however, to manufacture in lighter materials such as magnesium, aluminum, and carbon fiber with a sufficiently smooth surface to provide a good seal,” says Kaley. “The Stefa Mini Cassette Seal overcomes this problem. It attaches to a sleeve that goes over the rotating element, eliminating the need for expensive prepping of the surface of steel shafts and allowing customers to use lighter materials without any problems.”

This type of seal is particularly useful for cobots, where low weight is a key safety element. It increases energy efficiency in end-of-arm tooling on larger robots, such as those used for picking up cylinder heads and positioning them in an automotive plant. Well-suited to delta robots used for picking and packaging in factories, lightweight materials allow their arms to reach speeds of up to 300 picks a minute.

Longer run time
The second major use case, according to Kaley, is "anything that runs off a battery." The seals are ideal, for instance, for the axial flux motors that drive the wheels of AMRs, and in particular the wheels of the Automated Storage and Retrieval Systems (ASRS) that move around e-commerce warehouses and factories collecting and depositing products and components.

For an ASRS, a good seal is critical if it runs on a rack over areas where products or components are stored. "As an example, if a single drop of oil gets on the package of a 200 USD pair of shoes, it has to be thrown out," Kaley says.

Supporting precision
The final member of the Stefa Mini seal family, the Stefa Nano Radial Shaft Seal, is intended for use in precision manufacturing, such as in semiconductor cleanrooms or inside the production lines that build implantable medical devices.

"We're talking micron-level positioning and devices that can only be manufactured with robots. The dexterity and accuracy of a human are just not capable. We're not wired for that," Kaley says.

The Stefa Nano Radial Shaft Seal is available in diameters as small as 10 millimeters and can be used in robotic joints with oscillating and forward-and-reverse rotational movements, as, unlike many seals of a similar size, it can seal in both directions without any need for back-pumping.

An extension
The new range has generated strong interest since its launch. Kaley says, “Customers have completed trials on the Stefa Mini, with serial production already started. Others are considering the Nano for very small, extremely low-torque axial motors.”
Kaley though stresses that the Stefa Mini range is intended to extend the company's offering rather than replace Trelleborg’s existing radial shaft seals.
“It’s a strategic addition to the product portfolio,” he says. “We still have our larger radial shaft seals, Roto Variseals, and V-ring designs, for instance. Stefa Mini is really an addition to an already robust line of rotary seals.”

Given the ever-growing demand for smaller, lighter robots, it should be a welcome one.

Writer: Richard Orange
Photos: Shutterstock , Adobe Stock and Trelleborg