Design a Better Roller Screw Through Lean Engineering
Planetary roller screws provide significant benefits over other screw-type actuators in terms of performance, load capacity, duty cycle, and lifetime. Traditionally, however, those benefits have come along with significantly higher prices and lead times. As a result, the technology was typically saved for applications that had to have performance, no matter what the price. This article will explore the development of Tolomatic’s roller screws as a case study.
Along with manufacturing partner Motion Solutions, Tolomatic has released a family of U.S.-made roller screws that cut pricing and reduced lead times to as little as two weeks. The product line enables engineers designing general industrial automation equipment to gain the performance advantage of roller screws.
Planetary Roller Screws 101
A planetary roller screw combines characteristics of a standard screw-type actuator with the configuration of a planetary gearbox. Many variations exist, but the most general type consists of a nut that travels relative to the screw. Instead of recirculating balls, the interface between the threaded nut and the screw consists of a series of rollers arranged around the screw in a planetary fashion. The nut threads mesh with the roller threads, and the roller threads mesh with the threads of the screw.
In a planetary roller screw, the nut is lined with a planetary arrangement of rollers. The rollers reduce friction while significantly increasing contact area, enabling roller screws to support greater loads, higher duty cycles, longer lifetimes, and better accuracy and repeatability than other screw-type actuators. (Courtesy of Tolomatic)
The design significantly increases surface contact. As a result, a planetary roller screw can handle a much higher load in a smaller package than (for example) a ballscrew. This characteristic makes them very effective in applications with size or weight limitations. Roller screws are also much more robust than competing technologies, enabling them to handle very high-duty cycles and achieve long lifetimes.
Planetary roller screws deliver high accuracy and repeatability. They may carry a somewhat higher price than competing technologies, but it is important to make an apples-to-apples comparison. “You can get that level of performance from a ball screw, but you have to go to the next level, with precision grinding, special pre-loading, or other options,” says Aaron Dietrich, director of marketing at Tolomatic. “With a roller screw, it’s kind of inherently built in.”
The downsides of that higher surface contact area are increased heat generation, greater complexity, and higher cost. To further complicate matters, the lead times for roller screws have often extended to three or four months. As a result of these factors, planetary roller screws have traditionally been reserved for specialty applications with performance demands that can only be satisfied by roller screws. They would be more likely to show up in an aerospace application than a machine for stacking auto body parts, for example. Motion Solutions and Tolomatic partnered to create a planetary roller screw that could appeal to any industry.
Designed for Cost-effectiveness
For years, Tolomatic has built actuators incorporating roller screws. Once the company began to develop high-quality roller screws to meet its in-house actuator needs, it was a short step to releasing those products to the market. These roller screws could be applied to any robust and precise performance of a planetary roller screw to a wide range of industrial applications.
The manufacturing process was designed to yield cost-effective components on short lead time. They chose automation whenever possible to achieve a fast and efficient production time. The next part of the recipe is to maintain inventory of screws so that products can be assembled efficiently. “As an actuator company, we support a 15-day lead time with standard actuator products including with roller screws,” says Dietrich. “As a result, we actually stock fixed lengths of roller screw at any given point in time. So, not only is the manufacturing process efficient and cost-effective, but for the most part, we have the parts for our stock components sitting on the shelf.”
Improved Availability
Only a small number of suppliers worldwide produce these precision components, and there is limited overlap among product lines in terms of mechanical characteristics like flanges, hole patterns, etc. If a component becomes unavailable, the chances of finding an exact drop-in replacement from a different manufacturer are slim to none.
That was the problem faced by a tier-one semiconductor test equipment manufacturer when the roller screw currently used in its platform was discontinued. The company was facing a serious supply-chain issue, not only for the assembly of new machines but for the maintenance of existing equipment.
Motion Solutions worked closely with Tolomatic to modify one of the latter’s roller screw products and make it a drop-in replacement. The Motion Solutions manufacturing team examined the customer’s current product and then reviewed Tolomatic’s offerings. The Motion Solutions team handled the customization of the screw shaft and the nut, in addition to developing a lubrication solution. The component was modified so that it would integrate into the machine without the customer making any changes on their end. Once everything was approved, Motion Solutions built the subassemblies and shipped them directly to the customer.
Customization can be extremely useful for OEMs, but success hinges on properly defining the scope. Some changes, such as adding a flange or changing the length of the lead screw, are straightforward. Others, like changing the lead of the screw, involve retooling robotic equipment and can take many months to get right. In any case, it takes volume to make the exercise practical.
With a low-cost, highly available roller screw product line that is built in the United States, the rules of thumb for when to use a roller screw have changed. Machine builders have a new set of solutions they can use to increase performance, meet goals, differentiate their products in the marketplace.
Bill Saunders is the vice president of sales at Motion Solutions.
