The second in a short, 3-part series, Schaeffler’s Director – BU Production Machinery, NA, Craig Hooker gives Bearing Tips his take on trends he sees coming along for the industry.
Bearing Tips: Tell us about an application where engineers used your bearings to improve their design. What about this application is unlike anything possible 10 years ago?
Hooker: Schaeffler is currently in the process of launching a new product called a differential roller screw. This is product that is currently unavailable in the market place at the price point Schaeffler can provide. We are able to do this by designing for high volume, low cost manufacturing.
There are primarily 3 types of screws in the market today. If you need linear actuation and a screw is best (vs belt drive, rack and pinion, linear motor), it’s highly likely you use one of these 3 types. The first type is a lead screw. It uses a sliding motion between a threaded nut and threaded rod. This type of screw is most often the lowest cost screw but has poor efficiency and generates heat if the duty cycle is high. The second type is a ball screw. It’s similar to a lead screw except there are balls between the nut and threaded rod. The rolling motion of the balls as they re-circulate through the nut provide lower friction and thus higher efficiency. Although ball screws cost more, the higher efficiency and duty cycle are worth the trade off in many applications. The third type of screw is a roller screw. A roller screw uses planetary rollers that rotate around the threaded rod. In a traditional roller screw design, the planets have a helical thread that matches the pitch on the threaded rod. The planetary rollers have the advantages of providing many more points of contact as well as a line contact at each of those points (vs point contact in a ball screw). This results in much higher forces than ball screws (a multiple of 5x to 10x).
The efficiency of roller screws is also high (85-90%). The downside comes in the form of a high cost due to the fact they are ground to maintain the geometry critical to sharing the load across many contact points. The differential roller screw Schaeffler is launching will provide similar loads to the traditional planetary roller screw at prices similar to a ball screw. The efficiency isn’t quite as high as a traditional roller screw but is generally much better than a lead screw. There can also be some slippage between the rollers which creates lost motion over time. However, this can usually be accounted for by adding a simple feedback system.
This product has the potential to allow customers to create products that weren’t possible 10 years ago. An example is crimping tools. Today crimping tools typically use micro-hydraulics for very high forces or mechanical mechanisms (levers) for manual operation when low forces are required. The Schaeffler differential roller screw will allow manufacturers of this tool to create a product that has high forces but with lower weight, complexity, and cost compared to hydraulic units.