Barnstaple, England -Engineers use ball screws when they want efficient, reliable, and repeatable conversion of rotary to linear motion-characteristics that have made ball screws standard in a wide variety of applications. However, in today's higher load, longer travel machines, standard ball screws may vibrate in a transverse direction (due to their own weight), somewhat like a "jump-rope." But rather than eliminating the ball screw from consideration in long traverse, high-speed applications, engineers have to simply rethink the design. Instead of driving the screw directly, drive the nut instead. This design change overcomes the "jump-rope" issue with speeds of up to 200 m/min over a 2-3m traverse.
In a typical RN application, the carriage carries a motor-driven belt drive that attaches to a pulley mounted on the driven end of the ball nut.
Unfortunately, until now most rotating nut ball screws did not achieve the life of conventional designs. Why? Moving at faster speeds means completing more work in the same time. Consequently, a high-speed, nut-driven ball screw needs greater dynamic capacity to achieve the same life as a conventional screw-driven assembly. To increase life, Thomson IBL Company uses high-capacity, angular-contact bearings in its RN series rotating nut ball screws.
The RN series uses standard diameter and standard lead screws with a flange-type precision housing that mounts to a moving carriage. The stationary housing contains the preloaded rotating ball-nut assembly. In a typical configuration, the carriage would carry a motor-driven belt drive that attaches to a pulley that mounts to the driven end of the ball nut.
Turbine-like feed channels deliver lubricant to the recirculating ball bearings inside the rotating nut.
"There are like products available, however, these tend to fail in the field because they use integral bearing raceways between the rotating nut and the stationary housing. The raceways have lower load carrying capability than the ball nut they are supporting," explains Thomson IBL's Chief Engineer-Design and Development Dennis Walton. "In contrast, our rotating nut uses separate angular-contact bearings that match the dynamic capacity of the ball screw it carries, achieving longer life."
The biggest engineering challenge, according to Walton, was designing a lubrication system for the recirculating ball bearings inside the nut. "Centrifugal force due to nut rotation propels lubricant outwards once it leaves the feed pipe," explains Walton. "Our system was developed over many years to insure that lubricant penetrates down into the inner ball races to lubricate the screw itself."
Contact John DeGenove, Thomson Saginaw, Tel: (517) 776-5111; E-mail: [email protected] .