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Linear motion keeps movin' ahead

Linear motion keeps movin' ahead

As ball screw technology continues to achieve higher and higher accuracies and travel speeds at lower prices, Joseph Nook sees new opportunities for this venerable linear motion technology.

Design News: Despite competition from direct drive linear motors, ball screw technology seems to be holding its own in the marketplace. What are some of the factors that have contributed to the product's continued popularity and success?

Nook: Linear motor prices may come down in the future, but for pure capacity, the ball screw is still the most cost effective solution on the market today. Another advantage is the fact that balls screws can be made in extremely small sizes, which is a key attribute in applications where real estate is at a premium. And there is also an impressive experience and application base built up around the use of ball screws -- they have been around since before World War II -- which helps to keep the momentum going.

Much of the success of the ball screw has to do with our ability to continually improve the manufacturing process, which has led to ongoing improvements in both cost and lead accuracy. Take, for example, thread rolling -- it's an art form that has taken on a whole new perspective. Today, we can roll ball screws that approach the capabilities of precision ground ball screws. By putting a pre-loaded nut on that ball screw, we get comparable performance levels -- at a reduced cost.

Q: What are some other recent technical advances in ball screw technology?

A: One of the things that we've done is figured out how to roll hollow ball screws, which is a real technological breakthrough. The advantage of a hollow screw is that it allows the user to run lubricant or coolant right through the center of it. Also, hollowing out the screw reduces weight, changes the inertia, and leads to higher travel speeds in some cases.

We also now have the ability to manufacture the ball nut with the internal pre-load all in one step, which helps to minimize backlash. What we do is grind half the number of circuits on the ball nut then internally shift the lead and continue grinding the remaining circuits on a CNC grinding machine. This capability helps to improve the performance of the ball nut from a stiffness standpoint and results in lower torque values.

Q: Is there really such a thing as "zero" backlash?

A: Zero backlash probably needs to be defined a little better. I can produce a ball screw for a given application that has an operating load of, say, 100 lb. I can put a 30-lb preload into the assembly and eliminate the clearance between the screw and the ballnut. However, under load, some lost motion will be experienced, depending on the "spring rate" of the assembly and the load being generated when the load is reversed. Therefore, if we define backlash as "lost motion" of an assembly, then there is no zero backlash. Engineers must keep in mind the fact that minimizing backlash is very application dependent.

Q: Do you see the trend toward greater integration of linear motion components into a complete system continuing, and what are the benefits to design engineers?

A: Actually, I think we're beyond the trend phase today, it's now a business model for manufacturers. There's no question that the systems approach to providing solutions will continue to develop simply because of the tremendous time and cost savings associated with it. And what we're seeing is going well beyond the mere integration of mechanical components. In supplying complete solutions today, the mechanical people are getting married to the electrical people and creating alliances that bring powerful solutions to the end customer.

Q: Can you give us a sneak preview of some of the technology developments for ball screws looming on the horizon?

A: Certainly in the future we are going to higher travel speeds and accuracies, and I wouldn't be a bit surprised to see a smart ball screw. Obviously, such a concept poses specific challenges that still need to be worked out, but this industry has proven itself over time to be very creative and forward-looking.

'Ball screw cost, accuracy keep improving' Joseph H. Nook Jr. founded Nook Industries in Beachwood, OH, in 1969 to provide stocking and machining services for ball screws made by the General Motors Steering Gear Division. This early role enabled Nook to gain the technical expertise needed to enter into the design and manufacturing of linear motion and power transmission products. The firm then added manufacturing capabilities and expanded into linear technologies based on thread-rolling, including ball screws, acme screws, and worm gear actuators. Soon after, the company added linear bearing systems and components. Nook recently received recognition as a "Family Firm of Distinction" from Case Western Reserve University's Weatherhead School of Management.

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