Mechanical Options Match Needs

Lead Screws and Ball Screws Still Have an Important Role to Play in Linear Motion Applications

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We celebrated our 15th anniversary two years ago by republishing some of our more timeless content. It was really well-received, so we decided to do it again from time to time. Here’s an evidence-based article we first published in January 2008 that was, as it still is today, a reminder that the mechanical components of a solution often are just as vital as the accompanying electronics.

There’s an insistent drumbeat in Motion Control Nation that says the demise of electromechanical components, including the family of ball screws and lead screws is on a fast track. The well-touted advantages of direct drive motors for all but a few high-torque applications seem compelling.

As with many technology predictions, the talk of the demise of ball screws and lead screws for linear motion translation often neglects to factor in applications for which the approach is highly beneficial.

For example, ball screws continue to find favor with Westweigh Systems in Johannesburg, South Africa. The company worked with Tectra Automation to develop a fully automatic weighing and packaging machine for powder mills. It can fill and package up to 12 bags/minute, weighing 10–12.5 kg.

The solution rests on a highly automated system in conjunction with reliable performance from mechanical components. Linear guides and ball screw drives are directly attached to the servo motors and provide a sealed unit that minimizes the ingress of dirt. “Despite it being fully automated, we can offer it at a third of the cost of an imported unit,” says Bill Binckes, Westweigh sales engineer.

Noise Abatement

Meeting performance expectations isn’t always just about speed and accuracy. The designers of drug sample analysis machines at PerkinElmer Life Sciences, Waltham, Mass., will tell you that noise is a serious consideration in component selection. The company offers a broad range of automated systems for liquid handling and sample preparation for small academic laboratories and large labs engaged in clinical, biotech and drug discovery research.

Their noise problems came from the ball screws that play an integral part in the motion schemes of various machines that analyze customer drug samples. The ball screw was a 5/8-in.-diameter screw, a popular item produced in very high volumes. This is the same type of screw used in many automobile steering wheel systems. While the performance of the ball screw was fine, the noise it created was a nuisance.

Ball of Noise
Figure 1: The noise problems in PerkinElmer’s robotic liquid handling system, which came from ball screws that were an integral part in the motion, were eliminated by a switch to lead screws.
Photo by PerkinElmer Life Sciences

Fed up with the noise, as well as some of the maintenance and service issues necessitated by the ball screws—they constantly needed lubrication—PerkinElmer replaced the ball screws in several machines, specifically MiniTrak (Figure 1), a compact liquid handling model, and PlateTrak, a robotic liquid handling system designed for high-throughput screening and other drug discovery and biomolecular protocols.

Steve Schaefers, one of the manufacturing/production engineers involved with the machine designs, says his company did a thorough search for vendors to solve its noise problems. Investigating myriad competitive alternatives, they discovered that Kerk Motion Products was best-suited to their needs.

“We evaluated numerous prototypes and based our decision on results,” says Schaefers. PerkinElmer used Kerk’s B-series, non-ball lead screws with modified nuts constructed of acetal material with stainless steel screws coated with a proprietary TFE that Kerk claims increases lubricity and extends normal nut life by more than 300%.

“In a ball screw, noise emanates from the ball return tube,” says Andy Boyer, Kerk sales engineer. “The balls come out of the nut and go into the return tube. Basically, the balls move to the start of the circuit and then go back into the nut, and that process actually is very noisy.”

Boyer says it surprises a lot of people that a non-ball lead screw, which is a friction system, is quieter than a ball screw. “Most people think a ball screw is going to be really quiet and last forever,” he says. “While ball screws can be efficient in performance, they are markedly noisier than non-ball lead screws and require far more maintenance.”

Technical issues also arose because the Kerk nut design was a bit different, reports Boyer. “Our screws are constructed from a high-quality stainless steel, which is different than the ball screws previously used. Plus, for them to be properly mounted on their equipment, we had to modify the nut design.”

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