Taking the lead

Jan. 1, 2006
Leadscrews consist of a screw and its riding nut, which is wedged forward as the screw turns. Originally, leadscrews were assembled using low-quality

Leadscrews consist of a screw and its riding nut, which is wedged forward as the screw turns. Originally, leadscrews were assembled using low-quality screws from other fastener or screw designs so that a simple, one-piece nut only provided basic power transmission and rotary-to-linear motion conversion. Today, leadscrews feature technology with distinct advantages for motion control.

Modern materials keep friction very low (sometimes with a coefficient under 0.1) without any external lubrication. Self-lubrication is instrumental in combating environmental challenges. For example, cartoning and case packing regularly generate airborne paper dust, coating everything nearby. But because leadscrews can operate without lubricants, grease — which captures dust and creates a harmful, abrasive paste — is eliminated. They're also easier to tune and more predictable than timing belts. These two characteristics mean leadscrews are often maintenance-free. Also, there is lower particulate generation. Particulate generation is a byproduct of wear; by keeping friction to a minimum, wear is minimized. Also, lead screw wear tends to be steady and linear in nature while ball screws fail catastrophically when localized brinnelling or surface failure occurs on the ball or screw. The pitting and resulting debris triggers additional failure and heavy debris generation. Finally, ball screws rely on external lubrication. Lubrication breakdown, contamination and loss all are frequent causes for early failure.

Leadscrews also operate more quietly. Balls generate noise, especially traveling through the return tubes as they recirculate.

Other leadscrew characteristics include:

  • High helices for fast leads — greater than 100 mm per rev. No balls means lead screws are not constrained to available ball geometries and dynamics. (Getting balls to fit and roll in a high helix screw is a gating design parameter.) Second, thread rolling allows the manufacture of leads that cannot be cut or ground because the cutter will not clear itself. The cutter wipes out adjacent threads if the angle is to high.

    High helix, fast leads are possible in smaller diameters. Screws with 6-mm diameters and 25 mm-per-rev leads are very common in all types of equipment, including scanning, data storage, medical analysis, and semiconductor handling.

  • Very fine threads — as small as 0.5 mm per rev. Thread rolling, a cold forming process can produce very uniform, precise threads in high volume, at a fraction of the cost of machining, and without the problems of cutter wear.

  • Multifunction nuts. Nuts can include other functional components such as guide bushings, mounting features for other components, sensor flags, and gears. This results in part consolidation and reduced assembly time.

  • Easily customized nut designs. It is much simpler to incorporate a threaded hole in a custom molded or machined nut than it is to provide a full ball bearing recirculation circuit.

Particle emission ratesMECHANISM TEST VELOCITY (RPM) EMISSION RATE (PARTICLES PER MIN.) Stepping motors 900 to 1,200 0 to 1,000 Dc brush motors 3,000 to 7,500 0 to 30,000 Shielded ball bearings 600 to 1,200 0 to 4,000 Linear bearing 150 to 1,200 300 to 10,000 Linear leadscrew 50 ft/min. at 1,200 20 to 100 Ballscrew 50 ft/min. at 1,200 2,000 to 3,000 Sprocket and poly chain 600 to 1,200 4 to 300 Low cost ball screws often have imperfect thread forms that cause the ball to skid rather than roll. This behavior leads to accelerated wear. The smooth motion (and no impacting) of leadscrews means they actually generate less particulate than other wear components.

Low backlash with very light pre-load (in other words, low drag) is another benefit. For example, in some inkjet printer-movement systems, leadscrews move the print head on a linear slide. Because it is a high-speed application, antibacklash properties translate to excellent print quality. Any wear that does occur in the screw or nut is compensated by mechanisms that adjust automatically to maintain zero backlash over a long service life.

Polymer composite nut materials provide high strength (dynamic loads of 250 kg) and long life (over 750 million cm of travel). Plus materials can optimized for chemical compatibility, environmental constraints, wear properties, cost, and even color. They can also be molded to custom shapes for additional functionality. Nuts are sometimes made of metal. However, the advantages lubrication, noise, and cost benefits of composites are lost. That's why composites are used whenever possible.

Leadscrews vs. ballscrews

When should ballscrews be used? They're best when very high speed, lead accuracy, and load capability are important. But sometimes their cost and shorter life are prohibitive. Rolling-element ballscrews do have higher theoretical efficiencies than sliding-element leadscrews. That said, actual differences are often smaller because lubricant viscosity and manufacturing tolerances close the gap. In fact, these same efficiencies prevent ballscrews from offering self-locking leads. And at times, a system that does not backdrive is useful.

Rolled ballscrews are less expensive than ground versions, but have their own limitations; for instance, while load ratings are high, they require more maintenance and have shorter life and less design flexibility. In addition, they may cost several times that of a rolled leadscrew assembly.

In washdown environments, the unique materials possible for use in leadscrews allow total immersion in water and other fluids. Miniature leadscrews (with and without antibacklash compensation) generate precision motion in small packages unmatched by other technologies. In fact, modern high-accuracy screws and nuts 2 to 4 mm in diameter helped bring many of the latest data-storage drives and telecommunications equipment to market.

Often less than 80 mm long with both right and left-hand threads, these screws can have leads as fine as 0.65 mm. At the other end of the spectrum, leadscrews can have very fast, efficient, and accurate leads up to 100 mm per rev or more. This thread type is very successful in high-speed automation, including semiconductor handling, laser engraving, transportation door actuation, and valve actuation.

Rolled multistart threads also avoid what's called thread “drunkenness,” caused by pitch-to-pitch error of ground or cut multistart threads.

Imagine how many extra parts would be needed to perform certain functions if only standard nut configurations were available. Then, try to imagine getting this customization in a ball nut. Custom leadscrews reduce complex assemblies to single components. A nut incorporating the carriage and linear guide block is less expensive because it reduces component costs and simplifies of assembly and alignment.

In actuator units

Linear electric actuators are motor-driven screws. Traditionally, linear motion requires separate components to handle both driving and support or guidance. Actuators combine both functions in a single, coaxial component. Those with leadscrews consist of a precision-rolled leadscrew supported by sealed bearings in a concentric steel guide rail that drive an integrated nut or bushing. Load is attached to the screw end of its rod or screw and is unsupported. By eliminating the need for external rail-to-screw alignment, actuators simplify the design, manufacture, and assembly of motion systems. A linear actuator's coaxial design saves up to 80% of the space used by a two-rail system and usually costs less than equivalent components purchased separately. Thus, reduced labor and component costs contribute to substantial savings. In addition, built-in alignment between screw and rail prevents binding and promotes smooth operation.

Another benefit of linear models is the three-dimensional motion possible with a single actuator. When mounted vertically, they can be used to lift and rotate (often designated as Z-theta motion) simultaneously. With one motor driving the screw and a second rotating the rail, compact, self-supporting pick-and-place mechanisms are created. To reiterate, all alignment requirements are met within the actuator, so support and positioning are less critical than with traditional slide assemblies.

For more information, call Kerk Motion at (603) 465-7227 or e-mail the editor at [email protected].

On the case

On case-packing machines that move a product down a conveyor for packaging, it's important to move large product volumes quickly. RPT Motion, based in Quebec, Canada, provides custom modular linear motion systems. Over the last few years, the company has experienced increasing demand for case packers and other linear manufacturing solutions.

“So much of our business is driven by speed,” says Peter Ratcliffe, President of RPT Motion. “Because of globalization, we have observed an intense pressure to reduce downtime and thus, increase overall productivity.” A key segment of RPT's business consists of updating existing subsystems on machinery for increased production.

One challenging retrofit project was for a major tissue manufacturer whose operation bottlenecked at the case-packing stage. The manufacturer wanted to add a second case packer to overcome this. However, the existing system had a two-position lane changer powered by pneumatic cylinders that couldn't provide the four required positions or higher speeds. To service the four lanes and maximize time for product flow, a stroke in excess of 40 in. is required; plus, the lane changer indexes every 10 sec or less.

Says Ratcliffe: “We found bigger, heavier ballscrews with fast-enough lead, but they did not fit into the current assembly.” Instead, RPT teamed up with Kerk to design the required four-lane feed — two lanes in both case packers. Specifically, the new system uses Kerk's VHD-Series leadscrews, which have a lead to 3.625 in. in a smaller envelope. The servo system's speed accommodates the second case sorter and has subsequently eliminated the manufacturer's bottleneck — within the confines of the system's original dimensions.

Saving a bundle

Omega Design Corp., Exton, Pa., manufactures packaging equipment systems — plastic bottle and puck unscramblers, shrink bundlers, stretchbanders, and orienters. One of the company's products (the Classic Series of shrink bundlers) is in worldwide operation. This pneumatically-driven, PLC-controlled machine was designed for automatic shrink packaging of containers into predetermined bundle configurations. It helps eliminate the cost of cardboard boxes and creates a more secure package, which lends itself to further automation down the line, such as case packing.

With unique packaging on the rise, Omega needed to accommodate alternative package configurations. After incorporating several changes, the company focused on the product pusher, a pneumatic-driven actuator that pushed product into film or other packaging material. Inherent to pneumatic actuators is play and uneven motion control, causing inconsistent product flow through the pusher area, misaligned products, and machine stoppage. Air compression is also expensive.

After discussions with applications engineers, Omega decided to replace the pneumatic pusher with a high-speed Kerk RGS (Rapid Guide Screw) 10000, a cost-effective, screw-driven slide. Omega's engineers then worked with Kerk to incorporate a servomotor and a few additional components to create an intelligent motion system. “It was a simple swap-out,” says Devendra Shendge, a product development specialist with Omega Design. “The slide was retrofitted to the unit we were using and easily met the space constraints.” Speeds over 60 in./sec are possible, rivaling belts and cables while maintaining axial stiffness. The screw generates 150 lbf, while the original air cylinder only produced 80 lbf. Also: “Before it was just a continuous motion, 0-50 in./sec,” says Shendge. “Now we can accelerate or decelerate the machine. This is critical, because when you're dealing with unusual shapes and heavier mass, you can't just thrust them through the machine at top speed. You can damage the machine as well as the product.”

Omega first put the new Classic shrinkbundler on exhibit at the PMMI Pack Expo Show in Las Vegas, 2005.

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