Simplified assembly yields cost, production advantage.
Amtren Corp.’s Optical Systems Group, Montgomery, Ala., manufactures CD/R and DVD/R automated recording systems. John Klein, a design engineer with Amtren, was looking at designs to decrease cost and provide a better-quality product. When he and several other Amtren representatives saw the ScrewRail, they realized it could improve the performance of the duplicator products.
The ScrewRail was a new product for Kerk Motion Products, Hollis, N.H. Kerk had no documentation for it yet. Still, it sounded like it might provide the type of motion Amtren was looking to create.
Amtren’s design was revised to incorporate the ScrewRail. The new design was implemented within six weeks.
The ScrewRail is employed in one of the duplicator’s most critical operations — in a vertical orientation, in conjunction with the duplicator’s pickarm assembly.
Using the ScrewRail as the backbone of this operation — other than the actual CD/DVD trays, the pickarm is the only moving component — the pickarm grabs a CD/DVD from a stack of blanks, rotates it into position, and drops it into the CD/DVD drive, after which it picks up the CD/DVD out of the drive, then rotates it to an output stack. In some duplicator models, the CD/DVD may also be transferred to an accompanying printer.
The ScrewRail combines both drive and support/guidance functions into a single, compact, coaxial component. This eliminates the need for external rail-to-screw alignment, producing substantial cost savings over typical two-rail systems. It also delivers threedimensional motion from a single unit, allowing it to simultaneously lift and rotate when mounted vertically, crucial in Amtren’s application.
Amtren uses the 4000 series standard configuration. All ScrewRails feature TFE coating, eliminating external lubrication.
Besides providing Amtren with the ScrewRail, Kerk also redesigned the gear of the duplicator pickarm assembly. In the previous design, Amtren would purchase a machined gear, place it on the duplicator, and put in the setscrews, after which it would determine exactly where it needed to put the flag, drill holes, then align the entire assembly.
Kerk created a design for an injection-molded gear for Amtren with some custom machining, incorporating features that align to the rail and includes the flag positioning; all Amtren needs to do to mount the gears is press them onto the rail and snap a few parts into place, simplifying assembly and eliminating the need for alignment.
Taking the lead
Precision rotary-to-linear converters coupled with cost-effective motors help transform offices into print shops, doctor’s offices into labs, and manufacturing workers into bionic wonders. Still, ball and lead screws are not without vulnerabilities. Here Tom Solon, P.E., applications engineer at Kerk Motion Products, Hollis N.H., explains how to avoid a few common application pitfalls.
• Wimpy motors. Quite often, great designs fail because motors are selected solely for price. In the automotive world, the saying goes: There is no substitute for cubic inches. Maybe we should be saying, “There is no substitute for ounce-inches.” Budgeting for some extra torque covers a lot of sins.
As a cost-cutting measure, products in consumer markets (like computer peripherals) frequently employ motors with insufficient torque for desired drive parameters. It often becomes a compromise between meeting speed/load limit performance goals and combating friction resulting from equipment manufacturing tolerances.
• Insufficient structural integrity. Sheet metal is often used for equipment chassis. Unfortunately, manufacturing tolerances and lack of rigidity make sheet metal a poor choice for ensuring relative positions of multiple components in a quality motion system. The money saved in piece price is often lost in labor costs trying to get everything aligned. Flexing also compromises reliability.
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The solution here is to use stiffer backbone structure, whether it be metal plate, casting, structural plastics, or properly gusseted and reinforced sheet metal. An alternative is to use a self-contained actuator mechanism that includes drive and guidance elements.
• Long travel at high speeds can introduce resonance and vibration in large free spans. Equipment designers must slow things down, include heavy and expensive drives, or select specially designed actuators to combat this challenge.
• Noise can show up in a wide variety of applications, but is most prevalent when loads are light and mechanisms vibrate more freely. Rotary-to-linear converters are often blamed for noise; in fact, screws are often only sounding boards for vibrations introduced by stepper motors, belts, pulleys, bearings, and other drive train components.
• Sometimes designers pick two precision rail-and-bushing assemblies and a precision screw drive, and then spend hundreds of hours trying to get them all perfectly aligned so that nothing binds. Even if successful in engineering builds, these systems usually fail in production. Though some designers fall victim to the more is better frame of mind, these systems should only have one fully constrained axis.
The fix is to use components with as many degrees of freedom as possible. For example, a precision guide can constrain two axes, leaving the drive axis free; a second guide can then support load (without fighting the first guide for positioning.) Rotary-to-linear converters can provide axial positioning through the drive; ideally it is also unconstrained in the other two axes. This is one advantage of lead screws over ball screws; they give precise axial positioning while remaining compliant to lateral constraints.