Coupling knocks out transmission error

Newton, MA -To keep up with the latest imaging advances in digital optical scanning, large professional flatbed scanner manufacturers require more accurate drive systems. Such drive systems typically use stepper-motor-driven leadscrews in an X-Y table to move the scanner's CCD (charge coupled device) element across the image being scanned.

Slight speed variations in the CCD motion of one company's scanner, however, were, causing errors when capturing high-resolution images. These fluctuations resulted in color registration defects, creating an unacceptable banding effect across images. Although these bands were small (typically 30 microns), they were visible to naked eye.

Engineers traced the source of these errors to the flatbed drive system's helical beam coupling. Inconsistent motion transfer from one hub of the coupling to the other caused velocity errors that manifested themselves as position errors. Replacing the helical-beam couplings with patented Reli-a-Flex(TM)couplings reduced drive-transmission error, thereby eliminating the registration defects. The secret? A unique slot pattern with radial rather than helical slots, which provides torsional stiffness for transmission accuracy without inducing higher bearing loads on the coupled motors.

Transmission accuracy and radial compliance are the two most important things a coupling brings to a linear positioning system, according to Reli-a-flex's John Ricker. Coupling transmission error influences positioning accuracy and radial compliance absorbs bearing loads caused by misalignment, extending life. But until now, engineers have had limited choice between economical single-piece beam couplings and the more complex and expensive bellows couplings. While bellows couplings deliver torsional stiffness and radial compliance, beam couplings satisfy lean budgets.

"Traditionally you had to trade off torque for higher stiffness," Ricker says. "But with Reli-a-flex high torsional stiffness doesn't mean high bearing loads. Besides reducing transmission error nearly as well as a bellows coupling, the Reli-a-flex design betters other beam-type couplings in radial compliance." Ricker credits this to the use of radial, rather than helical slots, which maintain a constant angle of the centerpiece as the coupling flexes.

This particular slot pattern minimizes stress variation by maintaining a constant pivot length. The design pivot points travel back and forward axially without changing the centerpiece angle, resulting in uniform stress distribution and no eccentricity. "Similar-looking couplings may have a slot pattern which is a mirror image," says Ricker. "In this type of design the pivot points don't travel back and forward with a constant center piece angle. Instead they move in and out, and the centerpiece angle varies causing stress variations and less uniform rotation.

The Reli-a-flex coupling was designed and is manufactured at Reliance Gear Co. Ltd. (Huddersfield, UK). "When we decided to manufacture our own coupling," says Reliance Design Manager Alistair Durie, "we identified four critical performance criteria." These features include:

Torsional stiffness

Radial compliance

Transmission error

Torque capacity/reliability

"We established that in order to be better than traditional helical beam couplings we had to move away from the helical-type slot design," explains Durie. "This led to experimentation with various types of radial slotted designs. We used transmission error and torsional stiffness as the driving factors. Once we had a design that met our criteria, we then further optimized the geometry to play tunes with torsional stiffness, radial compliance, and transmission error."

Durie explains that solid couplings achieve the highest torsional stiffness, but they also yield the highest radial stiffness (lowest radial compliance). The Reli-a-flex design team created a slot pattern that gives high torsional stiffness and low radial compliance, i.e. without creating large bearing loads. "A combination of both the slot pattern and the high torsional stiffness, he explains, "achieves the kinematic superiority, with the lowest translational error in the industry."

In tests, the company compared its couplings with single- and multibeam designs from other manufacturers. Reli-a-flex found that its product had better than 10 times the stiffness of single spiral couplings in reversing applications, Ricker says. In one example, a Reli-a-flex coupling driving a 3,600-line encoder bidirectionally produced an error of one-half a count, while a four-beam spiral coupling driving the same encoder produced an error of two counts, and a single-spiral coupling produced an error of four counts. " What good is having a precision encoder," asserts Ricker, "if your losses inside the coupling cost you four or five encoder counts."

Reli-a-Flex couplings are made from low-inertia Grade 7075-T6 aluminum and are available in three sizes, each with two hub styles (set screw and integral clamp). Size 13 coupling (0.512-inch OD) bores include 0.125-inch, 0.1875 inch, and 0.250 inch. Size 16 (0.630-inch OD) has three bore sizes (0.1875 inch, 0.2500 inch, and 0.3125 inch) and the size 20 (0.787 inch OD) also has three bore sizes (0.2500 inch, 0.3125 inch, and 0.3750 inch). Peak torque capacities range from 138 inch-oz for the size 13 (clamp style) up to 457 inch-oz for the size 20 (setscrew style).

Additional details...Contact John Ricker, Reli-a-flex, 325 Duffy Ave., Hicksville, NY 11801; Tel: (888) 735-4235; Fax: (516) 771-0237; E-mail: rickerjb@aol. com; www.reliance.co.uk

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