Technology you can use 1654

DN Staff

June 7, 2004

6 Min Read
Technology you can use

Space Out: Plastic spacers separating recirculating balls inside a ball-screw nut prevent the bearings from contacting each other, keeping friction low to allow extremely low velocities.

SPACERS ALLOW SMOOTH MICRO-MOTION

These patented, compliant POM plastic spacers separate recirculating balls inside ball-screw nuts with very low friction. The result is fine and slow motion, on the order of single microns per second.

Without such spacers, intermittent short strokes or frequent oscillations push the balls into one another, increasing inter-ball friction and idling torque, which can produce instability in a servo loop. Spacers, such as a flexible ball chain, have been available for some linear guideways, but the higher speeds and complex ball-path geometry in a ball nut put extra loads on spacers, decreasing their reliability.

The plastic spacer resembles twin Belleville springs back-to-back that, when compressed slightly, cups and guides the balls during recirculation. But in the event of a jam, a hole in the center of the spacer permits adjacent balls to touch, limiting compression of the spacer to prevent it from cracking and prematurely failing. The spacer shape also traps lubricant to extend ball-screw life.

Contact:George Jaffe, Steinmeyer Inc. Tel (781) 275-6097; Fax (781) 275-4749; [email protected]http://rbi.ims.ca/3850-500

STEPPED ATTACHMENT SLEEVES QUIET VIBRATION, CUT COSTS

ConCentra ball and roller bearings feature a patented multistep sleeve attachment method that forms a near 360-degree interference fit on a fan shaft. The arrangement eliminates traditional radially non-symmetric retention using set screws, eccentric locking, or keyways. Thus high-speed fans using the method are less susceptible to imbalance and vibration.

Included in the Total ConCentra Shaft Solution are the SHL and SHQ bushings, which mount the fan wheel hub and drive pulley respectively. The bushings replace conventional bushings, taper lock bushings, or finished-bore components such as fan wheel hubs.

The only tool needed for bearing and bushing installation is an allen wrench. For simple component installation and removal, the wrench moves the stepped-surface concentric sleeve or bearing inner surface to ride over or off of the inner shaft-enclosing sleeve. The 1:3 stepped-taper surface profile is shallow enough not to self lock, as may happen with older-style, steeper (1:12) tapered attachments. All components are thus attached or removed with minimal force and without fretting damage to the shaft.

Smooth Running: Allen screws in the Total ConCentra Shaft Solution slide concentric stepped mating surfaces over one another to attache fan components. Without any radial imbalance in the attachment hardware, vibration is mitigated.

By eliminating the need to design special shafts for the conventional attachment methods that could require costly milling operations, SKF engineers also allow the use of less expensive keyless, commercial-grade, or hollow shafting.

The SKF method is designed for center-hung and overhung fans, in both horizontal and vertical applications, and is compatible with standard pulleys.

Contact:Dan Korona, SKF USA Inc. Tel (215) 513-4873; Fax (215) 513-4475; [email protected]http://rbi.ims.ca/3850-501

SCANNING ARCHITECTURE IMPROVES CONTAMINATION RESISTANCE

A new linear encoder with large elements ensures that sufficient light signals can get through any possible contamination for signal processing.

Precision linear encoders function by passing collimated light through fixed scanning reticle gratings and then through a moveable encoder grating scale onto fixed photocells. Processing the modulation in light intensity then determines position and velocity of the moving scale. Sealed encoders often have to function under harsh and dirty industrial conditions that contamination on optical components can degrade output signals, affecting measurements.

Both the single scanning reticle grating and single fixed photosensor in the Heidenhain sealed linear-scale encoder reduce contamination effects because of the single-field scanning process they make possible. Previous optical encoders rely on four smaller scanning gratings on the reticle in a 2 x 2 array, offset to one another by .25 of a grating period each. This array generates four sinusoidal signals, phase shifted by 90 degrees, needed to determine position and velocity. But contamination could possibly block most light through one or more of the individual gratings.

Large Area: The large-width areas of the single-field scanning encoser's reticle and photosensor allow correspondingly robust light signals to be detected.

Design engineers gave the single, large reticle grating in the encoder a slightly different period (more lines per unit length) than the grating on the moving scale. Because the line widths on both gratings are equal to their line spacing (and thus slightly different in line width as well as spacing), traversing the scale results in a varying light signal on the large photosensor that can be processed into the four phase-shifted signals needed for accurate measurement.

Contact:Chris Weber, Heidenhain Tel (847) 490-1191; Fax (847) 490-3931; [email protected]http://rbi.ims.ca/3850-502

READ HEAD PROCESSING TIGHTENS ENCODER RESOLUTION

The compact magnetoresistive read-head of the Linear Motor Encoder (LME) includes interpolation circuitry and a periodic reference mark. Trilogy Systems engineers took advantage of improvements in smaller chip packaging and compute power to put the processing electronics close to the read head. Thus they minimized any signal delays from the information on the magnetic encoder tape in reaching the processors. The arrangement also eliminates a separate interpolation box and connector, so cabling is direct to drive electronics. The result of the arrangement is resolution down to 1 micron, depending on quadrature used.

The magnetoresistive electronics are also more tolerant of alignment errors. Trilogy engineers say that for a typical air gap of about 0.8 mm, an encoder with optical alignment concerns may have an alignment tolerance of plus or minus 0.2 mm, whereas the LME has plus or minus 0.7-mm tolerance. The magnetic-tape-based encoder is more forgiving of dirt and contamination than an optically based device according to Trilogy.

LME is designed for high-speed, high-acceleration, and high -duty-cycle motion applications. With the company's linear motors, the fine stranded, high tensile strength copper flex cables are shielded for minimal EMI. Engineers configured the motors for minimal magnetic leakage, which along with the read head close to the magnetic tape eliminates any magnetic interference with encoder functions.

Contact:Susan Lavergne, Trilogy Systems Corp. Tel (281) 226-7750; [email protected]http://rbi.ims.ca/3850-503

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