A Better Belt Drive

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If you think belt drives have too much slop for your precision linear motion applications, think again. Bell-Everman Inc., a specialist in high-accuracy motion systems, has come up with a new kind of belt drive for applications that usually call for pricier linear motors, top-quality ball screws or rack-and-pinion drives.

Like many conventional belt drives, this ServoBelt actuator has a moveable carriage that rides atop a linear rail. But that's where the similarities end. The ServoBelt features a patent-pending design based on a pair of toothed, steel-reinforced polyurethane belts. The lower belt is static and bonded to a custom aluminum extrusion, while the upper "active" belt runs through a set of rollers within the carriage.

Along most of the rail length, the two toothed belts engage one another. In the carriage, though, the belts zip apart with the upper belt running over a servo-driven, toothed roller. This roller runs along the upper belt to move the carriage. A set of idlers, meanwhile, helps guide the upper and lower belts as they separate and re-engage as the carriage passes by. For ease of integration and configurability, Bell-Everman builds most of the drive's chassis and structural elements from off-the-shelf linear ball rails and aluminum t-slot framing from Bosch-Rexroth.

Michael Everman, Bell-Everman's chief technical officer and a principal in the firm, likens the new drive to a rack-and-pinion whose rack happens to be a toothed belt. "But this drive is zero clearance," he says, explaining that the conformance of the belt teeth to the drive roller teeth eliminates a potential source of backlash found on traditional belt or geared drives.

The unique design of the belt mechanism also means almost all of the belt – all but three inches or so – remain free of tension as the carriage goes on its way. "Only the portion of the belt between the rollers are under tension. There's no stretching anywhere else," says Everman. Keeping the belt mostly free from deformation contributes to a uniformly high stiffness that belt drives usually lack, and Everman says the ServoBelt is at least 10 percent higher than a conventional belt drive.

This stiffness gives ServoBelt a dynamic response and speeds associated not with belt drives but with more expensive linear motors and top-quality mechanical drives. Everman says, for example, the ServoBelt settles 10 to 20 percent faster than a conventional belt drive after a move. "It doesn't have the springiness that makes standard belt drives hard to tune," he says. As for speed, the standard ServoBelt offers acceleration of 4 g and speeds up to 4 m/s. An optional bearings upgrade will increase the speed limit to 10 m/s, which is well into linear-motor territory. 

The ServoBelt is far more accurate than a conventional belt drive and many rack drives too. Everman says the ServoBelt has an absolute bi-directional accuracy of +/- 60 microns and a unidirectional repeatability of +/- 10 microns. "A traditional belt drive's accuracy and repeatability would be measured in hundreds of microns," he says. With an optional linear encoder, the ServoBelt gets even better. Everman says it can be configured to achieve "sub-micron" accuracy levels previously possible with linear motors or the very best ball screws.

The ServoBelt covers a standard travel range from 0.15 to 50m. Between its five servo motor sizes and different rail options, the system can handle peak forces to 880N and continuous to 300N. Everman believes the ServoBelt will be most attractive on large format machines and material handling systems used in the packaging, wood working, metal cutting and aerospace industries. Most of these machines will likely require travel distances greater than 2m, benefit from independent carriages on the same axis and have accuracy levels that would have necessitated linear motors or high-end rack drives in the past. "What we've done is come up with something that performs like a linear motor with a cost-per-foot that's closer to a belt drive," says Everman.