by Carol Herriges, Serapid Inc., Sterling Heights, Mich.

One linear-motion actuator does away with magnetic parts so it works in applications sensitive to EMI and extra weight.

Linear actuators abound, but some applications need telescoping actuators — mechanical linear motion devices that extend into a volume beyond the machine base, drive housing and guides. There are many variations, but the most common telescoping actuators are based on interlocking segments of rigid chain, advancing spirals of helical band, segmented-cylinder spindle sets and zipper-type mating belts. In one way or another, all extend the linkages or coils into rigid columns that mechanically lock into a column that advances, pulls or lifts loads.

Telescoping actuators that use zipper-type linkages are sometimes called rigid-belt, zipper-belt or push-pull belt actuators. These are mechanisms that have two belts that come together like a zipper to form a rigid column. A motor powers two pinions (one slaved to the other) inside a casing. The pinions each engage a belt through small teeth on the belts’ backsides to pull the coiled belts out of a storage housing. The spacing of these belt teeth coincide with larger rounded studs on the belts’ fronts — studs that engage to make the belts form a column.

The belts wrap around the pinions and turn 90° as they travel through a channel that forces the belts’ studs to engage. To facilitate engagement, guide surfaces inside the pinion housing channel the belts around the bend.

Rigid-belt actuators can move 10 to 1,000 N or more. Standard rigid-belt actuator strokes are generally between 100 mm to several dozen meters—with or without guides. However, the addition of guides can increase load and stroke capabilities. Guide actuators can deliver force to 100 kg—moving 80 kg to 500 mm (for the largest applications) and 15 kg over 500 to 1,000 mm for more compact installations.

Why use rigid-belt actuators?
There are telescoping chain actuators capable of pushing and pulling thousands of kilograms a hundred feet or more, and these use articulating linkages to move loads in myriad directions. However, such telescoping actuators are less suitable for moving lighter-weight products in more delicate operations. That’s where rigid-belt actuators may be more appropriate.

In addition, unlike other telescoping actuators (including those based on chain linkages), rigid-belt actuators don’t necessarily need to have magnetic parts on their extensions. In fact, some rigid-belt actuators even have nonmagnetic drive housings. These incorporate polyoxymethylene (POM) belts plus other aluminum and stainless subcomponents, so they can go into applications that can’t have corrosive, magnetic or overly heavy machine sections made of metal—mobile construction, medical and pharmaceutical, and logistic and aeronautical designs, for example. More specifically, rigid-belt actuators are suitable for MRI tables, camera lifts on unmanned vehicles, cleanroom tables and platforms on aeronautical designs. Installation is simple, and the actuators can drive vertical and horizontal motion.

Plus, the actuators’ reinforced-plastic parts are cheaper and lighter than metal-based linkages, but maintain the same accuracy. As with any mechanical device with contacting parts, there is a small amount of play—not factoring in any safety coefficient that R&D applications usually require, elongation is up to 3 mm under 100 kg of loading, and compression is to 1 mm under 100 kg of loading. However, the weight of a belt (on average) is just 700 g per meter.

Custom options
Rigid-belt actuators can incorporate any kind of ac or dc motor, though for most setups, engineers pair them with a small servo or dc motor. It’s the engineer’s job to pick the most appropriate motor (and by extension, the control system) and then properly integrate controls, but some manufacturers offer support to make setup easier. Depending on the sizing, rigid-belt actuators move static loads up to 500 kg either horizontally or vertically. Push-pull speed is 20 to 300 mm/sec, and custom push-pull interfaces are possible. Designers can also fit the faces of rigid-belt actuators with plastic attachments to shield the application from the belt extremities.

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Serapid Inc.
www.serapid.us