Getting the Most from Belt and Rack-and-Pinion Driven Actuators
Economical and robust, the technologies can deliver surprising speed and accuracy over long travel.
In engineering, there is no such thing as the perfect solution, only the right solution for the application at hand. When it comes to linear motion at high speeds and over long distances, though, few solutions provide as good a fit as rack drives and belt drives. Certainly, linear motors can do the job, but even back before prices of rare-earth materials went through the roof, linear motors more than a few meters long could be cost prohibitive for all but the most demanding applications. Ball screw actuators provide very good performance but whip precludes their use beyond about 3 m, especially at higher speeds. The problem is that many industrial applications require runs of 30 m or more. In such applications, rack drives and belt drives deliver good performance, but only when properly specified. Here, we highlight what you need to know in order to design a system that will perform to expectations.
In a rack-and-pinion actuator, a toothed gear (the pinion) travels along a toothed track (the rack). The rack can be stationary and the pinion can travel with the motor, or the motor can be stationary and the rack can travel with the load. In theory, a rack-and-pinion actuator can handle any distance of travel. In reality, practical considerations impose constraints. The rack needs to be properly supported and aligned, for example (see figure 1). It cannot have excessive run out. More important, it must not exhibit tilt (torsion) or yaw, which can cause the carriage to stick partway along the length of travel. Controlling yaw is particularly important in the case of dual-rack designs. The two racks must remain parallel.
It’s an easy-to-understand technology but the devil lies in the details. Perhaps the biggest mistake is underestimating the amount of compliance involved in attaching a rack to a surface. “The rack takes the shape of the thing you're bolting it to, no matter how rigid you think it is,” says George Rabuzin, director of sales at the Drive Systems Group. “If you take a $100/m rack and mount it to an I-beam that runs along the side of your machine, your rail is going to conform to your I-beam. An I-beam does two things: It yaws and it runs up and down. You’ve got to take that beam and grind it until you have a flat surface or you’ll have lots of problems.”
Rack-and-pinion actuators must be adjusted for backlash. The best design for removing backlash features a pair of pinions preloaded with a 0.5º to 1º axial phase mismatch to keep them in constant tension relative to one another. The assembly gets pressed into the teeth of the rack, almost completely eliminating hysteresis.
Preloading a rack-and-pinion drive can be something of an art. If it’s too low, the teeth of the rack and the pinion may not fully mesh at high rates of travel, introducing backlash. If the preload is too high, at best it may overstress the bearings and at worst the teeth of rack and/or pinion may snap off. “People tighten it like the lug nuts on a wheel, until they break,” says Dan Lutz lead engineer at item North America (Akron, Ohio). “OEMs need to realize that you have to set the preload properly to get the maximum life.”
It’s important to remember that this technique transfers that force into the carriage. “You're trying to apply a little force to them to basically keep them nested,” says Lutz. “You may only have a few pounds but the force of your rack and pinion riding together is actually going to be converted into that as well, so you have to make sure you design for that.”
Beyond the performance constraints, cost is always a factor. As with any type of gear, this can vary depending on implementation. A soft-cut-steel rack provides an economical solution for forgiving applications. For situations requiring better performance, a helical ground rack matched with an angled pinion can deliver very high accuracy, but can cost more than an order of magnitude more. Adding a linear bearing to the rack increases stability and performance, but likewise adds to price, as well as introducing more maintenance tasks and additional points of failure.
Lubrication plays an essential role in prolonging the lifetime of the unit, enhancing it by as much as 40%. A variety of methods exist, from oil drips to felt pads that run ahead and behind the carriage. To avoid attracting dirt and other contaminants, the rack should be mounted upside down where possible, or beneath the machine surface.
The metal-on-metal nature of a rack drive limits speed. It also leads to increased wear, which cuts into lifetime. In the case of high-speed, high-repetition-rate applications, a better solution might be a belt-driven actuator.
The benefits of belt drives
Today’s belt drives have moved far beyond the older versions that achieved only limited accuracy and repeatability, and required constant maintenance to maintain tension. Modern belts maintain form factor by incorporating materials like Kevlar and metal. Precision-cut teeth on belts and pulleys enhance accuracy and repeatability (see figure 2). As a result, they can operate at speeds of 10 m/s while delivering accuracies of 100 µm or better.
To enhance performance, integrators should preload the belt to a force of 110% of that applied by the load. Basically the idea is to pre-stretch the belt by a few millimeters per meter. This ensures that load applied during operation will not add additional stress. With that degree of tensioning, a belt-driven actuator can be quite quiet, even when over 130 feet long. For applications like gantries, for example, belt drives provide very good solutions.
Belt sizing is essential to success. It’s possible to overdrive a motor for brief periods without causing catastrophic damage. Belts are not so forgiving. Belts typically have a working rating and then a tensile-strength rating. If a system exceeds the tensile strength of the belt, the belt gets elongated and cannot retract. “Now you’ve stretched the belt and that leads to a whole bunch of other problems,” says Rabuzin. “Now the teeth will start to get stripped off because they're not engaged on the pulley, so belt sizing is really critical. You have to understand what's going on.”
A certain amount of stretching over time is unavoidable. For applications like CNC machining that require regular homing, it may not be an issue. Still, too much slack can cause excessive belt wear. Especially in the case of a vertical axis, the load applies tension on one side of the top pulley but not on the other. This can result in a significant amount of slack around the bottom pulley, which can be a problem at high speeds. “You can have a lot of slack at the bottom if it's not tensioned properly,” says Lutz. “The belt can actually get through the reverse unit, double looped over, and ripped right off.” In general, belt-driven actuators should not be used in vertical applications, at least not without built-in fail safes involving brakes and/or clutches.
This brings up an essential point: careful design. It’s easy to get caught up in motors and drives and control schemes but for the best chance of success, design teams need to take actuators into account from the very beginning. Just knowing the stroke is not enough. OEMs need to know not just the amount of load but the type of forces involved —is it an offset load, cantilevered to the side? Is it an inertial load? Rather than starting with the motor and ordering the actuator, the best approach is to go in the opposite order. “You have to figure out your load, what you're going to put it on, and what it's got to hold,” says Lutz. “Only then should you determine what motor and gearbox you need.” Starting with the motor can lead to over specifying components for the application, which can add cost and have a ripple effect in the design. “It could be a difference of thousands and thousands of dollars because the design might be maxed out of something and then you need to jump up to a double linear motor or something like that. Sometimes there’s not enough thought going on.”
Questions to prepare for include the level of support, the direction of motion, speeds, accelerations, and bearing types. Unusual orientation can also impact the design by adding additional stresses. Start with as much information about the load in the application as possible and work with your vendor to ensure that the solution takes into account all possible problems. Used properly, belt- and rack-and-pinion-driven actuators can provide an effective solution for a range of industrial applications at a price point that will satisfy.