Over-Engineering Other than busted bearings, one of the negative consequences of crashes is over-engineering. Instead of trying to avoid crashes, the engineer will accept them as inevitable and try to design systems that will survive them. To do so, engineers may end up specifying bumpers or gas shocks. Depending on the type of shock absorber, these can cost as little as a few bucks and as much as $100 each. But install them on multiple axes, and those costs can add up — even before you factor in the labor and spare part costs. As popular as they are, bumpers and other shock absorbers are a bit like training wheels on a bike. Once someone knows how to ride, they come off. Likewise, linear motion systems that are properly designed and controlled can run safely without the expense of additional protection. Another crash-prevention strategy involves “sizing up” by choosing a heavy-duty linear component to survive a crash even when a standard-duty precision guide is perfectly suited for the job. By going up a rail guide size, as the thinking goes, the designer can achieve a great, crash- tolerant system. But the high cost of upsizing becomes apparent pretty quickly: This strategy can cost as much as 15 percent more than the previous size guide. That’s before the machine builder starts to make some accommodations. For example, a larger-size guide will need: • A larger drive mechanism, either ball screw- or belt-driven. • A larger motor. • A change in gearbox. • A larger steel box frame requiring more welding, more machining to ensure the surfaces are parallel and more mounting time and effort. The resulting elaborate system can end up costing tens of thousands of dollars more than transporting that same load with a system using a more-compliant linear guide designed for medium-precision applications. Not only does that great system result in additional design, machining and installation time and effort, those costs get added onto the total cost of the system. When a system costs significantly more than necessary, the resulting competitive disadvantage will mean lost sales. Favor Crash-worthy Bearing Designs While most crashes do occur on start-up, it is true that they occasionally occur well after a machine has been commissioned. Sometimes a loss of power can trigger a crash, or someone might inadvertently change a control setting. And truth be told, some machines are not run all that carefully. For these reasons, it can make sense for engineers to design with some crash-worthiness in mind. But what is the best way to do that? Engineers can choose to go with shock absorbers and over-engineered components, accepting those costs as the price of crash protection. Or they can favor linear bearings that inherently have crash-worthy construction. Not all bearings are created equal when it comes to surviving a crash. Recirculating ball systems with plastic end caps are notoriously susceptible to crash damage because the plastic tends to shatter. Should a single crash with this type of pillow block occur, you may find its ball bearings all over the floor. Bearings based on larger roller elements lack this particular Achilles heel. Their ball bearings and their raceway are contained within the roller element, not within a plastic cap. Rollon’s Compact Rail is a prime example of this type of bearing, and it will run just fine with a shattered end cap. And if it ever does experience any crash damage, its interchangeable rollers and pillow blocks allow it to be easily repaired without changing the rails. Another benefit that the large rolling elements of a precision guide like Compact Rail can offer is resistance to contamination, which can otherwise stop the rolling elements of high- precision systems in their tracks. Even if a contaminant does happen to mar the roller or the rail surfaces of the bearing, the large rolling element can keep on running. Plus, Compact Rail also features induction-hardened raceways to ensure reliable operation and long life for medium- precision applications. And when designers choose a high-precision guide to accomplish linear motion as part of a crash-mitigation strategy in their medium-precision applications, they’re likely creating much more work, delay and expense than necessary. That’s because profiled rail installation will require additional machining to create flat, straight and parallel mounting surfaces in order to prevent misalignment.
