36 n MECHANICAL June 2023 www.drivesncontrols.com Don’t let servo gearbox specs confuse you There is a debate in the servo gearbox industry over whether gearboxes should be benchmarked based on their rated or maximum input speeds. The former refers to the highest speed that gearboxes can reliably run at continuously, while the latter refers to the maximum speed a gearbox can reach on a temporary basis. Some gearbox suppliers benchmark according to rated rather than maximum input speeds in their documentation. Calculations based on rated speeds generally result in higher torque ratings, because maximum input speeds give a much more critical loading to gears, resulting in lower torque ratings. Pros and cons There are pros and cons to both approaches. Rated input speed gives a good indication of what the torque limits are, but potential users have to pay more attention when choosing their gearboxes, particularly if they plan to run them at high speeds. Maximum input speed offers information about what a gearbox is capable of, and demonstrates its full potential application. However, while most gearboxes will go above their nominal speed during a cycle, not all applications require gearboxes to run at maximum speed. Often, servo gear buyers are not aware of what they are looking at when it comes to the data provided by the supplier and this can lead to misunderstandings and confusion. New products benchmarked against rated input speed without context may look as though they have significantly higher torque capabilities than older products, or competitors’ products that have been benchmarked against maximum input speed. Small alterations to a product range can also appear to be completely new highperformance products. The servo gearbox supplier Apex Dynamics has always based its R&D testing and catalogue information on maximum input speeds. But it has recently announced that it is switching its benchmarking for newer products to rated input speed to provide greater clarity about the performance of the boxes. It says it has been looking closely at both forms of benchmarking, and wants to make sure that customers are aware of the difference when it comes to torque specifications, and that they understand the debate surrounding the two forms of calculation. Research Apex says its engineers have carried out extensive research comparing the performance of its AF planetary gearboxes with those of leading competitors based on both rated and max. input speeds. They found that the boxes outstripped many rival products no matter which standard was used. “When it comes to comparing brochures with different benchmarking data for torque, it really is comparing apples and oranges,” says Apex Dynamics’ UK sales manager, Andrew Parsons. “We have made the decision to bring our new product data in line with the new industry norms in order to meet customers’ expectations. However, we think it is important when people are comparing data that they are aware there are two ways in which it could be presented. “For example, the rated torque of Apex’s AP110 ratio 5.5:1 product is 435Nm, compared to 243Nm for a leading competitor’s product, when measured according to rated input speed,” he explains. “If the same comparison is done using maximum input speed, then the Apex figure is 370Nm, but the competitor figure is unpublished. Where the problem can arise is if a potential customer compares rated against maximum input speed without realising. If that happens, the customer has an incorrect picture about performance and cannot make an informed decision.” Apex Dynamics UK’s managing director, Mike Gulliford, adds: “We found most global companies are now benchmarking their gearboxes by rated input speed. We have made the decision to do the same in the best interests of our customers. We will continue to innovate, pushing the limits of planetary gearbox performance.” n There are two ways of benchmarking servo gearbox performance: rated or maximum input speed. This can be confusing to potential users. One supplier of these boxes has recently changed the way it benchmarks its products and wants users to be aware of the potential differences. You need to understand the differences between rated and maximum input speeds when choosing a servo gearbox
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