At first glance, the photo at the top is not appealing to any market — a pallet full of old gearmotors is not something one wants to think about after purchasing the necessary gearmotor/motor for their application. But think of it this way instead: these gearmotors were removed from their installation for a refurbishment project after being in service for 30 years. Sandia National Laboratories placed these gearmotors into service in their Heliostat Field in New Mexico in the 1970s. The gearmotors were used to position solar reflectors to concentrate light from all of the individual panels towards one point at the top of a tower. After 30 years, Sandia decided to upgrade the field with a new control system, and they decided to replace the still-operating gearmotors at the same time.
Maybe the product you’re designing doesn’t need to last 30 years, but if given a choice, don’t you want it to? To select the best integral gearmotor for a long-life application, one must go into more detail than just selecting the gearmotor based on speed and torque alone. This article provides five other criteria you should consider.
Before you consider what your application requires, you first need to establish your target for length of life. A simple “5- year” mark isn’t a good rule of thumb, because the calendar time isn’t what’s critical. It’s the operating time. In order to determine the operating time, you need to know the duty cycle of the application during operation, and the number of hours the machine operates during the day.
Here is an example of how to calculate the total operating hours. A gearmotor drives a transfer conveyor in short bursts, 1 minute at a time, with 1 minute off before repeating, over and over, during three 8-hour shifts, seven days a week. The designer needs a gearmotor that operates trouble-free for at least 5 years. How many operating hours is that?
(1 minute/cycle) × (1 cycle/2 minutes) × (24 hours/day) × (7 days/week) × (52 weeks/year) × 5 years = 21,000 hours.
Now that you know the life requirement for the gearmotor, you need to find a gearmotor that meets that target. Most integral gearmotor manufacturers don’t publish design life ratings, partly because it’s not always clear which is the limiting factor for the gearmotor life — the gears or the motor. Gearbox manufacturers are generally more forthcoming with their service life information, since they can focus on the gearing life only. A common number that appears is 25,000 hours nominal with a 1.0 service factor, based on an AGMA (American Gear Manufacturers Association) standard. Comparing that number to the 21,000 hours calculated above, it would seem that most gearmotors would meet the life requirement, assuming they all conform to AGMA standards. However, 25,000 hours is a nominal figure, and it’s a good bet that the designer wants more than half of the gearmotors to last 21,000 hours.
That’s why a lot of people use L10 or B10 life ratings. Torque ratings based on L10 life mean that 10% of the gearboxes operated at that load are statistically expected to fail before the L10 life, and 90% are expected to last longer. The L10 number is 1/5 the nominal number. So a gearbox with a nominal life of 25,000 hours would have an L10 life of 5,000 hours. Note that this number is much less than the 21,000 hours the designer in the example above was looking for, and it is based on a 1.0 service factor. According to AGMA guidelines, a conveyor not uniformly fed and operated more than 10 hours a day should have a 1.50 service factor. So the designer needs to adjust his original load estimate by multiplying it by 1.50. He should be looking for a gearbox with a torque rating 1.5 times what he thought he needed, and with an L10 life of 21,000 hours. And that’s just looking at the gearing, and not the motor. How can the designer modify his approach to integral gearmotor selection to achieve longer life? Here are the five criteria to consider.