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.
1. Gearbox Construction — The gearing type will probably be dictated to some extent by other factors. For example, worm gearing may be needed because self-locking is needed to keep the load in place after power is cut from the motor. Spur and/or helical gearing may be needed because a parallel shaft configuration is needed to accommodate space constraints. Since the one thing we’re focusing on is optimizing the life of the gearmotor, the gearing type doesn’t really matter, as long as it meets the life requirement and doesn’t result in a package size that exceeds the constraints of the machine design. If the package size does matter, then the gearing type does make a difference for getting the longest life in the smallest package.
Other construction aspects, like the orientation of the output shaft with respect to the motor shaft axis, can affect the life. Gearmotors typically are designed to be mounted with the gearbox level with the motor, and with the output shaft in a horizontal orientation. If an integral gearmotor is mounted in a position other than horizontal, the chance of leakage increases in both static and dynamic conditions as the shaft seals wear. Avoid mounting an integral gearmotor in an orientation where the gearbox is positioned above the motor. There is a shaft seal between the gearbox and the motor, but seals don’t last forever. Eventually the lubricant will find a path into the motor. This could interfere with commutation in a permanent magnet DC motor, the centrifugal switch in an AC split-phase motor, or the electronic commutator assembly in a brushless DC motor. If the drive shaft must point upward, then a right-angle gearmotor would be a better solution.
2. Gearing Torque Rating — As stated above, gearing torque ratings are established for a product to give a certain life, whether it is a nominal life of 25,000 hours, an L10 life of 5,000 hours, or something else. Let’s assume the designer in the example above is looking for a gearmotor that will drive a 50 lb-in load at 120 rpm for 21,000 total hours. First, he needs to multiply that by the 1.50 service factor, as noted above. He needs to look for a gearmotor rated 75 lb-in instead. The designer found a gearmotor in a catalog with those ratings, but learned that the L10 life is “only” 10,000 hours. What now?
The life of a gearbox increases if it is operated at less than its rated load. An inexact rule of thumb is that if the load on a gearbox is half of its rated torque, then the life will be four times the life on which the rated torque was based. The designer in our example turned the page of the catalog and looked for the next larger gearmotor that had the same speed of 120 rpm. He found a model rated 150 lb-in with an L10 life of 10,000 hours. Using this same train of thought, if he uses that gearmotor with a load of 75 lb-in (half the rating), then the L10 life would be about four times the 10,000 hours, or 40,000 hours. That meets his minimum requirement.
3. Motor Type — As with the gearing type, there will probably be other factors that dictate the motor type, besides optimizing the life expectancy. For example, a battery power supply most likely would preclude the use of an AC induction motor. The need for adjustable speed may eliminate a single-phase AC motor from consideration, or an installation in a hazardous location may make a brush DC motor a poor choice. We’ll disregard those considerations and assume that any motor type would work equally well in the application. Which one, then, is best for extreme long life?
Brush-type DC motors have an inherent construction feature that makes them particularly wear out faster than other motor types; namely, the brushes. Just as with the brake pads in a car, the brushes in a DC motor have to be replaced periodically in order to extend the life of the motor. In a car, you can’t replace the pads too many times before the rotor or drum wears out and has to be replaced. Similarly in a DC motor, you can’t replace the brushes too many times before the commutator wears out. Replacing brushes isn’t always an option if the motor is installed in a remote location. And the second set of brushes never lasts as long as the first set. For the longest life possible, either an AC induction motor or a brushless DC motor should be considered. They are essentially equal in terms of life, so the choice between the two will be determined by what power supply is available and what motion control characteristics are critical, such as start/stop frequency, speed regulation, and point-to-point positioning.
4. Motor Horsepower Rating — Now that our designer has a gearmotor with twice the torque rating than the application requires, he’s probably thinking that the motor on the other end of that larger gearbox is oversized, and he may start looking to see if that same gearbox is available with a smaller motor. Not so fast. Depending on the motor type and the gear ratio, the larger motor may be needed to obtain the extended life requirement.
DC motor types, both brush and brushless, run cooler at loads less than their rating. This is because their current draw is directly proportional to the load, and the temperature rise is directly proportional to the current draw. That is generally not the case with AC motor types. In fact, some AC motors actually run hotter at no load. How will the life of the motor increase if it runs cooler? Motor temperature affects lubricant life and insulation life. As a general rule, ball bearing or gear lubricant life is halved for every 25 °F (approximately 14 °C) increase in temperature. Generally, the motor insulating life is halved for each 10 °C increase in total temperature.
Before you start thinking about using an even bigger motor, consider that the motor may have already been oversized, even for the rated load, depending on the gear ratio. In order to take advantage of using common parts, manufacturers don’t always change the motor size to exactly match the needs of a particular gear ratio. In our example, the designer found a DC gearmotor rated 150 lb-in at 120 rpm. That gearmotor has a 3/8 HP motor and a 20:1 gear ratio. How can you tell if the motor is oversized or not?
Calculate what motor torque is required to produce the output torque rating using this equation: TMotor (oz-in) = TOutput (lb-in) × 16/(Gear Ratio × Gearing Efficiency).
The gearmotor in this example happens to be a parallel shaft type, with spur and/or helical gearing. These gearboxes generally have an efficiency of about 90% in the lower ratios. Solving that equation produces TMotor = 120 × 16/(20 × .90) = 107 oz-in. Is that 3/8 HP? Horsepower is a function of torque and speed. We can figure out the motor speed by multiplying the output speed by the gear ratio 120 × 20 = 2400 rpm. Now we can calculate the actual horsepower using the equation HP = TMotor (lb-in) × Speed (rpm)/63,025. Solving that equation produces HP = (107/16) × 2400/63,025 = ¼ HP, which is less than the motor’s 3/8 HP rating. This particular motor was oversized to begin with. There is no need to oversize a motor to extend its life expectancy.
5. Other Criteria — Some other commonly overlooked contributors to wear and tear on a gearmotor include excessive overhung loads, high ambient temperature, and water ingress. Observe the overhung, or radial, load limits of the gearmotors — the force that is applied to the output shaft, perpendicular to its axis. Exceeding the limit specified by the manufacturer will result in premature bearing failure.
Temperature effects on motor and gearing life were mentioned previously, but in the context of motor temperature rise due to the load. But there’s also the temperature of the ambient air surrounding the gearmotor. If the surrounding air temperature exceeds the rating on the gearmotor nameplate, it may be necessary to specify a larger motor to compensate. A larger motor would have a lower temperature rise under the same load as a smaller motor because it has a larger thermal mass from which to dissipate the heat.
Lastly, ingress of foreign material into the motor, particularly water, can reduce the life expectancy of a gearmotor. Observe the “IP” rating (Ingress Protection) on the motor, and make sure you have chosen a gearmotor with a protection rating that matches the actual application. If it’s going to be installed in a cleanroom, then IP00 should be acceptable. But if the gearmotor is going to be installed in a machine that is washed daily with a pressure hose, then it should have at least an IP65 rating. Water ingress can result in components rusting inside, condensate accumulating inside after the water evaporates, or an electrical short circuit.
By considering these five criteria, you can make sure that the gearmotor you choose for your next machine design will meet the life expectancy demanded for an extreme application.
This article was written by Terry Auchstetter, Business Development Manager at Bodine Electric Company (Northfield, IL). For more information, Click Here .