1. Understand what causes gearmotor failure.
- Excessive overhung loading (radial load on shaft) can destroy bearing support systems. Shaft fatigue failure is also common on overloaded shafts.
- Gearboxes that experience shock loads from large inertia loads or excessive acceleration/deceleration can cause gear tooth fracture.
- High thrust forces on shafts can exceed bearing capabilities, compromise press fits of components, and exceed structural housing strengths.
- Excessive torsional loadings on a shaft at keyways, cross-holes, and diameter changes are all potential failure locations.
- Gearmotors placed in environments not suited for them exhibit stress on the gearbox sealing package, lubricant issues, and poor heat dissipation. Properly protect the gearmotor to prolong its operating life.
- Gearmotors subjected to thermal cycling can experience condensation. Sometimes venting the motor and allowing the environment to stabilize is better than trying to keep the product totally enclosed and sealed.
- Lubricants should be specified for actual operating conditions. Adjustments can be made for extreme cold or hot applications instead of a more costly full temperature range.
- Motor mounting should be stable. Surfaces that can flex and cause misalignment will degrade product life.Engineering team, Bodine Electric
If a gearmotor is undersized (not enough power for the application), the typical warning sign is overheating. However, unless you check the temperature or actually feel the heat, it’s hard to detect before a failure. If the gearbox’s lubrication oil temperature rises high enough, it will cause a thermal breakdown of the oil, and once the lubrication system is compromised the gearbox will ultimately fail. Sometimes the undersized motor has to “work too hard” and it will actually burn out — a permanent and unrecoverable failure. Again, the warning signs are hard to detect: Sometimes you may smell the oil or motor if it’s overheating, or see paint discoloration. But unless the gearmotor is clearly visible, it will run until failure without you even realizing it.
An undersized gearmotor usually results from not understanding that both the motor and gearbox have separate thermal capabilities as well as efficiencies that, when combined together, result in a system output power that is different from what is typically published. Specifying and buying while only considering motor output and efficiency is a mistake.
Jon Roetman, Groschopp
2. Consider an integrated ac motor for optimal performance.
The best reason by far for making the switch to integrated ac gearmotors is performance. In a fully integrated gearmotor, the motor’s low-inertia rotor is specifically matched to the characteristics of the gear unit. This results in high dynamic capability, which is especially important for high stop/start cycling applications. Also, the majority of today’s gearmotors incorporate a high-performance brake, useful for applications requiring controlled load deceleration.
Another advantage of integrated gearmotors is that they’re designed to work well with inverters. Using a gearmotor and variable frequency drive (VFD) with closed-loop feedback (through encoders mounted on the motor shaft) makes indexing and point-to-point positioning applications possible. Electronic drives also provide fine-tuning of speed and control, incorporating features such as overload protection and adjustable starting torques. However, advanced features notwithstanding, the biggest gains in performance come from the unlimited combinations of motors and gear units.
Rich Mintz, SEW Eurodrive
3. Don’t forget to surf the web for helpful resources.
Whether it’s an industry association or a manufacturer’s website, the Internet is a great resource for gearmotor sizing and specification tips. Here are three to get you started:
American Gear Manufacturers Association (AGMA)
STP Design Search tool relates the “fit” of a motor or gearmotor to an application’s performance specifications; “fit” means closest match to speed, torque, motor efficiency, and gearbox efficiency.
Free software from SEW Eurodrive that helps users select a custom reducer or gearmotor
4. By design, gearmotors are built to outlast user-integrated motors and reducers.
A typical arrangement of the user-integrated solution is the quill mount reducer. These are generally less expensive and take up less space than a reducer with an external input shaft. But, they have no bearing to support the motor input, so the motor output bearing supports the reducer input shaft. The problem here is that in most motors, the bearing is a size 200 bearing, and it is not rated for the axial loads it will see during use. So, this bearing will fail prematurely. When it does, the resulting run-out and misalignment of the motor shaft will destroy the input seal to the reducer, and, very likely, damage the gearing.
It’s important to note that the so-called mating flanges of C-face motors and reducers generally don’t seal well, so contaminants (water, dirt, heat, chicken parts) will eventually find their way into the cavity between the motor and reducer. Here, the seal on the motor input may fail, because it’s subjected to this harsh industrial environment. This is not the case with gearmotors. The flanges of the motor and reducer are machined and factory sealed to prevent ingress of industrial goo. Likewise, the gearmotor’s internal components are designed to work together, and the environments and mechanical stresses to which they are subjected are known. Finally, one of the most hated tasks of both installation and preventive maintenance for user-integrated units is alignment. Misalignment causes failures; with gearmotors, the unit is delivered to the end-user perfectly aligned, and it stays that way.
Rich Mintz, SEW Eurodrive
5. Understand what gearmotor system power really means.
When specifying and selecting a gearmotor, it’s critical to know and understand gearmotor system power, and not just motor power. Typically, gearmotor manufacturers publish specified motor output power, which often does not accurately convey actual gearmotor output power; when specifying or selecting a gearmotor for an OEM application, relying on this published motor power specification (hp) alone can be deceiving. To fully optimize a gearmotor solution, calculate or obtain system output power, which represents the resulting gearmotor power after calculating the efficiency of both the motor and the gearbox.
Understanding system power and knowing separate motor and gearbox efficiency ratings also gives an engineer important data with which to make comparisons. For example, comparing efficiencies of a planetary gearmotor to a parallel shaft gearmotor allows product choices to be made based on overall efficiency, performance, and cost.
So, when selecting a gearmotor, determine if published power or output power specification is for the motor only, or if this specification also includes the gearbox/speed reducer efficiency. Any gearmotor vendor should be able to provide both motor and gearbox efficiency ratings; if it is not calculated as system power, calculate gearmotor system power by multiplying motor output power by the appropriate gearbox efficiency.
Ron Didier, Groschopp
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