n A gearbox input shaft failed because the alignment between the motor and gearbox couldn’t be maintained under load. Although this was a torque arm arrangement, this had a similar effect as a ‘soft foot’, which can be likened to sitting on a chair, and it rocks back and forth because the feet aren’t even. This can cause misalignment and vibration, which is a major cause of fatigue and early wear problems for powertrain equipment.


is relevant to alignment between the motor and gearbox especially. Typically, motors operate at high speeds, such as 1,750rpm or 1,150rpm. A gearbox is used to decrease the speed and increase the torque. Couplings are used to help compensate for both parallel and angular misalignment. Proper alignment will help reduce vibration and fatigue loads on power transmission equipment leading to a longer operating lifespan. The motor and gearbox must maintain acceptable alignment under load. Of course, as with any root cause machinery issues, we can see the evidence of misalignment in damage and wear to crane components, such as the aforementioned rails, wheels, motors and gearboxes, gear mesh, and even limit switches and triggers (a way of preventing motion from travelling beyond the operational zone). I have even seen crane wheels needing to be replaced every two years because they are wearing out due to misalignment.


Notably, misalignment issues are rarely caused


by installation. While the sources can greatly vary, most of the time they are from misuse. After all, crane manufacturers have a lot of incentives to get it right; if they don’t, they usually have to do some warranty work. Gear manufacturers also do a good job of making


sure the gears are properly aligned in a gear mesh. When gears fail, another issue is usually present, such as the foundation not being stiff enough to keep the machinery in alignment while under load. We have seen couplings get pushed outside of their operating range, which causes the gear mesh to start misaligning. When the gear mesh is misaligned, the gear teeth are not able to spread the


f Limit switch and trigger alignment is crucial for proper function of the crane.


r A gear mesh was not properly aligned. This caused uneven loading of the teeth and led to cracking.


transmitted force along the full length of the tooth. This concentrates the transmitted force into certain areas and leads to pitting and cracking. There are ways to detect misalignment before


it results in catastrophe. Typically, the crane manufacturer will provide an operations and maintenance manual; if the preventive maintenance programme is followed, these issues can be identified and fixed without any incidents. There is a large amount of metrology equipment that can be used to make very precise measurements. Coupling manufacturers also give good measurement instructions in their product literature. Therefore, most wheel and/or rail issues are (can be) discovered by preventive maintenance and routine inspections. Metal shavings are present in certain areas where wheel flanges are rubbing against the rail. Depending on the application, it can be quite loud when the cranes are in use. Sometimes during preventive maintenance, the crane can be moved when the environment is quiet and then noise is more likely to be heard. These issues can cause grinding against a rail and lead to wearing. If a rail system is supposed to be 100ft span, but there is a section where it bumps out a couple of inches, the outer wheel flanges will contact the rail and want to pull the rail in. Some preventive maintenance items can slip under the radar, however, such as oil samples from the gearbox. If metal shavings are consistently present in the oil analysis, this can be a clue that maybe the gears are experiencing accelerated wear. So, even something as simple as an oil analysis can provide a lot of insight into what is happening with a gearbox. ^


34 | Fall 2023 | www.ochmagazine.com


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