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"What defines heavily loaded gears? Isn't the load transmitted per square centimeter surface always within the established limits based on the safety requirements?"
Gears can be among the highest loaded components in any facility. They are used to transfer power to different angles, increase/decrease speed and improve the mechanical advantage of the driving component. While the load transmitted usually is well within the established safety limits, it is possible to load gears higher than what was originally planned.
The term "heavily loaded" is relative but can be applied to most low-speed gearboxes that are tasked with moving large equipment (long conveyors, large extruders, etc.) or pushing solid objects through a dye or extruder. Of course, this doesn't pertain to all gearboxes, but certainly these are the ones that experience the heavier loads.
Gear systems that are not operating in their defined speed or power could also increase the load on the gear surfaces. For instance, imagine you had a motor coupled to a gearbox, which in turn was coupled to a paddle stirring a mixture inside a vessel. The gearbox was selected for a certain torque and to handle a certain resistance from the paddle moving through the mixture. If you removed the original paddle and replaced it with a larger one, without changing the gearbox, this would increase the load on the gears and could subsequently cause excess friction and eventual failure of the gearbox.
Keep in mind that the selected oil for a gearbox is paramount to the life expectancy and operating efficiency of the gears. As load increases, so must the viscosity of the lubricant being used, regardless of whether it is oil or grease. The use of additional additives, such as extreme-pressure (EP) additives, or solid additives, such as molybdenum disulfide or borate, may be required to help support the loads.
The gearbox's driving condition also necessitates changes in the lubricant's additive package. A gearbox that is subjected to uniform/light driving conditions can utilize a rust and oxidation (R&O) inhibitor package, while those driven in a more sporadic or higher loaded manner may need EP additives.
Another downside to higher loading of gearboxes is the strain it puts on the driving component. Using the previous example, not only will the gearbox now be more loaded, but the motor tasked with turning the gear set will more than likely experience some of the additional load as well. This puts added strain on the motor, which causes it to run hotter, consume more energy and ultimately fail earlier.
So while gears are designed to handle more load than what is specified, it is a delicate balancing act to ensure you have the proper lubricant and the right driving components. This could make the difference in a machine running at peak efficiency and one that is struggling to meet the demands of the system.