Gear boxes are a complex subject in their own right. The equations of motion required to generate gear teeth are pretty complicated. And the issues associated with gear box reliability are even more complicated. The parameters of merit are precision and load capability. But cost is always a factor, and ultimately every system’s performance must be measured within the context of its life expectancy.
One of the most complex parts of the automobile is the transmission, which is a multistage gear reducer that “tunes” the speed range of the engine to the desired speed range of the vehicle at power levels of several hundred horsepower. What makes this so extraordinary is that the workings are almost entirely automatic. And the gearbox life expectancy is huge. I just sold a 15 year old car and it’s transmission system is still working perfectly.
Manufacturing processes associated with gear manufacturing have evolved to help deal with the various demands for performance at lower costs. The traditional method of gear cutting using machine tools generates accurate parts, but metallurgists found that the grain of the metal cut by machining caused weakening of the gear tooth. Powder metallurgy had been progressing to the point where it was more cost effective to mold gear profiles in sintered powdered metal and do only finish surfacing with machining processes. Later improvements in the process include the ability to load higher strength materials where needed in the design to produce higher strength parts at lower cost.
But as load requirements increase, all of the performance issues are magnified. And unique environmental conditions can play a part as well. In the current design of horizontal wind turbines, the gear box design is a critical component. The gear requirement at 2.5 megawatts is certainly a challenge, but adding the need for precision and and durability to survive 25 years of operation make the task incredibly difficult.
There are a couple of subtle aspects to gearbox operation that need to be considered. One is reversal stress. How does one calculate reversal stress? It’s the absolute value of the power, two times the power for simplicity, divided by the time period of the reversal. This is usually a really big number. And as the time allowed for the reversal decreases, the number goes up.
It doesn’t matter if the application is a servo motor system on piece of machinery or a gear increaser on a wind turbine. The situation is the same. It’s just more expensive when it’s a 30,000 pound reducer that’s 180 feet above the ground on a pole. But the principles are all the same.
Keeping the machinery running is a tough task regardless of the field. But monitoring the mechanical systems is key place to start. Next generation gear boxes will likely include electronics to monitor the loading and condition of the gearbox to prevent catastrophic failures.
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