Join Date: Dec 2012
Location: Cleveland, Ohio
Gear noise - or a "whine" as you are describing - is NOT indicative of a transmission failure. Gear noise is prevalent in many different types of geared systems including automobile transmissions, photo copiers, etc. If you really want to hear some gear noise, put just about any vehicle in REVERSE, get it up to 10-20 mph and listen to the gear noise roar! Modulate the throttle so that you can change the engine speed and change the frequency of the noise.
Gear noise is created by the undulations on the gear teeth profiles as the gears mesh together - these undulations cause vibrations - the vibrations travel from the gear teeth, to the gear blank, to shafts, to bearings, to housings, etc. The gear blanks, shafts or the transmission housings act like speakers and radiate the sound.
Transmission Error is the term used to define the angular displacement of the driven gear (gear) relative to the driving gear (pinion). Perfect involute gear teeth produce zero transmission error at no load. Once load is applied though, the gear teeth bend and Transmission Error occurs and there are other things that create Transmission Error. There is lots and lots and lots of scientific data that correlate gear noise to Transmission Error.
The "undulations" on the gear teeth are on the micron level (1 micron = 0.00004 inches) and you cannot detect them with the human eye - special measurement equipment is used. Gear engineers modify the gear teeth profile from a perfect involute for several reasons in an attempt to minimize Transmission Error and gear noise.
However, they typically cannot eliminate all gear noise. Some companies are better than others. Some vehicles within a given company are better than others. Just because a company labels itself as the "luxury" arm of an auto company, that does not mean that they are great at everything that they implement. I have seen bad gear designs that just needed to be thrown out they were so noisy. While others, were close to being quiet, and just needed to be tweaked slightly.
Automobile companies are very sensitive to price too! So do not expect the auto companies to just fix a bad gear design and pass the cost onto the consumer. Plus, some companies just have "average" gear designers that do not even know how to improve a given gear design.
The gear whine frequency varies with engine speed with a given gear ratio and this is known as the mesh frequency. As an example, if the pinion shaft is rotating at 120 rpm, it has a rotation frequency of 2 Hz. If the pinion has 100 teeth, the mesh frquency is (2 Hz)*(100)=200 Hz. The meshing is not a pure sine wave, so the mesh frequency also creates harmonic frequencies at 400, 600, 800, 1000 Hz, etc.
Once you notice gear noise, it is typically difficult to tune it out, but again, it is not indicative of a failure.