Hunting Tooth (Gears) 1

[Schuyler]

Can someone please describe what a gear hunting tooth is and also:

1. How does it effect the operation of the gear (i.e. wear, loading)?

2. How can you tell by the drawing if a gear has a hunting tooth (i.e. ratio with mating gear, etc.).

3. Any additional information on this subject would be greatly appreciated.

 

[sms]

Mathematically, the number of unique assembly phases (Na) in a given tooth combination is equal to the product of the prime factors common to the number of teeth in the gear and pinion. The numbers 15 and 9 have the common prime factor of 3. Therefore, three assembly phases exist. In the case of a hunting toot design, Na=1, so every tooth on the gear engages every tooth on the pinion at some point in time. This prevents wear patterns developing due to assembly, so the gears don't have to be match marked.

Here is a great article on the subject

http://www.bently.com/articles/691winter.asp

 

[MachineryWatch]

sms is exactly correct. Gear designers are usually looking for an Assembly Phase Factor (Na) = 1.0. This effectively minimizes the number of times a given tooth on one gear can mesh with a given tooth on the mating gear. There is a frequency in gearbox vibration analysis known as Gear Assembly Phase Frequency (GAPF) it is equal to the Gear Mesh Frequency (GMF)/Na. So you can see that any gear tooth count in a gearbox that results in an Na equal to greater than 1.0 can result in Fractional Gear Mesh Frequencies (FGMF) when a gearbox is becoming worn or suffers from other abnormalities such as gear misalignment or backlash misadjustments.

Hunting Tooth Frequency (HTF) can be generated in any gear mesh regardless of the number of teeth on each gear. The frequency generated is directly dependent on Na because HTF=(GMF)(Na)/(#TeethGear1)(#TeethGear2). So the higher a number Na is, the higher the resulting HTF. Typically HTF is a very low frequency (<10 Hz). This means that people doing analysis on gearboxes may miss the appearance of HTF in the low end of the spectral data, especially if they are concentrating on the high end (typically up to 3.5 X GMF). HTF may also appear as side bands around the gear RPM and/or around GMF. Since it is typically such a low frequency very high resolution spectra may be necessary to see the side band spacing.

The main thing HTF is showing us is that a higher level of energy is being created when a specific tooth on one gear comes into mesh with a specific tooth on a meshing gear. This can be a result of a foreign material passing through the mesh and damaging the engaged teeth only enough that the extra energy is created only when these teeth engage one another. It can develop in a similar way to false brinnelling in a rolling element bearing, i.e. the gear box is not operating but is locked in position for a long time while vibration energy from other machines or from shipping long distances causes the teeth that are in mesh to rub one another. Another condition I have personally witnessed is when a gearbox was in storage for a few years without the correct desicant protection. A tooth on each meshing gear was at the level of the lubricant surface where moisture remained on the surface of the lubricant due to surface tension. This led to an extremely slight corrosion. It was so slight that extensive visual inspections (plural) of the gear teeth did not reveal anything. The problem was only revealed after complete disassembly and inspection using advanced optical comparative technology to reveal very small abnormalities in dimensions and geometry. The Na was equal to 1.0 and the speeds were relatively low (and variable), so that the period of the energy ranged from a few seconds at the high speed end of the range to almost a minute at the low end of the speed range. The sound from the gearbox was very pronounced when these teeth meshed with one another. This gear mesh was transferring about 30,000 HP.

I hope this helps.

Skip Hartman

http://www.machinerywatch.com