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Setting part cleanliness specifications for gears

Setting part cleanliness specifications for gears

Setting part cleanliness specifications for gears

I am currently looking for a way to set a quantitative specification for maximum allowable size of foreign debris in our gear assembly. Does anyone know of calculations/methods of developing such spec? My concern is premature wear, noise, safety, etc due to debris in the gearbox.

Thank you!

RE: Setting part cleanliness specifications for gears

The most relevant factor for determining the max acceptable size of debris particles in a gearbox with oil lubed gears and rolling element bearings is probably the min operating thickness of the hydrodynamic oil film contacts. You want any debris particles in the lube oil to be small enough such that they can safely pass through the gear/bearing hydrodynamic oil film contacts. In my experience, something like a 3 micron limit would work very well.

Besides controlling cleanliness at assembly, you should also remember that the gears and bearings will generate quite a bit of metallic debris during the first few hours the new gearbox is operated. So your design should provide a way for the lube oil to flush this debris away from the gears/bearings and trap it in a filter device.

Lastly, when developing your process spec for controlling cleanliness of your gearbox, there are many sources of debris to consider. One source of debris that is often overlooked in the assembly of a gearbox is the metallic debris created from installing threaded fasteners. So I would recommend doing some testing to check for this type of debris being generated with your assembly process.

Good luck to you.

RE: Setting part cleanliness specifications for gears

i think it will be very hard to come up with a usable specification that not interferes with normal assembly and operating practice. in the past there have been indications that when reducing the particle count of the lubricant, wear on roller bearings could be reduced quite a bit. however, trying to carry this out in standard assembly was shown to be incompatible with normal production requirements. if you want to reduce the particle count you will need to assemble in a "clean room" and use "superclean" lubricant that is filled into the gearbox through a suitable filter. as soon as the gears start to operate however they will start generating lots of debris initially and still quite a bit after properly run in.

in theory these wear debris could be taken out by a suitable filter - the problem being that you need a large capacity filter with suitable filter size characteristics. since gears (unlike a properly designed hydraulic system that can be made clean with suitable filtering and can stay clean afterwards) gears will continue to add wear debris in a rate that a suitable equilibrium can not be achieved.

a better way to protect gears and bearings would imo be to include a magnetic plug to collect most of the metallic wear debris to keep them away from bearings and gears once "caught"

you should also be aware of the fact that both in gears and in roller bearings the lubrication is of the elastohydrodynamic variety - meaning that the lubricant film may be as thin as 0.1 micron at high loads. that means that minute sized debris still can do a lot of damage, so small that filtering them out with normal filtering equipment is impossible.

perhaps the best way to assure long gear and bearing life is to make sure that the gears are run in properly, which may call for the temporary use of a specialized running in lubricant. one other thing to consider is that since lubrication is of the EHD variety, complete separation of mating surfaces will never be possible at all speeds and loads and thus gears or bearings will fail eventually due to surface fatigue and the wear debris that result from it.

just one more remark: life of gears and bearings can be shortened exponentially when small amounts of water are present in the lubricant - even 0.1% can reduce the lifespan quite considerably.

RE: Setting part cleanliness specifications for gears

The question in the OP asking about "a quantitative specification for maximum allowable size of foreign debris in our gear assembly" does not have a simple answer. For example, the composition of the debris particles can be important. With a gearbox, metallic debris is usually of much greater concern than debris from cloth rags. Cleanliness standards typically specify the mean size and shape of debris particles collected in a given test sample, rather than just specifying a maximum debris particle size. Lube oil filters use a similar approach for describing their effectiveness called "beta ratio".

RE: Setting part cleanliness specifications for gears

I've seen some references to ISO lubricant cleanliness standards in rolling bearing literature. The grades or ratings had a 3 numbers in a row like 12/34/56
Perhaps like the stuff here-

I have no idea As to which "range" or "code" as shown on page 16 is appropriate, but I have a feeling those rascals over at AGMA probably do.

Do your gearboxes use circulating oil systems?

There is some discussion of many aspects of gearbox cleanliness, including cleanliness of the oil used at assembly and the use of a 3um filter in the test rig here -

There is even a section heading "Minimizing Built-in Contamination".
That sounds like maybe the area you are trying to address.

I believe at least for a while VW installed a very fine filter in the fuel lines of new vehicles that was to be removed or replaced at the the first, low mileage service. I'm sure the purpose was to catch stuff that even a German assembly area could not eliminate, but that would mess up the fuel injection system. I'm not sure if that was only true for the mostly mechanical KJet fuel injection.

RE: Setting part cleanliness specifications for gears

Here's a general recommendation from AGMA on lube oil filtration for industrial grade gear drives:
"Gear drives with pressurized oil systems should have a filter on the pressure side of the system to remove contamination particles. As a guideline, in the absence of specific manufacturer’s recommendations, the filter should be no coarser than 50μm absolute for gear drives with ball or roller bearings, and 25μm absolute for gear drives with journal bearings. In addition, a screen may be used on the suction side to protect the pump. This should be in combination with a filter and must have a coarse mesh to avoid flow restriction."

Here's a general recommendation from AGMA on lube oil filtration for aerospace grade gear drives:
"Fine filtration is becoming more common today for aircraft engine and gearbox systems for several reasons: longer bearing and gear life due to lower levels of debris, fewer or no oil changes and potentially longer filter life. Fine filtration requires a 3 to 5 micron absolute rating with a Beta factor of at least 100. Filter life has been found to increase after the initial system clean--up phase. Once the system is clean, it tends to stay clean."

So it seems the level of oil filtration depends on the application.

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