Grease Lubrication of Acft Mechanical Joints: Multiple Questions

[WKTaylor]

Folks...

Been asked to update a tech order section on grease lube of critical plain bearing and spherical bearing joints. During my research I've uncovered bits-n-pieces about grease lubrication of joints... but no concise publication or document for "general how-to-lubricate" joints.

USAF T.O.s and Boeng OMMs & AMMs & BACs have provided some insight, but all of these are obviously written for folks who have basic experience/knowledge/training... and do NOT include basics of lubrication to advanced data/info/concepts… all to insure a concise and consistent understanding of this more-complex-than-it-looks maintenance process among the acft technicians. There is lots of data out-there for other "hands-on maintenance processes" [safety-ing, cleaning, torquing, riveting, etc] that I have been able to ID as "the how-to". HOWEVER, there is this big hole around plain bearing lubrication practices. I can't go too-far into the subject here... but suffice it-to say there is a lot to know and understand to do the job right... especially when lube data is skimpy such as pointing to lube fitting with a grease type [ONLY] and no other info.

NOTE: that I haven’t even begun to discuss basics of mounting of mechanical components “wet” with grease [as Boeing call-it “butter-lubing”]; and lubing ball/roller bearings in-place; or gears, or ball-screws, etc…. Or cleaning-out contaminated/dirty/plugged joints/passages/lube-fittings, etc… or “grease 101 basics for mechanics”, etc…

Also... some real engineering questions:

Obviously grease type and initial temperature have a role. Cold grease is semi-solid and appears to act more non-Newtonian... whereas warm/hot grease acts more Newtonian. Obviously, grease intended for extreme cold tends to be more-fluid [low viscosity] in this range... whereas high temp grease can be virtually solid [brick-like] at extreme cold temps. So how does grease travel thru long passages and groves?... or is there a practical length/cross-section limit??? If that’s the case, then are more grease lube points "better"... or is an increase in the number of lube joints just an invitation for more to be missed by mechanics, already stressed to speed-up the work???

How large should grease grooves and holes/passages and ports be to assure adequate lube distribution under strenuous situations, such as lubricating at very low to very high temperatures... or very dirty/contaminated joints?????? What about basic groove configurations: single, multi-criss-crossing, multi-branching, spiral, etc?? And what about groove-shapes: rectangular, half-elliptical, half-radius, etc??

Are these factors related to bearing size... and/or total passage-length/Dia, grease viscosity and available pressures/flow rates, possibility of over-pressure, etc???

What effect do seals have on the process???

What about severe operation conditions, such as multiple days of continuous rain, snow, slush, deicing fluids, etc; or "Arizona-dusty desert days at extreme temperatures[static structural temps in excess of 160F].

Is the philosophy of “grease extrusion good-enough (for most joints)… or should it be grease extrusion all around… and should lubing be stopped before or after clean grease is observed.

GAAAAAAAHHHHH. HELP!!!! ENGINEERING/TECHNICIAN IDEAS/COMMENTS BASED ON EXPERIENCE, KNOWN DOCUMENTS ON THIS SUBJECT(??)…

Hope this makes sense.

Regards, Wil Taylor

 

[plasgears]

1. Grease, by its nature, is not a permanent lube, so replenishment is necessary. If some small reservoir can be built in, then more life is assured.

2. Assurance of filling is a question. I tried arranging a small overflow bleed hole remote from the filling point. A refinement would be an elastomeric bleed plug with slot that would open when full. It would remain closed normally.

3. The early proposed cruise missile engines had simple grease reservoirs under spring pressure for bearing lube. They provided lube long enough for the mission.

 

[Sparweb]

Have you ever seen those pictures in your everyday auto shop manuals of a man's hands working the grease into a bearing?

Bearing manufacturers (SKF, FAG, Torrington/Timken, etc.) probably have a lot to say about how their bearings should be grease, and with what, and under what conditions a certain grease can be used.

I've attached a segment of the Bell 212 helicopter MM, which has a section on Lubrication. Hope it helps. (1.0MB)

http://files.engineering.com/getfil...-6342dcf33311&file=BHT-212-MM-Lubrication.pdf

While on the helicopter topic, lack of grease in drive-shaft joints is a common cause of accidents. Either the parts get hot and vaporize their grease, or the grease isn't changed, or the grease was forgotten entirely. Lots of horror stories in TSB reports.

Steven Fahey, CET

 

[tbuelna]

wktaylor,

The lubricating component of grease is just oil. What makes it a grease are mostly the fillers and thickeners added to make it semi-solid, so that it tends to stay in place.

The most common cause of grease lubed joint problems is corrosion due to moisture intrusion and entrapment. This is especially common in joints that undergo regular heating and cooling cycles in humid environments (ie. an aircraft).

When re-lubing a greased joint, I would highly recommend injecting enough grease to ensure that any trapped moisture is displaced from the joint cavity. This may seem like a waste of grease, but any amount of grease is cheap compared to equipment failures due to corrosion.

Good luck.
Terry