Greasing outer wheel bearings

[vader8x8]

I have a 2010 Frontier 8x8. Its my first amphip since the 72 Attex I had as a kid. It has 750 miles on it now with no problems, and I am religeous about maintenance. Just wondering, can you over grease the bearings? (to many pumps, push a seal out) I grease bearings and lube chains every 10-12 hrs of use. I pump 6 or 7 shots of grease into the bearings each time. (just until I see fresh grease coming out of the seal) My service manual doesn't say how much to grease a bearing or when to stop pumping. Any suggestions on how much is enough?

[Mike]

Everyone says to be cautious of overgreasing so that you don't damage a seal but really, as long as you aren't applying a lot of pressure by really pumping the grease gun, you should be fine. I grease mine quite often as well until I see or hear the grease coming out.

[Rock Doctor]

I actually do it as a "Rule", pump till it comes out the seal. Often I get water squirting out before the grease comes out. It won't hurt these types of seals.


RD

[micmac]

Actually I think it's different for the different manufacturers. Argos have a wiper type seal so you can pump the grease until it comes out the seal. Max don't have that system and you can blow out a seal in the bearing by overgreasing. That's where the one to two pumps comes from

[mudNmallards]

The question of how much grease it enough ( or too much or too little) has plagued bearing users since the first bearing was made. The bearing industry answer is: there is no answer. ( they will not give one for warranty and liability issues because they make bearings- they don’t design the application the bearings are used in or what they are exposed to and that’s the determining factor)
The reason why is because any given bearing can have a thousand different applications and each one has a different lubrication type, volume and relubrication schedule depending on load, torque, RPM, environment/contamination, bearing type, running clearances, shaft fit, housing voids, heat and a few others. Short answer is- there is no “one size fits all” answer.
SKF gives the best rule of thumb answer for the question ( which usually serves as an adequate baseline) GP (grease replenishment)= .005*D*B (D=Bearing OD in mm, B= total bearing width in mm)
This ONLY accounts for bearing fill- NOT voids such as housing clearance, seal valleys etc.
That’s “how much” now for “how to”
For this to make sense, one needs to understand grease, what it does and what it does NOT do and how it works in a bearing.
Grease is a blend of regular oil with additives (just like in the bottle) with a base thickener. It SEPARATES in temperature inversions, pressure ( the rolling media squeezing it does it) exposure to contaminates and over time. (yes grease has an expiration date but most manufacturers will not release it but after sampling thousands of base stocks and reviewing the data- 2 yrs is about the maximum you get before too much base oil separates to ensure adequate lubrication or the additive package uses itself up) (us people in tribology have argued for these dates to be put on grease just like milk but so far to no avail)
So, what happened to the grease that used to be in the bearing?
Assuming there is no blown seal and the obvious answer is all the grease formerly in the bearing is now all over the place:
1) The grease has been churned to the point the oil has separated and the liquid has atomized and gone to atmosphere leaving a thicker base stock ( which actually impedes lubrication as well as damages a bearing) textbook over lubrication
2) The grease has been blended to “goo” and has settled to the bottom of the housing- textbook under lubrication
So- if either of these exist- the bearing must be physically cleaned before addition of any lubrication is added or:
1a) The thicker base stock ( thicker being defined as more dense than the grease in pristine condition- this does not mean hard carbonized- that’s the ultimate end)- what happens here is simple force and resistance, you pump the new grease in- it meets the harder grease, it flows around it and normally out of the seal( point of least resistance). End result- you think you greased the bearing but all you did was grease the surface of the old grease and further blew out the seal allowing more contamination to enter.
2a) The void got filled with new grease- what happens here is the depleted goo ( along with whatever contamination it is harboring) mixes with the new grease under load and changes its viscosity, contaminates it and reduces its life span by about 90% and its lubricating ability by about 99% in the first few hours of rotation. It would be more merciful to the bearing to just grease it with valve grinding compound and end it quicker.
How does grease lubricate a bearing?
No anti friction bearing is designed to run in grease- they are all designed to run in oil. That said, the application, cost, effort and other factors make grease the best choice. That’s a problem because oil cleans, cools and forms a hydrodynamic wedge. (things that make bearings last for decades) Grease only lubricates by boundary lubrication ( which is why they usually don’t outlast their oil lubricated clones). The outermost sides of the rolling media pick up small amounts and “push” it under the rolling media. It picks it up at the first open clearance (usually about the 3rd or 4th media from BDC and pushes it ahead of it until the tightest point (normally at BDC) where it is fully squeezed out. This is why “shielded bearings” generally outlast their open counterparts because the shielding ( which is NOT air tight nor is it a “seal” of any kind) ensures the proper amount of lubrication ( at the point of assembly)and maintains boundary lubrication.
Best practices for adequate bearing relubrication
The goal of bearing lubrication is to 100% replace the old grease with the proper amount of new grease. Does that happen in the real world?- hardly ever but you try to get as close as possible.
The argument of grease from the top or bottom is equally right and wrong depending on individual applications that have too many variables to cover but universally there should be a zerk and a release fitting ( has a spring loaded pin in it) so you can pump the new in and allow the old to escape without blowing out the seal.
I advise my clients to have an “A” and “B” lube supplier ( with compatible grease obviously) with different color grease dyes in it so if the color of this cycle is “red” and the other was “blue” then you pump the blue out until you see the red. ( this also lets you know if the grease is leaking out somewhere because the volume coming out should equal the volume going in- if it doesn’t- you know you have a problem and that’s the time to fix it.
Measure the amount of your guns stroke (in grams) on a paper towel and count the number of strokes ( using the formula)- that’s not wholly accurate because of external voids but it will alert you to bigger problems if the numbers do not add up.
ALWAYS rotate the bearing during lubrication ( or immediately after) with zerks removed- this allows the bearing (at zero pressure against seals) to distribute the grease for proper boundary lubrication, fills voids and will remove excess grease. It can also tell you if you have greased it enough because without X-ray vision, you don’t know if the grease you added actually got in the rolling media where it needs to be- this is the best way to tell without physical disassembly.
Unless you have a precision application requiring calibrated metered grease guns and ultrasonic lubrication along with vibration readings- the above best practices should greatly extend any bearings usable life and performance.
What kind of grease?
That question is as long as it is wide. For these vehicles ( basing this on my experience in the field from my own vehicles and those I have had to manage and maintain) would be a good NGLI #2 with EP rating, extra polymer ( what people call water resistant greases) with a standard additive package. There is no miracle lubrication so any SAE or API rated grease will do- what is important is how the bearing is maintained and purged/filled.
Too much and too little?
With the exception of heavy industrial equipment, nobody is going to fork over the money for temperature probes and accelerometers on these vehicles to monitor the bearings but if they did- here is what they would see.
Overfilled bearings would run outside of their heat range for a while. This will burn up the additive package and separate the oil from the base and cause scoring leading to reduced life. Also, there would be a bad “carpet” reading and probably inner and outer race peaks on vibration. This occurs because the mass of the grease is too much for the media to roll over so it “jumps”. This is like taking a case hardened hammer ( the media) and peening the races. The fatal result is the start of a brinelling and spalling of the surfaces. At that point the trap door has opened and its just a matter of time before the strength of the neck multiplied by the force of gravity meets the tensile strength of the rope. Once started- there is no stopping or saving it. ( thus is why too much lubrication is ALWAYS worse than too little)

[Bazooo guy]

When i grease a bearing i look at what comes out of it and listen for air.
If possible turn the bearing as you pump new grease in.
If the grease flows easy (not cold) it will push on the seal and some will squeeze out.
Every time my boat trailer goes in the water (or 300 miles) I pump grease into the bearings and water does come out.
If the grease that comes out is contaminated I pump untill its clean.
With an aatv i would think we should grease the outer axle bearings whenever they are submurged in water or mud.
If after a couple pumps the grease is clean its good. I dont think temp will be a factor on aatv axles unless you drive at high speed most of the time.