Help Understand Bearing Theory

In summary, the conversation discusses the topic of over greasing bearings and whether it is possible and how it can be done. It mentions the use of bearing packer tools and the importance of filling the bearing cavity with grease to prevent dry spots. It also mentions that over greasing can cause issues such as overheating and failure. The conversation also touches on the possibility of adding grease to a sealed bearing without removing it from the car, and the use of different types of grease for different bearings in different applications. The idea of installing a Zerk fitting into a bearing is also discussed as a way to avoid metal shavings entering the bearing during the drilling process.
  • #1
YoshiMoshi
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8
TL;DR Summary
Over Grease?
Hi,

Sorry if this is a stupid question, but I haven't taken fluid mechanics classes and mechanical engineering ones.

I understand that sometimes bearings get "dry" and you can regrease them. However I did some searching online and found that it's possible to over grease a bearing which can cause issues? Is this true in the automotive industry? I'm talking bearings that are not soaked in oil but need grease. For example ball joints, wheel bearings or pulley bearings.

If it's possible to over grease them, and you should just buy them when they get dry, then why do bearing packer tools exist for the purpose of adding grease to bearings? Moreover, if you should only have 20%-30% of the bearing cavity full of grease, then why do these bearing packing tools seems to 100% fill the bearing with grease?
1649983440775.png

I've even see the cone type packing tool that allows you to add grease into a bearing without even removing the seal.
1649983489512.png

This got me thinking. Is it possible to add grease to a sealed bearing without having to remove it from the car? Or has this tool or fastener still need to be invented? I see the cone packing tool is basically a threaded rod with a hole that goes all the way through it, then a hole on the side to let the grease out.
 
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  • #2
The bearing packer forces grease through a rolling element bearing for two reasons:
1) To push out old grease (or solvent if you cleaned the bearing first),
and
2) To fill the bearing with grease so there are no dry spots inside when it starts up.

But the bearing packer does not fill the entire cavity with grease. The cavity, the space between the bearing and the grease seals, is open for excess grease to push into when the bearing starts up.

It is possible for force grease past the seal of a sealed bearing, but the seal can be damaged in the process. Or to use a hypodermic needle through the seal, also damaging the seal. Or to drill a hole in the metal housing, screw in a grease fitting, and pump in grease. But that would put metal chips in the bearing.

Over greasing a bearing can cause overheating which causes the grease to break down and the bearing to fail.

I once drilled the ball and tie rod joints in my car to add grease fittings. It worked well, but each joint would only take a very small amount of grease. I had to add grease when it got harder to steer, which was every 1500-2000 miles.
 
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  • #3
YoshiMoshi said:
If it's possible to over grease them, and you should just buy them when they get dry, then why do bearing packer tools exist for the purpose of adding grease to bearings? Moreover, if you should only have 20%-30% of the bearing cavity full of grease, then why do these bearing packing tools seems to 100% fill the bearing with grease?
Because different bearings in different applications require different amounts of different types of grease.

Slow bearings that carry a high load will be completely filled with grease.

Faster bearings get hot, (so the grease flows), when they cool the internal airspace contracts and the bearing sucks in external air and moisture. That bearing must be filled with grease at the end of each period of operation or corrosion will occur during downtime.

If a grease nipple is fitted, there will be a way for excess grease to escape that will not push out the bearing seals. That is how you will know how much grease to inject.

A sealed bearing is not designed to last forever. You are expected to replace the entire assembly. You might flip out the seal and re-grease the bearing, but you need to know why it needs more grease. When sealed bearings wear the lip seal is often damaged. Are the seals worn? What grease should be used for the refill? How can you keep it clean during the re-fill?
 
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  • #4
jrmichler said:
The bearing packer forces grease through a rolling element bearing for two reasons:
1) To push out old grease (or solvent if you cleaned the bearing first),
and
2) To fill the bearing with grease so there are no dry spots inside when it starts up.

But the bearing packer does not fill the entire cavity with grease. The cavity, the space between the bearing and the grease seals, is open for excess grease to push into when the bearing starts up.

It is possible for force grease past the seal of a sealed bearing, but the seal can be damaged in the process. Or to use a hypodermic needle through the seal, also damaging the seal. Or to drill a hole in the metal housing, screw in a grease fitting, and pump in grease. But that would put metal chips in the bearing.

Over greasing a bearing can cause overheating which causes the grease to break down and the bearing to fail.

I once drilled the ball and tie rod joints in my car to add grease fittings. It worked well, but each joint would only take a very small amount of grease. I had to add grease when it got harder to steer, which was every 1500-2000 miles.
Interesting. So for bearings with rubber seals it's not possible to overfill them, because the excess grease will always get p ushed out if the bearing is overfilled, reducing it to the not-overfilled state? Sounds like it's really possible to overfill them then.

I also thought about putting a Zerk fitting into a bearing.
Some metal bearings have metal shields. I have seen YouTube Videos where they remove the metal shield to add grease. Would it be possible to remove the metal shield from the bearing, and then install a Zerk fitting this way? This would ensure that no metal shavings go into the bearing sense the drilling process would take place outside of the bearing.
I've also thought about drilling a hole through the outer race of a bearing. Looking online, people say that this is really hard steel, and you would need special equipment to drill a hole.

I'm a bit confused about over greasing still.

I thought about doing the same, and then I just went out and got new ball joints that came with Zerks. I think they will last a long time. Sealed ones can last for a very long time though. I still have some ball joints in like stabilizer bar link that are sealed with no Zerk fitting that have nearly 200,000 miles on them and they seem fine. Greasable ones should last even longer, outlive the car, I hope.
Baluncore said:
Because different bearings in different applications require different amounts of different types of grease.

Slow bearings that carry a high load will be completely filled with grease.

Faster bearings get hot, (so the grease flows), when they cool the internal airspace contracts and the bearing sucks in external air and moisture. That bearing must be filled with grease at the end of each period of operation or corrosion will occur during downtime.

If a grease nipple is fitted, there will be a way for excess grease to escape that will not push out the bearing seals. That is how you will know how much grease to inject.

A sealed bearing is not designed to last forever. You are expected to replace the entire assembly. You might flip out the seal and re-grease the bearing, but you need to know why it needs more grease. When sealed bearings wear the lip seal is often damaged. Are the seals worn? What grease should be used for the refill? How can you keep it clean during the re-fill?
So in the automotive industry, like a wheel bearing, or idler pulley bearing, these bearings can spin up to a few thousand RPM. Would this be considered low speed or high speed application?

I was thinking of maybe installing a Zerk fitting (not sure if possible) and excess grease would just pushed through the seal or hole were the Zerk is.
 
  • #6
YoshiMoshi said:
I was thinking of maybe installing a Zerk fitting (not sure if possible) and excess grease would just pushed through the seal or hole were the Zerk is.
I believe a Zerk fitting is commonly called a grease nipple. The nipple is closed by a spring loaded ball to prevent exit of grease, or entry of dirt and water from the external environment.

YoshiMoshi said:
So in the automotive industry, like a wheel bearing, or idler pulley bearing, these bearings can spin up to a few thousand RPM. Would this be considered low speed or high speed application?
There are many different types of automotive bearing lubrication schemes. You need to be much more specific; Make, model, year and bearing application.

YoshiMoshi said:
Interesting. So for bearings with rubber seals it's not possible to overfill them, because the excess grease will always get p ushed out if the bearing is overfilled, reducing it to the not-overfilled state? Sounds like it's really possible to overfill them then.
What do you mean by a rubber seal ? A separate external lip seal, or one attached to the outer
race of a sealed bearing. Take care that the grease does not push a lip seal out of the housing.

YoshiMoshi said:
Some metal bearings have metal shields. I have seen YouTube Videos where they remove the metal shield to add grease.
The steel guard or shield on a bearing is designed to protect the bearing from solid fragments in the oil pan, while allowing oil to flow about the internal bearing surfaces. It is not common to grease a shielded bearing.

You need more experience before you start inventing solutions to imaginary problems.

Ask yourself why a grease nipple is needed. Why did the engineer not design it with one?
Do not drill the hard bearing race to install a grease nipple. If you need a grease nipple, drill the housing that holds the bearings. Then you can change the bearing without having to drill it every time.

I just got a spindle bearing assembly from China with a grease nipple to the central cavity. So I wonder why it was fitted with two fully sealed bearings. Maybe it was designed originally for a one side sealed bearing. These days you can make one side sealed bearings by flipping out the seal on the inside, but why bother ?
 
  • #7
Baluncore said:
I believe a Zerk fitting is commonly called a grease nipple. The nipple is closed by a spring loaded ball to prevent exit of grease, or entry of dirt and water from the external environment.There are many different types of automotive bearing lubrication schemes. You need to be much more specific; Make, model, year and bearing application.What do you mean by a rubber seal ? A separate external lip seal, or one attached to the outer
race of a sealed bearing. Take care that the grease does not push a lip seal out of the housing.The steel guard or shield on a bearing is designed to protect the bearing from solid fragments in the oil pan, while allowing oil to flow about the internal bearing surfaces. It is not common to grease a shielded bearing.

You need more experience before you start inventing solutions to imaginary problems.

Ask yourself why a grease nipple is needed. Why did the engineer not design it with one?
Do not drill the hard bearing race to install a grease nipple. If you need a grease nipple, drill the housing that holds the bearings. Then you can change the bearing without having to drill it every time.

I just got a spindle bearing assembly from China with a grease nipple to the central cavity. So I wonder why it was fitted with two fully sealed bearings. Maybe it was designed originally for a one side sealed bearing. These days you can make one side sealed bearings by flipping out the seal on the inside, but why bother ?
Thanks for the help in understanding this.

My bad. Take for example any "light-weight" passenger vehicle. The speed of the bearing seems to effect the ability for it to be over greased or not? Such vehicles normally "red line" at around 6000 RPM and max out at 8000 RPM. Take for example a Toyota Camry.

What I mean by a rubber seal is this red thing here.
1650048310464.png

I thought shields were the metal inserts that some of them have like this one?
1650048359507.png

I'm trying understand the theory about bearings that need grease and aren't soaked in an oil bath, like inside of an engine.

I think grease nipples have gone away in the automotive industry. I prefer them. But I think most passenger vehicles no longer come with them. Ball joints that are 'sealed for life" last for a good time but not forever. They will get the vehicle over the warranty date so it's not the manufacturers problem to fix. I think the idea is that people don't necessarily like the cars to last longer, just to require last maintenance. I get aftermarket ball joints that have them when they go bad, so I never have to replace them again. Most people don't own their cars long enough I guess for a sealed ball joint or bearing to go bad. Although I guess "heavy duty" trucks still come with Zerk fittings on ball joints.

Do you have a picture of such a bearing with a Zerk fitting on the race? I have seen them on housings that house the bearing, but never on the bearing themselves.

I'm thinking of for example. Bearings on pulleys on passenger vehicles sometimes go "dry". This causes a squeaking sound that can be heard. People typically just replace the whole pulley instead of replacing just the bearing. I was thinking that it would be nice to simply just add grease periodically every oil change to the bearing, so you don't have this problem. However there is no easy way to do this, as there is no Zerk fitting. The current way to do this is to remove it from the vehicle, remove the bearing from the pulley and then add in grease.

Then I read online that you can overfill the bearing and cause damage. So I questioned if it was every a good idea to add grease to a bearing? I talking about an application of say a tensioner pulley in a passenger vehicle whose engine speed on the odometer typically stays between 2000 RPM - 4000 RPM but redlines between 6000 RPM-8000 RPM.

Would it be a good idea to add grease to such a bearing when it runs dry and starts to squeak? Or you can risk over greasing and cause damage in this application of it?
 
  • #8
YoshiMoshi said:
Such vehicles normally "red line" at around 6000 RPM and max out at 8000 RPM.
Yes, but those parts of the driveline are lubricated with oil baths, NOT grease.
Only the universal joints are greased, and they have very low surface velocities.

Most wheel bearings are greased, some are sealed, while others are assembled with external seals.

The differential ratio will be about 3.5 so wheel bearings do not run at the higher engine speed.

YoshiMoshi said:
I thought shields were the metal inserts that some of them have like this one?
You show an integral rubber lip seal that has an internal metal stiffener plate. That is typical of sealed bearings. It would have a part number with suffix -2R or -RR.
IIRC a steel shield would be suffix -Z or -ZZ.

YoshiMoshi said:
Do you have a picture of such a bearing with a Zerk fitting on the race? I have seen them on housings that house the bearing, but never on the bearing themselves.
You are the one who suggested drilling the bearing race. That is not done because the hole would damage and interfere with the rolling surface, and the bearing would no longer fit the housing.

YoshiMoshi said:
Then I read online that you can overfill the bearing and cause damage. So I questioned if it was every a good idea to add grease to a bearing? I talking about an application of say a tensioner pulley in a passenger vehicle whose engine speed on the odometer typically stays between 2000 RPM - 4000 RPM but redlines between 6000 RPM-8000 RPM.
On a 4 stroke engine, the camshaft turns slower than the crankshaft.
A timing chain tensioner will be oil lubricated with the chain.
A timing belt tensioner will have a sealed deep groove ball bearing. You are expected to replace the sealed bearing when you replace the timing belt.
 
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  • #9
YoshiMoshi said:
This got me thinking. Is it possible to add grease to a sealed bearing without having to remove it from the car?
Why would you want to add grease to a sealed bearing. It contains sufficient grease.

YoshiMoshi said:
Would it be a good idea to add grease to such a bearing when it runs dry and starts to squeak?
When a lip seal fails the lighter components of the grease can escape. The wear of the metal surfaces will result in more lip seal wear and movement. Once that failure cascade begins the entire sealed bearing should be replaced, rather than just adding more grease. Refilling a sealed bearing is not a sensible strategy if you want reliability and lower maintenance costs.

The tensioner pulley for a timing belt is in a dry environment. If you add grease to the sealed bearing the excess grease will certainly escape, and so damage the rubber timing belt.

The reason why the tensioner pulley is replaced with the bearing may be because the pulley wears against dust on the belt in the dry environment. It is also possible that the pulley is shrunk onto the bearing, something that is difficult for a mechanic to get right without a stable oven temperature and an assembly jig.
 
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  • #10
Baluncore said:
Why would you want to add grease to a sealed bearing. It contains sufficient grease.When a lip seal fails the lighter components of the grease can escape. The wear of the metal surfaces will result in more lip seal wear and movement. Once that failure cascade begins the entire sealed bearing should be replaced, rather than just adding more grease. Refilling a sealed bearing is not a sensible strategy if you want reliability and lower maintenance costs.

The tensioner pulley for a timing belt is in a dry environment. If you add grease to the sealed bearing the excess grease will certainly escape, and so damage the rubber timing belt.

The reason why the tensioner pulley is replaced with the bearing may be because the pulley wears against dust on the belt in the dry environment. It is also possible that the pulley is shrunk onto the bearing, something that is difficult for a mechanic to get right without a stable oven temperature and an assembly jig.
So I've seen videos on YouTube were they open a sealed bearing, brand new, just bought. It looks like there's hardly any grease in there. They all seem to come this way.

Then I've seen bearing packers, like the cone tool, were they then use this to pack in grease without even removing the red rubber seal and there's a whole lot more grease in there. Lots more.

I'm having a hard time understanding why packing tools exist if it over greases them?

I appreciate the help.
 
  • #11
YoshiMoshi said:
So I've seen videos on YouTube were they open a sealed bearing, brand new, just bought. It looks like there's hardly any grease in there. They all seem to come this way.
They contain sufficient high quality grease. That grease is clean, clear and hard to see. Why do you need to contaminate them to void the warranty ?

YoshiMoshi said:
Then I've seen bearing packers, like the cone tool, were they then use this to pack in grease without even removing the red rubber seal and there's a whole lot more grease in there. Lots more.

I'm having a hard time understanding why packing tools exist if it over greases them?
Packing tools should not be used on sealed bearings. Packing tools are used on bigger open bearings that do not have integral seals. Those bearings are usually conical tapered roller bearings that are assembled as separate parts.

It appears to me that you are giving YouTube content more credibility than the sewer deserves. Can you please provide a YouTube link that shows a sealed bearing being packed with grease "without even removing the red rubber seal".
 
  • #12
If you seriously want to learn about lubricating rolling element bearings, stop wasting your time on YouTube and go to a knowledgeable source - a bearing manufacturer. SKF is a major manufacturer of bearings, and they publish online an 1152 page engineering manual for their bearings. Link to that manual: https://www.skf.com/binaries/pub12/...lling-bearings---17000_1-EN_tcm_12-121486.pdf.

They have a chapter on lubrication starting on page 109. It's well worth the read. That manual has additional lubrication information in some of the chapters about specific types of bearings. The chapter starts with a flow chart for the type (grease or oil) of lubrication:
Lubrication.jpg

The next few pages discuss lubrication amount, lubrication intervals, relubrication procedures, and grease types. A partial excerpt:
Lube interval.jpg

While the entire bearing manual is an intimidating 1152 pages, the lubrication chapter is only 18 pages, plus a few more pages in the individual bearing chapters. It's written to be understood by people with a good high school education. You should have no trouble understanding it, but do expect to spend some time studying and digesting it.
 
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  • #13
Hey thanks for all of the input.


I've see this youtube video where they regrease a bearing without removing the seal.
The result is this on both sides.
1650150836205.png

While brand new bearings look like this.

1650151367381.png

I see that once a bearing goes bad and start making noise, the damage is already done and you should replace it. Does the first picture look over greased?
 
  • #14
YoshiMoshi said:
...
I've see this youtube video where they regrease a bearing without removing the seal.
The result is this on both sides.
As the picture below indicates, it is easy to force grease through the lip of the seal, by permanently deforming it toward the inside of the bearing.
After that happens, what will prevent dust, air and water from come inside and contaminate the freshly injected grease?

1106%200055%20-%2010000_tcm_12-137339.png


Much lubricant is not equivalent to good lubrication.
There is only a fine film of grease formed among the contacting metal parts that do the work, the rest is there for the free ride, mainly accumulating where the centrifugal effect quickly and permanently places it.
Additives like molybdenum are used to make that film stay put.

If old grease has gone bad due to contamination, thermal cycles and/or heavy oxidation, there is no point in leaving it inside, just to add more on top of it.
The proper procedure is to remove the old grease, to thoroughly clean the metal parts and to apply enough fresh grease to do the job.

Packing grease into the rollers of a conic bearing has been always done by hand, not to by-pass any seal, but to ensure that grease will reach deep inside the cage.

If built-in seals have been damaged or are worn, chances are that it is too late to save the metal parts.
If it is not, removing those old seals, cleaning and relubricating, seems to be better than injecting new grease and leaving the old one inside.
If no better option exists, you could re-install the old seals in place, but knowing that cleaning and lubrication will need to be done frequently after that point.
 
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  • #15
YoshiMoshi said:
I've see this youtube video where they regrease a bearing without removing the seal.
Thanks for the link; but it is NOT good advice.

Notice how the bearing spins easily and keeps spinning when first tested. There is no grease on the race and the seals are worn. The rattling sound it makes also tells you the races and cage are worn.

There is no question that the bearing is overfilled following the treatment. Overfilling the worn bearing with grease is a false economy, and a waste of grease. It will make a bigger mess as all that grease will come out again, then collect dirt.

Maybe that trick would work as a stopgap measure in a low speed application, but the cost of pulling down the equipment to temporarily fix a worn bearing is high when you consider that you will need to do it all again when you install a new bearing.
 
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Related to Help Understand Bearing Theory

1. What is bearing theory?

Bearing theory is a set of principles and equations that explain the behavior and performance of bearings. Bearings are mechanical components that support rotating shafts and reduce friction between moving parts.

2. Why is understanding bearing theory important?

Understanding bearing theory is important because it allows engineers and designers to select the most suitable bearings for a specific application and ensure optimal performance and longevity of the machinery or equipment.

3. What are the main types of bearings?

The main types of bearings are ball bearings, roller bearings, and plain bearings. Ball bearings use balls to reduce friction, roller bearings use cylindrical or tapered rollers, and plain bearings have a sliding surface between two parts.

4. How do bearings work?

Bearings work by reducing friction between two moving parts. They do this by using rolling elements (such as balls or rollers) or a sliding surface to support and guide the rotating shaft. This reduces the amount of force needed to move the shaft and increases its efficiency.

5. What factors affect the performance of bearings?

The performance of bearings can be affected by various factors, including load, speed, temperature, lubrication, and alignment. It is important to consider these factors when selecting bearings to ensure they can withstand the specific conditions of the application.

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