Maintenance-free spherical bearings lubrication 1

[mec00063]

Hi there!

I use maintenance-free spherical bearings for an aplication of very high loads (300mm diameter spherical bearings or larger are used) in a very dust laden environment (mining industry). I could, to avoid dust in the spherical bearing, protect is with grease and assembly it in an enclosure.

But in the manufacturer's catalogue it's written: "PTFE fabric spherical bearings must not be lubricated" and I'd like to know why.I wonder if it means that there is no need to lubricate or if the PTFE will be damaged? Does anybody has experience with this issue? Can contact with grease ruin the this kind of beaaring?

Regards,

Pedro

 

[Jboggs]

You asked, "Does anybody have experience with this issue"?

The answer is - yes! Guess who!

Here's a hint: they published a catalog with that warning in it.

They probably even included their phone number so people could call for more information. Maybe you could use it.

I know the answer to your question. Guess how I learned it. I called them!

Here's another good reason to get your information directly from the manufacturer rather than from a forum: its more reliable.

I'll also give you another tip: enclose the bearing in a housing with a small opening and feed a small amount of compressed air to it so you always have a very small positive pressure inside the enclosure. Then the only way dust can get to your bearing is to go against the air flow at the opening.

 

[mec00063]

Thanks for the quick answer Jboggs!

I strongly agree with you when you advise to get this kind of information from the manufacturer. But sometimes the manufacturer's front guy studied the item through the catalogues...the same they give us to select the bearings...and the information sometimes isn't very complete, and I believe this is one case. If I call them (I did) they say: Yes, you cannot lubricate it: Look at page xxx.... But what I'd like to know is: what happens? Will I loose 0,01% of the expected life of this bearing? Or will I get a steel on steel bearing within one year....and will have to change it?...I don't know...and this guy doesn't either...

I cannot use compressed air where it's instaled (but its a good idea!)

I've foud this information in a forum:

"
i myself wondered this and asked Aurora this is the replies, really is interesting!
PTFE lined rod ends should not need lubrication. In fact, lubeing a lined
rod end can hurt more than it can help.
The bearing liner wear face (the surface contacting the ball) is not pure
PTFE, rather it is a surface that contains a quantity of PTFE. As the ball
moves relative to the liner, the exposed PTFE is rubbed onto the ball, and
back to the non PTFE areas of the liner. Soon you will have coated the ball
and fully dispersed PTFE on the liner wear face.As this happens, the
friction goes down. Think of dipping a cloth in wax, and waxing your car. At
first, the cloth holds some wax, and the car is bare. As you rub the cloth
on the carS both the clothe and car become fully covered in wax. When you
lube the ball with oil or grease, you inhibit the balls ability to retain
the PTFE. Eventually, the PTFE will be removed from the liner, causing the
friction to increase.
I will caution that this may not apply to all makes of rod ends, but in
generall I'll stand by this.
Are you having specific problems? I'd be happy to adress specifics as
needed.
Regards,
John McCrory,
Race car product manager.
Non lined rod ends can be lubed with a high quality EP grease. Whatever you
use for wheel bearings or C.V. joints would be fine. In applications with
significant load, i.e. the steering or suspension, you are best to use a
lined rod end. No matter how well you lube a metal on metal joint, there is
no way to insure a lubricant film under load. A PTFE lined joint may require
a greater force to pivot un loaded, but under load will move more freely
than a metal on metal joint. I would be concerened about the bearing torque
only if it is a problem on the track - lack of steering return being an
obvious problem! Once you examine the joints on the car, let me know part
numbers and manufacture, and I may have more to recomend.
Regards,
John


"


What do you guys think? Do you agree with our friend John? Does any of you have seen a destroyed spherical bearing after greasing it?

 

[BearingDave]

Hello Pedro,

there are different types of maintenance free spherical plain bearings:
Steel/Bronze composite
Steel/PTFE fabric
Steel/PTFE composite

Based on the information I found, you should not grease the Steel/Bronze composite or the Steel/PTFE fabric types.

See the following link:

http://www.skf.com/portal/skf/home/products?maincatalogue=1&lang=en&newlink=3_0_43

During the first period of operation of steel/sinter bronze composite and steel/PTFE fabric bearings, there is a transfer of PTFE from the composite or fabric to the opposing steel surface of the inner ring. Any lubrication of the sliding contact surfaces would disturb this transfer and shorten bearing life. Therefore, lubrication of these bearings is not advisable and they have no relubrication facilities.

An initial lubrication and occasional relubrication of steel/PTFE composite bearings can extend the service life by a factor of at least two. SKF grease LGEP 2 or LGMT 3 (see table 1) are recommended for the lubrication of these bearings.

All SKF maintenance-free spherical plain bearings are supplied coated with a preservative. Bearings with the sliding contact surface combination steel/PTFE composite are also greased.

When operating conditions are such that enhanced sealing and protection against corrosion are required, it is recommended that the space surrounding the bearing is filled with grease. If the bearing arrangement is designed as shown in fig 4 it is possible to supply grease to the free space around the bearing without grease actually penetrating the bearing. Rust inhibiting, water-repellant lithium base greases of consistency 2 or 3, as for example, the SKF greases LGMT 2 and LGMT 3 should be used. On no account should greases containing molybdenum disulphide or other solid lubricants be used. Whether or not it is necessary to renew the grease during operation depends on the environmental conditions and ageing of the grease. Steel/PTFE composite bearings benefit from periodic relubrication of the bearing itself. The same grease should be used as for the surrounding space.