There is this well known procedure of re-greasing motor bearings while motor is running and leaving the drain plug opened for hours in order to let the old grease out.

Let's think if everything is OK here.
Assume a single shielded bearing with the shield toward the windings and with the housing cavity connected to the input port and drain port. I can see two problems here.

Firstly, pumping the grease until it starts appearing in the drain means that the housing cavity became 100% full. As bearing operates, it will of course expell some grease from its internals but the housing cavity is still going to be 100% full. Is it good? Probably not, as OEM normally recommends about 20-50% of the bearing volume itself and about same of the housing cavity volume to be filled with grease.

Secondly, as bearing is located in the dead end of the housing, the old grease inside the bearing itself will rather stay there, whereas new grease will replace the old one in the housing cavity only. Also does not sound right.

Am I wrong?

Thanks for you replies.

First an interesting point, SKF would recommend you install a single-shielded bearing with shield toward cavity rather than toward motor... the reason being that the purpose of the shield is to "meter" the grease flow and help protect the bearing from contamination. But most people do it the other way and that is fine as well.

Regarding your first point, I believe most people do not continue to add until grease comes out of the outlet port. At our plant we continue to add until EITHER #1 - we reach the predetermined amount added (let's say 1 ounce for 6313 bearing) OR grease comes out of the drain port... whichever one comes first. If grease does come out of the drain port we should evaluate some maintenance to remove that extra grease.... in all cases I know of on motors that would involve replacing the bearings... some centerhung pumps I know with easily accessible bearing caps can very carefully remove and repack grease without disturbing the bearing (be very careful of contamination).

Regarding your 2nd point - yes, there is nothing you can do to move that grease at the end but at least you can get some fresh grease nearby and hopefully the oil from that grease migrates into the bearing and tends to help keep things from drying out.

The fun thing about greasing, there are as many different methods and opinions as there are people involved in greasing.

Strange advice from SKF to install the shield toward the cavity. In that case you could rather use a double shielded bearing.

If the shield is installed against the cavity, why someone would grease it?

Your first question - it seems strange to me as well but that's the way they'd generally prefer it if you are installing a single-shielded bearing. I have heard it in person from our local rep and I think I have seen it in their literature. Perhaps from their standpoint they are not as concerned about the motor winding as about the bearing. If anyone can find that in their literature let me know. I will look tomorrow if I have time to see exactly what it says.

Your 2nd question - there are those that lubricate doulbe-shielded bearings as has been discussed here several times recently. It is certainly not a universally embraced practice.

lubricating of shielded bearings in most cases people do not know better. If there is a grease nipple someone will put grease. If you take out the grease nipple, an eager mechanic will assume that the nipple broke off and put another one.

Yes I agree that lubrication of double-shielded bearing is certainly not a well-accepted practice. I will mention the EPRI Large Electric Motor User's Group (NP7502) does recommend it and our plant does it, for better or for worse.

Hello Peter, we call double shielded the "eternally lubed" bearings. Sometimes "eternity" is just 3 years, 15+ years short of what people want. Some people here do not buy machines without nipples. Of if needed, nipples are added on day one. Repacking or opening a brg on the site to remove old grease is by far too risky. Dust and cigarette ash is like poison for a brg. What is the point in using a shielded bearing at all in a machine having regreasing design? You press in the amount prescibed and let the oil bleed into the bearings rolling surface. What is more to it? Regards Arne

I hope it was clear I was not recommending others to go out and lub double-shielded bearings, but my point it that it is not a black and white question.

I will pull together my thoughts and post a new thread on this subject.

Back on the subject of shield toward the cavity (vs toward the winding), I could not find the literature from SKF but I did find a recommendation from Heinz Bloch that the shield be placed toward the cavity and this arrangement permits relubrication. (refers to standard arrangement grease inlet and drain on same side of the bearing).

Here is a link to Heinz Bloch's article
http://www.machinerylubrication.com/article_detail.asp?articleid=575&relatedbookgroup=Lubrication2

"Single-shielded Bearings
Many bearing users consider the regular single-shielded bearing with the shield facing the grease supply (Figure 2) to be the best arrangement.


Figure 2. Single-shielded Motor Bearing with
Shield Facing the Grease Cavity

Experience indicates this simple arrangement will extend bearing life. This arrangement will also permit an extremely simple lubrication and relubrication technique. The shield serves as a baffle against agitation. The shield-to-inner-race annulus serves as a metering device to control grease flow. These features prevent premature ball bearing failures caused by contaminated grease and heat buildup due to excess grease. For other services where an open bearing is necessary, as in some flush-through arrangements, the shield can be removed in the field. "