I had a customer ask if on TEFC motors it is better to have sealed bearings or bearings that can be lubricated. They have some of each on a couple of roaster ovens. I have my opinions, but figure that some of you probably have better informed opinions (than mine) and wondered what your thinking is on this question.

Rusty,

This would be a good one for a quick and dirty poll. My vote, open re-greasable with re-lubrication driven by vib info. OEM recommendations based on load, total hours run, environment, and other variables are so subjective. Vib data is best in my plant. We deleted re-lubrication PMs on several criticals and have had a remarkable improvement in bearing health and longevity

I suspect that these motors are in a hot environment (roaster ovens), thus I would go for regreasable (shielded bearings).
Sealed bearings are good for small loads IMO 3-5 hp with speeds up to 1800 RPM

Wow, I think your going to open a can of worms here. Really depends on the customer. Can they handle properly greasing a bearing? noismakr's call is an example of over-lubrication causing early failure. The wrong grease can cause early failure. Under-lubrication causes early failure. In my case, most of my customer's maintenance departments can not handle the complexity of multiple machines, multiple greases, and varying greasing intervals. I usually vote for sealed bearings.

Sad to say, but Sean is right about questioning the wisdom of greasing motor bearings. Most of my clients (who tend to be small to medium sized operations) have improperly lubricated enough motor bearings to know that it isn't as easy as you'd think. If Noisemaker were my lube guy, I'd want open bearings and lube exactly like he suggests, but there just aren't that many guys like him around. They've all been laid off and now work as consultants.

You also need to mindful of what your motor shop is doing. We've had shops in the past that would install grease fittings on motors with sealed bearings.

I'm at a large chemical site. About a year and half ago we started a centralized lube group with responsibility for the whole site. Motor lubrication is part of their job. We went through the exercise of making sure the grease is compatible with what the manufacturer and rebuild shop uses, researched the amount to use and frequency based on size and service and set up the task accordingly.

However, I do agree that lubrication based on vibration data is the way to go assuming you are not getting a dampened signal. Sometimes it is hard to get a good signal to hear/see the high frequencies. Short of that you can hear with ultrsound when a bearing is getting dry. To train yourself you can listen while lubricating the bearing. The shift in sound is very evident. In fact with a little practice you can hear it with the old "screwdriver to the ear" trick.

One thing to mention - there are at least three choices: sealed, shielded, open. Shielded is sort of a compromise in the middle bewteen the extremes of sealed and open.

Bearing speed and size are important factors. We combine them by talking about D*N number (D = bore diameter in mm, N = speed in rpm). Some bearing OEM's provide speed limit for a sealed bearing which is lower than for than for the same bearing when shielded or open. This doesn't provide the whole picture because even for double-shielded bearing regreasing is not always effectively and is so would not suitable for very high D*N numbers where grease degrades rapidly (when grease reaches end of life, bearing starts screaming on vibration).

High speed, large bearing, continuous run, high load, hot environment - all push toward the need for greasing (open bearing). I think if you have double-shielded 6316 bearing at 3600rpm on TEFC motor (tends to run hot at inboard), you will have problems with grease life. We have problems with grease life even when we regrease our open bearings in this type of application.

Low speed, small bearing, intermittent run, normal temperautres - all push toward the selection of sealed or shielded bearing. A 6310 3600 rpm motor bearing on outdoor crane used only a few times per year - no brainer - sealed. Your washing machine - no brainer - sealed. Many other motors are good candidates even at continuous run.

A recommendation from one of Heinz Bloch's articles: "Regreasable shielded bearings are considered the best choise for light to medium loads in the D*N range from 108,000 to 300,000, and one can go with either sealed (non-greaseable) or shielded (regreasable) in the gray range from 80,000 to 108,000" [Above 300,000 open bearings are his recommended choice and below 80,000 sealed bearings are his recommended choice].

Also in contaminated environments or when subject to water spray, there is more incentive to lean toward sealed bearing if possible.

Standard caution - consulting OEM is always good if you have that luxury. Some motors may have unique construction which can affect the choice.

You can always try and see what happens. Bearing lubrication failure should give a reasonable warning if machine is bearing monitored.

This message has been edited. Last edited by: electricpete,

I visited a plant yesterday that has installed mist lubrication for their large motors. They claim higher reliability and cooler running bearings. It appeared there was a bit of a housekeeping issue. Is mist lubrication an alternative to greasing? I've never missed with it personally.

H. Bloch has a book on oil mist also. They can run cooler. The mist (oil/air) keeps containments and moisture out even while down.

oil mist for machine tool spindles is pretty much obsolete in the US due to environmental issues. Air-oil is the modern method. Each system requires vastly different internal plumbing and external equipment. SKF bought one of the main players a couple of years ago.

One of Kluber's fancy grease products might make "sealed/lubed for life" a more attractive alternative, although the sealed ball bearings on my 20 year old motorcycle were still in pretty good shape after 11 million revolutions or so.

Many years ago our plant electrical dept. decided that more motors were failing from overgreasing than lack of grease. They stopped greasing motors altogether and had the rebuild shop use sealed bearings for all motor rebuilds. This policy proved to work well for quite a while but with so many VFD drives in use now it seems like we may have to rethink the policy. The grease acts as insulation to help protect the bearings from electrical discharge machining. We have been having this problem with some of our VFD driven motors but are not sure if lack of grease was a factor. It seems that every paper you read about this issue talks about the shaft current having to overcome the insulating properties of the grease before it can discharge.

Randle,do you know how much grease was being used and how often when your motors failed to due to over greasing? We had several bearing failures in a relatively short time due to lack of lubrication. That prompted us to start/re-start lubing motor bearings.

11 million revolutions on a 3600 rpm machine is < 51 hours, just over 2 days. I think that wouldn't be good for an industrial machine.

I have two customers who use vibes each month to determine when to grease motors. Some need it more often than others. Both, however, follow something similiar to what EPete stated above about the size and service. Anything smaller than 5 HP automatically has sealed bearings in it. I think there are only two or three exceptions to this rule in the one plant.
When getting a motor rebuilt, they specify to pop the shield off one side, which is the side the grease comes into the cavity on.
I can't remember when we had a failure on a motor at either of these facilities due to a bearing crapping out on us from lack of grease.

We use to use ultrasound, but it was more aggravation than just taking vibes readings. Both facilities have a ultrasound grease gun, but quit using it also. They use the EnPac with Spike Energy overall to grease with.

fde:
I was at the same SMRP meeting. I don't think I liked the mess with the oil mist. There needed to be some way to turn the mist off if the motor wasn't running.

Dave

I have a different customer that uses an oil mist on some fan bearings and it makes a huge mess. They don't worry about "housekeeping" because I am the only one that ever gets close enough to the fans to be bothered with it. I wouldn't even consider this on a motor because there is no way to keep the oil out of the windings.

Great input. Perhaps I should have asked this question in two parts. The motors are 1800 rpm, mounted vertically, driving a fan via belts. They are mounted under the ovens, with the non-drive end up. We've really not had problems with them, but last month two (of four) showed bad bearings, and one is noticeably noisy (but not bad yet). I suspect washdown water infiltration, but it's not been a problem previously.

In my opinion, even though these are TEFC motors, if you direct water towards the shaft, there's a good chance you're going to put water in the bearings. Would using 'sealed' bearings tend to keep the water out, or would a 'greaseable' bearing allow you to grease the bearing and mitigate the water infiltration?

I am thinking that you for sure want a shield on the winding side because pumping grease into the windings is almost a given the way most people 'grease' anything (put enough in so you know it's going somewhere).

Rusty,
I deal with a lot of 40-200 hp motor and a few up to 400. I started as a field mechanic on equipment before moving into vibes. What I have found in my experience is bearings that can be lubricated are better only if they are lubed correctly (generic answer, I know). I have one site that refuses to grease due to bad grease practices previously. They were over greasing. The bearings failed within 3 years on 24 hour operation equipment. They have since stopped greasing motors. The motor bearings seem to last 5 years now (shielded bearings only, not sealed). Sealed may be the way to go in this case. I have another property that was greasing bearings by the rules (no ultra sound or helper tools involved) and suffered bearing failures after 5 years. The ultimate cause was found when the end bells were removed. The shields were still on. I agree that the shields need to be in place when the grease has a direct path to the windings, but, the side to the end bell needs removed to allow full grease flow. There are arguments on shields both ways so take it as you will. I always remove the end bell side shield. Both get removed if there are bearing retainer caps. To get back to lube vs. non lube, my customer who does not grease has had good success with the small amount of the Blue Polyurea grease that the bearings come with. The polyurea grease is superior in electric motors. If you go lube style, try to use that grease, fill the end bell, so the bearing has a fighting chance to survive, and then grease normal. This gives the new grease a chance to go direct into the bearings instead of "Filling the end bell" which I see far too much.
The TEFC motors unfortunately have a shaft on each end. This allows two entries into the motor. There are products designed to sling undesired contaminates from the entry point around the shaft. I don't know much but I have seen the brass ring style and it appears to work well. Is it possible to go with a totally enclosed only with no fan? This only leaves one entry. Also, I have installed battery powered auto greasers. They have a couple dip switches to set the time frame for the unit to push out the entire charge of grease. Seems to work pretty good on my applications. They move a little every day keeping fresh grease in there a little at a time.
Hope this helps.

As mentioned by Dan the newer mist (air/oil)systems do a better job of preventing the mess. There are health limits on how much of this stuff a worker can be exposed. Machinery likes this method.

Have you considered a moly based grease? They reject contaminates such as moisture better. I would stay away from lithium based greases on electric motors, i see shortened bearing life on that combo.