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Mr Fixit
12-13-2014, 09:52 PM
Hello Group,
I have just finished my RPC and the bearings felt ok but running it they have some noise. Gave them a shot of grease not much help. Older motor no history so I'm thinking new bearings.
SO, my question is, the name plate says 307 & 206 for bearing sizes. When I look them up I'm confused, I see 300 series and 200 series bearings then I also see 6300 series & 6200 series bearings, the differences I read are 6000 series are"Deep groove ball bearings" and the 200,300 series are "Cylindrical roller bearings" per this SKF sheet.
http://www.malloyelectric.com/wp-content/uploads/2013/07/Bearing-Handbook-for-Electric-Motors.pdf
The sizes or dimensions read the same. What is the correct choice for a 7.5hp Century motor being used as a RPC. I'm inclined to go with the 6000 series as they are a lot more available and cost less, may even have them in my stash somewhere. I haven't opened that motor yet got a project I need to finish first.
Look forward to your responses.

Mr fixit for the family
Chris:)

philbur
12-14-2014, 02:10 AM
You have no radial load to speak of so I think deep groove are the correct bearing. The link you provided points out that cylindrical roller bearings need a specific minimum radial load or bearing damage due to slipping may result.

Phil:)

becksmachine
12-14-2014, 06:44 AM
Bearing numbering systems are not necessarily universal across manufacturers.

That said, I am sure that your two bearings are;

206, 30mm bore, 62mm OD and 16mm wide

307, 35mm bore, 80mm OD and 21mm wide

With various prefixes and suffixes for seals, shields, grooves etc.

Dave

Rosco-P
12-14-2014, 09:30 AM
Curious, how old is the Century motor? Pull the bearing, get the part# directly from them, proceed from there.

Weston Bye
12-14-2014, 11:02 AM
I suppose I should go and do likewise.

However, the ancient motor I pressed into RPC service, though rumbly when running, spins freely, taking a couple of minutes to spin down when shut off.

I've refrained from fixing what ain't (totally) broke.

That said, it will probably fail when I in the middle of a hot job...

lakeside53
12-14-2014, 11:24 AM
I suppose I should go and do likewise.

However, the ancient motor I pressed into RPC service, though rumbly when running, spins freely, taking a couple of minutes to spin down when shut off.

I've refrained from fixing what ain't (totally) broke.

That said, it will probably fail when I in the middle of a hot job...

It may not be bearings at all. Single phasing a causes lot of vibrations which is why you should not "bolt" an idler down hard, but let it move on rubber mounts.


OP : They will be deep radials, most likely 6000 series. Best to take them out and compare. If your old bearings are open and you have grease ports, you can just ignore that and put in shields (not rubber seals). But... it may not be a bearing problem at all - just rpc vibration. Try isolating mounts or a rubber mat.

If you want to prove it, run it from true three phase and if it's fine, live with the single phase noises! I've had many that suck as idlers and are smooth a silk on 3P!

Mr Fixit
12-14-2014, 01:03 PM
Thanks group.
I had it bolted to the attic floor. Cut up some car tire rubber and it did quite a bit to quiet it down but it still has a low growl like rough bearings. I will tear it down this week and get the info off the bearings then put what is in there back.
Lakeside, if shielded by metal sides correct? Does the grease still get to where it needs to or do they not need greasing then?

TX Mr fixit for the family
Chris:)

lakeside53
12-14-2014, 01:10 PM
Put in shielded (ZZ for most manfs means both metal shields both sides). You will not need to re-grease them in your lifetime. Do put a small amount of grease in the end bells though - the bearings need to move with thermal expansion.

Try one more thing - wack (rubber hammer the end of the shaft axially left/right when running. Sometimes motor noise is just that the bearing is not lightly preloaded by the wave washer. Few rpcs are truly quiet, and are hard on bearings.

Rather than "car tire", buy some isolation mount rubber sheet used for HVAC. Car rubber is way too hard. You might need to experiment.

Mr Fixit
12-14-2014, 01:33 PM
Ok.
Tx Lakeside.

Chris

lakeside53
12-14-2014, 01:48 PM
BTW... while almost any bearing grade will work, there is a specific bearing grade for electric motors. It general falls between C3 (looser then normal) and Cn ("normal"), and are (were) selected for quiet running. Look for "electric motor quality" but ignore this if it's generic Chinese - meaningless.

ikdor
12-14-2014, 04:13 PM
The 307 without any letters before it is a deep groove ball bearing with a filling slot, so it has more balls (and thus loading capacity) than a regular ball bearing. (you pay for it with poor axial loading capacity, but there's not much of that in an electric motor)
The capacity difference with a 6307 is only 10% though, so it might not be worth the extra money. If it was a really old bearing it would not match the capacity of modern bearing steel, so in that case the 6307 would be equivalent.
The same story goes for a 206 versus 6206.

Make sure you get the same clearance class as the original bearings, often /C3 for electric motors, as the fits are based on that.

Igor