Bearing loss-density and motor bearings in general
Bearing loss-density and motor bearings in general
(OP)
Hello,
Motor bearings seems to be almost always 63-series type. Output shaft diameter seems to be dimensioning the bearings as far as I have been making arrangements. When modified lifetimes(Lhmr) of the bearings are considered motor bearings seems to exceeding easily "infinite life" of one million hours.
Q1: I have always chosen 63-series bearings because of their size - I have heard that for the same ID 63 runs cooler than 62. But is there some more scientific explanations? :) Maybe some rule of thumb for bearing (friction)loss-density when motor bearings are considered?
Q2: Nowadays it is very easy to get over 20year lifetime for bearing when Lhmr is considered. End customers and certifying bodies do accept Lhmr ratings also. I am a bit doubting that Lhmr value is realistic even if I know I have taken many factors into account and I use 50 000 hours as minimum value for Lh10. Have someone else been using some other rule of thumb numbers for Lh10?
Other comments about sizing motor bearing?
Thanks ,
-S
Motor bearings seems to be almost always 63-series type. Output shaft diameter seems to be dimensioning the bearings as far as I have been making arrangements. When modified lifetimes(Lhmr) of the bearings are considered motor bearings seems to exceeding easily "infinite life" of one million hours.
Q1: I have always chosen 63-series bearings because of their size - I have heard that for the same ID 63 runs cooler than 62. But is there some more scientific explanations? :) Maybe some rule of thumb for bearing (friction)loss-density when motor bearings are considered?
Q2: Nowadays it is very easy to get over 20year lifetime for bearing when Lhmr is considered. End customers and certifying bodies do accept Lhmr ratings also. I am a bit doubting that Lhmr value is realistic even if I know I have taken many factors into account and I use 50 000 hours as minimum value for Lh10. Have someone else been using some other rule of thumb numbers for Lh10?
Other comments about sizing motor bearing?
Thanks ,
-S





RE: Bearing loss-density and motor bearings in general
Choosing an L10 life is all about how the product will be used. For example, if designing a bearing for a domestic vacuum cleaner or dishwasher motor, an L10 life of 5,000 hours would be more than enough, because the product is only used intermittently. However a motor for a waterworks pump or vacuum pump would need an L10 of 50,000 or 100,000 hours due to it running continuously with reliability a critical factor. So it depends on the motor use. But 50,000 hours isn't a bad choice in general.
Note that bearings almost never reach fatigue life in practice, usually other factors like contamination or grease life kill it off first.
RE: Bearing loss-density and motor bearings in general
In electrical machines there is always heat coming from the rotor shaft to bearing also - might it be the thing why to choose oversize bearings? And in case of inverter driven motors there are insulated bearing housings used - so bearing and also other losses are put into bearing from the shaft - but the heat transfer from bearing to housing is limited because of the insulation on housing. So I think that a small bearing runs hotter because of its limited volume? Does this make sense to others? If yes, my original question is regarding to this effect - is there some rule of thumb value for loss-density of bearings? ... or is this too complicated way of thinking? :)
About L10:
For applications where I do make my designs is bearing lifetime requirement been always 20yeas equals around 175 000 hours. Bearing manufacturers and even end-customers accept Lhmr instead of Lh10, nowadays. Is electric motor industry just so conservative that Lh10 value is used instead? Lhmr = a1*a23*Lh10
RE: Bearing loss-density and motor bearings in general
A small roller bearing will have a lower speed rating than a larger ball bearing due to heat generation of all the line contacts.
There are many heat generators in ball bearing.
sliding in the contact ellipse, size of the contact ellipse ( adjustable via more open curvature in the races), how the cage/retainer is guided. Even the variations in grease characteristics ( not just quantity ) can make a real difference in heat generation.