which cage to use 1

[DUISENM69]

I have a choice to buy bearing for electrical motor 1500 RPM with brass cage or polyamid cage.

1) 7322 BECBM Angular Contact ball bearing single row "Brass Cage".
2) 7322 BECBP Angular Contact ball bearing Single row "Polyamide Cage".

What is the criteria I need to consider? I appreciate your advice.

 

[TPL]

We banned the use of polymide cages in our plant (as do most of the major oil and gas multinationals) simply because cage failures are almost completely undetectable using a walk around vibration monitoring program. Polymide is relatively soft and acts as a 'cushion' and impacts between rolling elements and bits of breaking up cage do not transmit to the bearing casing.

Once initiated, polymide cage failure accelerates quite quickly - when the rolling elements come in to contact the bearing failure is extremely quick, with a good deal of heat being generated, giving a potential fire hazard.

Polymide cages cannot be used at temperatures above 120 degC

We replaced all bearings with polymide cages with ones fitted with brass cages - failures are more gentle and easily picked up with a routine vibration monitoring programs. When a brass cage fails, it often releases huges number of tiny brass particles which discolour the lube oil - another indication of failure that is easily picked up. Brass is OK up to 300 degC but should not be used in an environment wher eit may come into contact with ammonia.

 

[JJPellin]

I work in a major oil refinery. We have also stopped using the non metalic cages. Our standard for angular contact thrust bearings is a machined brass cage. Several studies have shown that the brass cages have a considerable advantage over stamped steel and definately over the non-metalics. We do still use the polymide cage bearings in one application. The lower intermediate shaft bearing in certain models of Sundyne gearboxes uses this cage material. Sundyne insists that no other option can handle the speed. We are currently working to find an alternative so we can repalce this bearing, as well. For any bearing substition, it is a good idea to verify speed, temperature and load capability of the new bearing as compared to the service it is being placed in.

 

[diskullman]

The benefits of the polyamide cage: quiter, lighter, has some self lubricity, cleaner waste product (old grease) and much easier to make for the bearing manufacturer. It should not be a problem in an electric motor. That being said, and after a scan of the FAG manual for any benefits they can offer (none), considering the amount of failures & re-design of bearings back to steel & other metallic cages, I would definately buy the brass cage bearing.

Russell Giuliano

 

[JeanMicheling]

Japan used to put polyamide cage bearings in their machine tool spindles. They use it because they work better with vibration and in high speed conditions. I use polyamide cage once and I noticed that running temp was lower than phenolic cages. Actually, it's true for a certain speed but as you increase the speed over about 18000rpm phenolic cage runs cooler (according to NSK). Brass cage will give you high rigidity, high strength and high temp resistance. So to answer you questions, the criteria you should consider are :

-Speed, load, vibration, temp, noise, chemical environnement, lubrication and of course price.

NSK precision book gives a good general description of the different advantages of cages.

 

[Skogsgurra]

7322 is for a rather large motor. 700 - 900 kW, I think. I would not look at the price in such a case. Speed is not high. Sound level is probably not an issue. Preventive maintenance and the possibility to get an early warning before total bearing failure will probably be important.

Brass is usually chosen for such applications. I would chose brass. There are reports (rumours?) that polyamide fails catastrophically without much warning in the vibration spectrum.

Gunnar Englund

http://www.gke.org

 

[Mendit]

This is a vertical motor no doubt? We have been rebuilding motors for over 30 years and the polyamide cages have been a cause of many failures, some catastrophic. In all probability your bearing is oil bath lubrication, or it should be ? The upper bearings are the thrust and the lower bearing is a guide. Depending on your load if 2 bearings are used at the top there is a frequency generated between the 2 bearings which can cause the cages to break up. Brass is better at transfering heat away.

 

[Petri]

I have had some bad experiences with Polyamide Cage bearings in the past as well. I have read that Polyamide cages are effected by EP additives ( Sulphur ) in lubricating oils ( Hypoid ), with combination of elevated temperatures greater that 150 deg C, leading to material softening and could lead to ultimate failure.