Can Ceramic Bearings with Carbon Steel Raceways Become Electrically Conductive?

No need to post the same question 3 times, once will suffice.

-sorry, i'm in a foul mood today.

Hi RVZ717,

Am I missing something here? Where are the other 2 posts? I just don't see them. Just wondering...

Mike

They were probably removed by the Admins.

May I ask the reasons for your question please?

EG. Do you need such conductance for say a CNC machine to use a 2D sensor, or do you want the insulation to remain good for some other reason?

"leading to early failure due to electrical arcing"

"this problem or duration until it becomes conductive"

Then you need to set up a test procedure with some sort of resistance meter measuring across the bearings to follow the life plotted against resistance I would imagine....

The Hybrid bearing ceramic balls in steel races were initially developed for high speed applications not for non conductivity. There are some hybrids that would be be more suited for that type of application having thermoplastic races but with reduce load capabilities.

Over time yes the bearing would conduct. The lubricants in the bearing even if sealed are not in an air tight seal. Contaminants from the air could be mixed in the lubricant to cause it to conduct.

I understand the initial reason for using ceramics was speed. It has also become an alternative for electric motors and generators due to ceramic materials being so non-conductive. My original question (maybe not stated clearly enough) was intending to see if anyone was aware of any testing or evaluations of performance on ceramic bearings in the previously mentioned electrical applications.

I agree with your comment (or at least the logic) that the bearing would conduct over time. I am really looking for someone who has tested this type of application. If I want to consider the use of a very expensive ceramic bearing in motors/generators (more that 2x the cost of standard bearings) I would like to know what kind of difference in life I can expect. If the ceramic balls become conductive, it kind of defeats the purpose of using ceramics and spending considerably more for them. I apologize for not being clear enough about my logic/reasoning for asking the initial question.

Has anyone heard of any testing or documentation, that would give me some solid reasons or proven test data, for using ceramic bearings in applications where standard bearings seem to fail more frequently due to electrical arcing?

I would guess conductive particulates are the culprit. In series universal motors failure often occurs due to arcing caused by a trail of carbon dust. With wear over time conductive particulates would be released from the carbon steel. You could try a non-conductive alloy. Good luck.

Ceramic bearings are often made with a certain amount of metal powder infused into the ceramic, to prevent them from being too brittle. I'm not sure that this applies to ball bearings, but I guess it is possible, since the same properties would be desired as in bushing-type bearings. The metal used would normally be either high carbon steel or moly-chrome steel. This could mean that the bearings are actually conductive to begin with. This might be something worth checking into.

-Bob

If its a problem the bearing can be mounted in or on a suitable dense insulating material.

This will eliminate a conductive circuit and your problem.

I understand there are ways around having the arcing go through the bearing. I am aware of grounding rings, ceramic coatings around the outer ring surfaces or inside the housing to increase resistance are some of the alternative solutions to an arcing problem. Overlooking that and any influence metal particles in the grease or lubricant might have on the bearing, I still stand with my initial question.

I know theoretically, the impregnation of metal particles (from and during normal operation) should make the ceramic balls somewhat conductive. Has anyone tried to prove it? In order for it to be conductive would it require such a very high voltage, that for all practical purposes, it would still be considered an insulator?

I am not an electrical engineer and I don't know what kind of voltages are commonly built up inside motors/generators that want to go to ground. I am in no way, shape or form an expert on electrical fields and stray voltages in this type of equipment. Will these voltages have a high enough potential to use the metal particles on the surface of a ceramic ball to go to ground?

I am aware that certain types of motors have more problems with this than others and they seem to go through bearings much more frequently than others. The path of least resistance is the bearings. I understand there are many things that will cause bearings to fail but electrical arcing is so easily distinguishable that it makes me ask these questions.

I find this topic very interesting. Raises a lot of questions that sooner or later more people are going to ask especially since I believe the growth of motors and generators is going to grow very significantly if not exponentially in the world. Wind generators, electric cars, fuel cells, solar cells . . .

It is time for the engineers and creative thinkers in the world to start thinking about the future. My humble opinion.

Hi,

if you make use of the bearings so that no skidding occurs, then there will not be any such steel film on ceramic balls.

There is ample experience since now around 20 years with Si3N4 balls in high-speed ball-bearings in machine tools and high-vacuum pumps and aircraft.

But if you have a bearing assembly that is either designed with flaws or has not correct preload or has too fast start- or stop- acceleration or deceleration I would think that the phenomena of transfer-films may occur.

I saw these in non-rolling bearings, but I never heard about these in rolling bearings. But I saw severe damage done by too low preload or by too fast acceleration.

If you want to be sure: make an insulating ring of glass-fiber reinforced epoxy for mounting the bearing.

Pay attention to thermal behaviour as this will restrict the heat flow.

RHABE

With 20 years of usage and experience, I question how much of the requirement was because of electrical issues. Thermal issues, speed issues, weight issues I can easily understand but, how many of the applications called for a ceramic ball bearing specifically due to electrical and grounding issues in motors/generators? I kind of believe that electrical insulation was a secondary thought or even an unintended benefit when most were designed in. I wonder how many motor/generator manufacturers install ceramic ball bearings in their units. On high end products you would think they would capitalize on the benefits if they were that advantageous.

I have seen some bearings that were considered to have failed by ordinary wear but very likely this was electrically enhanced wear.

So how many of the designers and their bosses really know about electric failure of ball bearings?

RHABE

Good point! Maintenance people probably know more about this phenomenon than the design engineers!

My experience with Si3N4 balls in steel races of hybrid ceramic bearings only covers small sizes (8mm ID) and high speeds (15000>35000 /min) with Shell K12 oil, but has never disclosed any reduction in the insulation resistance.

More important has been the effect of the metal shields, which sometimes get lucky and bridge between inner and outer.

These tests revolved around cutter length gauging in miniature CNC machine spindles fitted with hybrid ceramic bearings, where we deliberately jumper the cutter to ground because of the constant and predictable high insulation value of the balls.

Mark Bingham

Hi,

these spindles are well designed and manufactured with adequate preload and accuracy.

Else there is very rapid failure or not adequate quality. Or both.

In a well designed bearing assembly there will not be any metal transfer from raceway to balls but the balls will be fully afloat on the elastohydrodynamic oil-film.

(Same as water on street and tires with low profile and/or speed too high).

How did you gauge cutter length? Contacting cutter to workpiece?

RHABE

Dear RHABE,

The method for gauging cutter location (hence its length as mounted) shows in

http://fourth-axis.com/z-gauge/

This is a special case, where an adjustable gauge is required, because this model of mill has two special characteristics:

1) Its firmware assumes the last Z-height manually selected is Z=0;

2) Its language has no equivalent to G92, therefore it is impossible to gauge to some standard height, then move to the required height, then reset the register with G92.

No intention to wander off-topic, just a direct answer to a direct question.

Mark Bingham

Logging in quite late and a lot of water has flown through the bearings .

However in my experience I have seen lot of carbons and other conducting materials deposited in the crevices of normal antifriction (roller bearings) over time, especially if they have been untouched for a couple of year.

I suspect that they migh be result of cracking of the oil, or normal contamination or may be some of the additives deposited (eg Moly sulph or so which has good electrical property)

never gone for chemical analysis of the cholesterol (I am exacly facing this problem now, when i opened one of the big machines after some 10 years of operation, i see black deposition everywhere - bearings, surfaces - and even the tubes are choked. Going through a massive cleaning exercise ), unfortunately no clot dissolving medicine for this.

Fortunately these are all metals - so no arcing problems.

But in your case, better to avoid the arcing by other means than to make bearing non conductive. go for some mechanism of rotor earthing .