Taking a Look at Ceramic Bearings

Full Ceramic bearings from Boca

Ceramic bearings in a motor (specifically) have come to my attention lately, and in particular, in a motor that was built as a “race” motor (I don’t know what that means) and by someone I don’t know anything about.  I decided to look into some of the information on ceramic bearings in general, and use on electric motors in specific.  Here’s what I got.

First, there are your full ceramic bearings, where the races and the balls are all ceramic, and there are your ceramic hybrid bearings where your races are steel and your balls are ceramic.  (Insert adolescent tittering.)  From Engineer’s Edge:

Ceramic bearings are typically constructed with a ferrous inner and outer ring or race with ceramic balls in the place of steel. Ceramic bearings offer many advantages over all steel bearings, such as higher speed and acceleration capability, increased stiffness, lower friction and more. Ceramic balls are also nonconductive. Ceramic bearings are available in all standard industry configurations such as, angular bearings, thrust bearing, pillow block bearing, needle bearings, and roller bearings.

Ceramic bearings balls are typically made from (Si3N4) ceramic silicon nitride and have greater hardness than steel balls resulting in longer ball life. Ceramic bearing balls have smoother surface finishes than most steel bearing balls. Thermal properties are also better steel balls which result in less heat generation due to friction at high speeds. To manufacture a extra fine surface finish on ceramic balls, the balls are elevated with a magnetic field and then polished with plasma stream. Ceramic bearings balls are rated at higher spin rates than steel bearing balls.

Ceramic bearing balls can weigh up to 40% less than steel ones, depending on size and material. This reduces centrifugal loading and skidding, so hybrid ceramic bearings can operate 20% to 40% faster than conventional bearings. This means that the outer race groove exerts less force inward against the ball as the bearing spins. This reduction in force reduces the friction and rolling resistance. The lighter balls allow the bearing to spin faster, and uses less energy to maintain its speed.

While ceramic hybrid bearings use ceramic balls in place of steel ones, they are constructed with steel inner and outer rings; hence the hybrid designation. While the ceramic material itself is stronger than steel, it is also stiffer, which results in increased stresses on the rings, and hence decreased load capacity. Ceramic balls are electrically insulating, which can prevent ‘arcing’ failures if current should be passed through the bearing. Ceramic balls can also be effective in environments where lubrication may not be available (such as in space applications).

Ceramic bearing balls require less lubricant and exhibit less lubrication degradation, which results in increased bearing life. Ceramic bearings manufactured from Si3N4 can operate at temperatures up to 1600F. Ceramics also are resistant to oxidation.  

Hybrid ceramic bearings from Boca

OK, now we’re getting somewhere.  There’s kind of an interesting forum discussion here, on Practical Machinist.  This is the most interesting comment, but there are a bunch of observations about ceramic use in various applications.  They also talk about the various marketing hype associated with ceramic:

“Ceramics can be much more wear resistant
They can be much slicker (higher lubricity)
They can be lighter in weight.
They conduct heat very poorly so they don’t grow with heat.
They can be tough enough to use as saw tips in saw mills.
The results you get depend on what you mean by ceramic. There are well over a thousand grades of ceramics currently being used.

C-3 and C-4 are most commonly grades of cemented tungsten carbide. If he is recommending tungsten carbide I think I would ignore it and go with a ceramic.”

Now, the RC space is where you see them used as upgrades on electric motors, however small.  Here’s a thread where you hear about problems with them failing:

I have had way better luck with normal steel bearings. The ceramic bearings spin better, but pretty much every engine bearing failure I have ever had has been a ceramic bearing.

…and here’s a quick list from a company who sells them, for edumacating their customers, I reckon:

LIGHTER. The ceramic ball is lighter then the steel ball. This means that the ball exerts less force outward against the outer race grove as the bearing spins. This reduction in outward force reduces the friction and rolling resistance. The lighter ball allows the bearing to rev up faster, and uses less energy to maintain it speed.

HARDER. The ceramic ball is harder then the steel ball. This gives you a bearing that will last up to 10 times longer then the steel ball bearing because it holds it’s finish better.

SMOOTHER. The ceramic ball has smoother surface properties then the steel ball. This means less friction between the ball and bearing races giving you a faster spinning bearing.

THERMAL. The ceramic ball has better thermal properties then the steel ball. The ceramic ball will not heat up like a steel ball. This helps avoid heat build up in the bearing, a primary culprit in skate and skateboard bearing performance degradation.

LUBRICATION. The ceramic balls are impervious to oxidation, chemicals, and require essentially no lubricant. The bearings are lubricated with a very, very small amount of Aral Aralub oil, not a gel or grease.

They wrap up with this caveat:

Solid Ceramic: The ceramic balls are solid ceramic silicon nitride. A cheaper ceramic bearing uses steel balls with a thin ceramic layer. The elastic and thermal property difference between steel and ceramic cause the ceramic to split and separate from the steel portion of the ball on these less expensive look a likes.

Avoid this type of ceramic ball bearing.

From another seller’s site, the Boca Bearing pages, here’s a quick bullet list on the differences between the full and hybrid.  Definitely check out that page, it’s the most informative and succinct source of detailed information I could find.

Full ceramic:

• Will not corrode or rust
• 2/3 the weight of steel
• Resistant to resist acids, alkali, blood, salt and water
• Extremely low coefficient of friction and thermal expansion
• Lower friction results in less energy consumed
• Operating Temperature from as low as (-85°C) up to (+ 900°C)

Hybrid ceramic:

• High loads, high speeds and extreme temperatures are factors
• Increased bearing life and decreased down time
• Minimal need for lubrication
• No cold welding between ceramic balls and races
• Multi-purpose use in variety of machines and different production lines
• High efficiency, low weight and small size

Real data is pretty hard to find, on the efficiency question.  I was able to find this story on MachineDesign.com with some actual test charts, originally from Champion Bearings.  Not an academic study, but the efficiency claims are pretty remarkable.  “In tests, all-ceramic bearings consumed less than 25% of the power used by ceramic-metal-hybrid and all-metal bearings.”

The recurring thread?  If you’re going to go with a ceramic bearing, there’s a huge range of quality, and if you skimp, you’re likely going to regret it, and it can be a costly mistake. In terms of the benefits for an electric motor?  My personal opinion is that the weight savings and efficiency will be negligible on a motorcycle, and the cost is prohibitive.