Ball Bearing Limiting Speeds

Well, I am happy to be back here at PTE! It has been a few years since I have been able to write casually due to some legal structures in my company. Coincidentally, those barriers were removed not long before PTE Sr. Editor Matt Jaster reached out to me to ask if anything had changed with my disposition. Additionally, I recently started working on a couple of topics with ABMA (now managed by AGMA), which I won’t get into just yet, and then discovered through Matt that AGMA and PTE have also joined ranks. What a great combination, very exciting developments in bearing land, indeed!

While we have been out of contact, COVID has been running almost in parallel with the unprecedented electric revolution in the automotive industry. When the COVID shutdowns started, I thought we had—maaaybe—five years before everything looked like a post-nuclear apocalypse out of a movie. Somehow, in a page out of bizarro-land, this electrification wind sprint started around the same time as COVID and made us all busier than ever!

Electrification has really brought ball bearings back into focus as the primary bearing in our drive systems. Some boxes have tapers on the slower shafts for stiffness and others are using a ball/cylindrical combo for efficiency. Regardless, we all face the same challenge on the primary shaft in dealing with the potential of 18,000–20,000+ rpm speeds coming out of the motor. Plenty of applications run 20,000 rpm; what makes the automotive motor unique is, in addition to speed, we are driving huge torques, frequent torque reversals and a huge range of temperatures both internally and geographically. Of course, this all needs to be suited for high-volume manufacturing as well. Adding full ceramic balls, a PEEK cage and a high precision classification is a really easy way to run greater than 20,000 rpm all day but is an expensive option. One bearing alone could hurt the cost competitiveness of your gearbox. In the case where a single bearing can change the landscape of your project, it is worth taking a little time to understand exactly what the drivers of our speed limitations are.

Due to the extensiveness of this subject, we will only discuss catalog Limiting Speed today which is a mechanical based speed rating. Thermal-based speed ratings are rooted in ISO 15312 and DIN 732 which we will cover next time. Those ratings discuss speed limitations solely based on the ability to keep the bearings thermally stable and make no assumptions about mechanical limitations. 

While pinning down the origins of the original limiting speed is vague, it is loosely based on a metric called DmN, a simple calculation of bearing pitch diameter ((inner diameter + outer diameter) / 2)) x rpm. This value is completely mechanical and mostly concerns the integrity of the cage, though as we will discuss, the cage is influenced by everything else. The catalog limiting speeds are based on a DmN of 500,000 for sealed bearings and between 600,000–750,000 for open bearings. For example, the top line in Figure 1: [(25+37)/2] x 18,000 = 558,000 for grease and 682,000 for oil.

The catalog is considered a safe, continuous operating speed. Somewhere between 700,000–1,000,000 DmN is considered moderate speed, which is still probably OK, but we must start dialing in clearances, making sure we don’t have large misalignments, our component tolerances are reasonably good, and we have a good supply of lubrication. When we go over one million, this is considered the high-speed region. Again, an off-the-shelf bearing may be fine in the low one-millions, but now our shaft, housing and absolute position tolerances should be very good (in the IT5/6 region which we will discuss another time) we also need to start thinking about cage strength, cage materials, fillers – glass and carbon fiber, and the dynamic mechanisms that we are going to discuss below.  

Let’s talk about forces acting on the cage. If you are wondering where these cage forces are coming from, you are not alone. If the bearing is perfectly aligned and the balls are turning with the rings, there should not be a lot of external forces, right? In many cases, the forces are negligible and why you find very thin plastic cages in many consumer-type products. When we get into higher speeds (beyond the catalog rating), the small forces can develop into forces large enough to compromise the cage integrity.

Outside of the obvious centrifugal force, there is a push-pull mechanism on a cage that is developed from the internal clearance of the bearing. If the bearing has zero clearance, as with a set of preloaded angular contact bearings, then the ball would be in perfect rolling contact 360 degrees around the raceway and there would be no tendency for the balls to slow down or speed up around the raceway. However, when we are dealing with a single-row deep groove ball bearing, we have an internal radial clearance to deal with. While this clearance is small, at high speeds it becomes a factor.