When a machine is failing it typicaly begins with an ever-so-subtle whine. Then, escalating to a rattle of a complaint requiring action. Typically, it’s not the shaft or the motor. It’s the bearing.
Bearings can be thought off as just metal rings. They’re not. They’re precision instruments. They determines how reliable and long-lasting your equipment will be. Ignoring specs doesn’t save you money. It postpones a breakdown that is bound to happen.
How to Choose the Right Bearing
To break that down some more, here’s a chart with dimensions for deep groove ball bearings (above). As you can see, it begins with anatomy. An inner race spin while the outer race remains stationary. A cage holds a bunch of steel balls between these two. The geometry is precise. How does the load distribute on the balls? It are determined by the raceway radius and the contact angle. Too large an angle and the load concentrate on one side or the other. This leads to premature fatigue.
Typically the rings is produced from AISI 52100 steel. They are hard enough to stand up to deformation but tough enough to absorbs shock. Most folks think all ball bearings is created equal. They’re not. Ball bearing difference comes down to how perfect the rounds of the balls are and how tight they’re machined into the rings.
So what does this mean? This comes down to something called the ABEC rating system. And it’s likely one of the most misunderstood specs out there. Chances are if you look at a set of bearings they’ll say ABEC 1, 3, 5, 7 or 9. People often assume higher number is better.
Nope. Here’s the deal: ABEC 1 is meant for general purpose uses such as washing machine drum. This means that the rings aren’t necessarily perfectly aligned with each other and allow for a little bit more runout. It’s less expensive to make and has more forgiveness for alignment issues. On the other hand, ABEC 7 or 9 is considered high precision applications. These have microscopic tolerances. Put an ABEC 9 bearing into a dirty environment and it will fail sooner then an ABEC 1 bearing. No forgiveness for mistakes whatsoever. So match precision to application, that’s the trick.
Another thing where your gut isn’t a good guide is load capacity. Each series has both a dynamic load rating and a static load rating. Dynamic load is what you can put on it when it’s turning. Static load is when the bearing is not spinning but is still loaded. From there you have to calculate your expected lifespan based off those numbers. Go past the dynamic load and you’ll get cracks underneath faster. Those cracks result in spalling, that means little pits in the raceway. They don’t go away once started and those bearings are ruined.
The last part of the puzzle is lubrication. One thing you’ll notice is this is overlooked…until it’s too late. According to the guide, fill the bearing housing with grease, leaving 30-50% of capacity empty. It seems strange to leave an empty space but think about what happens when you overpack something. Heat occurs. Then the heat kills the viscosity of the lubricant.
Think about your machine’s environment. Is it hot? Standard mineral greases can break down if you run hot. Synthetics are better for handling the heat, but more expensive. Lastly the clearance class comes into play here. Running a high speed motor in a hot environment and using a standard clearance will cause the bearing to seize because the metal expand to take up the extra space inside the bearing. C3 clearance means extra room.
A lot of times people pick the largest bearing they can fit. Choosing the correct bearing is not about selecting the largest one. It is about picking the right balance between speed, load, temperature, and precision. Measure the shaft, determine the load, look on the chart where those intersect and there you go.
The correct bearing dissapears into the back of your mind. You don’t even think about it anymore. If a piece of machinery is running silently, then somebody paid attention to detail.
