“This bearing’s running a little hot.” We’ve all heard that, and we’ve probably all had the same response: “OK, but you’re going to need to give me a little more than that. Which bearing, and how hot?”
There’s no point in checking temperatures if we don’t have a good idea of the normal operating range, the range where we should get concerned, the range where we’ve got a problem on our hands and the range where it’s already out of our hands and we’re in damage control mode.
The normal operating range for bearings is <180 F, with 140-160F being the sweet spot for a number of bearings and applications. Even with that piece of information at our disposal, we need to know what we’re talking about what we take that measurement.
Depending on the configuration and placement of the bearing, the temperatures we check may be the housing or the outer ring. As much as possible, we want to know the temperatures at the outer ring. The outer ring is part of the bearing itself, so its temperature will reflect the overall health of the bearing better than the housing. To a certain extent, the outer bearing temperature is a lagging indicator of what’s happening deeper inside the bearing, where the bearing is more likely to fail if temperatures go high out of sight. The housing, therefore, even more so.
When we see a bearing temperature go above the normal operating range, one of the first things we should look at is the lubrication. The viscosity of the lubricant and the temperature of the bearing interact. If a low-temperature grease has been used, it may not have the necessary viscosity to reduce friction. Friction goes up, temperature goes up. If the bearing has been over-greased, the grease pressure may be too high, causing temperature to go up. Conversely, under-greasing will increase friction, also causing temperatures to go up. Many of the same considerations apply to the oil. Is the temperature of the bearing driving a change in oil viscosity? Or is the oil sump not cooling the returning oil sufficiently, sending the oil back at a higher temperature and lower viscosity than the bearing needs to stay healthy?
While you’re trying to figure all this out, the temperature of the bearing may still be rising, possibly crossing from the concern zone to the problem zone. The team may need to bring the temperature down while troubleshooting.
Safely cooling the bearing is another reason everyone needs to know what the temperatures are actually saying. Applying cool water or some kind of cooling pack to the housing can bring down the temperature on the gauge, but it won’t do much for the internal temperatures of the bearing. In fact, it could make them worse: by cooling the housing while the internals heat up, the outer components contract while the inner components expand and, well, you can see what that goes.
Applying water directly to the bearing is another tempting but dangerous option. If the internal bearing temperatures are near the danger zone, the shock of cold water could cause immediate and catastrophic failure. The “best” case scenario is that the water interferes with the lubrication, causing a rapid increase in friction and likely failure.
The safest approach, then, is to use cooling fans. They allow for an indirect, gradual reduction in temperature. Remember, this about buying time and keeping things safe. A fan is not a long-term solution.
Whatever the normal operating range of your bearings, a swing of 50F is a reason to take action. Smaller but frequent changes in temperature, or an unusual temperature reading in conjunction with any other abnormal indications (noise, smell, temperature or other log readings elsewhere in the system, reductions in system output) are an early warning sign that something is going on with the bearing. Or, the bearing may be a lagging indicator of a bigger problem that’s been just below your attention until now.
Bearing health is a critical aspect of mechanical safety and efficiency. The response to “This bearing’s running a little hot” could be the difference between a short interruption and extended downtime.