You're likely greasing your bearings on a schedule. The maintenance log is clean, but yet the bearings keep failing.
The culprit is probably sitting in your grease gun.
Over-lubrication causes as many premature bearing failures as neglect, and it's far more common in plants that have a maintenance program than ones that don't. When there's a schedule, techs follow it, and when that schedule says "grease every two weeks," most people don't question it.
What Over-Greasing Actually Does Inside a Bearing
A bearing housing shouldn't be considered an endless reservoir as there's a designed void space, typically 30–50% of the cavity, that lets grease distribute across the rolling elements and form a thin protective film.
Push more grease past that point and it has nowhere to go. Pressure builds. The excess churns against the rolling elements, generating heat instead of reducing it. We've seen housing temps spike 20–30°F from a single over-greasing event.
That heat attacks the base oil which accelerates oxidation, breaks down the thickener, and degrades the grease faster than normal operating conditions ever would. The bearing isn't running hot because it's failing. It's failing because it's running hot.
And this one doesn't get enough attention: bearing seals aren't designed to handle internal pressure. Excess grease blows past them, strips them from their groove, or warps them permanently. Once a seal is compromised, contamination gets in. Contamination is the leading cause of bearing failure in food processing and general fabrication environments.
How to Calculate Your Bearing Lubrication Interval
Most plants inherit a relubrication schedule from whoever set up the PM system years ago. That schedule usually came from a manufacturer's general guidelines or, let's be honest, "that's how we've always done it."
The standard formula for relubrication interval (in operating hours) is:
K × (D^0.5 / N) × F
- K = 500 for ball bearings, or 1,000 for roller bearings
- D = bearing bore diameter in mm
- N = shaft speed in RPM
- F = correction factor for temperature, contamination, shaft orientation
For a Dodge SCED-207 pillow block with a 35mm bore, running at 900 RPM in a clean, moderate-temperature environment, the calculated interval is actually 500–600 operating hours, not the 336 hours a bi-weekly schedule delivers, which means you're servicing it nearly twice as often as necessary.
Most plants are over-maintaining their bearings by 30–50%. That's not just wasted grease. It's seal damage, accelerated wear, and early replacements that didn't need to happen.
One more rule worth knowing: on any new bearing installation, plan your first preventive maintenance inspection within the first 50–100 hours of operation. After that, intervals should be calculation-driven, not calendar-driven.
Three Signs You're Over-Lubricating Right Now
You don't need a thermal camera to suspect a problem. Check for:
- Purge grease visible at the seal or shield face. That's pressure finding the path of least resistance.
- Elevated bearing housing temperature without any load change. If the application hasn't changed and the bearing is running hotter than usual, grease churning is a likely cause.
- Dark or degraded grease at every PM interval. Fresh grease should purge close to what went in. If it's coming out black or brown before the next service, you're cooking it.
If your techs consistently feel resistance when pumping in new grease, the housing is already full. That resistance is the bearing housing giving you feedback and pushing through it is how you blow a seal.
Getting Grease Volume Right
Volume matters as much as interval. The standard approximation for grease volume per service:
G = 0.114 × D × B
Where G is in ounces, D is bearing OD in inches, and B is bearing width in inches.
For most standard pillow blocks, that's 0.5–1.5 oz per service. A standard lever-style grease gun can deliver 1–2 oz per stroke depending on the gun and the back-pressure. One full stroke on a small bearing may already be too much.
Count your strokes. Know how much your gun delivers per stroke. Most techs don't actively look into things such as this, but it's often worth finding out.
An additional item to note: refrain from mixing grease types that aren't compatible with one another, just because the colors look similar (believe it or not, this happens). Lithium-complex and polyurea greases are a common problem pair. When they mix, the thickeners can react and liquefy the lubricant entirely, stripping protection from the rolling elements. Check a grease compatibility chart before switching brands on any bearing already in service. If you need to switch, ensure you're purging completely first.
Frequently Asked Questions
What is the correct bearing lubrication interval for industrial bearings?
It depends on bearing type, bore diameter, shaft speed, and operating conditions. The formula K × (D^0.5 / N) × F gives you a calculated baseline. K = 500 for ball bearings, or 1,000 for roller. Most standard industrial applications land in the 400–800 operating hour range, not the bi-weekly schedules many plants run.
How do you know if a bearing is over-lubricated?
One of the clearest signs are grease visible at the seal face, elevated housing temperature without changes to the application itself, and seeing degraded grease at every service interval. If you feel resistance pumping in new grease, the housing is already at or beyond capacity.
Does over-greasing cause bearing failure?
Yes. It generates heat through churning, damages seals by pressurizing the housing interior, and accelerates grease degradation. Over-lubrication often contributes to a fair amount of premature bearing failures in plants running scheduled PM programs, particularly those that grease on a fixed calendar rather than looking at actual run hours.
How much grease should you add to a bearing per service?
Use G = 0.114 × D × B as a starting point, where D is bearing OD in inches and B is width in inches. For most standard pillow blocks, that's 0.5–1.5 oz per service event, often just one careful stroke from a lever-style grease gun.
Can you mix different bearing greases?
Not without checking compatibility first. Lithium-complex and polyurea are a well-known incompatible pair. Mixing them can cause the lubricant to liquefy and lose its protective film entirely. If you're switching grease types, purge the old grease fully before introducing the new product.
If you're dealing with repeat bearing failures despite regular greasing, the greasing itself may be part of the problem (which we have a great line of offerings for). Our team has worked with manufacturers across West Michigan for 25 years, and we're happy to talk through your PM intervals and whether your relubrication schedule makes sense for your equipment. Reach out here, if that's the case, we would love to help.
Written by the MRO-PT Team, 25 years of mechanical power transmission and MRO experience serving manufacturers.