Bearing failure is a leading cause of spindle breakdowns. The truth is, it’s easy to take bearings for granted. Out of sight, out of mind. But bearing systems are at the heart of every spindle and can’t be forgotten. They’re highly engineered and manufactured to hyper-precise tolerances. As such, they require strict maintenance protocols and ignoring these protocols can rapidly lead to otherwise avoidable bearing failure and costly spindle repairs.
In the best-case scenario, bearing failure puts your spindle out of action until it can be swapped out, leading to unproductive downtime and high repair/replacement costs. That’s the best case: in the worst case, failed bearings can do tremendous damage on the way out, potentially ruining the spindle. Repair costs are now in the several thousand-dollar range, deadlines are missed, and everyone gets frustrated – customers, your employees, and management.
Fortunately, the bearings in modern spindles fail in four distinct, identifiable stages. This is good news because — with the right attitude about maintenance and with a good predictive monitoring system in place — you can closely track your bearings’ health and schedule repair or replacement prior to failure.
Stage 1: Normal Operations
Because of the immense physical stresses they’re subjected to, bearings’ first stage of failure typically falls well within normal operations parameters. At this stage, vibrations at ultrasonic frequencies — from 1,200K to 3,600K cycles per minute (CPM), or 20,000 to 60,000 Hertz (Hz) — indicate that lubrication in the bearings is beginning to thin and bearing-to-raceway contact has begun.
Check lubrication levels, increasing or amending as necessary, and continue production.
Stage 2: Micro-Pitting
Left unmitigated, decreased bearing lubrication leads to increased friction, which contributes to the development of micro-pitting on bearings as well as raceways, which in turn further increases friction. Bearings exhibiting advanced micro-pitting will generate resonance detectable in the range of 120k to 480k CPM (2,000 to 8,000 Hz).
Though plant owners and managers are often hesitant to do so, it is at this point that bearings in highly critical machinery should be replaced, ideally at the next reasonable opportunity.
Stage 3: Increased Wear
Bearings at stage 3 of failure display plainly visible imperfections if disassembled and observed. In terms of vibration, harmonic frequencies appear, as well as sidebands, which are additional modulator frequencies that bracket the primary frequency.
At this point, both critical and non-critical equipment should be shut down for immediate bearing replacement.
Stage 4: End of Life
Evident in the reduction and ultimate loss of standard vibration frequencies, plus the rise of random vibration spikes (particularly in the lower frequencies), stage 4 bearing failure indicates that your bearings have entered the end of their usable life.
At this point, failure is imminent. Ideally, no equipment should ever be pushed to this point.
Vibration Monitoring and Predictive Maintenance
As we’ve alluded to, deciding when to repair or replace bearings in equipment can be a bit of a balancing act.
In reality, it is common to find plant owners and managers who push their equipment right up to the breaking point, thinking (incorrectly) that this is the best way to get the most out of their spindle — their machinery goes down less often, but the repair costs and downtime costs can be immense.
In a perfect world, every plant manager replaces bearings at the first reasonable opportunity after bearings enter stage 2 — this requires equipment to be brought down more often (relatively speaking), but for shorter durations and at dramatically lower costs. A realistic solution is, perhaps, somewhere in the middle.
Whatever your philosophy, it is important to know the status of your bearings’ health at all times. The only way to do this effectively is with a well-designed network of vibration sensors connected to high-quality vibration monitoring and analysis software, such as our VibePro spindle monitoring system. With this type of predictive maintenance program in place, you can easily find a safe, cost-effective balance between production and maintenance.