Lubrication and Failures
Part of Gear Making
Recognizing lubrication failures in gear systems and establishing maintenance routines to prevent them.
Why This Matters
Lubrication failure is the single most common cause of premature gear and bearing failure in early machinery. A gear system that is well-designed, well-made, and properly aligned will fail quickly if its lubricant is allowed to run out, degrade, or become contaminated. Conversely, a gear system that is only moderately well-made will give decades of service if consistently well-lubricated.
The challenge is that lubrication failures are often invisible until significant damage has occurred. A gear running dry sounds different — harsher, hotter, with a metallic ringing — but this change may be missed if the machinery is unattended or in a noisy environment. By the time a lubrication failure is obvious, the gear surface is already scored, the bearing is worn, and the repair is significant.
Systematic lubrication maintenance — scheduled oil replenishment, condition inspection, and early recognition of warning signs — is one of the highest-return maintenance practices in any mechanical system. This article covers both how lubrication fails and how to prevent and respond to those failures.
How Lubrication Fails
Starvation: The lubricant supply runs out. In an oil cup or grease-packed bearing, the lubricant is gradually consumed (lost to evaporation, centrifugal throwing, leakage, or absorption into porous materials). When the supply runs out, the bearing or gear runs dry.
Prevention: scheduled refilling on a fixed timetable, not waiting for visible symptoms. In high-duty applications, check daily. In low-duty applications, check weekly or monthly.
Thermal degradation: Heat from operation breaks down the lubricant chemically. Animal fats oxidize and polymerize; mineral oils oxidize more slowly. Degraded lubricant becomes either too thick (carbonized or polymerized), losing its flow properties, or too thin (cracked and volatilized), losing its film-forming ability. In extreme cases, the lubricant carbonizes to a hard varnish that blocks lubrication passages.
Signs: dark color, thick consistency, strong burnt odor, gritty texture. Prevention: maintain adequate lubricant quantity (thin films overheat faster), reduce load if operating temperatures are excessive, change lubricant on a schedule rather than waiting for degradation signs.
Contamination: Water, abrasive grit, or chemical contaminants enter the lubricant. Water emulsifies in oil and greatly reduces its film strength; it also promotes corrosion. Grit circulates between gear flanks, rapidly abrading both surfaces.
Signs: milky appearance (water contamination), gritty feel between fingers (abrasive contamination), unusual discoloration. Prevention: seal gear housings from water ingress, keep the environment around machinery as clean as possible, change lubricant after any known contamination event.
Incorrect viscosity: The lubricant is too thin or too thick for the application. Too thin: film breaks down under load, direct metal contact occurs. Too thick: lubricant cannot flow into the contact zone quickly enough at low temperatures, or generates excessive churning drag at high speeds.
Signs: rapid wear despite lubrication (too thin), excessive heat generation and sluggish operation (too thick). Prevention: match lubricant selection to operating conditions.
Channeling (grease lubrication): When a grease-lubricated bearing first starts, the grease in the path of rolling elements or sliding surfaces is displaced to the sides. If the grease is too stiff or cold, it does not flow back to re-lubricate the contact. The bearing runs dry in the contact zone while surrounded by grease.
Prevention: use the correct grease consistency for the operating temperature. Rework grease into contact zone during initial startup of newly lubricated equipment (run machine at low load for first few minutes to warm and work the grease).
Recognizing Lubrication Failure in Progress
Temperature: A bearing or gear contact running without adequate lubrication generates significantly more heat than a well-lubricated one. Learn to check operating temperature by touch (a well-run bearing should be warm, not hot) or by holding a drop of water to the housing — a hiss indicates overheating.
Sound: A dry bearing sounds harsh, scratchy, or with a rhythmic metallic knock. Normal running is smooth and relatively quiet. Any sudden change in sound warrants investigation.
Vibration: As wear progresses in a dry-running bearing, increased clearance allows shaft movement. The shaft wobbles, causing gear misalignment, which causes abnormal contact and further noise and vibration. Vibration that was not present before is always a warning.
Lubricant appearance on inspection: When checking lubricant level or condition, observe its appearance. Black, gritty, or watery lubricant needs immediate replacement.
Increased power consumption: More power is needed to drive machinery with inadequate lubrication, due to increased friction. If the water source or hand power source seems to be working harder than usual for the same machine operation, check lubrication.
Emergency Response to Lubrication Failure
If a bearing or gear is found to be running dry:
- Stop the machine or reduce load immediately.
- Allow to cool before adding fresh lubricant — adding cold oil to a severely overheated bearing can cause thermal shock cracking in cast iron housings or hardened components.
- Inspect the surfaces once cool. Look for scoring (parallel scratches along the contact direction), discoloration, and any deposits.
- Clean out degraded lubricant thoroughly before refilling. Old degraded or contaminated lubricant mixed with fresh lubricant reduces the effectiveness of the fresh.
- Inspect for dimensional damage — measure the shaft and bearing bore. Wear during dry running may have increased clearance significantly, requiring replacement.
- Refill with fresh lubricant of the correct type.
- Run under reduced load for at least one hour after restart, monitoring temperature and sound.
Establishing a Lubrication Schedule
The most important single action for machinery longevity: establish a written lubrication schedule and follow it without exception. The schedule should specify:
- Which lubrication points exist (by location and name)
- What lubricant each takes
- How often each point is checked and replenished
- Who is responsible
Post the schedule at the machine or at the maintenance storage location. Review it after any repair — a repaired component may have different lubrication requirements than the original, or the repair may have disturbed previously adequate lubrication.
An ounce of lubricant applied on schedule is worth a pound of gear metal replaced after a dry-running failure.