Relubrication intervals calculated for the duty you actually run, not the chart

Every lubrication route in every plant starts the same way: somebody opens the manufacturer’s chart, reads off a figure for the bearing size and speed, writes it into the maintenance system, and the interval is never revisited. Then the fans near the dryer keep failing at eight months and the conveyor pulleys outlast everyone’s expectations, and nobody connects either outcome to the schedule. A bearing relubrication interval taken straight from a chart is a starting point derived under laboratory conditions, and the correction factors that turn it into a real number for a real machine are neither obscure nor difficult. They are simply skipped, because applying them requires knowing the operating temperature, the load ratio and the shaft orientation, and nobody has written those down either.

What the base interval actually assumes

The published relubrication interval for a given bearing type, bore size and speed is derived from grease life testing under a specific and quite favourable set of conditions: a horizontal shaft, an operating temperature at the bearing outer ring of around seventy degrees, a load well below the bearing’s rating — typically under a tenth of the dynamic capacity — clean surroundings, no vibration beyond the machine’s own, and a good-quality lithium complex grease with a mineral base oil of appropriate viscosity. That combination describes a well-installed electric motor in a clean plant room and very little else. Every departure from it shortens grease life, and the departures multiply rather than add. A fan running hot, vertically mounted, in a dusty area with a belt drive imposing significant radial load is not at seventy percent of the chart figure; it is closer to fifteen. Applying the factors is not pedantry, it is the difference between a schedule that prevents failures and one that merely records visits.

Temperature is the multiplier that breaks every schedule

Grease life halves for roughly every fifteen degrees of operating temperature above the seventy-degree reference, and that relationship is exponential rather than linear, which is why it dominates everything else. A bearing running at one hundred degrees has around a quarter of the grease life the chart suggests. At one hundred and fifteen it has an eighth. The base oil oxidises, the thickener structure breaks down, the oil bleeds out and the residue hardens into a varnish that obstructs the rolling elements rather than lubricating them. What makes this dangerous in practice is that few plants know their actual bearing temperatures. The design figure in the manual is not a measurement, and an infrared reading on the housing exterior can be twenty degrees below the outer ring. Ten minutes with a thermometer at each critical position, taken when the machine is hot and working rather than at start of shift, produces the single most useful input to the whole calculation. Positions running above ninety degrees need either a shortened interval, a synthetic grease with a higher temperature capability, or an engineering fix to the heat source.

Speed factor, bearing type and why rollers need more

The speed factor combines bore diameter and rotational speed into a single number, and it captures the fact that a large slow bearing and a small fast one stress grease very differently. Beyond that, bearing type carries a correction that surprises people used to thinking of all rolling bearings as equivalent. A ball bearing is gentle on grease because the contact is a point that rolls cleanly. A cylindrical roller bearing works the grease harder through line contact and roller end sliding against the ribs, and typically needs relubrication at around a third of the ball bearing interval for the same size and speed. Spherical and taper roller bearings are harder still, with substantial sliding at the roller ends and in the case of spherical rollers a degree of skewing that shears the grease continuously, so intervals can drop to around a fifth. A heavily loaded 22213-K-M-C3 in a crusher feed conveyor is a genuinely demanding lubrication case, and treating it on the same schedule as the motor driving it guarantees one of the two is wrong.

Vertical shafts, vibration and contamination

Three further factors each roughly halve the interval, and they are the ones most often present in the equipment that fails. A vertical shaft cannot retain grease the way a horizontal one does; it drains downward away from the upper bearing and packs into the lower one, so vertical positions need both a shortened interval and, ideally, a grease with better mechanical stability. Vibration from an adjacent process — screens, crushers, hammer mills, compactors — mechanically works the grease and accelerates oil bleed, which is why bearings on vibrating equipment often need continuous or near-continuous lubrication rather than periodic. Contamination, whether dust, process material or water, is the third: any ingress means grease must be purged frequently enough to carry the contaminant out, and in a washdown environment the interval becomes a function of the washdown schedule rather than of running hours. Where two or three of these apply together, the corrected interval can fall so far below the chart figure that a single-point automatic lubricator becomes the only practical answer.

How much grease, and the damage from giving too much

The quantity for a top-up is calculated from the bearing outside diameter and width, and it is far smaller than intuition suggests — usually a few grams for a medium industrial bearing, not the half-cartridge that gets delivered when somebody with a grease gun is told to be thorough. Over-greasing is not a harmless excess. A bearing cavity packed solid has nowhere for the rolling elements to displace grease to, so they churn it continuously, and churning generates heat, which oxidises the grease, which raises the temperature further. Motors are particularly vulnerable because excess grease driven past the inner bearing cap reaches the windings, where it degrades the insulation. The correct fill for the bearing itself is complete coverage of the rolling elements, with the surrounding free space filled to somewhere between a third and half depending on speed, and a high-speed bearing needs less rather than more. Counting strokes on a calibrated grease gun rather than pumping until resistance is felt is the practical control, and it costs nothing to implement.

Purge paths, relief plugs and sealed-for-life positions

Grease added to a housing has to displace the grease already there, and if it has nowhere to go the housing pressurises and forces grease past the seals. Every housing designed for relubrication has a relief path, and in the great majority of plants that relief plug is still in place, painted over, because removing it before greasing and replacing it afterwards is an extra step that gets dropped. The result is a lubrication programme that damages seals at every visit. The correct procedure is to open the relief, grease with the machine running where it is safe to do so, allow the purged grease to discharge, run for a period, and then close the relief. Where a position is genuinely sealed for life, adding grease is not merely useless but harmful, since forcing grease past a contact seal permanently deforms the lip and lets contamination in afterwards. Sealed units belong on a replacement schedule rather than a lubrication schedule, and marking them clearly at the machine prevents a well-meaning operator from undoing that decision.

Grease selection is part of the interval, not separate from it

The interval and the product are a single decision. Base oil viscosity has to suit speed and temperature — too thin and the film fails, too thick and churning losses raise the temperature — and it is chosen from the bearing’s speed factor rather than from a general preference. Thickener type governs mechanical stability, water resistance and temperature capability, and determines compatibility with whatever is already in the housing, which is the point at which many lubrication programmes quietly destroy themselves. Mixing incompatible thickeners can produce a mixture that softens to the consistency of oil and runs out, or hardens to a block, and neither outcome is recoverable without a full strip and clean. Consolidating a plant onto fewer grease grades is almost always worthwhile for this reason alone, provided the consolidation is done against the demanding positions rather than the easy ones. The consequences of getting it wrong are set out in our collection of grease compatibility disaster stories, and the broader shift in lubricant specification across European plants is covered in our piece on sustainable industrial lubrication.

Building a route that survives contact with the plant

A theoretically correct schedule that nobody can execute is worth less than a rough one that gets done. Practical routes share a few features. They group positions by interval into a small number of bands — weekly, monthly, quarterly, annual — rather than carrying forty distinct intervals, because a route with forty intervals will be followed for two months and then abandoned. They label the grease type at the point of application, not only in the system, so the person with the gun cannot use the wrong product. They specify quantity in strokes of a named gun rather than in grams, because nobody weighs grease. And they carry a small number of positions on condition monitoring instead of a fixed interval, typically the expensive and inaccessible ones, where a temperature or vibration trend justifies the extra effort. Building that monitoring into an existing round is straightforward, as described in our note on setting up a vibration monitoring route. Where a position keeps failing despite a correct schedule, the cause is often not lubrication at all, and electrical erosion in inverter-driven motors is the classic imposter, as covered in our piece on shaft currents and fluting.

Recalculating a plant’s intervals properly is a job of a few days, not a few weeks, and it produces a result that usually surprises everyone: a handful of positions need lubricating far more often than they currently are, a larger number need it considerably less, and the total labour barely changes. The positions that shift furthest are the hot ones, the vertical ones and the roller bearings under real load, which are also the positions that generate most of the unplanned stops. The work needs three pieces of information per bearing — measured operating temperature, actual speed, and an honest description of orientation, load and contamination — and none of them requires an instrument more sophisticated than a thermometer. What it does require is accepting that the chart figure was never meant to be the answer, only the place the calculation begins.