Setting Lubrication Intervals Correctly: A Practical Guide
Lubrication intervals are determined systematically based on the dn value, operating temperature, load, and ambient contamination. Correctly dimensioned intervals are the foundation of preventive maintenance: they prevent wear, extend the service life of bearings, gearboxes, and guides, and reduce long-term costs significantly. The most common mistakes arise from intervals that are too long or too short — under-lubrication causes wear and bearing damage, over-lubrication causes overheating and accelerated wear.
This practical guide shows how to set lubrication intervals systematically, based on DIN 51519 (viscosity classification), manufacturer recommendations (e.g. SKF, Schaeffler), and proven industrial practice. After working through it, you will be able to create a reliable, machine-specific maintenance plan quickly.
Key takeaway: Lubrication intervals are determined by speed, load, temperature, and environment. Rolling bearings with dn ≤ 100,000 receive NLGI 2 grease every 6–12 months, gearbox circulating oil is changed every 10,000–20,000 operating hours, and linear guides are relubricated every 500–1,000 operating hours. Documenting everything in a maintenance plan is essential.
5 Functions of Lubricants
- Friction reduction: Separates metallic surfaces and reduces wear
- Heat dissipation: Carries frictional heat away from the contact zone
- Corrosion protection: Forms protective film against moisture and aggressive media
- Contamination sealing: Prevents particles from penetrating the contact zone
- Power transmission: In hydraulic systems and fluid couplings
Cost of lubrication failure
Studies show that 40–50% of all rolling bearing failures are attributable to lubrication problems. An unplanned machine shutdown caused by a bearing failure costs typically 5–10 times more than preventive maintenance including lubricant costs. Lubrication and bearing-replacement costs are key inputs to a full total cost of ownership analysis of the drive train.
Key Influencing Factors
The lubrication interval depends on the following factors:
- Speed (dn value): Higher speed → shorter interval
- Temperature: +15°C doubles aging speed → halves interval
- Load: Heavier load → shorter interval
- Contamination: More contamination → shorter interval
- Lubricant quality: Synthetic lubricants → longer interval
- Component design: Sealed vs. unsealed bearings
dn Value and Lubrication Intervals for Rolling Bearings
| dn Value [mm × rpm] | Typical Interval | Notes |
|---|---|---|
| < 50,000 | Every 2,000–5,000 h | Low-speed drives |
| 50,000–150,000 | Every 1,000–2,000 h | Standard industrial bearings |
| 150,000–300,000 | Every 500–1,000 h | Higher speeds |
| > 300,000 | Oil lubrication required | High-speed bearings |
Rolling Bearings: Practical Approach
For a practical example: a motor with a 40 mm bearing bore running at 1,450 rpm in an ambient temperature of 40°C:
- dn = 40 × 1,450 = 58,000 → standard interval range
- Bearing temperature ≈ 60°C → correction factor 0.5
- Normal load → no additional reduction
- Clean industrial environment → minor reduction
- Result: relubricate every 700–900 operating hours
For the full formula-based calculation using the SKF method: Calculate relubrication intervals.
Gearboxes: Oil Change Intervals
Industrial gearboxes follow a typical maintenance pattern:
- New gearbox (break-in period): First oil change after 200–500 operating hours – metallic abrasion particles from the break-in period are removed
- Mineral oil: Oil change every 2,000–5,000 operating hours or annually
- Synthetic oil (PAO/ester): Oil change every 10,000–20,000 operating hours or every 3–5 years
Oil analysis (TAN value, viscosity change, wear metals) provides reliable information about the actual oil condition and can extend or shorten intervals based on actual condition rather than fixed schedules.
Linear Guides: Travel-Distance-Based Intervals
Roller guide systems such as LinRol/LinTrek and other linear guides with balls or rollers require regular relubrication. Typical intervals are based on operating hours rather than calendar time:
- High-load, fast traverse axes: Relubricate every 500 operating hours (e.g. machine tools, gantry robots at 80% utilisation)
- Standard linear guides (medium load, normal): Every 1,000 operating hours
- Low-load positioning axes: Every 2,000 operating hours possible (e.g. laboratory tables, printing system travel)
- Ball screws (lifting spindles): Analogously 500–1,000 h depending on load and speed
Grease quantity per lubrication event: follow manufacturer specifications (typically 0.5–2.0 ml per lubrication nipple). Over-greasing forces old grease out of the guide and attracts contamination.
Further guidance on lubricants, lubrication points, and maintenance cycles is provided in the detailed lubrication guide for roller guides.
Automatic Lubrication Systems
Three common system types:
- Single-point lubricators: Battery-powered, mounted directly on the lubrication nipple. Set time or impulse-controlled.
- Multi-point systems: One pump supplies multiple lubrication points simultaneously via distribution blocks.
- Progressive distributors: Distribute lubricant successively; failure detection through monitoring of the last lubrication point.
Maintenance Plan Structure
A proper lubrication maintenance plan contains for each lubrication point:
- Lubrication point designation and location
- Required lubricant (designation, NLGI class / ISO VG)
- Lubrication interval (hours or km)
- Lubricant quantity per event (ml or g)
- Access (lubrication nipple type, location)
- Record of lubrication events (date, person, observations)
Checklist and Common Mistakes
7 Most Common Lubrication Mistakes
- Using the wrong lubricant type
- Applying too much grease (over-greasing)
- Not cleaning lubrication nipples before greasing (contamination introduction)
- Mixing incompatible greases
- Ignoring actual operating conditions (especially temperature)
- No lubrication documentation
- Not adjusting intervals to actual operating conditions
From design to enquiry: procurement notes
- Cost driver: Unplanned bearing failures cost many times the price of the lubricant. A single bearing replacement including downtime typically exceeds an entire year of lubrication costs.
- Standard vs. special-purpose lubricants: For standard applications (dn value below 100,000, ambient temperature), NLGI 2 lithium complex grease from commercial suppliers is sufficient. High-temperature, cleanroom, or food-contact applications require specified special lubricants that should be requested directly from the manufacturer.
- What a request should include: NLGI class, thickener type (e.g. lithium, polyurea), viscosity class (ISO VG), operating temperature range, and desired package size (cartridge 400 g, pail 5 kg). For gear oils: state ISO VG class, mineral or synthetic (PAO/ester), and any existing manufacturer approvals.
- TCO aspect of central lubrication systems: Automatic lubrication systems significantly reduce manual maintenance effort on machines with many lubrication points. The investment typically pays back through reduced downtime and consistently maintained intervals.
- Lubricant compatibility when changing suppliers: Not all greases are miscible. When switching suppliers, check the thickener types, and if in doubt, clean the bearings before refilling to avoid incompatibilities.
Frequently Asked Questions about Lubrication Intervals
The most common causes are: 1) Insufficient lubrication quantity (too little grease/oil), 2) Incorrect lubricant type (wrong viscosity or NLGI class), 3) Lubrication intervals too long (lubricant aged or contaminated), 4) Incompatible lubricants mixed, 5) Lubricant contaminated by water or particles. Most failures can be prevented by following the manufacturer's lubrication specifications.
The common rule of thumb: fill the bearing with 30–50% of the available space in the bearing housing with grease. Too much grease leads to churning losses, heat generation, and grease ejection. Too little grease leads to insufficient lubrication. For precision bearings, follow the manufacturer's specifications – often just 30% fill level.
The dn value is the product of bearing bore diameter d [mm] and rotational speed n [rpm]. It is a measure of the peripheral speed of the rolling elements and determines the required lubricant viscosity. For high dn values (> 300,000 mm×rpm), lower-viscosity lubricants or oil lubrication is required. Low dn values tolerate higher viscosities.
As a general guideline: first oil change after 200–500 operating hours for new gearboxes (break-in period); thereafter every 2,000–5,000 operating hours or annually, whichever comes first. Synthetic gear oils allow significantly longer intervals (up to 20,000 operating hours). High thermal or mechanical loads shorten the intervals. Regular oil analysis provides definitive information about actual oil condition.
For hard-to-access lubrication points, high relubrication frequencies (more than weekly), or critical applications where lubrication failure causes costly downtime, automatic systems are highly cost-effective. The investment typically pays off within 1–2 years through reduced maintenance time and fewer lubrication-related failures.
About the Author
Thomas Albrecht
Head of Procurement · Procurement
Expert in lubrication technology and maintenance planning for industrial mechanical engineering.