Relubrication Intervals: Formulas & Examples
Introduction
Relubrication intervals for rolling bearings are calculated from rotational speed, bore diameter, bearing type, and three correction factors (temperature, load, environment) using the classical empirical formula t_f = k_f x (14·10⁶ / (n x √d) − 4 x d). Practical values range from a few weeks (hot, contaminated, high-speed) to several years (cool, clean, slow-running) depending on operating conditions.
This tutorial walks you step by step through the calculation with real-world examples. We show how to apply correction factors correctly and turn the formula into a practical maintenance schedule.
Key takeaway: The classical engineering approximation for the relubrication interval is t_f = k_f x (14·10⁶ / (n x √d) − 4 x d) x f_T x f_L x f_U in operating hours — with bearing-type factor k_f and correction factors for temperature, load, and environment. It provides indicative values for maintenance planning; the binding specifications are the diagrams published by bearing and grease manufacturers (e.g. SKF, Schaeffler).
Basic Formula for Grease Service Life
The Classical Empirical Formula
For grease-lubricated rolling bearings, an empirical formula has become established that has been used in lubrication engineering literature for decades and aligns well with the relubrication diagrams of bearing manufacturers:
This is a numerical-value equation — the units are fixed:
- n = bearing rotational speed in revolutions per minute (rpm)
- d = bearing bore diameter in millimetres (mm)
- k_f = bearing type factor: 10 for deep groove ball bearings, 5 for cylindrical roller and needle roller bearings, 1 for spherical roller, tapered roller and thrust bearings
- f_T = temperature factor (based on bearing temperature, not ambient temperature)
- f_L = load factor (ratio of equivalent bearing load P to dynamic load rating C)
- f_U = environmental factor (contamination, moisture)
- t_f = relubrication interval in operating hours
Simplified Example Without Correction Factors
Under reference conditions (bearing temperature ≤ 70 °C, light load, clean environment: f_T = f_L = f_U = 1) the formula gives the basic interval. Example deep groove ball bearing with d = 50 mm at n = 3,000 rpm:
Approximately 4,600 operating hours correspond to around half a year in continuous operation — a realistic value that aligns with manufacturer diagrams for this bearing size.
Structure and Validity Limits of the Formula
The first term (14·10⁶ / (n × √d)) reflects that the interval decreases with increasing speed and bearing size (more rolling contacts, more working of the grease). The subtracted term 4 × d accounts for the additional grease stress in larger bearings. The formula applies to standard lithium greases, bearing temperature up to 70 °C, horizontal shaft, and speeds below the limiting speed; for a vertical shaft the interval is halved.
Note: The formula provides indicative values for maintenance planning — as an accessible calculation method. Binding specifications for specific applications are the relubrication diagrams and data of bearing and grease manufacturers (e.g. SKF diagram based on n·d_m, Schaeffler grease service life F10) and the machine manufacturer’s recommendations.
Influencing Factors and Correction Factors
Temperature Factor f_T
Temperature is the most significant factor influencing grease service life. Chemical reactions (oxidation, thickener degradation) roughly double for every 10–15 °C rise in temperature.
| Bearing Temperature | f_T (temperature factor) | Application example |
|---|---|---|
| ≤ 70 °C | 1.0 | Reference range, standard industrial application |
| 85 °C | 0.5 | Warm operating environment, motor bearings |
| 100 °C | 0.25 | High temperature, foundry |
| 115 °C | 0.12 | Upper limit for standard lithium greases |
| > 120 °C | — | High-temperature grease or oil lubrication required |
Rule of thumb: For every 15 °C of bearing temperature above 70 °C, the relubrication interval is halved. Below 70 °C, conservatively use f_T = 1.0. Important: bearing temperature is typically 10–30 °C above ambient temperature due to self-heating.
Load Factor f_L
Bearing load determines friction and thus indirectly the service life. Higher load = more heat generation = shorter service life.
- Light load (P ≤ 0.1 C, where C = dynamic load rating): f_L = 1.0 (reference)
- Medium load (0.1 C < P ≤ 0.15 C): f_L = 0.8
- Heavy load (P > 0.15 C): f_L = 0.5
- Impact load or strong vibration: f_L = 0.3 — additionally check grease suitability
Environmental Factor f_U
Contamination, moisture and salt exposure reduce grease service life through ingress of foreign matter and corrosion:
- Clean operating environment (production area, well-sealed): f_U = 1.0
- Moderate contamination (dusty but protected): f_U = 0.5–0.7
- Extreme contamination (mining, damp outdoor bearings): f_U = 0.2–0.3
Worked Example 1: Rolling Bearing in Electric Motor
Task
An electric motor has SKF 6309 deep groove ball bearings (bore diameter d = 45 mm, dynamic load rating C = 81.9 kN) and runs at n = 1,800 rpm. Operating conditions:
- Average bearing load: 5 kN (approx. 6% of dynamic load rating, light load)
- Ambient temperature: approx. 60 °C (production area, well-ventilated)
- Lubricant: lithium complex grease NLGI 2 (standard)
Step-by-Step Solution
Step 1: Calculate basic interval (deep groove ball bearing → k_f = 10; √45 ≈ 6.71)
t_basic = 10 × (14,000,000 / (1,800 × 6.71) − 4 × 45) = 10 × (1,159 − 180) ≈ 9,790 h
Step 2: Determine correction factors
- f_T: Bearing temperature ≈ 85 °C (60 °C ambient + approx. 25 °C self-heating) → f_T = 0.5
- f_L (light load, P/C ≈ 6%): f_L = 1.0
- f_U (clean operating environment): f_U = 1.0
Step 3: Calculate relubrication interval
t_f = 9,790 × 0.5 × 1.0 × 1.0 ≈ 4,900 operating hours
Step 4: Convert to calendar time
- Motor runs approx. 8 h/day, 5 days/week = 40 h/week ≈ 2,080 h/year
- t_f = 4,900 h / 2,080 h per year ≈ 2.4 years calendar time
Practical Recommendation
Theoretically, relubrication would not be required for just over 2 years. In practice, however: grease ages even at standstill (oxidation, oil separation). Recommendation: relubricate with NLGI-2 lithium grease at least every 2 years as part of the annual/major maintenance — and visually inspect the grease at every inspection: dark or contaminated → replace early.
Example: Linear Guide
For linear guides, the relubrication interval depends on the travel distance rather than rotational speed. Typical values for profiled rail guides:
- Light load, clean environment: every 50–100 km travel distance
- Normal industrial conditions: every 20–50 km
- Heavy load or contamination: every 5–20 km
- CNC machining centres (coolant): every 1–5 km or use automatic lubrication
If you are using TEA roller guides, see our maintenance guide for LinRol and LinTrek systems. Product details are available on the LinRol/LinTrek product page.
Grease Change vs. Relubrication
There is an important distinction between relubrication (adding fresh grease) and grease change (completely replacing the old grease):
- Relubrication: Adding a defined quantity of fresh grease without removing old grease. Used during normal maintenance intervals.
- Grease change: Complete removal of old grease and refilling with fresh lubricant. Required when the grease is degraded, contaminated, or a different grease type is being used.
Warning: Never mix incompatible greases!
Mixing lithium and calcium soap greases, for example, can cause sudden lubricant breakdown. When changing grease types, always perform a complete grease change.
Practical Tip from TEA:
From procurement practice, the biggest hidden cost trap in relubrication is not the price of the grease but compatibility during top-up: if you mix greases with different thickeners (for example lithium with polyurea or calcium sulfonate), the resulting blend can saponify, bleed, or harden — and the interval you painstakingly calculated then counts for nothing. So define one standard grease per lubrication point (for most rolling bearings a lithium-complex grease of NLGI 2 is sufficient), label it unambiguously, and list it as a fixed item in the spare-parts/maintenance package — this prevents unintended mixing across multiple maintenance cycles and supply batches. When switching to a different grease, wash out the old lubricant completely beforehand rather than simply topping up.
Automatic Relubrication Systems
For hard-to-access lubrication points, high maintenance frequency, or critical applications, automatic relubrication systems (lubricators) offer significant advantages:
- Single-point lubricators: For individual lubrication points; driven by electromechanical mechanism or gas pressure
- Multi-point systems: Supply multiple lubrication points via distribution blocks simultaneously
- Progressive systems: Distribute lubricant successively to many points; failure detection built in
TEA Recommendations and Practical Checklist
Relubrication Interval Calculation: Checklist
- Collect bearing data: bore diameter (d), rotational speed (n), dynamic load rating (C), manufacturer specifications
- Calculate dn value: dn = d [mm] × n [rpm]
- Record operating conditions: temperature, load (in % relative to C), environment (clean/dusty/damp)
- Determine correction factors: f_T, f_L, f_U from tables or empirical values
- Calculate relubrication interval: t_f = k_f × (14·10&sup6; / (n × √d) − 4 × d) × f_T × f_L × f_U [operating hours]
- Convert to operating hours: t_f,eff = t_f × average daily operating hours
- Set calendar interval: calculated hours ÷ average operating hours per month
- Document maintenance plan: record interval with date and maintenance person