Calculating Relubrication Intervals: Formulas and Practical Examples
Insufficient or incorrect lubrication is one of the most common causes of premature bearing and guide failure. Yet relubrication intervals are often set by gut feeling rather than calculation. This guide shows how to calculate relubrication intervals systematically and adapt them to your specific operating conditions.
Basic Formula
The basic formula for calculating the relubrication interval of rolling bearings (a simplified engineering approximation based on manufacturer recommendations, e.g., SKF, Schaeffler) is:
tf = 1000 / (d × n) × fT × fL × fU [hours]
Where:
- tf = Relubrication interval [hours]
- d = Bearing bore diameter [mm]
- n = Rotational speed [rpm]
- fT = Temperature correction factor
- fL = Load correction factor
- fU = Environmental correction factor
Influencing Factors
Temperature Correction Factor fT
| Bearing Temperature | Factor fT |
|---|---|
| ≤ 55°C | 1.0 |
| 55–70°C | 0.5 |
| 70–85°C | 0.25 |
| 85–100°C | 0.125 |
| > 100°C | Special grease required |
Load factor fL: For normal loads (P/C ≤ 0.1): fL = 1.0. For heavy loads (P/C = 0.1–0.2): fL = 0.5. For very heavy loads: fL ≤ 0.25.
Environmental factor fU: Clean environment: fU = 1.0. Normal industrial environment: fU = 0.5–0.7. Heavy contamination, water ingress: fU = 0.2–0.5.
Worked Example: Rolling Bearing
Given: Deep groove ball bearing 6309
- Bore diameter d = 45 mm
- Speed n = 1,800 rpm
- Bearing temperature 65°C → fT = 0.5
- Normal load (P/C = 0.08) → fL = 1.0
- Clean environment → fU = 1.0
Calculation:
tf = 1000 / (45 × 1800) × 0.5 × 1.0 × 1.0 = 0.00617 hours × correction = ~6.2 hours
Practical recommendation: relubricate every 500–700 operating hours (based on manufacturer-specific tables for this bearing type).
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 centers (coolant): every 1–5 km or use automatic lubrication
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 lead to sudden lubricant failure. When changing grease types, always perform a complete grease change.
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
8-Step Checklist for Relubrication Intervals
- Determine bearing bore diameter (d) and rotational speed (n)
- Measure actual bearing temperature (not estimated!)
- Calculate load ratio P/C from application data
- Assess environmental conditions (cleanliness, moisture)
- Apply correction factors and calculate basic interval
- Reduce calculated interval by 20–30% as safety margin
- Record lubrication events in maintenance log
- Verify actual bearing condition at first maintenance – adjust interval if needed
Frequently Asked Questions about Relubrication Intervals
The relubrication interval specifies how often a bearing or guide must be re-greased to ensure reliable lubrication. It is calculated from the operating conditions (speed, temperature, load) and is stated in operating hours. In practice, a safety margin of 20–30% is applied to the calculated value.
If lubrication is insufficient, the lubricant film breaks down. Metal-to-metal contact between rolling elements and raceways leads to increased friction, temperature rise, wear, and ultimately bearing failure. In the worst case, the bearing seizes completely – causing costly downtime.
Each 15°C increase in bearing temperature approximately halves the relubrication interval. At 70°C operating temperature, the interval is about 50% shorter than at 55°C. This is why precise knowledge of the actual bearing temperature – measured with a thermometer, not just estimated – is important for reliable calculations.
Grease is by far the most common choice for rolling bearings – it is easy to apply, stays in place, and protects against contamination. Oil is preferred for very high speeds (dn value > 300,000), large amounts of heat dissipation, or continuous lubrication systems. For most industrial applications, grease is the right choice.
Yes, high-quality synthetic greases with longer service life (indicated by longer calculated intervals) can extend relubrication intervals compared to mineral-based greases. However, always use the grease specified by the manufacturer or calculate the interval specifically for the selected grease using the manufacturer's data.
Über den Autor
Thomas Albrecht
Head of Procurement · Procurement
Expert in industrial lubrication technology and maintenance planning for mechanical engineering components.