Electric Motors Lubrication guide; Lubrication of electric motors

Electric Motor Bearing Lubrication: Complete Guide for Engineers

Introduction

Electric motor bearings are critical components that enable smooth rotation of the shaft. Proper lubrication ensures friction reduction, corrosion protection, and extended motor life. Lack of lubrication or incorrect greasing can result in unnecessary downtime, overheating, and premature motor failure.

This guide explains:

• Why bearing lubrication is essential

• Types of greases used in motors

• Key selection criteria for motor greases

• Correct regreasing intervals and quantities

• Industry best practices to maximize motor reliability

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Why Electric Motor Bearings Need Lubrication

The main functions of bearing grease in an electric motor are to:

• Reduce friction and wear between moving parts

• Protect against corrosion in harsh environments

• Act as a seal against dust, dirt, and contaminants

Without proper lubrication, bearings overheat, degrade, and eventually seize.

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Grease Lubrication in Electric Motors

Composition of Grease

Grease is a semi-solid lubricant made of three key ingredients:

1. Base Oil – provides lubrication (mineral oil for general use, synthetic oil for high/low temperature applications).

2. Thickener – holds the oil in place and prevents leakage (e.g., lithium, calcium, polyurea).

3. Additives – enhance performance (anti-wear, anti-corrosion, oxidation inhibitors, dyes).

👉 Common motor greases:

• Lithium-complex greases – high dropping point (~500°F), good for high temps.

• Polyurea-based greases – excellent oxidation stability (e.g., Polyrex EM, Unirex N3).

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Selecting the Right Electric Motor Grease

When choosing grease, consider these six factors:

1. Viscosity

   • Typical: 500–600 SUS @ 100°F.

   • Must match motor load and speed.

2. Consistency (NLGI Grade)

   • NLGI 2 is the most common for motors.

   • Ensures proper pumpability and film thickness.

3. Oxidation Resistance

   • High ASTM D 3336 rating is preferred for long life at high speeds.

4. Anti-Wear Properties

   • Use EP additives only if thrust load exists.

   • Avoid EP greases in standard motors (they shorten grease life).

5. Dropping Point

   • Higher is better (≥ 500°F).

   • Lithium complex and polyurea greases perform well.

6. Shear Stability

   • Tested under ASTM D 217.

   • Should soften no more than 1–1.5 NLGI grades after 100,000 strokes.

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Adding Grease to Electric Motor Bearings

• Access points: Provided at motor ends.

• Right balance: Too little grease → metal-to-metal contact; too much grease → overheating and leakage into windings.

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Re-Greasing Intervals

• Double-sealed bearings (ZZ) – “lubricated for life,” usually no regreasing.

• Open or single-shielded bearings – require periodic regreasing.

Factors influencing frequency:

1. Operating Temperature – oxidation doubles every +10°C above 49°C.

2. Running Hours – continuous duty requires more frequent regreasing.

3. Grease Quantity – larger housings last longer.

4. Bearing Speed (Dn or Ndm value) – higher speeds → more frequent regreasing.

5. Vibration – accelerates grease breakdown.

6. Contamination – dust, dirt, or water ingress shortens grease life.

👉 Typical interval:

• From 2 weeks to 2 years, depending on speed, size, and load.

• Light to medium duty motors (continuous operation) → at least once a year.

• Reduce interval by half for every 10°C rise above recommended temperature.

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Calculating Re-Greasing Frequency

Formula (Method 1):

$$\text{Frequency (hrs)} = \Bigg[\frac{14,000,000}{(\text{rpm})(\sqrt{\text{Bearing ID (mm)}})} - (4 \times \text{Bearing ID (mm)})\Bigg] \times F_{bearing} \times F_{temp} \times F_{contam}$$

Where:

• $F_{bearing} = 10$ for ball, $5$ for cylindrical, $1$ for thrust bearings

• $F_{temp} = 1$ (<160°F), halve value for every +20°F

• $F_{contam} = 0.1–1.0$ depending on contamination (motors usually 1.0)

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Determining the Correct Quantity of Grease

Method 1: By Bearing Speed

• ≤50% limiting speed → fill ½ to ⅔ free space.

• >50% limiting speed → fill ⅓ to ½ free space.

Method 2: By Formula

$$\text{Grease (grams)} = \text{OD (mm)} \times \text{Width (mm)} \times 0.005$$ $$\text{Grease (oz)} = 0.114 \times \text{OD (in)} \times \text{Width (in)}$$

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Housing Fill Guidelines

• 30–50% fill – Standard, prevents overheating.

• 50–75% fill – For slow-speed motors or limited regreasing options.

• Full pack – For dirty environments (with pressure relief method).

⚠️ Overpacking leads to overheating and grease churning.

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Best Practices for Electric Motor Grease Application

✅ Always clean grease fittings before application.
✅ Use the same grease type to avoid incompatibility issues.
✅ Regrease while motor is running (when possible) to ensure even distribution.
✅ Purge old grease by opening drain plugs during regreasing.
✅ Maintain records of lubrication schedules.

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Conclusion

Proper selection, application, and regreasing of electric motor bearing grease significantly extend motor life, reduce downtime, and improve reliability. By following the correct intervals, quantities, and best practices, engineers can ensure maximum efficiency from their motors.

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⚠️ Disclaimer

This article is for educational and technical reference only. Always follow manufacturer guidelines and grease supplier recommendations for specific motor applications. Incorrect lubrication practices can cause bearing or motor failure.

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