The reliability and lifespan of an electric motor are intrinsically linked to the health of its bearings. Proper lubrication is the single most critical factor in ensuring bearings operate as intended, preventing premature failure and costly unplanned downtime. This guide breaks down the essential principles of bearing lubrication, covering both grease and oil applications for various bearing types, including rolling element (ball and roller) and sleeve bearings.

Part 1: Grease Lubrication for Rolling Element Bearings

Grease is the most common lubricant for rolling element bearings in electric motors. Success depends on applying the right amount of the correct grease at the proper frequency.

Determining Grease Relubrication Intervals

The frequency of relubrication is not a one-size-fits-all parameter. It is highly specific to the application and depends on a range of factors:

• Operating Temperature: Higher temperatures degrade grease faster.
• Motor Shaft Speed (RPM): Higher speeds require more frequent lubrication.
• Bearing Size and Load: Larger bearings and heavier loads increase stress on the lubricant.
• Hours of Operation: Continuous operation requires a more rigorous schedule than intermittent use.
• Environmental Conditions: The presence of contaminants like dust or moisture necessitates shorter intervals.
• Vibration Levels: High vibration can accelerate grease deterioration.

For motors operating under normal conditions, a standard relubrication interval chart is an invaluable tool. For more demanding situations, these intervals must be adjusted. As a general rule, reduce the standard interval by 50% for conditions such as:

• Vertical motor mounting
• Heavy belt loads
• Operation in hostile or contaminated environments

Ball and Roller Bearing Relubrication Intervals Chart

Figure 10-18: Grease relubrication intervals for normal operating conditions as a function of bearing speed and bore size.

The 7-Step Grease Lubrication Procedure

To ensure grease is applied effectively and safely, follow these steps:

1. Prepare the Motor: Relubrication should be performed with the motor shaft stationary but while the motor is still warm.
2. Clean the Inlet: Locate the grease inlet fitting and thoroughly clean the area around it to prevent contamination. If the motor uses pipe plugs, replace them with appropriate grease fittings.
3. Open the Relief Port: Remove the grease relief plug and clean out any hardened grease that may be blocking the exit path. This is crucial to prevent over-pressurization.
4. Add the Lubricant: Add the recommended volume of the correct type of grease.
5. Use the Right Tool: Always use a hand-operated grease gun to control the volume and pressure applied.
6. Purge Excess Grease: With the relief plug still removed, run the motor for about 30 minutes. This allows excess grease to be purged, preventing overheating from over-packing.
7. Finalize: Shut down the motor, replace the relief plug, and wipe off any excess grease.

A Note on Shielded Bearings

While sealed bearings cannot be relubricated, shielded bearings allow for a small amount of replenishment. Empirical evidence shows that some oil from the new grease will eventually "find its way" past the shield and into the ball or roller area. While this effect is limited, it provides some benefit. The shield also serves the vital purpose of restricting foreign materials from entering and damaging the bearing.

Grease Compatibility: A Critical Consideration

Mixing incompatible greases will lead to rapid bearing failure. The thickeners in different grease types can react, causing the lubricant to break down and lose its oil-retaining properties. The only certain way to avoid this is to use the same grease for relubrication. If a change is necessary, ensure that both grease manufacturers have documented that the products are compatible.

Part 2: Oil Lubrication for Bearings

Oil Level for Ball and Roller Bearings

• Horizontal Shafts: For motors with horizontal shafts, the oil level should be maintained at the approximate center of the lowest rolling element when the motor is not operating.
• Vertical Shafts: In vertical applications, the oil level should ensure approximately 50% submergence of the rolling elements.

Oil Change Frequency for Rolling Bearings

The oil change interval is heavily dependent on operating temperature:

• 50°C (120°F) or less: Replace oil once a year.
• 80°C to 100°C (175°F to 210°F): Replace oil at least quarterly.
• Critical Equipment: For mission-critical machinery, perform an oil analysis at least quarterly to determine the optimal time for an oil change.

Selecting Oil Viscosity for Horizontal Rolling Bearings

Oil viscosity is selected based on the operating temperature and a speed factor known as dmn.

• dm = Bearing Pitch Diameter = (Bearing ID + Bearing OD) / 2
• n = Shaft Speed in RPM
• dmn = Speed Factor = dm x n

Calculation Example:

• Bearing: 6210 (ID = 50 mm, OD = 90 mm)
• Speed (n): 3600 rpm
• Conditions: 90°C operating temperature, normal load

1. Calculate dm:
   dm = (50 mm + 90 mm) / 2 = 70 mm
2. Calculate dmn:
   dmn = 70 x 3600 = 252,000
3. Select Viscosity:
   Using the dmn value and the operating temperature, refer to the table below. For a dmn of 252,000 and a temperature of 90°C, the recommended oil is an ISO VG 56 or VG 68 turbine oil.

ISO Viscosity Grades for Horizontal Rolling Bearings

Note: Values in [brackets] are for heavy/impact loads.

Oil Viscosity for Vertical Rolling Bearings

For vertical motors, selection is simplified and is based on ambient temperature and bearing type. If the motor lubrication plate specifies synthetic oil, DON'T substitute another oil.

Important: If the motor nameplate specifies a synthetic oil, do not substitute with a mineral-based oil.

Lubrication for Sleeve Bearings

• Oil Level: If manufacturer instructions are unavailable, the general guideline is that oil rings should be immersed by approximately 1/4 of their circumference or 20% of their diameter.
• Lubrication Frequency: If available, use relubrication intervals based on manufacturer instructions, or use the guide below:

Part 3: Bearing Temperature Limits

Monitoring bearing temperature is a key indicator of health. The following limits are applicable to all bearing types (ball, roller, and sleeve):

• Normal Operation: Up to 80°C (176°F)
• Alarm: 90°C (194°F)
• Trip (Shutdown): 100°C (212°F)