Industrial Motors / Industrial Bearings / Machinery Lubrication / Motor Monitoring

Get your bearings: Electric motor lubrication 101

How often should electric motors be lubricated? How can you be sure you’re using the right amount? Here’s your primer.

By Thomas Bishop, EASA

How often your facility’s electric motors should be lubricated—and how much lubricant they should receive—depends on several factors. When you’re considering electric motor lubrication frequency and quantity, take into account the following:

  • Is the lubricant grease or oil?
  • Is the bearing type sleeve, ball, or roller?
  • If it is a ball bearing, is the enclosure open, shielded, or sealed?

Let’s delve into these considerations further by breaking them down into the following categories:

  • Grease lubrication frequency and quantity for ball and roller (rolling element) bearings.
  • Oil lubrication frequency and level (relates to quantity) for ball and roller bearings.
  • Oil lubrication frequency and level (relates to quantity) for sleeve bearings.

Grease lubrication of sleeve bearings is extremely rare, and sealed bearings cannot be relubricated, so those topics will not be addressed in this article.

Grease lubrication frequency and quantity for ball and roller (rolling-element) bearings

The frequency of relubrication is application- and product-specific, depending on parameters such as:

  • Operating temperature
  • Motor shaft speed
  • Bearing size
  • Bearing load
  • Hours of operation
  • Environmental conditions/contaminants
  • Vibration levels

If motor manufacturer instructions are not available, determine the appropriate grease relubrication intervals for motors under normal operating conditions using Figure 1, which is based on information provided by NSK Bearing Corp. (Note: Grease relubrication intervals for normal operating conditions can be read as a function of bearing speed and bore. This diagram is valid for bearings on horizontal shafts in stationary machines under normal conditions. The curves represent the internal diameter of the bearings in mm.) For conditions such as motors with vertical mounting, belt load, or use in hostile environments, reduce the interval determined from Figure 1 by 50%.

To use Figure 1, do the following:

  • Find the motor rpm on the horizontal axis.
  • Draw a vertical line from the motor rpm to the curve that represents the bearing inside diameter (in millimeters), or the next smaller dimension.
  • From that point, draw a horizontal line to intersect the hours of operation (relubrication interval) for the applicable bearing type.

Example: Determine the lubrication frequency of a 6314 bearing operating at 1,780 rpm in a normal environment. The inside diameter in millimeters of the bearing is five times the last two digits of the bearing number. Thus the bearing ID is 70 mm (5 × 14). Note that there is no curve for 70 mm; thus, use the curve for the next smaller size (i.e., 60 mm). The 60 mm curve intersects with the 1,750 rpm value at about 11,000 hours (using column for type A) for a ball bearing. If the motor were operating in a hostile environment, the lubrication interval would be reduced from 11,000 hours to 5,500 hours (0.5 × 11,000). If the bearing were a cylindrical roller bearing with the same bore size, such as an NU214, the lubrication interval in a normal environment would be about 9,000 hours (using column for type B).

Relubrication quantities. Figure 2 shows guidelines for the grease quantity to be used during relubrication.
Example: The relubrication quantity for the 6314 ball bearing will be calculated. The outside diameter is 5.91 in. (150 mm) and the width is 1.38 in. (35 mm). The lubricant quantity G is 0.90 fluid ounces (0.11 × 5.91 × 1.38), or 26 milliliters (0.005 × 150 × 35).

Lubrication procedure. Ball and roller bearings may be lubricated with the motor running or stationary. However, lubrication while the motor is stationary but at or near operating temperature is recommended. Here are the steps to follow:

  1. If lubrication must be performed while the motor is running (not recommended), make certain to observe all safety precautions.
  2. Locate and clean the grease inlet area.
  3. If equipped with a pipe plug, replace it with a grease fitting. For increased reliability, replace grease (Zerk) fitting with a shutoff style fitting having a release pressure limit of 5 to 20 psi (35 to 140 kPA) to help prevent damage to the bearing when adding grease.
  4. If there are no provisions for grease fittings or pipe plugs, the bearings are not intended to be relubricated. Thus, it is not recommended to remove end brackets to relubricate bearings.
  5. Remove relief drain plug or pressure relief vent fitting.
  6. If grease is caked inside the relief hole, clean with a wooden stick or suitable tool. If severe caking appears at the relief hole, run the motor until the bearing housing is warm, permitting a free flow of grease to exit the housing.
  7. Add the recommended volume of the recommended lubricant using a hand-operated grease gun.
  8. Run the motor for one-half hour with the relief drain plug removed before replacing the plug. For increased reliability, replace drain plug with pressure relief vent fitting rated 1 to 5 psi (7 to 35 kPA) to continually purge excess grease, and eliminate the one-half hour waiting period for lubricant purge.
  9. Replace the pipe plugs, if applicable, and wipe off excess grease.

Note: Shielded bearings allow for a “small” amount of relubrication but with little effect, depending upon the clearance between the inner race and the shield. This clearance varies among manufacturers from 0.003 in. to 0.015 in. (0.08 mm to 0.38 mm). Empirical evidence shows a certain amount of oil from the lubricant will “find its way” into the ball area, but the shield will limit the amount of foreign material that can enter and cause damage.

Oil lubrication frequency and level (relates to quantity) for ball and roller bearings

For horizontal shaft applications, the oil level should be maintained at approximately the center of the lowest rolling element (see Figure 3), according to the oil gauge, when the motor is not operating. For vertical shafts, the stand-still oil level should be maintained at approximately 50% submergence of the uppermost row of the rolling elements (see Figure 4).

The interval at which lubricating oil should be changed varies depending upon operating conditions, oil quantity, and type of oil used. In general, for oil bath lubrication where the operating temperature is 50°C (120°F) or less, oil should be replaced once a year. When the operating temperature is between 80°C - 100°C (176°F - 212°F), oil should be replaced at least every three months. For critical equipment, it is advisable that lubricating oil be analyzed at least every three months to determine when oil replacement is necessary.

Oil lubrication frequency and level (relates to quantity) for sleeve bearings

The clearance between the shaft journal and the bearing bore is critical with sleeve bearings. Any short-term, metal-to-metal contact can increase the bearing temperature, and the associated “wiping” can quickly degrade the bearing, possibly causing catastrophic failure. To maintain sleeve bearing clearances, follow the lubrication guidelines in Table 1.

Select relubrication intervals based on the motor manufacturer’s instructions (if available). Otherwise, use the intervals provided in Table 1. Frequent starting and stopping, damp or dusty environments, extreme temperatures, and other severe service conditions warrant more frequent oil changes than shown in Table 1. Contact the motor manufacturer regarding oil change intervals for specific situations, or regularly check the oil for contaminants or discoloration and replace it as needed.

Another way to determine oil replacement intervals is to have a laboratory analyze oil samples periodically. Tip: Take oil samples with the motor shut down to avoid removing too much. When replacing the oil, fill the reservoir to the level that’s normally shown on the sight glass.

Sleeve bearing oil level. If oil level information is available from the motor manufacturer, follow it. If not, as a general guideline, the oil rings should be immersed to approximately one-quarter of their circumference or 20% of the diameter of the rings (see Figure 5).

Too low a stationary level means the oil level is dangerously low when some of the oil is in play (in the bearing, dripping down the inside of the chamber, etc.). Too high an oil level means increased friction between oil and rings. The rings will turn more slowly, supplying less oil to the bearing. Adding oil with the machine at rest is preferable to when the machine is operating. Overfilling when running can initiate a siphoning effect of oil through the labyrinth passages, thus leading to chronic oil leakage.

One last note about temperature limits that is applicable to ball, roller, and sleeve bearings: The suggested bearing temperature limit for normal operation is 80°C (176°F), alarm at 90°C (194°F), and trip at 100°C (212°F).