In a high-power electric motor, the vast majority of failures originate not from the windings but from bearings that are not properly lubricated. Especially in cast-iron-housing motors of IEC 160 frame size and above, the bearings must be re-greased regularly throughout the motor's service life. This is precisely where the concept of the grease nipple and re-greasable bearing comes into play. While factory-filled, permanently lubricated sealed bearings are sufficient in small frames, in larger frames the grease inside the bearing degrades over time, loses its consistency and must be renewed. In this article we examine in detail why a grease nipple is mandatory on large-frame motors, what factors determine the lubrication interval, how to select the correct grease, and how to make an error-free choice at the ordering stage.

Why Are Re-Greasable Bearings Required in Large Frames?

Bearings in electric motors fall into two fundamental categories: sealed bearings that are lubricated for life, and open bearings that can be re-greased in the field by means of a grease nipple. In small and medium frame sizes, that is, generally up to the IEC 132 frame, the grease filled at the factory is sufficient for the motor's practical service life; these bearings are chosen as the sealed type and require no maintenance. However, as the frame grows, the bearing size, the amount of grease it holds and the motor's expected operating hours all increase. In this case the life of the factory grease becomes far shorter than the life of the motor itself.

In a large-frame motor, the bearing both carries higher loads and rotates for a longer period. The grease oxidizes over time, the oil within it separates out, and the soap structure that gives it its consistency breaks down. This deterioration leads to the thinning of the lubricating film and to metal-to-metal contact; the result is excessive heating, noise and ultimately bearing failure. For this reason, in frames of IEC 160 and above the bearings are equipped with a grease nipple through which grease can be injected from the outside, and they are re-greased at defined intervals.

The Difference Between Sealed and Open Bearings

A sealed bearing is a product made leak-tight on both sides with felt or metal shields, filled with grease at the factory, and into which no subsequent intervention is possible. An open or re-greasable bearing, on the other hand, is designed with grease channels, an inlet nipple and, in most cases, a drain plug. This arrangement allows the old grease to be expelled and replaced with fresh grease. In large motors this feature is a critical advantage in terms of being able to perform maintenance without dismantling the motor, because disassembling a large motor in order to change a bearing is both costly and time-consuming.

What Does the Lubrication Interval Depend On?

How frequently a re-greasable bearing must be greased is not a fixed value; it is calculated in hours based on several fundamental factors. This interval can vary over a wide range, from a few hundred hours to several thousand hours, depending on the motor's operating conditions. Maintenance carried out without first determining the correct lubrication interval will either suffocate the bearing by greasing it too often, or cause it to lose its lubricating film by greasing it too rarely. Both extremes shorten bearing life.

The principal factors that determine the lubrication interval are as follows:

  • Speed (number of poles): The faster the motor turns, the more quickly the bearing fatigues and the faster the grease degrades. For this reason, the lubrication interval is shortest in 2-pole motors (approximately 3000 rpm) and longer in 4-pole and 6-pole motors.
  • Frame size: As the frame grows, the bearing diameter increases; large-diameter bearings require more frequent greasing because both the peripheral speed and the grease load rise.
  • Operating temperature: High temperature rapidly shortens grease life. Every appreciable rise in bearing temperature leads to a significant shortening of the lubrication interval.
  • Grease type: The base oil and soap structure of the grease used determine its resistance to high temperature and load; a high-quality grease provides a longer lubrication interval.
  • Ambient conditions: Dusty, humid or chemically laden environments contaminate the grease, shortening its life and requiring more frequent lubrication.

When these factors are combined, two motors of the same power rating can have completely different lubrication intervals. For this reason, the lubrication chart provided by the manufacturer or the lubrication information on the motor nameplate should always be taken as the basis. If a large 2-pole motor operates continuously and at high temperature, its lubrication interval is at its shortest point, and the maintenance plan must be built accordingly.

Why Is It Shortest in 2-Pole Motors?

Because 2-pole motors rotate at around 3000 rpm, the peripheral speed of the bearing is at its highest level. High speed increases the mechanical stress on the grease and accelerates the separation of the oil from its base structure. In addition, high speed generates more heat in the bearing, which further shortens grease life. When these two effects are combined, large-frame 2-pole motors are the motors that need to be greased most frequently among all the pole options. Keeping the lubrication interval long in these motors is the most common cause of bearing failure.

Selecting the Correct Grease

In lubrication, the correct grease type is just as decisive as the correct interval. In electric motor bearings, lithium-complex soap-based greases of NLGI 2 or NLGI 3 consistency grade are generally used. The NLGI grade expresses the consistency, that is, the firmness of the grease; NLGI 2 is preferred for standard conditions, while NLGI 3 is preferred at higher speeds and temperatures, or in vertically mounted motors, to prevent the grease from flowing away. Lithium-complex-based greases remain stable across a wide temperature range and, thanks to their high dropping point, preserve the lubricating film even in hot bearings.

The fundamental rules to be observed in the selection and use of grease are as follows:

  • Do not mix different bases: When lithium-complex grease is mixed with a polyurea or calcium-sulfonate-based grease, a chemical incompatibility may arise; the grease loses its consistency and its lubricating property breaks down. If the grease is to be changed, the bearing must first be cleaned.
  • Avoid over-greasing: Stuffing the bearing full of grease causes the grease to be continuously kneaded and heated, leading to overheating of the bearing. The correct amount is always the value specified by the manufacturer.
  • Work clean: Dust or dirt pushed into the grease nipple is carried directly into the bearing. The tip of the nipple must be cleaned before greasing.
  • Choose the correct consistency: Where vertical mounting or high temperature is involved, choose NLGI 3; under standard conditions, choose NLGI 2.

How Is the Greasing Operation Carried Out?

Proper greasing consists of more than simply injecting grease into the nipple. In large-frame motors, there is usually a drain plug on the rear side of the bearing. The function of this plug is to allow the old, degraded grease to exit when fresh grease is injected. If the drain plug is kept closed, the fresh grease that is injected accumulates in the bearing cavity, builds up pressure and, by forcing the seals, can cause the grease to escape toward the winding side. For this reason, it is good practice to open the drain plug during greasing.

A proper greasing operation typically follows these steps: first the drain plug is opened; the motor, if possible, is greased while running at low speed, so that the fresh grease is distributed evenly within the bearing and the old grease is expelled through the drain hole. The amount of grease specified by the manufacturer is injected slowly; sudden and excessive pressure is avoided. After the operation is complete, the motor is run for a while to expel the excess grease, and then the drain plug is closed again. This disciplined method ensures that the bearing is left neither starved nor suffocated.

Monitoring the Bearing Temperature

In large-frame critical motors, the most reliable way to determine whether lubrication has been carried out correctly is to monitor the bearing temperature. For this purpose, PT100 type temperature sensors are installed in the bearing housings. The PT100 sensor gives early warning of abnormal rises in bearing temperature, making visible problems caused by both insufficient and excessive lubrication. A temporary rise in temperature immediately after greasing is normal; however, a persistent and rising temperature trend is the harbinger of a problem in the bearing. This monitoring is one of the most effective ways to prevent unplanned stoppages. For the relationship between efficiency and the correct operating point, our content on asynchronous motor rotor bar material explains another factor that determines the heating behaviour of the motor.

Making the Right Choice at the Ordering Stage

When ordering a motor, the matter of bearings and lubrication is often overlooked, yet this choice directly determines the maintenance cost and the service life of the motor. The general rule is clear: in motors up to approximately the 132 frame, a sealed, lifetime-lubricated bearing is sufficient and no additional lubrication equipment is required. In large-frame motors of 160 and above, a re-greasable bearing should be ordered together with a grease nipple and, preferably, a drain plug. This choice allows the motor to be maintained in the field without dismantling.

The points to be observed when ordering are as follows: the motor's frame size, number of poles and expected operating regime should be specified; for continuous and high-speed applications, a re-greasable construction must definitely be requested. For large motors on critical lines, a bearing temperature sensor (PT100) should also be included within the scope of the order. For correct power and speed matching, our content on 55 kW electric motor selection explains how to verify the frame and mounting type from the nameplate. For the correct choice in pump applications, our content on centrifugal pump motor selection also provides complementary information.

To procure a motor quickly with the correct frame, poles, lubrication equipment and protection class, the electric motor prices offered by HEM Motor provide technical support in selecting the correct bearing and lubrication configuration for large-frame motors. On critical lines, delivering the large motor with the correct speed and frame quickly from a strong stock minimizes the risk of unplanned downtime.

Frequently Asked Questions

Does every motor need to have a grease nipple?

No. In small and medium motors up to approximately the 132 frame, the factory-fitted sealed, lifetime-lubricated bearings are sufficient and no grease nipple is required. The grease nipple and re-greasable bearing become mandatory in large-frame motors of IEC 160 and above, because in these motors the life of the factory grease is shorter than the life of the motor, and regular renewal is necessary.

Does over-greasing damage the bearing?

Yes. Filling the bearing with more grease than necessary causes the grease to be continuously kneaded and heated, leading to overheating of the bearing; this situation can damage the seals and cause the grease to escape toward the winding side. For this reason, the amount of grease specified by the manufacturer should always be used, and during greasing the drain plug should be opened so that the old grease is expelled.

Which grease should I use, and can different greases be mixed?

In electric motor bearings, lithium-complex-based grease of NLGI 2 or NLGI 3 consistency is generally used. Greases with different soap bases should not be mixed, because the chemical incompatibility can degrade the consistency and lubricating property of the grease. If the grease type is to be changed, the bearing must first be cleaned of the old grease and completely filled with new grease.