Bearing Configurations for Large Electric Motors

Today, we delve into the bearing arrangement in medium and large motors, as well as the characteristics of these bearings.

1. Bearing Arrangement in Large Motors

The primary function of bearings is to transmit load while facilitating rotation. Therefore, whether it’s a small motor or a large one, the only difference lies in the load that the bearing needs to transmit. Consequently, this results in variations in the bearing’s size or form.

Hence, large motors predominantly employ bearings where the rolling elements have line contact. Examples include cylindrical roller bearings, spherical roller bearings, and spherical roller thrust bearings.

The image above on the right depicts a bearing configuration for a large horizontal motor. The fixed-end bearing on the left typically employs a spherical roller bearing, also known as a self-aligning roller bearing.

The floating-end bearing shown in the top left image uses the same self-aligning roller bearing or a toroidal roller bearing.

For large vertical-axis motors, a type of bearing that may be unfamiliar to motor engineers might be used. The floating-end bearing shown in the figure above is a self-aligning roller bearing that we are quite familiar with; for motors that are insensitive to alignment, cylindrical roller bearings can also be chosen.

The fixed-end opts for a self-aligning roller thrust bearing, a type that resembles tapered rollers, but also possesses self-aligning capabilities.

Looking at these two configurations, when choosing bearings for large motors, load-bearing capacity is a significant consideration. Line-contact bearings, due to their larger contact area between rolling elements and raceways, boast a correspondingly higher load-bearing capacity, making them suitable for high-load applications.

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However, the bearing’s load capacity and speed performance are two sides of the same coin; in conventional bearing design, a higher load capacity inevitably leads to a decrease in speed performance.

Here, I’d like to focus on discussing two bearings, the spherical roller (or as we prefer to call it, self-aligning roller) bearing, and the spherical roller thrust bearing.

2. Spherical Roller Bearings (Self-aligning Roller Bearings)

As the name implies, these bearings are self-aligning. But what does “self-aligning” mean? It doesn’t refer to an adjustable center size or load center of the bearing itself, but rather, it means that the bearing can function normally in applications with a certain degree of eccentricity without affecting its performance.

The theoretical state of rotating machinery is that the trajectory of the rotating axis is a straight line.

However, under actual working conditions, firstly, there exist machining and installation errors among various parts of the motor; secondly, the gravity of the motor and the magnetic pulling force are at play; thirdly, the motor’s rotating shaft is not an ideal steel body, and bending and deformation are inevitable.

These factors may cause the actual trajectory of the rotating shaft to resemble a spindle or an hourglass shape.

A deformed rotating shaft will generate additional internal loads on the bearings. For instance, when cylindrical rollers or deep groove ball bearings are chosen, apart from external loads including rotor weight and magnetic pulling force, the bearing also endures what is referred to as “internal loads”, which are additional loads caused by shaft deformation.

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These loads are ones that designers may not have taken into account, or theoretically, cannot accurately calculate as part of a series of loads. Consequently, these “unconsidered” loads acting on the bearing may lead to premature bearing failure, or other wear exceeding design expectations.

However, the primary function of self-aligning series bearings is to neutralize eccentricity within the bearing, allowing the bearing to retain its normal functionality even under eccentric conditions (within a certain range).

This series of effects is achieved through the design of special rolling elements and the contact surfaces of the outer ring raceways.

Simply put, the outer ring raceway of such a bearing is part of a “great circle” through a sphere.

This type of bearing exhibits excellent eccentric adjustment capabilities, but is one of the few designs with two rows of rolling elements.

Therefore, while the bearing can bear both axial and radial loads, the ratio of loads in both directions determines the initial conditions for load distribution across the two rows of rolling elements.

3. Spherical Roller Thrust Bearing

As the name suggests, the primary role of this bearing is to bear the thrust load, i.e., the axial load. Its other functions are similar to those of a spherical roller bearing, operating normally under eccentric conditions without affecting the bearing’s functionality.

However, the rotational performance of the thrust bearing itself is inherently low. Due to design considerations, the rotational performance of this bearing significantly declines, making the control of the bearing’s speed limit particularly stringent. Consequently, lubrication conditions require careful consideration.

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Let’s delve a bit deeper into the speed limit of the bearing. Regardless of whether we are selecting cage designs, choosing lubrication, or even calculating the re-lubrication interval, we will encounter the matter of determining the bearing’s speed limit (Recommended limits for speed).

Here, we introduce a speed limit A, also known as the ndm value, which is the product of the bearing’s operating speed and the pitch diameter – essentially, the pitch diameter’s linear speed.

Additionally, we should encounter another term, referred to as the bearing factor bf.

When determining the re-lubrication interval for bearings, we use the product of bf and A as the basis for selection. Taking the SKF spherical roller thrust bearing as an example, its bf value is 4, one of the highest among all types of bearings.

Moreover, under different load conditions, the recommended speed limit for this bearing is quite low compared to other bearing designs, due to the bearing’s unique design and specialized application.

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