What types of bearings are used in HVACR motors?
There are lots of different applications in HVACR systems that utilize motors. These include direct drive fans and blowers, belt-drive fans and blowers, pumps, actuators, and gear drives to name a few.
When the motor and equipment manufacturers determine the bearings to be used in the motor, some of the considerations include the application, load, conditions, environment, operating temperatures, direction of load, required life, operating speed, speed controls, noise levels, end play, lubrication, and exposure to moisture and condensation.
In the HVACR industry, motors are typically designed with ball bearings or sleeve bearings. There are special bearing designs that may occur, but we’re going to talk about the most common designs.
Sleeve bearings are common in HVACR applications up to about 1 horsepower. At that rating and beyond there is typically too much load for the sleeve bearing and bearing and motor life expectancy is shortened. The load can be axial (parallel to the shaft) or radial (perpendicular to the shaft). A fan blade is an example of an axial load. A belt-drive application is an example of a radial load. Improper belt tension is a common cause of premature bearing failure.
There are two types of sleeve bearings frequently used: self-aligning and rigid.
The self-aligning sleeve bearing is designed so that it can move when installed in the end bells of the motor. This feature allows for realignment of the bearing if, for example, the shaft is inadvertently hit or bumped during installation. Lubrication is packed around the bearing using Permawick. The bearing material allows the oil to completely permeate the bearing, providing a film of oil between the shaft and the bearing.
Self - Aligning Sleeve Bearing
The rigid sleeve bearing is pressed into the end bells of the motor. Once installed, it does not move. Permawick is also packed around the rigid sleeve bearing. This bearing, however, does not allow for the oil to permeate through the bearing material. A wick is placed into a window of the bearing, which allows oil to be drawn into the bearing. The shaft then rides on a film of oil when operating.
Rigid Sleeve Bearing
For proper lubrication in a sleeve bearing, the shaft needs to turn at a minimum of 500 RPM. Because of this, ball bearing motors should be used with speed controllers and variable frequency drives (VFDs). Another characteristic of sleeve bearing motors is that they have end play in the shaft. That is, the shaft can be slightly moved in and out. This allows for the expansion of washers in the bearing system as the motor heats up during operation.
Ball bearings are designed to handle larger loads. The bearings are pressed onto the motor shaft. When the motor is assembled, the end bells are pressed over the bearings. Grease is inserted around the balls of the bearing. The grease lubricates the bearing as the inner race turns with the motor shaft during operation.
A ball bearing has mechanical parts that move during operation. The sleeve bearing does not. These mechanical parts make noise that is not present in a sleeve bearing. Because of this, sleeve bearing motors are often used in applications where noise levels are critical. Furnace blower motors have typically been sleeve bearing in an effort to reduce noise levels through the ducts.
When asked “what bearing provides longer life, a sleeve bearing or a ball bearing?”, most people would answer ball bearing. However, when loaded properly, a sleeve bearing provides longer life. But some loads are too much for the sleeve bearing to handle.
When selecting replacement motors, the bearing design needs to be considered. Most motors in HVACR are designed for specific applications. By choosing them according to their application, you can rest assured that the bearings are designed for that application, resulting in long motor life.