This presentation covers:
- How to recognize the symptoms and determine the presence of damaging currents
- Causes of damaging currents (e.g., machine dissymetry and VFDs)
- Testing to confirm and assess the magnitude of shaft and bearing currents
- Solutions to eliminate or control shaft and bearing currents (e.g., insulators, isolators and ceramic bearings)
"We rebuilt a 75 hp electric motor recently. It ran fine in the service center, but the customer reported high bearing temperatures shortly after installing the motor. The bearings failed after only a few hours at full load." The first response for most of us is to suspect an alignment problem. But there is another possibility that should be considered. An electric motor must have room for thermal expansion of the shaft, or bearing life will be severely reduced. The endplay of a ball bearing motor plays an important role in bearing life.
Have you ever repaired a sleeve bearing motor, only to have the customer complain that it leaks oil? Perhaps the motor had a history of oil leaks, and the windings were oil-saturated when you dismantled it. Two-pole machines are especially notorious as chronic oil leakers. The first step toward correcting an oil leak is to identify the cause. A good place to start is to determine whether the motor has a forced-oil system. If so, check for a metering plate in the oil supply line. The typical metering plate (see figure) has about a 3/32 diameter orifice to meter the volume of oil. Often installed in a pipe union, the metering plate is easily lost when the motor is removed from service. The repairer rarely gets the forced-oil system with the motor. The customer does not recognize that little piece of metal that looks like a conduit knockout, and that tiny hole cant possibly be for oil flow. So it gets thrown away .
Have you ever had to deal with chronic drive end ball bearing failures with a V-belt application? This article will take some of the mystery out of how to determine the load on a bearing, and how to increase the bearing capacity when necessary.The focus will be on bearing loading due to belt pull with V-belt drives. How to modify a motor to accept a cylindrical roller in place of a lower capacity ball bearing will also be detailed. The calculation of bearing loading may at first appear to be a daunting task due to the many variables involved. However, taken a piece at a time,the calculations are rather straightforward. An example will be used to illustrate this point.
Welcome to the age of predictive maintenance technologies. More and more of our customers are using tools such as vibration analysis to assess the health of their rotating equipment. Many of our customers are using this technology to assess new and rebuilt rotating equipment once it's installed and running. This serves two main purposes:
- It demonstrates the quality of the newly acquired/repaired equipment (taking the burden off the supplier/service center should the equipment vibrate once it's installed).
- It provides a baseline for trending. Unfortunately, these initial vibration readings can be pushed into an "alarm status" by many customer-related issues such as poor coupling alignment and/or machine installation. This is why it's so important for today's repair facility to provide the customer with "baseline" vibration data gathered during its final test run, providing evidence that the rotating equipment ran within general vibration guidelines before being shipped. Vibration frequency analysis can expose many mechanical and electrical problems in an electric motor. The purpose of this article is to discuss one of these: Bearing Defects.
There are a number of ways that the shaft of an electric motor can become magnetized in service. The most likely culprit is electric current through the motor and shaft, either from internal dissymmetry, welding or from a variable frequency drive. It can also be caused by electrical faults in the system, or even a lightning strike. If enough current does pass through the shaft, then it can remain magnetized, even after it is taken off line. The problem is that when a shaft is magnetized, it can further lead to bearing failures, unless something is done to eliminate the residual magnetism. The first reason for bearing failures is that the residual magnetism can cause shaft currents, which can quickly lead to bearing failures. But in addition, a magnetized shaft will attract bits of metal to the bearings. This reduces bearing life because it damages the bearing surfaces.
There are two main types of load that act on the bearings of a motor - radial and axial.
- Radial - A radial load is defined as aload that is applied perpendicular tothe shaft. An example of a radial load would be an overhung load, such as with asheave.
- Axial - An axial load, also referred to as thrust, is a load that acts parallel to the shaft on which the bearing is mounted. Just the rotor weight of a vertically-mounted motor will cause a downward axial load on the bearing. The type and magnitude of the load will determine what type of bearing should be used in the application. If the wrong type of bearing is used, it could lead to a motor failure in a short period of time. This article will address many of the common types of bearings used in electric motors, the types of load for which they are designed, their applications, and their enclosures.
There are many reasons for the use of precision grade bearings. The selection of type and size are specific to the application. Some of the determining factors are load ratings, both radial and axial, operating speeds, temperature and the degree of accuracy during operation. Bearings are made to exacting tolerances and are rated in various levels of accuracy. These standards were established by the Annular Bearing Engineers' Committee (ABEC) for the American Bearing Manufacturers Association which has been the benchmark in the industry since the early 1900s. The ABEC scale classifies different accuracy and tolerance ranges into classifications 1, 3, 5, 7 and 9. These classifications or grades are issued to a bearing after a series of vigorous measuring and testing procedures have been completed. With higher ABEC grades, an increase in running accuracy, speed and performance can be achieved.
One key method to improve the reliability of bearings is through proper lubrication. We should always strive to use the best available lubricant in the motors and other rotating equipment we repair. What if you find a situation where you've made a repair and the bearings are failing after only a few months of service? What has gone wrong?