Replacing bearings and motors
May 29, 2015
In every feed mill there are aspects regarding the maintenance of equipment that will help you avoid misapplications of bearings or motors. Here are a few thoughts on the subject.
Bearings and Seals
Every rotating shaft has some type of bearing supporting it. These bearings may be of several types depending on the application. Bearings have two purposes. The first is to support a radial load in supporting the shaft from its own weight or the sideways pull on the shaft due to a v-belt drive or similar load. The second is to be able to resist thrust loads where necessary and keep the shaft from moving along its length.
Bearings come mounted inside of several types of housings including pillow blocks, flange mount and other types. Some bearings use open-type housings and others are in sealed housings. Sealed bearing housings use seals fitted against the outside of the bearing housing and shaft. These seals are designed to seal against the rotating shaft surface and the bearing housing to keep lubricants in and foreign materials out of the bearing.
One of the biggest causes of bearing failure in sealed bearings is overlubrication. The bearing should be lubricated as noted in the instruction manual using the proper type and grade of lubricant. Dealers can tell you what type of lubricant to use if the brand recommended in the instruction manual is not available. Lubricant expands as it heats up during use of the equipment so over greasing can also add to the problem.
It is often felt that a sealed bearing requires greasing frequently, but this is not the case. Many seals are ruined by overlubricating a bearing. If lubricant is visible easing out around the shaft and seal, the bearing has too much lubricant and the seal has been broken.
If you are concerned about whether you have used too much lubricant in a bearing, remove the grease fitting (zerk) from the bearing housing and operate the machine for an hour or two. If there is too much lubricant in the bearing, it will expel the excess. After this is done, put the lubricant fitting back in place and resume normal operations.
Most bearing manufacturers offer a large number of bearing types, housings and capacity ratings for different uses and applications. When bearings fail on a piece of equipment, it is best to replace them with an identical bearing with the same housing and load rating capacities. Bearing interchange manuals are available showing the various brands of the same type that will interchange with the original bearing. Housing size dimensions and shaft diameters that will fit the shaft aren’t the only criteria for choosing a replacement bearing. Radial load capacity must also be checked.
As an example, many years ago a bearing on one of the roll shafts failed on a small 9-inch diameter by 30-inch roller mill used to grind grain. The bearing was mounted in a pillow block housing. The brand of bearing used when the roller mill was originally made wasn’t readily available, but a bearing interchange manual showed another brand that had the same housing dimensions was readily available from the local supply house. This bearing was installed and the roller mill put back into service.
It wasn’t long before the new bearing failed. Upon deeper investigation, it was found that although the new bearing and housing assembly was an exact fit for the earlier failed bearing, it did not have the same radial loading capacity for the shaft. Ultimately, the original bearing brand was found to have a much higher radial load capacity needed for the loads imparted by the roller mill roll shafts. Once a new bearing of the original manufacturer was found and installed, there was no further trouble.
When a motor fails on a piece of equipment, the first thing you look for is a replacement. Using the nameplate data on the failed motor, it is easy to order a new one to replace it. This is an opportune time to install one of the new premium motors to take advantage of its higher efficiency.
But, you may try to replace it with a used one you have on hand. It is important that this used motor will work and is appropriate for the application. You will need to compare the burned out and replacement motors nameplates to see if the replacement is OK. The nameplates should show that the horsepower, speed, operating voltages and phases are compatible.
Another item that may or not be identical on the nameplate is the frame number for the motor. Commonly used motors today are “T” frame motors, while older or special motors having the same horsepower, speed, operating voltage and phases may be “U” frame motors. The U frame motors are physically larger and will not mount or fit the same as a comparable “T” frame motor. The motor mounting bolt-hole pattern is different between the motors. Other physical dimensions also vary between the two frame types.
Over the years there have been three changes in the way motor frame numbers are assigned. The original designation was a 3 number code that was used for motors with 1 horsepower or greater up until 1954. In 1952, NEMA (National Electrical Manufacturers Association) re-rated motors based on advances in available materials and construction practices and added a “U” suffix to the frame number. In 1964, another re-rating occurred and the suffix was changed from “U” to “T,” which we still use today. With advances in materials and insulation quality, the same horsepower motor has become housed in smaller and smaller frame sizes.
As an example, a standard TEFC (Totally Enclosed Fan Cooled) 40-hp 1,800 rpm motor was originally housed in a size 405 frame prior to 1952. With the 1952 re-rate, it was housed in a 364U frame, and since 1964 it has been housed in a 324T frame. Since the frame numbers are different, even though the horsepower is the same, these motors are not of the same physical size and mounting bases have to be modified to install them.
Another thing to consider when installing a replacement motor is the NEMA design class. Different motors having the same horsepower and speed can have different starting currents, torque ratings and other variables. The four NEMA designs have unique speed-torque-slip relationships, making them suitable to different type of applications. Selection of a particular motor for an intended task must take these parameters into account. The four NEMA designs have unique speed-torque-slip relationships, making them suitable to different type of applications: design types are A-D.
Motors of NEMA design B are normally used in grain processing operations. These motors are suited for a broad variety of applications, requiring low starting current and normal starting torque – common in conveyors and bucket elevators. Design A or C will work in some applications. Descriptions and further information on these applications can be found on the internet by looking up NEMA.
Sometimes a motor frame will end in two letters such as a 324TS (40 hp 3600 rpm). The “S” means that the motor is designed for direct coupling only to the piece of equipment. Do not use a motor whose frame number ends with an “S” for a v-belt or chain drive. The bearings in these motors are not designed to take sideways pull required with a v-belt or chain drive connection (See table on page 64 for a listing of the various motor frame sizes by horsepower and speed).
Another thing that must be considered when replacing a motor is in what type of atmosphere the motor operates. If it is outdoors, it is common to use a TEFC-type enclosure. But if it is in a dusty or hazardous location, the motor needs to be rated for that atmosphere.
Hazardous area classifications are by division. Division 1 locations have ignitable concentrations of dust in the air, most or all of the time, during normal operations. Division 2 locations are not likely to have ignitable concentrations of dust in the air during normal operations. A motor operating in this type of atmosphere will need to also have a UL (Underwriters Laboratories) nameplate indicating in what type of hazardous atmospheres it may be used. Motors for Division 1 locations in grain processing facilities are rated Class II (combustible dust) Group G (grain dust) and will have an UL label listing that the motor is for IIG locations. Class IIG motors are also considered explosion proof. Motors for Division 2 locations will have nameplate data saying they can be used in those locations.