A small percentage of bearings achieve their application design life, and in pump and motor applications, bearings are the most susceptible components to premature failure. The cost of a failed pump or motor add up quickly, but end users can take several precautions that help maximize bearing life.
In most cases, the cost of the equipment repair is less than 8 percent of the overall expense of the failure—while downtime costs usually account for an estimated 90 percent.
Pump users frequently ask how long certain bearings should last. The answer depends on the amount of information that can be provided at the time the question is posed. Life calculations can be run based on furnished application data, but it is difficult to accurately predict bearing fatigue.
Bearing life is commonly measured using an L10 or L10h calculation, which is a statistical variation of individual bearing life that is most often communicated as life in hours or revolutions. A bearing's L10 life, loosely interpreted from International Standards Organization (ISO) and American Bearing Manufacturers Association (ABMA) standards, is based on the lifespan of 90 percent of a group of identical bearings in a given application. In a nutshell, it is a calculation of how long 90 percent of the bearings will last in that given application.
What makes this equation impractical for determining definite bearing life is that it is based on the load capacity of the selected bearing, the actual application loads, the bearing type (ball or roller) and the rotational speed (in revolutions per minute [rpm]) of the application. The L10 life calculation does not consider temperature, lubrication and other key factors related to pre-service damage that are crucial to achieving the designed application bearing life. Proper storage, treatment, handling, installation and maintenance are simply assumed. Predicting bearing fatigue without consideration of these variables is problematic. As a result, an estimated 10 percent of bearings meet or exceed their calculated fatigue life.
Limit Pre-Service Damage
Proper storage and handling can limit pre-service damage to bearings. Standard bearings are predominantly produced from 52100 steel, a highly refined steel material that can be prone to oxidation. If end users do not handle bearings like they do other precision equipment, the longevity of the bearing can be compromised. Bearings are typically packaged at the factory with a Ferrocote or a thin coat of preservative oil. Handling the bearings with bare hands or wiping off the preservative can reduce corrosion-resistance. If a thin coat of preservative or oil is not re-applied, damage can occur. Bearings should be kept in an area free from vibration to avoid false brinelling, which is a phenomenon characterized by localized material wear or damage that occurs as a result of frictional vibrating contact between surfaces.
Figure 1. The graphic shows an example of a bearing being heated through induction heating (Graphics courtesy of the author)
Proper mounting is essential to bearing life. The correct mounting methods may include induction heaters, presses or bearing mounting impact tool kits. \uc0\u8232 Using the appropriate shaft and housing seat diameters is crucial to optimal life and performance. Common ball and roller bearing applications with a rotating shaft/rotating inner ring should have an interference shaft fit. In most cases, standard ball and roller bearings have a radial internal clearance built into the bearing. The clearance will be reduced during installation onto the bearing journal to accommodate the interference shaft fit and prevent negative clearance (pre-load). The slightest oversized or undersized shaft can shorten bearing life by more than half. The same is true for housing fits. End users should consider referencing shaft and housing fit charts for ball and cylindrical roller bearings used in general and electromechanical repair applications.
The basic shaft and housing fits on these charts are accurate for general applications and based on standard Annular Bearing Engineering Committee (ABEC) precision grade, normal operating temperatures and normal loads. However, for special applications that may include high-speed, high-heat, outer ring rotation or any non-standard design, check with the original equipment manufacturer (OEM) to obtain their fitting practices. The OEM designed the unit, so taking time to research these specs is important.
Proper storage, handling and installation can eliminate the cause of about 30-35 percent of failed bearings, and appropriate preventive maintenance can drastically extend equipment life.
Apply Correct Lubrication
Lubrication issues account for up to half of all failed bearings. Failures may result from insufficient or excessive lubrication, improper lubrication methods, incompatible lubrication, incorrect viscosity and contamination. Lubrication separates contact surfaces, reduces friction and protects against corrosion. Proper lubrication can also seal equipment from the ingress of contaminants and, in the case of circulating oil, offer heat displacement.
Table 1. Adding incompatible greases can result in a rapid reduction of the grease life, which translates to accelerated bearing failure.
Many pump applications use oil for the superior lubricating properties needed for high speeds. Oil lubrication serves as a filter and has additional advantages in terms of the life of the lubricant. One negative of using oil is that it can be difficult to effectively and easily seal. This is where sealing grease is generally used because it is less burdensome. There are two options for using grease with bearings. One option is to use an enclosed (sealed or shielded) bearing that is pre-filled with the appropriate amount of grease by the manufacturer. Users may also choose an open bearing that will require replenishment. The overwhelming majority of enclosed pre-filled bearings are ball bearings as opposed to roller bearings because grease life is longer with ball bearings.
Because sealed-for-life bearings are only sealed for the life of the grease, one might think that an open bearing, in an application where grease can be replenished, would survive much longer. That is not always the case. Proper upkeep requires the correct re-lube intervals, the correct amount of grease, a compatible grease and protection against foreign contaminants.
Some manufacturers may provide re-greasing interval charts, which are usually based on the bearing bore size, operating speed and bearing type.
A general way to determine how much grease to replenish is to multiply the outside diameter of the bearing in millimeters by the bearing width in millimeters, then multiply by 0.005. This will give you the re-lubrication amount expressed in grams.
Most of these re-greasing graphs or charts are based on an operating temperature of 160 degrees F or below. Depending on the bearing manufacturer, suggested re-lube intervals should be reduced by half for every 25-27 degrees F above an operating temperature of 160 F.
Additionally, a further reduction in re-greasing intervals may be warranted in potentially contaminated applications and in vertical applications. If re-lubrication detail is not known, it is best to first check with the pump or motor manufacturer. If they cannot provide a recommendation, contact the bearing distributor or OEM.
When adding lubrication, first determine what grease is in the equipment, and make sure to add a compatible grease. All greases are made of three main components: a base oil that can be synthetic or organic, a base thickener, and any additives used for desired enhancements. Pay close attention to the base thickener. Many types of thickeners are used, including calcium, clay, sodium, aluminum complex, lithium and urea compound (polyurea).