As Ollie Jones wrote nearly 60 years ago, “Love Makes the World Go ‘Round.” While our world rotates because of an emotion, industrial equipment needs help from something without feelings: ball bearings.
Despite being inanimate, ball bearings will let you know when they’re in trouble; they’ll squeak, vibrate, overheat, and generally will make a nuisance of themselves. Maintaining your ball bearings is typically a routine task, although there are some bad ideas regarding bearings circulating throughout the manufacturing sector. Providing excellent solutions to these bad ideas, J/E Bearing and Machine shares 11 examples of ball bearing myths and their solutions:
- It’s okay to hammer a bearing into position if needed. A direct hammer blow leaves dents in the raceway that cause noise and dramatically reduce bearing life. If installation is difficult, first check the shaft diameter and look for burrs, dirt or corrosion. If needed, use a press to slide the bearing on. Apply pressure equally on the face of the inner ring to avoid damaging the raceways and rolling elements.
- Off-the-shelf TGP shafting is the best option. It’s more important to know the shaft’s tolerance range to ensure it meets the spec for diameter and roundness. Review the manufacturer’s recommendations and measure/specify the correct diameter.
- It’s fine to hand-tighten set screws one at a time. Under tightening allows the bearing to slip on the shaft. Over tightening distorts the raceway or cracks the inner ring. Tighten the first setscrew to half the recommended torque, the second setscrew to the full torque, then go back to the first setscrew and apply full torque.
- Bearings should not be hot to the touch. Normal bearing operating temperatures range from 27 to 66 degrees C, but some applications run higher or lower. Most bearings are rated for -29 to 121 degrees C, but special grease, seals or heat stabilizing processes allow them to operate at higher temperatures. Bearings normally run hotter at start-up or right after re-lubrication because excess grease increases drag and friction. Spikes up to 10 degrees C are normal at start-up, and -1 degree C after re-lubrication. Steady-state temperatures resume as the rolling elements purge excess grease through the seals.
- Bigger bearings are always better. They may show a higher fatigue life, but if the load does not achieve the minimum requirement, rolling elements skid along the raceway. This causes high temperatures, excessive wear, lubrication breakdown and failure.
- Sealed/lubed-for-life bearings will last forever. Bearing life depends on the grease, which is affected by the operating conditions (speed and load) and environment (temperature and contamination). Grease life improves with enhanced seals, proper installation and proper selection.
- Re-lubrication once a year is sufficient. Start by reviewing the manufacturer’s lubrication recommendations, but actual intervals may vary quite a bit, depending on load, speed, temperature or environmental conditions. Applications with higher speeds, temperatures or heavy contamination sometimes require weekly or even daily relubrication. By contrast, a mounted ball bearing in a lightly loaded, low-speed, clean environment may do fine with 12- to 24-month intervals. Certain applications may need to be monitored and lubrication intervals/amounts adjusted accordingly.
- Re-lubrication replenishes grease that temperature breaks down or deteriorates. Pumping in new grease also helps flush away contamination.
- Always add grease until it purges from the seal. Doing so will probably fill the bearing cavity, which increases operating temperature and may create enough pressure to blow out the seal. However, in low-speed or dirty conditions where contamination may easily enter the seals, filling a bearing may help improve performance. Application experience will dictate when the entire bearing cavity should be filled.
- Add grease if a bearing makes noise. Noise indicates internal damage has likely occurred. Adding grease may provide temporary relief, but a noisy bearing should be closely monitored and replaced at the first opportunity. Be sure to investigate the root of the failure.
- Any grease will do. Greases do differ. Some may be incompatible because of the different thickeners (soaps) used. For example, many electric motors use a polyurea thickener while some mounted ball bearings use lithium complex thickeners. These greases are borderline compatible, but depending on the actual make up, may not work together. Grease types will also be incompatible based on the viscosity or type of oil in the grease.
- Just shoot grease through the fitting. It’s better to always clean grease fittings and the grease gun tip. Put the tip in an oil bath or protect it with a plastic cover. A plant’s uptime and OEE may “turn” on bearing health. If you are falling short of desired operational life, ask the bearing manufacturer to assist you with selection and troubleshooting.
Protect Bearings from Voltage Discharge
In addition to being the victims of various myths, ball bearings are also very susceptible to voltage discharges that are created by a few sources. Jay Carlson, writing at the IEN website, explains how easy it is to burn your ball bearings.
(Ed. Note: the link at IEN is no longer valid, so I’m linking to the article via waybackmachine.org. I created a PDF version of the article which you can view here. Thanks to Mike Skoglund for pointing out this problem.)
Ball bearings in electric motors support and locate the rotor, keep the air gap small and consistent, and transfer loads from the shaft to the motor frame. When a stray current in a machine uses a bearing as its path to ground, the resulting damage is referred to as electric arc bearing damage. The most common causes of electric arc bearing damage include asymmetry in the motor’s magnetic circuit; unshielded power cables; and fast-switching variable frequency drives (VFDs).
Once electric arc bearing damage has begun, excessive vibrations, increased heat, increased noise levels, and the reduced effectiveness of the lubricant will contribute to shorten a bearing’s service life. The extent of damage to bearings will depend on the amount of energy and its duration. However, the effect usually will be the same: pitting damage to the rollers and raceways, rapid degradation of the lubricant, and premature bearing failure.
Carlson explains that arcing results from a difference in potential between the bearing housing and the shaft. The amount of voltage varies because of the size of the ball bearing, type of bearing cage, and the design of the seal. There is a higher chance of arcing if the bearing is equipped with pressed steel shields because the air gap between the electrically conducting shield and the bearing ring is the insulation.
How Damage Results
When an electric current passes through the contact zone of a bearing’s rolling elements and raceway, the energy of the electric discharge generates heat, causing localized melting of the surface. The effect on a bearing is almost like a series of small lightning strikes, which melt and re-temper internal bearing surfaces. The outcome is that some surface material flakes away and spalls out to create noise in the bearing and potentially shortened service life.
Cratering is perhaps the most commonly experienced effect of electric arc damage. This type is characterized by molten pit marks (invisible to the eye). A dull gray surface of the rolling element will send a visual warning sign of cratering to telegraph that bearing deterioration is present.
Another telltale warning sign will present itself as characteristic fluting(or washboarding) patterns in the raceways of bearings. Fluting is caused by the dynamic effect of the rolling elements continually moving over the micro-“craters” and etching a rhythmic pattern into the running surfaces of a bearing’s races. Noise and vibration from the bearing increases and, eventually, the deterioration will lead to complete bearing failure.
Should electric arc bearing damage be suspected, bearings should be replaced and proper insulation should be provided to prevent electric currents from passing through: Hybrid ball bearings (which substitute ceramic balls for steel rolling elements) offer an advanced and practical solution.
There is, Carlson writes, another solution to the voltage discharge problem: using hybrid bearings, which include exchanging the steel rolling elements for ceramic balls.
Hybrids incorporate rings made from bearing steel and rolling elements manufactured from bearing grade silicon nitride. Because silicon nitride has high resistivity, hybrid bearings provide ideal insulation from electric currents both in ac and dc motors. Hybrid bearings further possess a higher speed capability and can sustain longer service life than all-steel bearings in most applications for a variety of reasons.
There are several key characteristics of hybrid bearings when comparing them to all-steel bearings.
- Lower density: Silicon nitride balls are 40% less dense than similarly sized steel balls, reducing centrifugal force and friction. This means higher speeds, less weight, lower inertia, and more rapid starts and stops. In short, the bearings can run faster and cooler.
- Higher hardness: Ceramic balls are harder than both steel and most potential particle contaminants. This means the bearings can eliminate contaminant particles either by crushing them or pressing them into the (softer) steel rings, where they can be rendered harmless.
- Lower friction: Silicon nitride’s low coefficient of friction enhances wear resistance to enable the bearing to run cooler even under poor lubrication conditions. This means better lubrication, less noise, and lower operating temperatures.
- Higher modulus of elasticity: Ceramic rolling elements have a 50% higher modulus of elasticity than steel. This means increased bearing stiffness and reduced deflection under load to promote reliable performance.
- Lower coefficient of thermal expansion: Ceramic rolling elements have a thermal expansion of only 29% of similar steel rolling elements. This means less sensitivity to temperature gradients for more accurate pre-load control.
- Lower maintenance and energy costs: Maintenance costs can quickly multiply if a bearing must be changed frequently and an extension in the service life of a bearing without increasing maintenance costs can contribute to reductions in the overall operating cost of equipment. Less friction adds up to lower energy costs.
- Extended service life: Most bearings are designed into applications based on loading conditions and do not take into account factors such as lubrication, contamination, and maintenance. Without proper attention to these external factors, a steel bearing rarely reaches its optimized design and service life. The properties of ceramics combine to hold the promise of service life up to 10 times that of a standard steel bearing.
- Extended grease life: In environments imposing high demands on the bearing lubricant, standard bearings experience surface wear due to insufficient lubricant film and bearings can fail if the initial grease charge is not replenished within an acceptable timeframe. Hybrid bearings run cooler and can operate with thinner lubricant films, so there is less aging of the grease and re-lubrication intervals can be longer for increased service life compared with standard bearings in the same operating conditions.
- Lower operating temperatures: The heat generated in bearings is attributed to viscous friction between the balls and raceways. The source of the loading is both external and internal, and little can be done to reduce the external loads. However, since ceramic balls have only 40% of the density of steel balls, less centrifugal load is generated by the balls and the internal friction is lower. This translates to cooler running for the same operating conditions (or, if applicable, a higher rotational speed while maintaining the same temperature).
- Reduced wear from contamination: In contaminated environments solid particles create dents in the rolling surfaces and raised edges around those dents, causing noise and premature wear as steel balls roll over those surfaces. The harder ceramic ball material puts contaminants in their place.
- Reduced wear from vibration: In equipment exposed to static vibration there is an inherent risk of false brinelling (wearing away of the surfaces within the ball and raceway contacts), which can eventually lead to spalling and premature failure. Lighter-weight ceramic balls minimize the potential for false brinelling.
Experienced product and service partners can serve as reliable resources to help keep users current about these and other remedial solutions for electric arc bearing damage.
As you can see, properly maintaining and troubleshooting your ball bearings is a complex task. By following your motor manufacturer’s maintenance instructions, you not only can extend the lifespan of your equipment, but you also ensure that you will not break any warranty issued by the manufacturer.