A Real-World Approach To Improved Lubrication, Part II: Electric Motor Lubrication
EP Editorial Staff | May 29, 2012
Attention to these best practices and troubleshooting techniques can help deliver the TLC your motors deserve.
(Author’s Note: Much of the information in this series is based on the practical knowledge of real-world lubrication professionals. Once such expert is Mark Kavanaugh who has over 42 years of experience in large manufacturing operations, and is currently responsible for coordinating the lubrication of thousands of pieces of rotating equipment in a refinery. Mark is certified as a CLS, MTL I and MLA II.)
Electricians, oilers, lube technicians and operators hold the key to electric-motor reliability: It’s their ability to be proactive about their equipment. That means working to prevent motor failure with proper lubrication practices and early identification of potential problems through proper troubleshooting techniques.
Electric motors are a major component of every plant—possibly accounting for up to half of a site’s rotating equipment. They are also among the least-understood and least-appreciated equipment categories. Many articles have been written on electric-motor lubrication, in particular grease lubrication. Here (in the second article of a seven-part series), we discuss the three major motor-lubrication strategies: oil, grease and oil mist.
Oil is used with many large motors. Horizontal or vertical in design, such units can be bath- or pressure-circulation-lubricated. Sump capacities range from three quarts to several gallons.
Fig. 1. This horizontal motor is lubricated with an oil bath maintained by bottle oilers.
Figure 1 shows a horizontal motor that’s lubricated with an oil bath maintained by bottle oilers. Many larger motors have sleeve bearings and are lubricated by a flinger or slinger ring in an oil bath. For sleeve bearings applications at speeds of 3600 RPM, ISO 32 rust- and oxidation-inhibited oil (R&O) is recommended. At speeds of 1800 RPM and lower, ISO 68 oil is recommended. In areas with high ambient temperatures, some plants use an ISO 68 for all normal-speed equipment and ISO 100 for very low speeds.
Fig. 2. Vertical motors with top thrust bearings, like the unit shown here, are normally oil-lubricated.
Figure 2 shows a vertical motor with a top thrust bearing—such units are normally oil-lubricated. Most common thrust-bearing types are angular-contact or spherical-roller in nature: They’re flooded. The bottom bearings are usually sleeve or rolling-element types. Recommended lubricants are R&O ISO 32 and ISO 68 for ball-type and ISO 150 for spherical rollers.
A major indicator of oil-lubricated electric-motor problems is an increase in equipment temperature. During a motor inspection, it’s important to measure that temperature increase, preferably with an infrared non-contact thermometer (commonly referred to as a heat gun). The operator should be aware of what the normal temperature range for a specific motor is—maybe 150-160 F—and report major variances of 10-20 F degrees so that proper condition-monitoring tools can be applied to identify the problem.
Proper use of the heat gun is a must. Consistency on where/from what distance to take a temperature reading is crucial. On some heat-gun models, sighting in where two bisecting infrared dots merge will establish an accurate, repeatable distance from the target. Be sure to increase the reading by 10-15 F degrees to account for the motor’s housing thickness. Direct oil-temperature readings should be taken from the sight glass or bottle oiler. Common problems that lead to overheating include:
- Low oil level
- Inoperative slinger ring
- Insufficient bearing clearance
- Shaft damage
- Oil-seal rubbing on shaft
- End thrust on bearing face
- Plugged oil passages in a circulating system
- Lubricant contamination
If slinger rings are used, they need to have the right oil level to operate properly. The ring is normally 1.5 times the diameter of the shaft and the oil level needs to be between 1/8” – 3/8” from the inside bottom of the ring. Equipment misalignment will cause slinger rings to be cocked and not function properly. A better alternative is to attach a flinger directly to the shaft to properly lubricate the motor bearings. The oil level associated with a flinger isn’t as critical as with a slinger ring—and is usually close to ½” from the outside bottom of the flinger.
Fig. 3. Horizontally mounted, grease-lubricated motors are more common than vertical motors (which, in some cases, can also be greased).
Many articles have been written on the grease lubrication of electric motors. This section will summarize some of the best practices from different plants.
Horizontally mounted, grease-lubricated motors (like the unit shown in Fig. 3) are more common than vertical motors (which can, in some cases, also be greased). The most common bearing found in horizontal motors is the single-row deep-groove type. In some cases cylindrical rollers are used. Some of the major questions to be addressed in the greasing of electric motors bearings are:
- Should the motor be stopped or running during greasing?
- What type of grease should be used?
- How much grease should be added?
- How frequently should the motor be greased?
- What is the proper procedure in greasing a motor?
Some OEMs recommend greasing motors while stopped; others recommend greasing as a motor runs—most plants take the latter approach (running). Greasing stopped motors is utilized in some plants during shutdowns.
New or rebuilt motors are greased stopped—until grease emerges from the vent plug to be sure both the supply and vent lines are filled before operation. Upon startup, the vent plug should be open for at least 30 minutes to expel any excess grease.
The most popular grease for electric motors is now a low-noise polyurea thickener NLGI 2 with a mineral viscosity of 100-120 cSt. These greases are also usually low-bleed, and some have synthetic PAO as the oil. Many people still use an NLGI 2 lithium-complex thickener with a synthetic or mineral oil. Unfortunately, different grease thickener types often have compatibility problems—for example, some polyurea and lithium thickeners are incompatible. To avoid incompatibility problems during an electric-motor rebuild, it’s critical to use the same grease as the plant will be using to lubricate it.
There are a number of ways to determine the proper amount of grease to add to electric motors. OEMs publish tables on the proper amount to add in cubic inches, cubic centimeters, ounces and grams based on the bearing number or frame size. For example, 1.8 in3 is equivalent to one ounce by weight of grease. The proper amount to add can also be calculated based on the formula: Gb = DB/10 (where Gb = ounces of grease, D = bearing outside diameter in inches, and B = bearing width in inches).
Once the calculation has been made, the number of shots per ounce needs to be determined based on the type of grease gun being used. You have several options for this:
- Use a grease meter that attaches to a grease gun and measures the amount dispensed in ounces and cubic inches.
- Use a postage scale and weigh out ten shots and convert to shots per ounce for that particular grease gun.
- If available, fill an old 35mm film canister with grease and count the strokes. This is about one ounce of grease.
Because there is a large variance in strokes per ounce for different grease guns, try to use the same type for all of your electric motors. A good idea is to color-code your grease guns. Put a colored rubber sleeve around a gun, and attach a rubber zerk cap of the same color to the electric motors on which you’ll use the gun.
How frequently a motor should be greased is related to factors such as speed, ambient temperature, horsepower, severity of operation and load configuration. You should initially follow the OEM’s guidelines and modify based on your conditions and experience.
It should be noted that vertically mounted electric motor bearings are greased twice as frequently as horizontally mounted bearings. Table I lists general guidelines for greasing frequency.
Table II shows frequency guidelines for NEMA continuous-duty electric motors that a large manufacturing operation determined for itself.
Procedures for greasing electric motors properly have been the topic of many articles—suffice it to say that there are many different ways that constitute “proper.” The strategies presented here are based on the procedures used in several successful industrial lubrication programs.
In many respects, bearing configurations determine a proper greasing procedure. The following guidelines should be considered:
- Small sealed-for-life bearings should never be greased. To prevent accidental/inadvertent greasing, remove or plug the zerk. (Such greasing has been known to cause motor failure.)
- Double-shielded bearings usually are not greased. Although not recommended, if you elect to grease these bearings, be sure to do it very slowly; use half the amount of grease calculated; and reduce the interval by half.
Most electric motors are either open- or single-shielded on the inboard side. The PROPER PROCEDURE described below focuses on these bearing configurations.
Keep in mind that too many motors are over-greased, which can lead to premature bearing failure. Work done by John Underwood at DuPont has shown that when running motors are greased properly, very little lubricant (if any) escapes from the purge plug.
The PROPER PROCEDURE is to calculate the correct number of shots and to add no more. If grease comes out of the plug before you reach the calculated number of shots, STOP GREASING. (Be sure to remove the purge plug and clean out the line with a wire brush before you begin applying the grease.)
Ultrasonic technology—for helping determine the correct amount of lubricant and frequency of application—is another helpful tool in the proper greasing of motors.
- If using ultrasound, attach the probe to a safe location near the bearing. If the audible level is less than the established baseline, no lubrication is required. As an example, two plants I am familiar with have readings of 40-50 decibels for their typical level and no lubrication is required at or below these levels.
- Remove grease purge plug, if accessible, and clean out hardened grease with a wire brush. If a Gits cup or Alemite relief-type valve is in place, this is done automatically.
- Wipe zerk and grease-gun tip with a lint-free rag.
- Safely discharge one stroke from gun into the rag.
- Slowly add the correct amount of grease.
- Calculate the number of shots based on bearing dimension.
- Inject the calculated number of shots.
- Discontinue if, at any time, grease emerges from vent.
- Discontinue if resistance is encountered.
- One petrochemical operation has used the following procedure successfully for many years.
- Remove purge plug and clean out vent.
- Install a plastic tie-wrap until it touches the bearing.
- Slowly add grease until tie-wrap moves slightly.
- If the calculated number of shots is reached before movement of the tie-wrap, discontinue greasing.
- With ultrasound probe properly attached, slowly add grease.
- Continue to add grease as sound level decreases and until its level starts to rise.
- If an audible rise doesn’t occur, discontinue greasing when the calculated number of shots has been reached or if grease comes out of the vent plug.
- Remove probe from motor.
- Calculate the number of shots based on bearing dimension.
- Once the greasing has been completed, let the motor run at last 30 minutes to expel any excess grease before reinstalling the purge plug.
Who performs the greasing of electric motors varies from plant to plant. Some sites will only permit electricians to do it; others will allow it to be done by operators and lube technicians.
Some plants that utilize ultrasonics have one designated person to do all the greasing. Such a strategy offers the advantage of consistency in the greasing program—if possible, this approach is highly recommended.
It’s interesting to note that when ultrasonic technology is employed in the greasing of electric motors, in many cases the amount of lubricant used is significantly less than the calculated amount. What this means is that even with calculated amounts, over-greasing is still occurring. That said, electric-motor ball bearings simply don’t require much grease.
Oil-mist lubrication has been a staple in the refining and petrochemical industries for over 50 years. This technology offers a number of advantages in the lubrication of electric-motor bearings:
- Significantly cooler-running bearings (15-30 F)
- Clean once-through lubrication
- Greatly reduced bearing failures
- Trouble-free lube system (no moving parts)
- Alarm systems that monitor oil level and flow rate
One study, conducted over a three-year period, evaluated oil-misted and greased electric motors and recorded associated failures. The results were noteworthy: The greased motors in this study had 40 failures out of 16 motors. The misted units had just two failures out of 400 motors.
Fig. 4. One oil-mist system can lubricate equipment installed over a distance of 600 linear feet. (Source: Lubrication Systems Co. [LSC])
Figure 4 illustrates a typical oil-mist system. One system can lubricate equipment over a distance of up to 600 linear feet. The system consists of the following components:
- Central oil-mist console
- Distribution network
- Oil-mist collection containers
The mist that’s generated is one part oil to 200,000 parts air, with the oil particles less than three microns in size. A reclassifier converts the mist to a light film of oil that lubricates the bearing. The typical system operates at 10-20 inches of water-column pressure—most try to run at 20, which is less than one PSIG.
Oil-mist systems typically use ISO 68 polyalphaolefins and diesters. Since the clearances are very tight in the reclassifier orifice, any wax buildup from the oil at cold temperatures will plug them. Many plants in warmer climates use an ISO 68 or ISO 100 Group II mineral oil with low-paraffin content.
Fig. 5. In a typical oil-misted electric-motor application, dry mist flows through a reclassifier to produce wet mist that’s deposited as a light lubricant film on the bearing. (Source: LSC)
Figure 5 illustrates how oil-mist technology works on an electric motor: The dry mist flows through a reclassifier to produce wet mist deposited as a light lubricant film on the bearing.
To avoid potential problems with oil mist, the motor-bearing housing should be refitted with labyrinth-type bearing protection (such as Inpro/Seal bearing isolators or Iso mag seals). Bearing and stator housings must be fitted with open drain lines that are piped to the oil-mist collection pot. Motor service leads must be sealed in the conduit at the junction box to prevent the mist from traveling back to the switch house.
The following is a useful checklist for operators of oil-misted electric motors:
- Daily… Check for green light on top of mist console. Perform visual mist inspection at bearing housings and collection pots. Check mist console to verify mist header pressure, mist quality, air pressure, air temperature, oil temperature, reservoir oil level and supply-drum level.
- Weekly… Drain collected oil from header drain legs, manifold drops, collection pots and bearing-housing sight glasses. Check distribution system for leaks or sags in the header piping.
- Quarterly… Ensure PM and inspection of the entire system by the vendor or qualified inspector.
Although electric motors are critical to the operation (and, ultimately, the profitability) of most plants, in some facilities they’re one of the most neglected equipment categories. Don’t let this be the case at your site—especially when it comes to lubrication.
There are countless opinions on how to properly lubricate motors. This article has attempted to encapsulate a range of best practices employed by some of the most successful industrial lube programs. One of its key points and most valuable takeaways is this: Just because a little lubrication is good, does NOT mean more is better.
Greasing a motor until your chosen lubricant comes out of the purge plug will not improve the health of your equipment. This technique—which, sadly, is still used in too many facilities across industry—can result in severe over-greasing and, in many cases, shorter motor-bearing life. In fact, it would almost be better NOT to grease at all than to use this method.
The May/June installment of this series will cover proper pump-lubrication and troubleshooting techniques. The focus will primarily be on centrifugal pumps. LMT
Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training for operations around the world. Email: firstname.lastname@example.org