Friday, August 1, 2014

Electric Motors Lubrication guide; Lubrication of electric motors

 Electric Motor Bearing Lubrication
Proper lubrication of moving part is very essential so as  lubrication of electric motor bearings is essential to maintaining them in peak operating condition which will leads to reducing unnecessary motor downtime.

Grease Lubrication
Motor bearings greasing is done because grease has properties as a lubricant of its simplicity of application and unique characteristics.

The primary functions of an electric motor bearing grease are to:
• Reduce friction and prevent wear
• Protect bearings against corrosion
• Act as a seal to prevent entry of contaminants

Grease is a semi-solid lubricant composed of following ingredients
(i)             Base oil
(ii)            Thickener and
(iii)           Additives.

 These components are combined in complex chemical reactions under controlled temperatures and pressures.
 The base oil used in greases may be
(i)             Mineral
(ii)            Synthetic.

Mineral oils are adequate for most electric motor bearing applications. However, synthetic base oils may be required for extreme temperature applications or where longer re-greasing intervals are desired.

Main function of  the thickener is that it serves as a carrier for the oil and prevents it from leaking out of the application. Some common thickeners include metallic soaps that can be composed of calcium, lithium, sodium, aluminum or barium and complex metallic soaps such as lithium-complex. A thickener increasingly employed in electric motor bearing lubrication is polyurea, Unirex N3.
Polyrex EM greases utilizes a polyurea thickener.

As with many lubricating oils, additives are frequently used to impart special properties to the grease.

Commonly used additives include, corrosion inhibitors, anti-wear or extreme pressure agents, oxidation and corrosion inhibitors, pour point depressants, lubricity agents, and dyes or pigments.

Choosing the Right Electric Motor Grease

The following criteria may be used as typical indicators of a good electric motor grease:
(i)              Viscosity:
 The typical mineral oil viscosity in an electric motor grease is in the range of 500 to 600 SUS at 100°F .Oil viscosity should be appropriate for the load and speed of the application at operating temperature. This will help to insure maximum protection and component life. Your electric motor builder may provide a specific recommendation.

(ii)             Consistency:
A grease’s consistency or firmness is stated in terms of its NLGI (National Lubricating Grease Institute) grade, which ranges from 000 to 6. The consistency of a grease should be appropriate to the application, as it affects pumpability and ability to reach the areas to be lubricated. A NLGI 2 grade grease is the most commonly used in electric motor applications.
(iii)            Oxidation Resistance:
 Electric motor greases should have outstanding resistance to oxidation. This extends the life of bearings running at high speeds and high temperatures. ASTM D 3336 High Temperature Grease Life test results give a good indication when operating under extreme conditions. Choose a grease with a high ASTM D 3336 oxidation life.
(iv)               Anti-Wear:
Unless a motor is mounted so there is a thrust load on the bearings, it is generally advisable to use a grease without extreme pressure (EP) additives. EP additives can shorten the life of the grease and should not be recommended where they are not needed. On the other hand, bearings designed to handle heavy thrust loads may require a grease with an EP additive.

(v)            Dropping Point:
The dropping point gives an indication of the temperature at which the grease will melt or the oil will separate from the thickener. Due to the high temperatures that can be reached in an electric motor bearing, a grease with a high dropping point is frequently desirable. Lithium complex greases and polyurea-thickened greases both have dropping points of approximately 500°F or higher.

(vi)            Shear Stability:
 ASTM D 217 Cone Penetration of Lubricating Grease test measures the consistency of the grease after it has been worked 100,000 strokes. An electric motor bearing grease should soften no more than 1 to 1.5 NLGI grades in this test. An electric motor bearing grease that softens more than that may leak out of the bearing with age.

Adding Grease to Electric Motor Bearings
Greasing of the motor bearings is done through points provided at motor bearing ends. It is important to know when to do greasing of motor and what quantity will be required for greasing of motor as lower amount of grease and extra amount of grease will leads failure of motors.

Re-greasing Intervals
Electric motors utilizing double shielded or double sealed bearings also known as double Z bearings, which are typically of the lubricated-for-life design, usually do not require re-greasing.
On the other hand, all others, those being open or single shielded or sealed bearings, should be re-lubricated periodically to replace grease that has deteriorated, leaked away, or become contaminated. Generally, operating conditions will dictate the re-lubrication interval required.
All greases deteriorate at some rate, even under moderate operating conditions. The principal causes are oxidation, excessive oil bleeding, and mechanical working. At high temperatures, oil evaporation may also be a factor. Oxidation eventually increases the oil viscosity and hardens the soap. Some oil bleeding is desirable, but too much reduces the ability of the grease to maintain an effective lubrication film. Mechanical working, or shearing, may change grease properties such as consistency, making the grease less suited to the application. Excessive oil evaporation may harden the grease. Deterioration often ends in hard, dry, deposits that can neither lubricate bearings nor protect them against contaminants.

Operating and other factors that influence relubrication frequency include: temperature, continuity of service, quantity of grease in housing, size and speed of bearing, vibration, exposure to contaminants, effectiveness of seals, and the grease’s suitability for the particular service.

1. High grease temperatures increase the oxidation rate, doubling it for every 18°F (10°C) rise above 120°F (49°C). High temperature also tends to increase the rate of bleeding and evaporation of the oil. Additionally, grease tends to soften as temperatures increase and may become fluid enough to leak out of housings. Other things being equal, operating at high temperatures will require more frequent relubrication, or the use of a high temperature grease.

2. Continuity of service means hours of service per day or other time unit. A grease continuously subjected to deteriorating factors will need replacement more often than the grease in a bearing used only occasionally.

3. A large quantity of grease in a properly designed housing will last longer than a small quantity in a proportionally smaller housing. The small quantity will be reworked more often than an equal portion of the large quantity and will not benefit from reserve capacity (including more oil and additives). Under moderate conditions, however, a small quantity of grease in a factory lubricated sealed or shielded bearing may last a long time, perhaps several years.

4. The Dn value of a bearing (bore diameter in mm x speed in rpm) is proportional to the linear speed of the rolling
elements and may be used as a guide to determine relubrication frequency. In bearings operating in the Dn range of 150,000 to 200,000 or more, grease in the path of the elements is severely worked and heated. Such bearings require more frequent relubrication even with correctly selected grease that does not slump excessively. Some bearing manufacturers use Ndm (speed in rpm x pitch diameter of the bearing) instead of Dn. This method produces somewhat higher reference values, but considers the effect of rolling element size and the bearing’s cross section dimensions.
5. Vibration causes grease to feed more freely into the rolling elements’ path, where it is worked and heated excessively. This reduces grease life, especially in high speed bearings. Churning and shearing in bearings “mills down” some greases, which become fluid enough to leak excessively. Either factor means more frequent relubrication.

6. More frequent relubrication usually will be required if the grease is marginal in any major characteristic — oxidation, bleeding, pumpability, antiwear and antirust properties, or mechanical stability.

It is not a simple matter to decide when and how often to relubricate. Generally, the decision reflects experience and the machine builder’s and grease supplier’s recommendations.
Relubrication intervals for most rolling element bearings range from two weeks to two years although for many it is once a year during scheduled maintenance shutdowns. At the lower extreme, bearings running at or near their speed limits may require relubrication as often as every six to eight hours.

It is important to regrease on an appropriate schedule so that the old grease remains soft enough for purging. Bearing or equipment manufacturers recommend relubrication intervals based on operating conditions and type of grease. Typically, light to medium duty electric motors, that run continuously, will require at least annual relubrication. Reduce the relubrication interval by half for every 10°C above the nominally recommended temperatures.

Two commonly used methods for determining the correct  relubrication frequency follow.

1. The first utilizes the following equation:
Frequency (hours) = {[14,000,000/(shaft rpm)(Bearing ID)1/2
mm] — [(4)(Bearing ID) mm]}{F bearing type}{F temperature} {F contamination}
F bearing type = 1.0 for spherical or thrust bearing, 5.0 for cylindrical bearing, 10.0 for ball bearing
F temperature = 1.0 for under 160°F, divide by two for everyc20°F above 160°F
F contamination = 0.1 to 1.0 depending on the level of contamination—motor bearings normally 1.0

2. The second method utilizes the following graph for determining relubrication frequencies:

Determining the Correct Amount of Grease
Determining the correct amount of grease for an electric motor bearing is one of the most important steps in initial greasing and in regreasing of the bearings. An insufficient amount of grease could lead to bearing failure due to lack of lubrication. On the other hand, over-lubrication can also lead to bearing failure and cause problems due to migration of the lubricant in to the windings. One of the two methods following is frequently used for determining the quantity of grease to be added to a bearing:
1. 1/2 to 2/3 of the free space in the bearing — when operating speed is less than 50% of the limiting speed of the bearing.
1/3 to 1/2 of the free space — when the speed is more than 50% of the limiting speed of the bearing.
2. Another method of determining the appropriate quantity of grease to fill the bearing is determined by the following equation. This is a simple method of calculating the amount of grease needed for a standard application.
Quantity of grease (g) = Outer bearing diameter (mm) X bearing width (mm) X 0.005, or
Quantity (oz) = 0.114 X (bearing OD) in X (bearing width) in
It is common practice to pack the bearings as well as the bearing housing with grease. In addition to holding the bearing in place, the bearing housing also acts as a grease reservoir. The following may be used as a guide to filling the housing with grease.\

• 30% to 50% fill
Typically used. For very high speeds the lower limit should be used in order to reduce churning and overheating of the grease. Overpacked bearings tend to overheat, and to overheat even more at higher speeds.
• 50% to 75% fill
For slow speeds, or in the absence of other methods of regreasing, fill the housing 50% to 75% with grease. After the housings are packed and the motor started, the rolling elements will push the excess grease from between the races into the housing, leaving only the thin lubricant film needed to minimize friction and wear.
• Full pack
 A particularly dirty environment may call for the housing to be completely filled, but the bearing itself will only contain enough grease for lubrication. The pressure relief method will also produce a full pack.

One full pack method begins with the bearing filled with grease and the housing 75% full, leaving just enough space to receive the excess grease pushed out by the rolling elements. If the housing were actually packed full, the grease between the rolling elements could not escape and would be severely worked.