Lubrication update for electric motors

By Heinz P. Bloch, PE, Process Machinery Consulting

Australia’s largest oil refinery, BP’s Kwinana, is among the best-of-class refineries using oil mist lubrication on machinery with rolling element bearings. In its process pumps, the refinery has enjoyed the benefits of oil mist technology for many years; a recent report mentioned an MTBF (mean-time-between failure) somewhere between 114 to 120 months. As of early 2019, they still haven’t had a lubrication-related bearing failure since 1999. The facility’s ACHE (air-cooled heat exchanger) fan bearings haven’t been touched since installation in 2004. The same 15-years-and-still-going feedback was obtained for plummer block bearings in BP’s Kwinana’s ID (induced draft) and FD (forced draft) fans.

API610, the most widely used pump standard in the petrochemical and refining industries, includes experience-based recommendations for lubricant application, and one of these relates to oil mist. The API standard asks for oil mist to be routed through the bearings (Figure 1).

Although intended for pumps, this same recommendation will work equally well for electric motor rolling element bearings.

The resulting diagonal through-flow route guarantees adequate lubrication, whereas oil mist entering and exiting on the same side might allow some of the mist to leave without first wetting the rolling elements.

Through-flow is thus one of the keys to a successful installation. This article addresses how oil mist lubrication is widely used for electric motors.

Major electric motor manufacturers, including the former Reliance Electric Company (Cleveland, Ohio), were fully aware of this fact. Representing “best technology,” their mid-1970s bearing housings were configured for through-flow.

Moreover, a very wide oil mist suitability range is documented for electric motors. Industrial giant Siemens has, years ago, published technical bulletins showing pure oil mist as a superior technique for electric motors ranging in size from 18 to 3000kW.

Bearing size constraints and synthetic lubricants for electric motors

Decades of experience confirms the success of oil mist for rolling element bearings in the operating speed and size ranges found in motors for process pumps.

Since about 1960, empirical data have been employed to screen the applicability of oil mist. The influences of bearing size, speed and load have been recognised in a rule-of-thumb oil mist applicability formula, limiting the parameter “DNL” (D = bearing bore, mm; N = inner ring rpm; and L = load, lbs) to values below 10^9, or 1,000,000,000.

An 80mm electric motor bearing, operating at 3,600 rpm and a load of 600lbs, would thus have a DNL of 172,000,000 – less than 18 per cent of the allowable threshold value.

As of 2019, approximately 50,000 oil mist lubricated electric motors are operating flawlessly in reliability-focused user plants.

Capitalising on this favourable experience, the procurement specifications for both new projects and replacement motors (with rolling element bearings) at many of these plants require oil mist lubrication in sizes 15kW and larger.

Although it was well known that synthetic lubes reduce friction, little quantitative work had been done before 1980. Morrison, Zielinski and James1 quantified how diester-based lubricants reduce the frictional power losses of industrial equipment; their findings are summarised in Tables 1 and 2.

Power loss per bearing (KW)
L = 8.9 KN (2000 lbf)	Oil Sump	Oil Mist
MIN 68	0.271	0.192
SYN 32	0.254	0.169

Table 1: Overview of power loss with different oils and application methods1.

Change	Δ Power loss per bearing	Total reduction
Sump:MIN 68 to SYN 32	0.017	6%
Mist: MIN 68 to SYN 32	0.022	8%
Sump MIN 68 to Mist MIN 68	0.080	29%
Sump SYN 32 to Mist SYN 32	0.085	31%
Sump MIN 68 to Mist SYN 32	0.11	38%

Table 2: Overview of power loss and loss reduction percentages with different oils and application methods1.

The potential cost savings through power loss reduction are quite substantial. It has been estimated that industrial machines consume 31 per cent of the total energy in the US2.

As much as five per cent of the mechanical losses of these machines could be avoided through a combination of improved equipment design and lubricant optimisation.

Motor sealing

Motor sealing and mist drainage are well understood. Although oil mist will neither attack nor degrade the epoxy insulation on electric motor windings manufactured since the mid-1960s, mist entry and related sealing issues merit inclusion in this overview.

Regardless of motor type, i.e. TEFC, X-Proof or WP ll, cable terminations in junction boxes should not be made with conventional electrician’s tape.

The adhesive in this tape will last but a few days and then become tacky to the point of unravelling. Inferior products are replaced by superior materials; these are often Teflon®-based. For termination leads (“T-Leads”), competent motor manufacturers use an irradiation cross-linked polymeric insulation system that is highly resistant to oil mist.

To date, irradiation cross-linked polymeric insulation systems have consistently outperformed the many other “almost equivalent” systems.

Similarly, and while it must always be pointed out that oil mist is neither a flammable nor explosive mixture, it would be unsightly to allow a visible plume of mist to escape from the junction box cover.

The wire passage from the motor interior to the junction box should, therefore, be sealed with 3M Scotch-Cast Two-Part Epoxy potting compound. Doing so will exclude oil mist from entering the junction box.

Finally, it is always good practice to verify that all electric motors have a small (3mm) weep hole and that XP-motor drains are given closer attention.

The latter are furnished with either an explosion-proof rated vent or a suitably routed weep hole passage at the bottom of the motor casing or lower edge of the motor end cover. Intended to drain accumulated moisture condensation, the vent or weep hole passage will allow coalesced or atomised oil mist to escape.

Note, however, that explosion-proof motors are still “explosion-proof” with this passage.

Reasoning on the issue should convince us that a motor with its interior slightly pressurized (to 0.1–0.3 psi above ambient) by non-explosive oil mist cannot ingest any explosive vapours from a surrounding atmosphere.

The suitability of oil mist for Class 1, Group C and D locations was specifically re-affirmed by Reliance Electric in July of 2004.

TEFC vs WP ll construction

On TEFC (totally enclosed, fan-cooled) motors, there are documented events of liquid oil filling the motor housing to the point of near contact with the spinning rotor.

Conventional wisdom to the contrary, there neither were, nor will there be, detrimental effects with the oils used in normal industry; the motors could have run indefinitely.

TEFC motors are suitable for oil mist lubrication by simply routing the oil mist through the bearing, as has been explained in a comprehensive text on lubrication.

There are numerous other references, including API610. No special internal sealing provisions are needed with pure oil mist filling a TEFC motor so long as the pressurised mist keeps dirty atmospheric air from entering.

On weather-protected (WP ll) motors, merely adding oil mist has often been done and has generally worked surprisingly well. In this instance, however, it was found important to lead the oil mist vent tubing away from regions influenced by the motor fan.

Still, weather-protected (WP ll) electric motors do receive additional attention from reliability-focused users and knowledgeable motor manufacturers.

Air is constantly being forced through the windings and an oil film deposited on the windings could invite dirt accumulation. To reduce the risk of dirt accumulation, suitable means of sealing should be provided between the motor bearings and the motor interior.

Since V-rings and other elastomeric contact seals are subject to wear, low friction face seals are considered technically superior. The axial closing force on these seals could be provided either by springs or small permanent magnets3.

Also, many modern motors use advanced rotating labyrinth seals with closure O-rings that travel axially4.*

As is so often the case, the user is asked to make intelligent choices. Some low friction axial seals (face seals) may require machining of the motor end caps, but long motor life and the avoidance of maintenance costs will make up for the added expense.

Double V-rings using Nitrile® or Viton® elastomeric material are sometimes used because they are considerably less expensive than face seals.

Sealing to avoid stray mist stressing the environment

Even when still allowed under prevailing regulatory environmental regulations (e.g. OSHA or EPA), air quality and environmental concerns make it desirable to minimise stray oil mist emissions.

It is helpful to recall that state-of-art oil mist systems are fully closed, i.e. are configured so as not to permit any mist to escape. The various bearing housings are sealed with the magnetic seals incorporated in the motor end bells in Figure 1; alternatively, advanced rotating labyrinth seals could be installed5.

Combining effective seals and a closed oil mist lubrication system has, for many decades, represented a well-proven solution. The combination not only eliminates virtually all stray mist and oil leakage, but makes possible the recovery, subsequent purification, and re-use of perhaps 97 per cent of the oil. These recovery rates make the use of more expensive, superior quality synthetic lubricants economically attractive.

For many years PAO and diester-based “synthetic” lubricants have proved to embody most of the properties needed for extended bearing life and greatest operating efficiency.

These oils excel in the areas of bearing temperature and friction energy reduction. It is not difficult to show relatively rapid returns on investment for these lubricants, especially when the system is closed and the lubricant is being reused after filtration6.

Closed systems and oil mist lubricated electric motors give reliability-focused users several important advantages:

• Compliance with current and future environmental regulations
• Extended bearing life and reduced electric motor maintenance budgets
• The technical and economic justification to apply high-performance synthetic oils

PAO and diester-based “synthetic” lubricants embody most of the properties needed for extended bearing life and greatest operating efficiency. These oils excel in the areas of bearing temperature reduction (Figure 2) and friction energy reduction (Figure 3).

 

A composite plot of different changes and power reduction percentages is given in Figure 4. It is not difficult to show relatively rapid returns on investment for these lubricants, especially for closed systems where the lubricant is reused after filtration.

Converting grease-lubricated electric motors to pure oil mist.

When converting operating motors from grease lubrication to oil mist lubrication, consider the following measures in addition to the above:

• Perform a complete vibration analysis. This will confirm or rule out pre-existing bearing distress and will indicate if such work as re-alignment or base plate stiffening is needed to avert incipient bearing failure
• Measure the actual efficiency of the motor. If the motor is inefficient, consider replacing it with a modern high-efficiency motor, using oil mist lubrication in line with the above recommendations. This will allow capture of all benefits and will result in greatly enhanced return on investment
• Evaluate if the capacity of the motor is the most suitable for the application. “Most suitable” typically implies driven loads that represent 75 per cent to 95 per cent of nominal motor capacity. The result: Operation at best efficiency
• Note that converting an overloaded, hot-running electric motor to oil mist lubrication will lead to marginal improvement at best.

The required volume of oil mist is often expressed in “bearing-inches” or “BIs”.  A bearing-inch is the volume of oil mist needed to satisfy the demands of a row of rolling elements in a one-inch (~25mm) bore diameter bearing. One BI assumes a rate of mist containing 0.01 fl. oz., or 0.3ml, of oil per hour.

Certain other factors may have to be considered to determine the needed oil mist flow and these are known to oil mist providers and bearing manufacturers. The various factors are also extensively documented in several references; they are readily summarised as:

• Type of bearing. The different internal geometries of different types of contact (point contact at ball bearings and linear contacts at roller bearings), amount of sliding contacts (between rolling elements and raceways, cages, flanges or guide rings), angle of contact between rolling elements and raceways, and prevailing load on rolling elements.
• The most common bearing types in electrical motors are deep groove ball bearings, cylindrical roller bearings and angular contact ball bearings
• Number of rows of rolling elements. Multiple row bearing or paired bearing arrangements require a simple multiplier to quantify the volume of mist flow
• Size of the bearings, related to the shaft diameter –inherent in the expression “bearing-inches”
• The rotating speed. The influence of the rotating speed should not be considered as a linear function. It can be linear for a certain intermediate speed range, but at lower and higher speeds the oil requirements in the contact regions may differ from straight linearity
• Bearing load conditions (preload, minimum or even less than minimum load, heavy axial loads, etc.)
• Cage design. Different cage designs may affect mist flow in different ways. It has been reasoned that stamped (pressed) metal cages, polyamide cages or machined metal cages might produce different degrees of turbulence. While different rates of turbulence may cause different amounts of oil to “plate out” on the various bearing components, the concern vanishes when oil mist is applied in through-flow mode

Using the right bearing and a correct installation procedure

Very significant increases in bearing life and overall electric motor reliability have been repeatedly documented in the half century since 1960.

Of course, oil mist cannot eliminate basic bearing problems. It can, however, provide one of the best and most reliable means of lubricant application. Bearings must be:

• Adequate for the application, i.e. deep groove ball bearings for coupled drives, cylindrical roller bearing to support high radial loads in certain belt drives, or angular contact ball bearings to support the axial (constant) loads in vertical motor applications
• Incorporating correct bearing-internal clearances
• Mounted with correct shaft and housing fits
• Carefully and correctly handled, using tools that will avoid damage
• Correctly assembled and fitted to the motor caps, carefully avoiding misalignment or skewing
• Part of a correctly installed motor, avoiding shaft misalignment and soft foot, or bearing damage incurred while mounting either the coupling or drive pulley
• Subjected to a vibration spectrum analysis. This will indicate the lubrication condition as regards lubricating film, bearing condition (possible bearing damage) and general equipment condition, including misalignment, lack of support (soft foot), unbalance, etc.

Sealing to avoid stray mist stressing the environment

Closed systems and oil mist lubricated electric motors give reliability-focused users several important advantages:

• Compliance with current and future environmental regulations
• Convincing proof that oil mist lubrication benefits electric motors and the maintenance budget
• The technical and economic justification to apply high-performance synthetic oils

Since the 1980s, modern additives technology has further strengthened wear protection and offers reduced energy consumption with other synthetic base oils and without requiring reductions in viscosity. All are worthy of your consideration.

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