Lubricant Selection and Application for Gears

Selecting lubricants for enclosed and open gear drives.

By LLOYD (TEX) LEUGNER

Great care must be taken when selecting, applying and caring for gear lubricants. When troubleshooting gear drive problems the lubricant type and additive compounds used must be considered. This discussion looks specifically at lubrication for both enclosed and open gear drive types.

Enclosed Gear Drives: The majority of gear sets are used in enclosed gear boxes and the lubricants used are subjected to very severe service (Figure 6). Typically, the lubricant is thrown to and from gear teeth, shafts and support bearings in the form of mist or spray provided by either splash or pressure fed systems (Figures 7 & 8).

Lubricants applied in these atomized conditions are subjected to the oxidizing effects of air with the violent churning and agitation of the lubricant by the rotating gear sets. This action also raises the temperature, which further increases the rates of oxidation. Sludge and varnish deposits may result, which in turn reduce oil flow or interfere with heat removal in oil coolers.

These conditions may in turn reduce viscosity to the point where boundary lubrication between gear teeth (asperity contact) may occur, raising the temperatures even further. Eventually these conditions, combined with fluid friction, will cause lubricant failure, gear and bearing damage, or both.

The final operating temperature of the lubricant is a function of three conditions; fluid friction, heat generated by any tooth contact and the ambient temperature surrounding the gear case. As an example, a temperature rise of 50°C (90°F) of the oil, combined with an ambient temperature of 15.6°C (60°F), will result in a gear drive operating temperature of 66°C (150°F).

As a general rule, oil temperatures of enclosed gear drives should never exceed 87.8°-93.3°C (190°-200°F) and the best service will be obtained at temperatures in a range of 50°-60°C (120°-140°F). Where high-temperature conditions occur, oils of lower viscosity with excellent oxidation stability might be considered, or oil coolers installed. Where severe high-temperature or high-load conditions exist, polyalphaolefin (PAO) synthetic lubricants may also be considered.

In addition to the temperatures encountered and the methods of lubricant application, the lubricant selected for enclosed gear drives depends upon several additional considerations. These include gear type, speed, surface finish of gear teeth, reduction ratio, load characteristics and operating conditions such as contamination levels.

For example, the recommended lubricant used in heavily loaded herringbone or spiral bevel gears with pitch line velocities of 5-15 m/s (1,000-3,000 ft/min) operating in ambient temperatures of 10°-35°C (50°-95°F) might be AGMA 5 EP, equivalent to an ISO viscosity of 220, unless the gear manufacturer makes a specific recommendation.

In other applications used in lightly loaded, high-speed gearing using journal support bearings, rust and oxidation inhibited (R&O) lubricants might provide satisfactory service. Where moderate loads or changes in drive direction are applied, an R&O oil with anti-wear additives may be successfully applied.

In worm gear drives, compounded oils containing 7-10% fatty acids might be used successfully, unless the operating temperatures continually exceed 60°C (140°F). In these cases, successful lubrication has been achieved with synthetic polyalkylene glycol (PAG) lubricants in the ISO viscosity range of 460. These synthetics have a low friction coefficient with very high viscosity indices, which result in reduced sliding friction and lower oil temperatures.

With regard to multiple-reduction gear ratios, the first reduction operates at the highest speed and so requires the lowest oil viscosity, while subsequent reductions operate at lower speeds, thereby requiring lubricant of higher viscosity.

Since the low-speed gear in a gear set is usually the most critical in the formation of an EHL film, a viscosity selection compromise is necessary and an oil of higher viscosity would be applied, so long as the use of such a lubricant does not cause the temperature to increase excessively.

Whenever the temperature increases during these arbitrary selection processes, select an oil with the next lower viscosity, but monitor the oil for any increase in the rates of wear.

Beyond these general gear lubricant recommendations, more detailed lubricant specifications and recommendations may be obtained by referring to the American Gear Manufacturer’s Association (AGMA) standard 9005-D94. This AGMA standard provides recommendations for gear oil viscosity selections based on the pitch line velocity of the final reduction gear for helical, herringbone, straight bevel, spiral bevel and spur gear drives.

The pitch line velocity can be calculated using the following equations:

Vt = (π np d)/60,000 Where Vt = Pitch line velocity in m/s
np = Pinion speed in rpm
d = Pinion pitch diameter in mm

OR

Vt = (π np d)/12 Where Vt = Pitch line velocity in ft/min
np = Pinion speed in rpm
d = Pinion pitch diameter in inches

An example of how this standard is used is outlined in Table 1. It illustrates both ISO and AGMA ratings and how they correspond.

In addition to standard 9005–D94, gear drive operators should also be familiar with the AGMA viscosity classifications and how these relate to ISO Viscosity Standards as in Table 2.

It is also important to note that when AGMA lubricant recommendations are applied, it would refer to an ISO 150, extreme pressure (EP) gear oil as an “AGMA 4EP.” If it was referring to an ISO 150 non-EP gear oil, the reference to EP would be dropped (e.g. AGMA 4).

To summarize, the lubricant characteristics for enclosed gears are as follows:

a) Correct viscosity at operating temperature.

b) Adequate low-temperature fluidity to ensure proper circulation at the lowest anticipated startup temperature.

c) Excellent oxidation stability, rapid ability for water separation, with good anti-rust properties.

d) The correct anti-wear or extreme pressure additive levels, depending upon gear system application.

e) Compatibility with sealing materials used on input and output shafts to eliminate (or reduce) oil leakage.

Open Gear Drives: Originally lubricants used for open gear drives included such products as asphaltics (residual oils) and these are of three types -- 'straight' residuals, 'compounded' and 'solvent cutback.'

While still in use in certain applications and parts of the world, most modern lubricant manufacturers now offer non-asphaltic, solvent-free, multi-purpose greases and oils that are applied to open gearing by gear dipping, drip, brush or spray methods.

As greases, these lubricants may have NLGI consistency grades of 0 (zero) or 00 (double zero) with worked penetrations of 370 to 415 and oil viscosities of 680 to 1000 cSt.

As lubricating oils, synthetic polyalphaolefin (PAO) extra heavy duty EP lubricants, with ultra-high viscosities of up to 46,000 cSt at 40°C, are now available. It should be noted that many large, slow-speed gear drives that operate at speeds of under 100 rpm are lubricated as open gears, even though they may be enclosed in a protective housing to keep out dirt, water and other foreign material. These gears will be found on such equipment as draglines, hoists, cranes and shovels and many gear designs can be found in these applications.

(Editor's Note: For a complete discussion of gear designs and other gear lubrication issues, refer to the Troubleshooter's Guide to Gear Drives, Machinery & Equipment MRO, December 2006, pg. 15, or view the article online at www.mromagazine.com.)

Industrial Lubrication magazine contributing technical editor, Lloyd (Tex) Leugner, is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of maintenance and lubrication problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca .

Article posted December 2006.

Table 1

Ambient Temperature

Pitch line velocity of final reduction  -40–10°C (1) -10 + 10°C +10 to +35°C +35 to +55°C
15–25 m/s ISO 68  ISO 68  ISO 150 ISO 320
(3000–5000 ft/min) (AGMA 2S Synthetic) (AGMA 2 Mineral Oil)  (AGMA 4 EP Mineral Oil) (AGMA 6 EP Mineral Oil)
 
Table 1
(1) All AGMA recommendations that include the letter "S" indicates a synthetic lubricant.

Table 2

AGMA System  ISO System
AGMA 0 ISO 32
AGMA 1 ISO 46
AGMA 2 ISO 68
AGMA 3 ISO 100
AGMA 4 ISO 150
AGMA 5 ISO 220
AGMA 6 ISO 320
AGMA 7 ISO 460
AGMA 8 ISO 680

Figure 6

Fig. 6: In enclosed gearboxes, lubricants are subjected to severe-service use.

Figure 7

Fig. 7: The lubricant is thrown to and from gear teeth, shafts and support bearings in the form of mist or spray provided by either splash or pressure fed systems. Images courtesy of Philadelphia Gear Corp.

Figure 8

Fig. 8: The lubricant is thrown to and from gear teeth, shafts and support bearings in the form of mist or spray provided by either splash or pressure fed systems. Images courtesy of Philadelphia Gear Corp.

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