LUBRICATION OF GEARS

The purpose of lubricating gears is as follows:

1. 1. Promote sliding between teeth to reduce the coefficient of friction (m).
2. 2. Limit the temperature rise caused by rolling and sliding friction.

To avoid difficulties such as tooth wear and premature failure, the correct lubricant must be chosen.

20.1 Methods of Lubrication

There are three gear lubrication methods in general use:

1. Grease lubrication.
2. Splash lubrication (oil bath method).
3. Forced oil circulation lubrication.

There is no single best lubricant and method. Choice depends upon tangential speed (m/s) and rotating speed (rpm). At low speed, grease lubrication is a good choice. For medium and high speeds, splash lubrication and forced circulation lubrication are more appropriate, but there are exceptions. Sometimes, for maintenance reasons, a grease lubricant is used even with high speed. Table 20-1 presents lubricants, methods and their applicable ranges of speed.

The following is a brief discussion of the three lubrication methods.

20.1.1 Grease Lubrication

Grease lubrication is suitable for any gear system that is open or enclosed, so long as it runs at low speed. There are three major points regarding grease:

1. Choosing a lubricant with suitable viscosity. A lubricant with good fluidity is especially effective in an enclosed system.
2. Not suitable for use under high load and continuous operation. The cooling effect of grease is not as good as lubricating oil. So it may become a problem with temperature rise under high load and continuous operating conditions.
3. Proper quantity of grease. There must be sufficient grease to do the job. However, too much grease can be harmful, particularly in an enclosed system. Excess grease will cause agitation, viscous drag and result in power loss.

20.1.2 Splash Lubrication

Splash lubrication is used with an enclosed system. The rotating gears splash lubricant onto the gear system and bearings. It needs at least 3 m/s tangential speed to be effective. However, splash lubrication has several problems, two of them being oil level and temperature limitation.

1. Oil level.
   There will be excessive agitation loss if the oil level is too high. On the other hand, there will not be effective lubrication or ability to cool the gears if the level is too low. Table 20-2 shows guide lines for proper oil level. Also, the oil level during operation must be monitored, as contrasted with the static level, in that the oil level will drop when the gears are in motion. This problem may be countered by raising the static level of lubricant or installing an oil pan.
2. Temperature limitation.
   The temperature of a gear system may rise because of friction loss due to gears, bearings and lubricant agitation. Rising temperature may cause one or more of the following problems:
   - Lower viscosity of lubricant.
   -Accelerated degradation of lubricant.
   -Deformation of housing, gears and shafts.
   - Decreased backlash.
   New high-performance lubricants can withstand up to 80 to 90°C. This temperature can be regarded as the limit. If the lubricants temperature is expected to exceed this limit, cooling fins should be added to the gear box, or a cooling fan incorporated into the system.

20.1.3 Forced-Circulation Lubrication

Forced-circulation lubrication applies lubricant to the contact portion of the teeth by means of an oil pump. There are drop, spray and oil mist methods of application.

1. Drop method:
   An oil pump is used to suck-up the lubricant and then directly drop it on the contact portion of the gears via a delivery pipe.
2. Spray method:
   An oil pump is used to spray the lubricant directly on the contact area of the gears.
3. Oil mist method:
   - Lubricant is mixed with compressed air to form an oil mist that is sprayed against the contact region of the gears. It is especially suitable for high-speed gearing.
   - Oil tank, pump, filter, piping and other devices are needed in the forced-lubrication system. Therefore, it is used only for special high-speed or large gear box applications. By filtering and cooling the circulating lubricant, the right viscosity and cleanliness can be maintained. This is considered to be the best way to lubricate gears.

20.2 Gear Lubricants

An oil film must be formed at the contact surface of the teeth to minimize friction and to prevent dry metal-to-metal contact. The lubricant should have the properties listed in Table 20-3.

20.2.1 Viscosity of Lubricant

The correct viscosity is the most important consideration in choosing a proper lubricant. The viscosity grade of industrial lubricant is regulated in JIS K 2001. Table 20-4 expresses ISO viscosity grade of industrial lubricants.

JIS K 2219 regulates the gear oil for industrial and automobile use. Table 20-5 shows the classes and viscosities for industrial gear oils.

JIS K 2220 regulates the specification of grease which is based on NLGI viscosity ranges. These are shown in Table 20-6.

Besides JIS viscosity classifications, Table 20-7 contains AGMA viscosity grades and their equivalent ISO viscosity grades.

20.2.2 Selection of Lubricant

It is practical to select a lubricant by following the catalog or technical manual of the manufacturer. Table 20-8 is the application guide from AGMA 250.03 "Lubrication of Industrial Enclosed Gear Drives".
Table 20-9 is the application guide chart for worm gears from AGMA 250.03.
Table 20-10 expresses the reference value of viscosity of lubricant used in the equations for the strength of worm gears in JGMA 405-01.



Table 20-3 The Properties that Lubricant Should Possess

No.	Properties	Description
1	Correct and Proper Viscosity	Lubricant should maintain a proper viscosity to form a stable oil film at the specified temperature and speed of operation.
2	Antiscoring Property	Lubricant should have the property to prevent the scoring failure of tooth surface while under high-pressure of load.
3	Oxidization and Heat Stability	A Good lubricant should nor oxidized easily and must perform in moist and high-temperature environment for long duration.
4	Water Antiaffinity Property	Moisture tends to condense due to temperature change, when the gears are stopped. The lubricant should have the property of isolating moisture and water from lubricant.
5	Antifoam Property	If the lubricant foams under agitation, it will not provide a good oil film. Antifoam property is a vital requirement.
6	Anticorrosion Property	Lubrication should be neutral and stable to prevent corrosion from rust that may mix into the oil.

Table 20-4 ISO Viscosity Grade of Industrial Lubricant (JIS K 2001)

ISO Viscosity Grade	Kinematic Viscosity Center Value 10-6m²/s (cSt) (40ºC)	Kinematic Viscosity Range 10-6m²/s (cSt) (40ºC)
ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG ISO VG 2 3 5 7 10 15 22 32 46 68 100 150 220 320 460 680 1000 1500	2.2 3.2 4.6 6.8 10 15 22 32 46 68 100 150 220 320 460 680 1000 1500	More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than More Than 1.98 2.88 4.14 6.12 9.00 13.5 19.8 28.8 41.4 61.2 90.0 135 198 288 414 612 900 1350 and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than and less than 2.42 3.52 5.06 7.48 11.0 16.5 24.2 35.2 50.6 74.8 110 165 242 352 506 748 1100 1650

Table 20-5 Industrial Gear Oil

Types of Industrial Gear Oil		Usage
Class One	ISO VG    32 ISO VG    46 ISO VG    68 ISO VG   100 ISO VG   150 ISO VG   220 ISO VG   320 ISO VG   460	Mainly used in a general and lightly loaded enclosed gear system
Class Two	ISO VG    68 ISO VG   100 ISO VG   150 ISO VG   220 ISO VG   320 ISO VG   460 ISO VG   680	Mainly used in a general medium to heavily loaded enclosed gear system

Table 20-6 NLGl Viscosity Grades


Table 20-7 AMGA Viscosity Grades

AGMA No. of Gear Oil		ISO Viscosity Grades
R & O Type	EP Type
1   2   3   4   5   6   7  7 comp   8  8 comp       8A comp   9	2 EP 3 EP 4 EP 5 EP 6 EP 7 EP 8 EP   9EP	VG 46   VG 68   VG 100   VG 150   VG 220   VG 320   VG 460   VG 680   VG 1000   VG 1500

Table 20-8 Recommended Lubricants by AGMA

Gear Type		Size of Gear Equipment (mm)		Ambient temperature ºC
				-10 ... 16	10 ... 52
				AGMA No.
Parallel Shaft System	Single Stage Reduction	Center Distance (Output Side)	Less than 200 200 ... 500 more than 500	2 to 3 2 to 3 3 to 4	3 to 4 4 to 5 4 to 5
	Double Stage Reduction		Less than 200 200 ... 500 More than 500	2 to 3 3 to 4 3 to 4	3 to 4 4 to 5 4 to 5
	Triple Stage Reduction		Less than 200 200 ... 500 More than 500	2 to 3 3 to 4 4 to 5	3 to 4 4 to 5 5 to 6
Planetary Gear System		Outside Diameter of Gear Casing	Less than 400 More than 400	2 to 3 3 to 4	3 to 4 4 to 5
Straight and Spiral Bevel Gearing		Cone Distance	Less than 300 More than 300	2 to 3 3 to 4	4 to 5 5 to 6
Gearmotor				2 to 3	4 to 5
High Speed Gear Equipment				1	2

Table 20-9 Recommended Lubricants for Worm Gears by AGMA

Types of Worm	Center Distance mm	Rotating Speed of Worm rpm	Ambient Temperature, ºC		Rotating Speed of Worm rpm	Ambient Temperature, ºC
			-10 ... 6	10 ... 52		-10 ... 16	10 ... 52
Cylindrical Type	£150 150 ... 300 300 ... 460 460 ... 600 600<	£700 £450 £300 £250 £200	7 Comp	8 Comp	700< 450< 300< 250< 200<		8 Comp
						7 Comp
Throated Type	£150 150 ... 300 300 ... 460 460 ... 600 600<	£700 £450 £300 £250 £200	8 Comp	8A Comp	700< 450< 300< 250< 200<	8 Comp

Table 20-10 Reference Values of Viscosity Unit: cSt/37.8°C

Operating Temperature		Sliding Speed m/s
Maximum Running	Starting Temperature	Less than 2.5	2.5 ... 5	More than 5
0ºC ...  10ºC	-10ºC ... 0ºC	110 ... 130	110 ... 130	110 ... 130
0ºC ...  10ºC	More than 0ºC	110 ... 150	110 ... 150	110 ... 150
10ºC ...  30ºC	More than 0ºC	200 ... 245	150 ... 200	150 ... 200
30ºC ...  55ºC	More than 0ºC	350 ... 510	245 ... 350	200 ... 245
55ºC ...  80ºC	More than 0ºC	510 ... 780	350 ... 510	245 ... 350
80ºC ...  100ºC	More than 0ºC	900 ... 1100	510 ... 780	350 ... 510