1. Sliding motion produces more wear than rolling motion. Worm gear moves in sliding motion and cause temperature rise.
2. A lot of wear will occur during startup, shutdown, low speed or high load conditions. During this time, machine is operating in boundary lubrication regime when oil film is not thick enough to overcome the surface roughness of the contacting metal surfaces, and friction is at its highest level.
3. Components of lubricants:
i. Lubrication oil(typical): approx. 90% base oil, 10% additives
ii. Compressor oil: approx. 99% base oil, 1% additives
iii. Engine oil: approx. 80% base oil, 20% additives.
iv. Grease: approx. 70% base oil, 10% additives, 20% thickener.
Note: Grease lubrication is only satisfactory up to about xx oC.
4. ISO Viscosity Grade reports viscosity at 40degC. Dynamic (aka absolute) viscosity (cP or mPa s) tells how much force is needed to make a lubricant flow, Kinematic viscosity (cSt or mm2/s) tells how fast a lubricant flow when a force is applied. Lubricant viscosity is affected by contaminants (e.g. water or dirt particles) and oil ageing.
5. Viscosity Index (VI) of lube oil tells us the rate of viscosity change due to a temperature change. VI is determined by measuring the kinematic viscosity at 40 & 100 oC and compare with two reference oils of VI = 0 & 100.
6. Typical Viscosity Index (VI) of Lube Base Oil:
a. Traditional mineral oil (Group I base oil) – 95 to 100
b. Hydrotreated mineral oil (Group II base oil)– typically above 100
c. Hydrotreated mineral oil (Group II+ base oil) – Group II base oil that has a slightly higher VI of approx. 115, Group II+ is not officially recognized by API though.
d. Hydrocracked base oil (Group III base oil) – >120
(Note: Group I, II & III base oils are considered mineral oils as they are derived from crude oil). However, Group III base oil is considered as synthetic base oils for marketing purposes as new hydrocarbon structure is created during the hydrocracking process.
e. Polyalphaolefin (PAO) synthetic oil (Group IV base oil): up to 140
(Note: It is a pure chemical that is not refined from crude oil)
f. Group V base oil (e.g. polyalkylene glycols (PAGs), synthetic esters, etc)
Note: Sulfur content of Group I is > 0.03%, Group II & III is < 0.03%. The quality of a lubricant is gauged by examining how much the viscosity will decrease with increasing temperature. Lube oils with high VI experience lower degree of viscosity decrease as the temperature increases. This property makes high VI oils much more stable and therefore more desirable.
7. Factors to be considered when selecting oil lubricants:
- Equipment manufacturer’s recommendations (e.g. min., optimal & max. viscosity). The most important property of lube oil that determines the bearing life is viscosity. Bearing speed and bearing pitch diameter are used to determine the proper oil viscosity for a bearing.
- .Ambient temperature. If the ambient temperature is increased, an oil that is higher in viscosity is generally recommended. Conversely, if the ambient temperature is lowered, a lighter oil is recommended.
- Operating temperatures of equipment. A xxoC increase over ambient would be expected in a worm gear set. For high temperature (>xxx oC) applications, the preferred method of lubrication is circulating oil with a heat exchanger. Viscosity Index (VI) diagram to be evaluated for the entire operating range. Synthetics oils have the longest oxidation life among all types of industrial gear oil.
- Rotating speed (oil to be used for lubricating high speed bearings). If the speed of a bearing is increased, an oil that is lower in viscosity is generally recommended.
- Additives such as antioxidant and detergent are added to lube oil to remove existing debris, prevents new deposit from forming in engine, and combat harmful effects of ethanol that cause engine corrosion over time.
- Loading or abnormal pressure
- Type of oil lubrication (e.g. boundary, fluid film lubrication, etc.)