eSource Reduction Gears II
Reduction gears are present throughout all industries utilizing machines to do work. Gears may be arranged as simple gear trains in single reduction, with a driver gear and a driven gear and a number of idler gears. Where greater speed reduction is required, one or more intermediate gears are employed. They consist of sets of two gears of different diameters mounted on the same shaft, and serve to reduce the speed of the train in proportion to the diameter of the gears.
Power input from electric motors, steam, hydraulic or gas turbines are generally used in applications where uniform torque is desired. This permits uniform gear tooth loading. With reciprocating engines variable torque is produced, with variable gear tooth loading as a result. This may require a higher viscosity lubricant. Gear motors are used as speed reducing units between electric motors and the driven equipment. They may be designed for single reduction gearing as part of the motor, or when double or triple reduction is desired, they may be mounted separately.
If we examine the surface of a highly polished metal plane under a microscope, we see that the surface which we thought was flat is actually a series of peaks (asperities) and valleys (troughs). When one plane surface is brought into contact with another, there are few actual contact points. These correspond to the limited number of high asperities on one surface which are in contact with corresponding asperities on the other. We therefore define friction as the force resisting motion when the asperities of these surfaces in contact are moved relative to each other. Friction will vary depending on the materials and load applied. It is always present whenever motion exists, and is undesirable in the operation of bearings, gears, and other machine elements. The type of friction described above is dry, solid sliding friction, which requires considerable force to overcome. If one, or both, surfaces is cylindrical or spherical, this is described as dry, rolling friction. Another type of friction is fluid friction, which occurs when the molecules of a fluid move past each other. They generate significantly less friction than solid surfaces because much less force is needed to overcome the cohesive forces of the fluid. Friction always produces heat, more with solid sliding/rolling friction and less with fluid friction. The most important function of a lubricant then, is to prevent metal-to-metal contact, or to minimize friction between metal surfaces in motion with respect to each other. Lubrication is the principal of supporting sliding load on friction reducing film - the film substance is a lubricant – to apply the substance is to lubricate.
When a lubricant is injected between two metal surfaces in motion relative to each other, the lower cohesive forces of oil are substituted for the higher metallic sliding/rolling friction generated on the unlubricated surfaces. The oil film which is formed separates the two metal surfaces. There are three regimes of lubrication, described as Fluid Film – Mixed Film – Boundary (Figure 1).
The proper viscosity grade of straight mineral oil will provide adequate film thickness for full film lubrication in industrial gears that are not heavily stressed. However, gears sometimes experience high unit loadings, low or variable sliding speeds, or shock loading. This may limit the oil film to thin layers which will not prevent metal-to-metal contact, and most of the load is carried by the asperities rather than a continuous oil film. This condition, called boundary lubrication, may cause film breakdown with consequent hot spots on the metal and seizure or welding. Mixed film lubrication is intermediate between full film and boundary lubrication, and occurs when part of the load is supported by fluid film and part by the asperities. In such cases where the asperities come into contact, straight mineral oil must be modified with various additives to supplement the boundary lubrication properties of the oil. These may be designed to enhance oiliness by adding vegetable or animal fatty acids as in compounded oils, or provide mild anti-wear properties, or adding chemically reactive extreme pressure (EP) additives. EP additives are activated by high pressures and temperatures, where normal anti-wear agents would fail. They react strongly with the metal surface, forming a soft chemical layer that wears off and prevents welding of the surface asperities. Used especially for low-speed and shock-loading gear box applications, EP additives are mainly sulfur phosphorus compounds. The type of additives used depends upon the application.
The viscosity of the oil, the oil’s thickness or thinness, can be defined as the fluid’s resistance to flow and is directly related to the oil film strength. An oil’s viscosity must be thick enough to fill the valleys and cover the asperities of both opposed surfaces under load, yet not so thick as to increase drag, and generate excess heat due to fluid friction. Viscosity is impacted by temperature, both ambient and operating temperatures. As the temperature of an oil increases, its viscosity thins out, and as the temperature decreases, the viscosity tends to thicken. Viscosity determines the amount of fluid friction and drag, more for higher viscosity oils (thick) and less for lower viscosity oils (thin).
The lubrication of enclosed industrial gears requires highly refined, high quality oils within the viscosity range recommended by the manufacturer. They must reduce friction and wear (which may be done through either a physical or chemical barrier preventing metal-to-metal contact), remove heat, remove contaminants, prevent rust and corrosion. The oils must be noncorrosive to gears, free of grit or abrasive matter, and resistant to oxidation or sludging. They must have good demulsibility and defoaming characteristics.
Factors to be considered when selecting lubricants are gear type, surface finish, gear speed, gear loading, drive type, lubrication delivery system, temperature range, and operating conditions. If speeds are high and loads are light, a lower viscosity oil may be used. Where speeds are low and loading is heavy, contact time is longer and a higher viscosity oil should be used. Colder ambient temperatures require lower viscosity and lower pour points. Gear units working under varying load conditions require an oil which will maintain its viscosity and film strength at the higher temperatures caused by heavy loads, and yet not be too viscous under lighter loads. Tooth type has significant influence in lube oil selection. The relatively low sliding velocities found in spur, herringbone and bevel gears may not require anti-scuff/EP compounds (Table 1). The American Gear Manufacturers Association (AGMA) specification AGMA 9005-E02 provides the end user, original equipment builder, gear manufacturer and lubricant supplier with guidelines for minimum performance characteristics for lubricants suitable for use with enclosed and open gearing which is installed in general industrial power transmission applications. It provides recommendations for selecting lubricants based on current theory and practice in the industry.
Table 1: Types of Lubricants Used With Various Gear Applications
Recommended viscosity and lubricant chemistry is often found on the gear nameplate, where the manufacturer has indicated the preferred viscosity and whether the oil should be R&O, EP, Synthetic, or Compounded. For example, “Lubricant: ISO 220, EP” would indicate an ISO Viscosity Grade of 220 with an Extreme Pressure (EP) additive formulation. In older gearboxes, the same information may read “AGMA 5 EP”, which refers to the AGMA gear oil classification system that was in use for decades.
Effective lubrication of gears and bearings depends to a great extent upon the method of application. Splash, or Bath, lubrication is widely used for in gear sets enclosed in oil-tight housings which serve as the oil reservoir or sump. In this system, the lower gear dips into the reservoir and carries the oil to the meshing points of the gear train. In some splash systems there are troughs which capture some of the oil spray and redirects it onto bearings. Splash systems are reliable and require very little attention. Pressurized circulating systems are used for medium and heavy gear sets where large volumes of oil are required. In wet sump systems, the pressurized oil is pumped directly from the internal oil reservoir to the lubrication points. Oil returns to the sump by gravity for recirculation. Dry sump systems are supplied with pressurized oil from a circulating pump which draws from an external oil reservoir and delivers cooled and filtered fluid under pressure to the gear train. In both circulating systems, the oil may be filtered and otherwise conditioned before reapplication. Large drives and pinions are oiled through a series of sprays mounted on a pipe manifold, and are spaced to ensure complete coverage. The spray heads direct oil onto the incoming mesh.Part III discusses reduction gear maintenance and oil analysis.
David Doyle, CLS, OMA I, OMA II