For gearing equipment owners and operators, the ultimate goal is to achieve a return on their investment; this is done by both maximizing the output, reliability and efficiency of their machinery, and minimizing downtime and operating costs.
Continued reliability, successful operation and long life of power transmission equipment largely depend upon the constant supply of lubrication oil of proper quantity, quality and condition. The lifeline of the gearbox is its lubrication system, critical for supporting the drive under all modes of operation.
The purpose of a gearbox lubrication system is to provide an oil film at the contacting surfaces of all working components to reduce friction and wear. In addition, the oil serves to remove and dissipate heat from where it is generated, preventing gearing component temperatures from rising to excessive levels. Other lubrication functions include the transfer and/or removal of wear particles, as well as the filtration of rust and corrosion and any other undesirable contaminants.
However, failure of the lubrication system to perform any one or more of these functions may result in premature equipment failure.
Understanding the role and importance of a lubrication system in the overall life of a gearbox serves as a foundation for understanding the needs for maintaining such an effective system. And that is what this article aims to doprovide maintenance professionals with the tools to properly understand the lubrication needs for extending overall life of their gearbox. This article examines the number of lubricant types available, as well as the systems used to supply such lubricant throughout a gearbox. In addition, proper maintenance functions are provided for sustaining a functional, effective lubrication system.
Understanding lubrication. Lubrication can be defined as the control of friction and wear between adjacent surfaces by the development of a lubricant film between them, called an elastohydrodynamic (EHD) oil film.
EHD film thickness between gear tooth surfaces is quite small, usually less than 1.25 micrometers (0.00005 in.). Oil film thickness is significantif the adjacent surfaces are not fully separated, the EHD film leaves local areas of contact between those surfaces, making them vulnerable to surface fatigue.
Viscosity is a characteristic of fluids to resist flowing freely. It is one of the most important characteristics of a lubrication fluid. Lubricating oil viscosity changes appreciably with temperature, and is generally stated at two temperatures: 40°C (100°F) and 100°C (210°F). Viscosity is usually expressed in terms of the time required for a standard quantity of a fluid at a given temperature to flow through a standard opening.
Fatigue life of contacting components of a gearbox, such as gear teeth and bearing rollers, is determined by a complex combination of speed, load, lubricant temperature, clearance and alignment. The lubricant's role in this interaction is determined primarily by speed, viscosity and temperature. The effect of these factors on the fatigue life of elements can be dramatically altered at higher temperatures with lower viscosity, and thinner resultant oil films. Selecting the correct lubricant for any application requires a careful study of expected operational and environmental conditions.
Gear lubricants. Several factors must be considered before choosing a gear lubricantthe unit's operating speed and load, temperature range and lubricant availability, to name a few. However, the most important parameter in selecting a lubricant is viscosity. High-speed units produce an acceptable oil film at the tooth contact area even with a low-viscosity oil; at lower operating speeds, a thinner oil film is generated, requiring more viscous oils to separate contacting surfaces. Still, often a gearbox will contain both high- and low-speed gear meshes. In these cases, a compromise must be obtained (though in such cases, performance of these gear meshes may be reduced).
There are two basic types of lubricants used in gear drive systems: petroleum-based mineral oils and a general category known as synthetic lubricants.
Petroleum-based lubricants. Petroleum-based mineral oils are complex mixtures derived from refining crude oil. Petroleum products have been found to excel as lubricants in most applications. Mineral oils are usually compounded with different chemical additives to improve specific properties such as increased lubricant life, resistance to rust and oxidation and even increased load-carrying capacity.
High-load oils, called extreme-pressure (EP) gear lubricants, contain selected additives that increase the load-carrying capacity of gearing by forming a film on the metal that provides component separation under higher load conditions. EP lubricants are ideal for use when severe operating conditions are anticipated. Often these lubricants will contain more than one chemical additive for load capacity enhancement over a wide temperature range, most commonly compounds of phosphorous and sulfur. However, EP gear lubricants should not be used in gear units containing an internal backstop or an internal friction clutch unless the lubrication types used have been specifically approved by the gearbox manufacturer.
Synthetic lubricants. Synthetic lubricants consist of base fluids manufactured by chemical synthesis or molecular restructuring to meet specific physical and chemical qualities desired for certain operating parameters, such as high-temperature thermal and oxidation stability, low-viscosity variation over a broad temperature range, low-temperature capability and/or long service life.
Care must be taken when synthetic lubricants are substituted for previously utilized lubricants. Compatibility with other gearbox components like rubber lip seals, rubber O-ring seals and housing paint must be established. Synthetic lubricants can be up to four times more costly than petroleum-based oils, and are thus generally reserved for problem applications such as extremely high or low temperatures, equipment subjected to frequent overloads and equipment with a marginal lubrication system.
The largest class of today's synthetic lubricants is the estersmaterials containing the ester chemical linkage. Esters have wide operating temperature ranges and high viscosity indicesthus permitting low-temperature operation, as well as providing good lubrication characteristics at high temperatures. A lubricant's viscosity index is a measure of how much that oil's viscosity varies with temperature.
Another class of synthetic lubricants is the synthesized hydrocarbonsthese lubricants contain many of the advantages of esters (to a lesser extent), but have a similar structure to mineral oils, making them compatible with mineral oils while not being detrimental to seals and paints (esters have low compatibility with some polymeric materials such as those used in seals and paints).
Lubricant viscosity selection. In general, the lowest viscosity oil capable of forming an adequate oil film at all operating conditions should be chosen. However, in practice, the lubricant chosen is often a compromise between the requirements of the various lubricated componentssuch as gears and bearingsand the particular application requirements such as large ambient temperature differentials.
Lubrication systems. There are two types of gearbox lubrication systems in use: splash lubrication systems and force-feed lubrication systems. The intent of both types of systems are the same, to distribute oil to each component of the gearbox sufficient for lubricating and cooling that component yet minimizing heat generation by oil churning.
Splash lubrication. Splash lubrication systems require that the gearbox be filled to a predetermined lubrication oil level. Rotating gear elements within the gearbox must dip into the oil and "sling" it into troughs, pockets or directly to bearings and gear meshes requiring lubrication and cooling oil. Feed troughs are employed to capture oil that is "slung" onto the upper gearbox housing wall by a dipping gear element. This oil drips into the trough which, in turn, distributes that oil to the bearings. Such systems are better suited for gearboxes containing rolling-element bearings than those with journal bearings, which require far more oil.
A splash lubrication system requires at a minimum, oil troughs, bearing oil pockets, an oil fill location, an oil drain and a breather. In cold ambient temperatures, an immersion heater should be provided in the sump. Cold starting temperatures can cause oil viscosity to be too high to properly distribute oil upon startup.
Splash lubrication systems are far simpler and less expensive than force feed, but are applicable only to low-speed gear units. As shaft operational speeds increase, the heat generated in the gearbox becomes excessive, requiring an external, force-feed system to supply larger volumes of lubricant to lubricate and cool gearbox components. In addition, higher-speed units require oil to be precisely introduced at the gear and bearing interfaces; this is accomplished through strategically placed jets to properly lubricate the gear meshes and dedicated bearing oil supply lines.
Temperature control/thermal rating. The second primary function of the lubrication system is to provide heat removal. Adequate cooling is necessary to maintain oil viscosity control and oil quality. Conversely, for every gear drive there is a thermal rating; the average power that can be transmitted continuously without overheating the unit and without using any special external cooling method. If the thermal rating is less than the mechanical ratingthe load a gearbox can transmitadditional cooling supplied by a force-feed lubrication system is required.
Auxiliary cooling can be used in combination with splash lubrication to increase the thermal rating of a gearboxfor instance, air can be forced past the radiating surfaces of the gear casing by strategically placed fans internal to the gearbox and located on a high-speed pinion shaft. In addition, the unit can be cooled by a water jacket; water passages are built into the gear housing, usually at the high-speed end, and heat is carried away by a cooling water flow that is isolated from the lubrication oil sump.
To operate a gear unit at maximum efficiency, auxiliary cooling schemes should include thermostatic controls so that the oil is not cooled unnecessarily. Operating with too cool a lubricant increases churning losses. Adding cooling fins to increase the surface area of the gearbox casing can increase the heat transfer from the gear casing to the ambient air.
Force-feed lubrication. In a typical force-feed lubrication system, a shaft- or a motor-driven oil pump draws oil from the gearbox sump through a suction pipe. The oil is directed from the pressure side of the oil pump through a filter to cleanse the oil, and through a cooler employed to cool the oil. A pressure relief valve is typically located before this filter to protect the system from too high an operating pressure. If the filter becomes clogged, the relief valve will permit the unfiltered oil to bypass the filter so the gearbox will continue to receive lubrication and cooling oil albeit unfiltered (unfiltered oil is better than no oil). Another relief valve is often located at the inlet to the gearbox to limit the oil feed pressure if the system contains both shaft- and motor-driven pumps, and both are running at the same time. A shaft-driven oil pump is driven by one of the rotating gear shafts of the gearbox. Some lubrication systems will include both motor- and shaft-driven oil pumps. The motor-driven pump can be activated prior to gearbox startup to supply full oil flow requirement to the gearbox prior to shaft rotation until the attached lube oil pump is running at a speed sufficient to supply full lubrication oil flow to the gearbox, during gearbox coast-down or as a backup in case of failure of the main shaft-driven oil pump.
Check valves are located so that the main pump does not pump through the auxiliary system and that the auxiliary pump does not pump into the pressure side of the main oil pump. A bypass is provided at the cooler serving as both a pressure relief valve and/or a thermostatically controlled valve set so that the pressure drop across the cooler is limited during times when the oil is cold; additionally, temperature and pressure sensors are located at various critical points throughout the system.
Relatively little oil is required for lubrication using a force-feed system provided it is properly applied. The bulk of the oil flow is required for cooling the gear tooth flanks and bearings. For demanding high-speed applications, gear tooth meshes are sprayed on either in-mesh or out-mesh sides or, in some instances, both sides.
System components. A typical force-feed lubrication system consists of the following major components:
Pumps. The gearbox oil pump delivers a given quantity of oil over a wide range of oil temperatures and viscosities. In addition, the gearbox pump must be capable of priming itself and overcoming pressure drops in the line between the oil reservoir and the pump suction port.
The most common method of lubricant delivery is the positive-displacement lube-oil pumpthese pumps deliver a given quantity of fluid with each pump rotor revolution. A positive-displacement pump's output is directly proportional to its operating speed and offers practically constant oil flow at any particular speed regardless of downstream conditions.
Gearbox lubrication pumps can be mounted to the unit and driven by one of the gearbox shafts, or independently mounted with an electric motor or other prime mover driving. When the pump is shaft driven, oil flow will vary directly with shaft speed. In a gear pump, as the gears rotate, fluid is trapped between the gear teeth and the case, and is carried around the pump casing to the pump discharge oil port.
Filtration. Gearbox lubrication systems are subject to contamination due to a variety of causesinternal component wear generates particles washed into the oil stream; foreign particles find their way into the system during assembly, maintenance and everyday operation. These contaminants, if uncontrolled, will cause wear and even failure of bearings or gear elements.
Lubricant cleanliness is a major concern when looking to maximize geared equipment service life. The lubrication filters play a key role in ensuring that abrasive particles are removed from the system.
In addition to the filtration of fluids, the lubrication filters often incorporate a bypass for clogged element conditions, a magnetic drain plug to collect metallic particles and a visual and/or electrical cleanliness indicator.
In force-feed lubrication systems, the oil must be supplied through a filter media that is compatible with the lubricant, meets the viscosity requirements without excessive pressure drop and removes particulate matter consistent with the rotating equipment design.
The oil filter should be located on the pressure side of the pump so warmer, less viscous oil is being filtered. Filter elements can be either cleanable and reusable or disposable. Cleanable filter elements are usually made of wire mesh, with cleaning commonly accomplished using an ultrasonic liquid bath.
Coolers. In force-feed lubrication systems, the oil inlet temperature to the gearbox is controlled by passing hot sump oil through a cooler. Such a cooler must be capable of achieving the required oil temperature drop when exposed to the maximum ambient air temperature anticipated for the application. However, a generous temperature margin should be applied during design to account for cooler deterioration.
The two types of coolers used are liquid-to-liquid and liquid-to-air. In oil-to-water (liquid-to-liquid) coolers, hot oil gives up heat to cooler water, resulting in cooler oil and hotter water. Where water is unavailable, radiators are used to blow cooling air over oil tubes. However, air-to-oil (air-to-liquid) coolers require larger envelopes than oil-to-water coolers; in addition, hot days will limit the amount of cooling a radiator can achieve.
Oil reservoir. The oil reservoir may be integral with the gearbox or separately mounted and connected to the gearbox by piping. The oil level in the reservoir will vary from a maximum when the unit is shutdown and oil has drained from lines and components to the minimum permitted during operation.
At shutdown, when lines and components such as coolers and filters drain back into the reservoir, the oil level will be higher than the maximum operating level; thus, the reservoir tank must have sufficient volume to accommodate the drain backflow and still retain some air space at the top. To ensure complete draining for cleaning and oil changes, the unit should be fitted with a drain connection located at or near the bottom of the sump. The oil pump suction line should be located slightly above this reservoir bottom so that any sediment on the bottom is not pulled into the pump suction line.
Oil return lines should be piped into the reservoir near the maximum operating level away from the area around the pump suction connection so that the incoming oil must travel the maximum distance to the pump suction. By maximizing this dwell time, the oil has more time to lose any entrained air before it is again circulated through pumps, filters and coolers. To facilitate reservoir inspection and cleaning, sufficiently large access openings must be provided.
Breather. The gearbox breather is used to vent pressure that may be built up in the gearing unitsuch pressure may result from air entering the lubrication system through seals or the natural heating and cooling of the unit. When a cold gearbox starts up, the heat generated during operation will cause air pressure to build within the gearbox housing.
Piping. The lubrication system piping is intended to distribute lubrication oil in accordance with system design requirements and should be as simple and direct as practical. The piping connections for a gearbox can cause problems at assembly and startup since often the responsibility for supplying piping and lubrication system components is split between the gear manufacturer and user. If this is the case, care must be taken to avoid piping complications at installation.
It is good practice to have only one external oil feed connection with all other oil passages placed inside the system casingthis means that any slight leaks in the piping connections will be internal and harmless. This also means there will be less chance of damage during installation. In all, the piping arrangement must be carefully designed to minimize pressure drops and leak sources.
Lubrication monitoring. In providing reliable service, the lubrication system must incorporate sufficient sensors to allow continuous and complete system condition monitoring.
Resistance temperature detectors (RTDs). RTDs allow continuous temperature monitoring at key locations, such as oil supply and drain temperatures, as well as sump temperature. RTDs should be of the duplex type to obtain redundant readings for accuracy or to supply a backup.
Temperature switches. Temperature switches are typically used as a trigger to alarm or shut down the gearbox prime mover when excessive temperatures are experienced, and can be permissives for cold temperature startup. These switches are typically located in the main oil reservoir and lube-oil supply lines.
Pressure switches. Pressure switches are used as a mechanism to trigger the operation of auxiliary back-up pumps, as well as to initiate a signal for system shutdown when pressure is lost at the main oil inlet to the gearbox.
Flow switches. Flow switches are used as a trigger to signal loss of flow below the minimum oil demand requirements of the system.
Water detectors. Water detectors act as a means to detect the presence of unwanted water in the lubrication oil system.
Lubrication system maintenance. Given the integral role that the lubrication system plays in overall gearbox life and longevity, it must be continually maintained so that the system is functioning at peak performance. It is important to develop a systemic inspection method, condition verification and documentation to avoid any unexpected lubrication system failures, and ultimately equipment damage. The following are areas of concern for maintaining a properly functioning lubrication system.
Cleanliness. Dust, dirt, grit and wear particles in the lubricant supply must be kept to a minimum. Filters and strainers should be serviced regularly to avoid circulating contaminants within the oil, as well as to avoid excessive pressure drops that can reduce the quantity of oil supplied to the gear drive.
Lubricant condition. The service life of a lubricant is negatively affected by a number of factors, including high temperatures, water and/or emulsions, solid contaminants and operating environment. An oil sample should be drawn from the oil sump at scheduled intervals and analyzed by the lubricant supplier or a reputable maintenance provider. The lubricant supplier should be consulted for typical oil changeout limits for the particular oil used.
Sensor/switch settings. An annual check of all switches and sensors should be performed to verify operation as per lubrication system schematic specified settings. System vibration and environmental conditions can alter settings, ultimately affecting critical timing and initiation of sensor functions.
Auxiliary pump function. Pumps and other motorized accessories should be checked at scheduled intervals to verify operability, proper oil delivery, pressures and motor power draw. Relief valve settings should be checked to ensure that the required oil delivery is supplied to the gear drive at the proper pressure.
Flow and pressure check. Flows and pressure drops at the cooler, filters and inlet to the rotating equipment should be routinely monitored and recorded to identify any adverse trends that might be developing.
Cooler condition. An annual check of cooler condition is important to maintain cooler efficiency. Water-cooled heat exchanger coolant ports should be checked for any fouling or blockage. All sacrificial anodes should be replaced. Air-oil cooler core fins should be checked and cleaned of any dirt buildup that would affect heat transfer efficiency.
Breathers. Oil breathers should be checked frequently since they will become dirty. Any blockage in the breather could potentially lead to leakage elsewhere in the drive to relieve pressure.
Visual component check. The entire lubrication system should be checked daily for all indicator gauge readings, pipe connections, vibration, bolted connections, oil leaks or seepage, loose accessories and wiring connections.
Sound levels. The operating sound level of the pumps should be routinely noted. Any increase in sound level could indicate the presence of air in the lube system, blockage at the pump intake, air leaks in the pump shaft seal, worn or loose parts in the pump, filter blockage or high oil viscosity from the pumped fluid being too cold.
Greased points. Some motors and pumps are equipped with greased bearings that must be lubricated at manufacturer recommended intervals. HP
| The author|
Jules DeBaecke is vice president of engineering for Philadelphia Gear Corp. in King of Prussia. He is responsible for maintaining and enhancing Philadelphia Gear's leadership role within the industry as world-class engineering experts. Mr. DeBaecke joined Philadelphia Gear in 1981 as engineering design manager for the Synchrotorque (hydroviscous clutch) and Marine Divisions, as well as manager of production engineering for all products. Since then, he has also held positions as product manager for the marketing and sales of marine, synchrotorque, test stand, material handling and high-speed product lines; and marketing manager. Prior to 1981, Mr. DeBaecke was employed by the Naval Ship Engineering Center, Philadelphia Division, where he was responsible for research, development, test and evaluation of US Naval main propulsion equipment, as well as fleet machinery maintenance worldwide for this equipment. At the project level, he was recognized as the US Navy's clutch expert for equipment installed on naval vessels including gas turbine-driven cruisers and destroyers, nuclear submarines and aircraft carriers. Mr. DeBaecke holds a degree in mechanical engineering from Drexel University in Philadelphia, Pennsylvania.