Low Temperature and Viscosity
Low ambient temperatures affect the flow characteristics of a lubricant. Dropping below the pour point and the higher viscosity not only restricts oil flow to bearings and other machine elements, but also translates into high startup torque. As a result, machines often cannot start or excessive friction causes a complete failure.
Industries and transportation in northern parts of the United States, Europe and Canada are vulnerable to harsh outdoor conditions during winter months. Avoiding high churning and splash losses at low temperatures in gear boxes; developing remedies for more efficient lubrication of bearings and lubricated joints; and implementing a reliable and low- maintenance technology that enables vehicle wheel bearings to safely operate over wide temperatures require careful lubricant selection.
Fortunately, specially compounded mineral oils or synthetics are available that match cold flow requirements.1 In difficult cases, heating is needed for piping, reservoir and filters. In other cases, grease or self-lubricating materials may reduce or even eliminate troublesome low-temperature problems.
The low-temperature limit for starting an oil- lubricated machine is often specified by the pour point of the oil. This is the lowest temperature at which oil will flow when chilled under prescribed laboratory conditions (ASTM D97). With most mineral-based industrial oils (designated as turbine, hydraulic, industrial and machine oils), this pour point corresponds to the temperature that freezes the paraffin molecules of the oil into a white crystalline wax that will eventually immobilize the overall oil.
Pour point additives that suppress this gelling effect of the wax are used in many automotive oils as well as in industrial lubes. Although gelling is reduced by these long-chain additive molecules, individual wax particles separating out of oil at low temperatures may still plug filters and impede circulation.
With their low paraffinic content, wax free synthetic and naphthenic mineral oils can be further cooled to a lower pour point. At this point the viscosity becomes so high (usually about 100,000 centistokes, cSt) it will eliminate any visible oil flow in the pour point test.
While pour point establishes one low-temperature operating limit, other demanding requirements for low viscosity appear in critical machinery and lube system flow areas such as suction and drain piping, pumps and filters. This viscosity limit represents the highest viscosity at which oil flows and properly lubricates in a system. At temperatures below this limit, the elevated oil stiffness interferes with adequate lubrication and related hydraulic functions in a machine.
Corresponding pour point temperatures and viscosity limits for representative mineral oils in Table 2 provide a basis for matching low-temperature needs. For a large industrial electric motor, for instance, a heavy turbine oil (ISO viscosity grade VG 68) calls for a low-temperature limit of 21°F for the 2,000 cSt machine's viscosity limit, as specified in Table 1. To operate in outdoor temperatures down to 0°F, either heaters would be necessary to bring the temperature above 21°F for starting or the user should switch to a light turbine oil of VG 32 cSt. While synthetic polyalphaolefin (PAO) or ester-type oils might be considered because of their superior low-temperature properties, careful consideration should be given to the possible deleterious effects on electrical insulation, paint and rubber seals.
Multigrade Oils. Multigrade oils have been developed to improve low-temperature startability and to enhance pumpability. A 5W-30, for example, offers low-temperature automotive engine protection in cold engines by exhibiting low viscosity equivalent to SAE 5 oil; whereas in hot engines its viscosity increases to SAE 30. A 20W-50 oil works well for aircraft engines, where 20W oil provides faster and easier starting in winter and its 50 viscosity protects the engine against metal-to-metal contact when the aircraft is operating under normal conditions. Similarly, gear manufacturers recommend 75W-90 oil for adequate splash lubrication of gear teeth.
These multigrade automotive oils are normally avoided in industrial applications where 15 to 20 percent of the additives are tailored to the demanding conditions in internal combustion engines, which may introduce foam, emulsions and shortened service life. Nevertheless, some high-performance hydraulic and circulating industrial oils are specially compounded with similar additives to lower the pour point and provide improved viscosity/temperature characteristics.
Compact arrangement using an oil supply close to lubricated bearings and other machine elements often simplifies startup and operation at low temperatures. This can include wick oiling and immersion in an oil bath to bring oil directly in contact with bearing surfaces to facilitate low-temperature starting. Oil rings are another possibility.
Wick Oiling. Oil in saturated wicks, as in fractional horsepower electric motors and traditional railroad car axle bearings, suffers from immobilization at and below the pour point temperature. In railroad axle bearings, oil wicking was generally satisfactory even down to the pour point temperature. Also, after starting an electric motor, its normal electrical heating quickly increases the oil wick temperature to enable adequate lubrication.
Oil Bath. Bearings immersed in oil baths involve a broad range of viscosity limits. Pivoted-pad thrust bearings in large vertical motors and generators are limited to a maximum viscosity of about 2,000 cSt (approximately 20°F above the pour point of medium turbine oil). Higher viscosity at lower temperatures or with more viscous grades of oil will not provide adequate oil feed through the gaps between individual thrust pads. A similar low-viscosity limit is expected for flow into and through the feed grooves in sleeve bearing designs.
Higher viscosities are tolerable, however, for less demanding performance requirements in small units with limited flow restrictions. Industrial gearbox units are perhaps most tolerant of high viscosity in low temperatures, at the expense of higher power losses. Some premium gear oil specifications call for a maximum viscosity of 135,000 cSt down to -30°F to cover low-temperature applications. The low-temperature limit for gear lubrication is typically specified as 10°F above the pour point rather than as a limiting low-temperature viscosity.2
Oil Rings. For oil rings hanging from a rotating shaft to lift oil from a bath for delivery to bearings in electric motors and line shafts, the maximum viscosity drops to about 1,000 cSt. When viscosity increases to this limit, the friction driving the ring from the thickening oil on the upper surface of the shaft continues to overcome the increased drag in the lower oil bath. Below this low-temperature limit, however, a thickening sheath of stiff, cold oil picks up on the ring. Contact of this oil with the sides of the ring slot in the bearing can cause erratic performance.
The greatest demands for low viscosity come from turbine generators for electrical power generation, large compressors, turbomachinery systems, steel and paper mills. These lube systems typically handle 1,000 to 2,000 gallons and more of oil. The following are common sensitive points for limiting oil flow at low temperatures in large systems.
Pumps. Pumps are susceptible to damage from excessive local pressure drops that cause cavitation at the suction. In a matter of minutes, this process can damage pump flow passages, bearings and seals. The following steps can be taken to minimize excessive pressure drop and cavitation at the pump suction.
- Keep piping to the pump straight and sized for a flow velocity of three to five feet/second or lower.
- Submerge the suction of centrifugal pumps.
- Aim for a maximum (calculated) intake pressure drop of two psi for positive displacement pumps at the lowest oil feed temperature (highest oil viscosity).
Feed and Drain Lines. Feed lines are typically designed for oil flow rates of five to 10 feet/second. Drain lines are commonly sized to run half-full to allow space for foam and the escape of air, both entrained in the oil and dragged along by the flow. For large industrial oil circulating systems, about one foot/second is a common full-drain design velocity at a limiting low-temperature viscosity in the 40°F to 65°F range.3 Any higher viscosity accompanying lower temperatures endangers low oil feed along with oil backup and overflow in the drains.
Filters. Pressure drop through oil filters at rated operating conditions is commonly in the five to 10 psi range. Being proportional to oil viscosity, a temperature reduction of 25°F to 35°F would double this pressure drop for oil flowing through a filter. To minimize this increase in flow resistance, filters should either be installed in a warm enclosure or equipped with a high-pressure actuated bypass.
Lubricating large turbine generators has complex viscosity requirements. For instance, initial circulation of oil through the system piping requires a reservoir temperature above 55°F and a viscosity of 100 cSt or less. Temperature must then be maintained below 100°F (above 32 cSt viscosity) to avoid bearing wear at the low speeds of five to 10 rpm for several hours when the shaft is placed on a turning gear during turbine warmup. Finally, a reservoir temperature of 120°F is held for what is typically the rated operating condition.
When considering applications involving bearings, gears and other machine elements, initial calculation of minimum oil film thickness may be needed to identify the minimum viscosity grade and type of lubricant to be used at the rated operating conditions. Using the maximum viscosity for cold starting conditions, it is then possible to choose whether heating of the oil reservoir or other lubrication system elements is needed.
Smaller Lube Systems (generally up to 50 gallons). Most electric motors and their driven equipment, as well as automobile and truck engines, undergo fewer oil flow restrictions. Accordingly, they allow higher limiting viscosities (Table 1) and corresponding lower temperatures.
In extreme situations, automotive engines can start at temperatures down to -25°F, and even lower with multigrade oils such as SAE 10W-30 and 5W-30. Also, aircraft jet engines can be started and operated in 65°F ambient temperature with synthetic oils.
Problems associated with starting a car at low temperatures often result in bearing starvation and associated metal-to-metal contact leading to damage and excessive wear. This is due to the lubricant being stiff because of the high initial viscosity, making it difficult to flow through the pipes and onto the bearing surfaces. In addition to starvation, a stiff lubricant can also cause an increase in the skidding of rolling elements in a bearing. In radially loaded bearings, the rollers positioned in the unloaded part of the bearing tend to slip where the clearance is greater. This results in a higher acceleration in various rollers and the reduction of lubricant film thickness with its concomitant increase in the subsurface stresses and reduction in bearing life.4,5 According to Östensen6, a general remedy involves using lower viscosity oil with a heating device. Alternatively, an oil dilution system for pumping a small amount of fuel into the oil may be considered to provide lower viscosity oil during engine startup.
Table 3. Typical Low-temperature Limit for Greases
Ball and roller bearings are currently being used in many applications because of their simpler housing designs, lack of a need for a lubrication system, lower starting friction and lower cost than traditional plain sleeve bearings. Low temperatures are generally accommodated by selecting an appropriate grease that will allow startup at cold temperatures with the torque available in the machine and without undue skidding of the ball complement.
The limits in Table 3 are set primarily by the viscosity of the oil component that makes up 80 to 90 percent of the grease composition. Conventional industrial greases made with mineral oils in the order of 100 cSt viscosity at 40°C (104°F) can be used down to -25°F to -30°F (where viscosity of the oil phase reaches about 100,000 cSt) in most industrial electric motors and other devices where moderate torque is available.7
In lightly loaded bearings, ball skidding may set a more demanding low-temperature limit. In one evaluation of motor-generator failures on suburban railroad passenger coaches at 0°F, the inner rings of the ball bearings were observed to spin at full speed under light loads within a cluster of bearing balls immobilized by the stiff grease. Normal bearing operation became possible after switching to a low-temperature diester grease which also eliminated ball skidding, internal bearing wear and previously encountered failures.
High friction torque from stiff grease at startup in cold climates drops with decreasing base oil viscosity. Greases formulated with lower viscosity mineral oils in the range of 25 to 30 cSt at 40°C, for instance, allow operation to about 20°F lower temperature than standard industrial greases using 100 cSt oils. Increased evaporation rates of these lower viscosity oils, however, reduce grease life when operating at temperatures above 160°F to 170°F. As indicated in Table 3, greases employing synthetic oils can be used to -100°F and below.
The channeling nature of the grease also has a pronounced influence. Lindenkamp and Kleinlein 8 observed that after only one minute of ball bearing operation the torque dropped to only 30 to 60 percent of the starting torque. This effect is dependent on the amount of grease in the bearing: the greater the amount of grease, the higher the break-away friction torque. ASTM standard D14789 can be consulted for description of torque measurement in grease-lubricated ball bearings at low temperatures.