How Grease Keeps Bearings Running
Engineers must understand that sleeve-bearing operation to specify the right grease for longer bearing life.
Grease is thickened oil. Sound simple, right? But Lubricating bearing with grease is much more complex than that, in both benefits and drawbacks.
Grease lubricates cast bronze, aluminum-bronze, or tin-bronze sleeve bearings up to 15 in. in size with surface speeds up to 20fpm. The friction modifying lubricant film it provides minimizes wear at low speeds, under shock loads, during start-and-stop cycling, and while reversing direction.
Grease is more stable, requires less maintenance, and leaks considerably less than conventional oils. It also lets users do away with elaborate oil-supply systems. And extreme-pressure and antiwear additives, as well as graphite and molybdenum-disulfide
Powders, are improving the performance of greases.
However, predicting the performance of grease-lubricated bearings is more complex than for their oil-lubricated counterparts. Grease has both solid and liquid phases, and engineers must consider both, as well as how they work together at specific temperatures and shear rates.
Grease behavior can be particularly hard to predict when the shaft or bearing undergoes low-amplitude oscillation. Due to its higher viscosity, grease may not get replenished as readily in the bearing load zone during oscillation, leading to wear particles, galling, scuffing, and fretting.
Grading greases
The National Lubricating Grease Institute (NLGI) classifies greases into grades by stiffness from 0,00 to 6 where 6 is the stiffest. Stiffer greases are more mechanically stable under high shear forces and low speed and under shock loads. However, the stiffer the grease, the harder it is to lubricate the bearing surface using channels from a central lubrication system.
Plain bearings usually use Grade 0 through 2. Softer grades, 0 and 1, are easier to feed to rows of machine elements, but offer less mechanical stability than Grade 2 grease. Nevertheless, they adhere to sleeve bearing surfaces and handle shear forces from oscillatory motion better than regular, unthickened oils.
Thickeners can give greases high dropping points --- the temperatures at which they begin to drip and lose their ability to keep the oil phase in suspension and maintain higher apparent viscosity. Dropping-point ratings are higher than maximum operating temperatures because few grease recover their structural capabilities once cooled from dropping point.
Thickeners, which usually make up about 10% of a grease by weight, may be simple or complex soaps or nonsoap based.
Simple soaps, formed by neutralizing a glycerine-organic acid complex with a base like lithium hydroxide, provide dropping points in the 180 to 190°C range. Complex soaps use both long and short-chain organic acids to get more cross-linking and higher dropping points of 240°C and above.
Fine clay particles or nonmelting organic powders like polyurea can replace soaps for a grease with no melting point for high temperature use.
While thickeners are limited by dropping points, the oil components of greases also suffer at high temperatures. Heat speeds oxidation and evaporation, eventually hardening the grease and increasing oil viscosity.
Temperatures like those encountered in arctic mining, around – 50°C, on the other hand, increase greases’ apparent viscosity, making rotation more difficult and boosting the torque required to start the machine. In addition, at low temperatures, greases can be to stiff to work with central lubrication systems to form a lubrication film on the bearing surface. Low temperature limits for greases vary from – 75 to 0°C, although the exact limit depends on the type of oil and, secondarily, the thickener content.
Low-viscosity synthetic oils --- such as polyalphaolefins, esters, and highly refined American Petroleum Institute (API) Group 3 oils --- with high viscosity indices (VIs) work best in these cold environments.
No matter the operating temperature, it is apparent viscosity that determines how well a grease will work. Grease is a non-Newtonian fluid, so its viscosity depends on both temperature and shear rate. And both oil and thickener phases contribute to total grease viscosity.
Choosing a target viscosity for plain bearings means balancing lubrication-system dispensing rates and the ability to form lubricating films. Lower viscosity greases work better with central lubrication systems while higher-viscosity greases stay in place better to form lubricating films and reduce the bearings’ contact with asperities, microscopic peaks on the shaft surface.
In practice, however, it is usually the central grease-lubrication system that sets the viscosity target. Oil-viscosity grades in the range of 150 to 460 cSt at 40°C are typical.
Because of these lower viscosities and the high rate of asperity contact in plain bearings, greases still need additives to cut friction and control bushing wear, especially in oscillating applications. Engineers often turn to greases with higher viscosity base oils and with additives designed for extreme pressure, low speeds high loads, oscillation, and high temperatures.
While engineers may choose oils and thickeners partly based on test and performance ratings, filed experience is the ultimate guide to meeting load, speed, power loss, and temperature requirements.
Predicting performance
It’s hard to generalize the performance characteristics of greased sleeve bearings. But engineers can best estimate how bearings in one application will perform by looking at temperature, regreasing needs, torque wear, load capacity, coefficient of friction, and temperature rise from similar bearings in other machines.
Load capacity is lost as shaft-surface sliding speed increases. W. A. Glaeser and K. F. Dufrane calculated that load capacity drops from 5000 psi at 10 fpm, based on projected bearing area at a maximum temperature of 300°F. For increased reliability, engineers should limit design loads on greased sleeve bearings to 250 to 500 psi.
Coefficient of friction in sleeve bearings vary widely depending on whether a full separating, or hydrodynamic, film of lubricant forms during operation. When it does, where there is a large volume of grease or higher sliding speeds, coefficients of friction can fall in the 0.01 to 0.02 range.
However, in most applications that use grease, the shaft slides in its bearing bore too slowly to generate a hydrodynamic film.
In heavily loaded pin-bushing joints, for example typical operating speed is under 10 rpm and the surfaces fall in the boundary lubrication regime where there is considerable asperity contact. Film thickness cannot accommodate the entire load. As a result the friction coefficient is typically 0.08 to 0.16.
Excessive temperature rise is a major concern in any bearing. Overheating leads to excessive degradation of grease and premature scuffing failures. Heat generation is directly proportional to the product of coefficient of friction, load, and speed.
Extensive experimental bearing tests at the LSU Center for Rotating Machinery show that bearing temperature rises gradually to a steady level where the bearing operates satisfactorily. However, under severe operating conditions, like those with high loads and speeds, temperature increases exponentially over time until the bearing fails without ever reaching a steady-state temperature.
In oscillating bearings, the angle of oscillation, also called the swing angle, affects temperature. (See the accompanying diagram.) Increasing the oscillation angle by 10° boosts the steady-state temperature by 20°C.
Regreasing rates
High operating temperatures dry grease through evaporation and oxidation of the oil. Grease can also be lost by creep. In general, the higher the operating temperature, the more frequently bearing surfaces need to be regreased. For bearings with surface speeds above 10 to 20 fpm, engineers usually use continuous-feed systems to supply NLGI grade 00,0 or 1 greases.
How do engineers determine the appropriate grease-feed rate? Here’s an empirical relation to predict the right feed rate:
Q = 3.5 x 10-5 x L x D x N0.3
Where L = bearing sleeve length in mm, D = bearing diameter in mm, and N = bearing speed in rpm.
For example, to calculate the required feed rate Q in cm3/hr of fluid grease for a 4-in.-long sleeve bearing with a 4-in. diameter (101.6-mm long x 101.6-in diameter) operating at 200 rpm:
Q = 3.5 x 10-5 x 101.6mm x 101.6mm x 200 rpm0.3
Q = 1.8 cm3/hr = 0.11 in3/hr
For elevated temperatures, use the accompanying graph (see “Regreasing time versus operating temperature”) to figure out the maximum operating time between regreasing. For example, a bearing working 8hr/day may require weekly regreasing if the bearing temperature is about 220°F, but may only need to be regreased monthly if it operates at 100°F.
To confirm that these grease feeds are appropriate, an engineer might initially set the lubrication system to deliver 25 to 50% more grease. Then he can gradually reduce the feed rate while watching for an undesirable rise in friction or bearing temperature that would indicate a lower limit for the replenishment rate.
In addition to the feed rate, the bearing bore may need grooves to distribute the grease.
Axial grooves are common in continuous, unidirectional-motion applications. Central circumferential grooves help spread grease on oscillating bearings, especially where load direction varies. And figure-eight-shaped grooves are ideal for continuously rotating bearings under heavy loads.
Bearing suppliers can often recommend customized grooving for special applications. MD