How Grease Keeps Bearings Running: A Deep Dive for Engineers

In this BOCA Innovation newsletter, we will explore the benefits of well-greased bearings. If you’re involved in the engineering world, especially in machinery, you know that keeping things running smoothly is crucial. Let’s talk about something that might seem simple but is pretty complex: grease in bearings.

What’s the Big Deal About Grease? Grease is thickened oil. Sound simple, right? But lubricating bearings with grease is much more complex than that, with 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 20 fpm. Its friction-modifying lubricant film minimizes wear at low speeds, under shock loads, during start-and-stop cycling, and when reversing direction.

The benefits of grease: It’s way more stable, needs less upkeep, and doesn’t leak nearly as much as regular oils. It also has the benefits of extreme pressure and antiwear additives, plus the perks of graphite and molybdenum disulfide!

Predicting how grease-lubricated bearings will perform can be a bit trickier than with oil-lubricated ones. That’s because grease is made up of both; solid and liquid phases, so engineers have to take both into account. They also need to think about how these phases interact at different temperatures and shear rates.

Grease behavior can be particularly difficult to predict when the shaft or bearing undergoes low-amplitude oscillation. Due to its higher viscosity, grease may not be 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, low speed, and 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 Grades 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.

SIDEBAR-- BOCA's KRYTOX GPL fluorinated grease with an operating temperature of -50°C to 180°C is safe in all reactive gases, including oxygen, chlorine, fluorine, and HF. It's also resistant to oxidation and won't form coking. Additionally, it won't react with acids, bases, or other harsh chemicals.

Predicting Performance

It’s tough to make broad statements about how greased sleeve bearings perform. However, engineers can get a good idea of how bearings will hold up in a specific application by checking out factors like temperature, regreasing intervals, torque wear, load capacity, coefficient of friction, and temperature increases from similar bearings used in other machines. It’s all about gathering data/insights from what’s worked before

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 the 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.

The Coefficient of Friction?in sleeve bearings varies widely depending on whether a fully 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 the 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. Increasing the oscillation angle by 10° boosts the steady-state temperature by 20°C.

Choosing the Right Grease When choosing grease for your bearings, consider the viscosity you need. Lower viscosity greases work with central lubrication systems, while higher viscosities help reduce contact wear. But remember, these choices often depend on the specifics of your lubrication system.

Final Thoughts Navigating grease lubrication in sleeve bearings is crucial for maintaining machinery efficiency and longevity. Experienced engineers know that hands-on testing and field experience are essential in perfecting this balance.

So next time you grease those bearings, remember there’s a whole lot more going on than just slapping some grease in there! No to worry! The BOCA team is here to help you with your selection.

Let’s keep the conversation going—what has been your experience with grease and bearings? Share your thoughts in the comments!