Nye Lubricants Inc.
Edited by Victoria Reitz
When faced with the challenge of reducing cost without sacrificing quality, damping grease can be a useful tool in a design engineer's bag of tricks. Damping greases are specialty lubricants that reduce wear and seal out dust and moisture like traditional greases, but are primarily used to control motion and noise in mechanical and electromechanical devices.
Think of a zoom lens on a 35-mm camera. The "velvet feel," virtually silent operation, and fact that the lens doesn't coast out of focus are all the work of a damping grease on the focusing threads.
Damping greases were first formulated to economically build fine tolerances into microscopes, telescopes, and binoculars. Though available for more than 60 years, damping greases didn't see much use beyond optical instruments and that's because of their limited low-temperature capabilities. At room temperature they worked well, but they became so viscous at low temperatures they restricted rather than facilitated movement. That changed in the mid-1980s when several broad-temperature synthetic damping greases were developed. Switch manufacturers, especially automotive suppliers for whom –40°C is the low-temperature norm, were among the first to take advantage of them. A small amount of damping grease on switch detents softens that plastic-on-plastic clicking, which can easily be perceived by consumers as poor quality. Further, damping grease gives a smooth feel to hand-actuated parts of a switch — without the expense of fine engineering tolerances.
How damping greases work
All greases are made by mixing an oil with a thickener, typically soap, clay, silica, or a synthetic material like polytetrafluoroethylene (PTFE). Thickeners hold the oil in place. When grease shears — by a rotating shaft, sliding lever, or rolling-element bearing, for example — thickeners release oil to lubricate moving parts. The difference between standard and damping greases is shear resistance. In fact, damping grease performance depends more on its internal structure than on ability to reduce friction between mating surfaces.
The oils used for damping greases are usually viscous, high-molecular-weight synthetic varieties which give much higher internal shear resistances than standard greases. Most standard greases have a slick, even buttery consistency. Damping greases, on the other hand, are sticky. It takes some force to move objects through damping grease. This internal shear resistance lets damping greases "damp" or control motion and noise. The higher the shear resistance, the greater the damping.
Damping grease prevents contact between mating surfaces but makes it necessary to apply some force to move those surfaces in opposite directions. Therein lies the "magic" of damping grease. Parts move within the grease and do not come into contact, so there is little if any noise or wear. And the damping characteristics limit free motion when that force is removed.
Greases in the real world
Consumers frequently judge quality by how a device feels and sounds. Automotive manufacturers have entire engineering groups devoted to eliminating buzzes, squeaks, and rattles, and in search of that "perceived quality" feel. Damping greases help meet these tactile and acoustical design goals. High-viscosity synthetic hydrocarbon oils are typically the base oils of choice.
Damping greases are frequently used in potentiometers that have a rotating shaft with a hand-operated control knob. As the shaft turns, it moves a contact finger on a conducting surface to control resistance in a circuit. These devices are usually small, so the slightest rotary motion can significantly change the electrical result. In most cases it is also impossible for the steadiest of hands to keep from coasting past a desired setting. Therefore precise settings are a design goal. A small amount of damping grease on the shaft is one solution. Not only does this enable precise hand settings, it delivers that velvety, high-quality "feel." And it all comes without expensive mechanical solutions to control any free motion.
Noise, not motion control, is sometimes the primary concern. Case in point: a squealing worm gear in a household mixer. An offthe-shelf grease chosen by a major appliance manufacturer was "channeling," that is, getting pushed aside by the gears, and not slumping back into the gear teeth, causing a shrill, metal-on-metal squeal. The decision was made to custom-formulate a lubricant by experimenting with various amounts and types of thickener and base oil. In formulating lubricants, engineers have to get the right "apparent viscosity," a measure of how stiff a grease remains under shear, and the right "kinematic viscosity," which is the viscosity of the base oil alone. In the mixer's case the noise was squelched with a "pourable damping grease." It had the internal shear resistance to quiet the gear, but was soft or pourable enough to slump back into the gear teeth and not channel.
Most components that use synthetic hydrocarbon damping greases experience low to moderate shears or, worst case, high shear for short periods of time. But what happens to the mechanical stability of damping grease after extended, high-shear exposure? Most damping greases lose their mechanical stability.
It's important to distinguish grease's rheological behavior from its shear thinning. Most greases shear thin as a function of shear rate or time, but recover their original viscosity when shear ceases. This is not the case when high-viscosity, synthetic hydrocarbon damping grease is subjected to extended high shear. Instead it undergoes a nonrecoverable reduction in viscosity, a permanent rheological change. Simply put, it loses its damping characteristics.
The culprit is the gellant, not the blend of synthetic hydrocarbon base oils. When the same high-molecular-weight oil blend is mixed with PTFE, the new grease returns to its original viscosity after 153 hr of continuous high shear. Interestingly, shearing the grease actually improves its damping characteristics because the shearing mills the PTFE. This shear behavior has led to the recent commercialization of a series of damping greases for high-shear components that need motion and noise control.
Applications are already surfacing. DaimlerChrysler specifies one of these PTFE-thickened damping greases for its tilt-steering gearing. Visteon, an automotive supplier based in Dearborn, Mich., uses it with favorable results in an adjustable steering column. Another supplier, Delphi Automotive of Troy, Mich., chose a light version of this shear-stable grease to fix a warranty problem with tie-rod boots, an application where the grease's rust-prevention additive is a plus. And, Lear Corp. of Southfield, Mich., changed from a more traditional damping grease to the PTFE-thickened version, noting that the new grease gives its switches a "silkier feel."
In the world of office automation, especially printers and copiers, high-shear damping greases promise to play a major role in both noise and cost reduction. Although the shear rate of gearing in printers and copiers is not as demanding as in tiltsteering columns, shear time in these constantly moving components presents equally demanding challenges. Add in the need to keep office machinery quiet and lowcost, and high-shear damping greases become more important. Damping greases take dollars out of design by removing mechanical complexity. Parts used for motion control and noise suppression can often be replaced with low-cost damping grease.
Selecting a damping grease
Properly chosen damping greases must retain damping qualities throughout the temperature range of the application. Synthetic hydrocarbon greases are suitable for –40 to 125°C. Silicone-based greases work in temperatures ranging from –60 to 200°C. Because of potential contamination problems, however, silicone-based greases are not recommended for optical and electrical applications.
Material compatibility must also be tested. For example, some synthetic hydrocarbons weaken polyethylenes, polystyrenes, polyvinyl chloride plastics, and some low-density elastomers. This means long-term compatibility problems may arise. Although material suppliers offer compatibility charts, the best way to ensure material and lubricant compatibility is through life testing.
Damping greases are also selected for the "feel" designers want. Generally, the more delicate the device (the lower the torque), the lighter the grease. Engineers can choose from various grease consistencies, from light to ultraheavy. For example, the volume control on a radio calls for a lighter grease, whereas the release mechanism on a parking break needs a heavier grease. Generally, damping grease can be custom-formulated to achieve the specific feel the engineer wants.
High-viscosity, silicone-based damping greases are a good fit when there are material compatibility or high-temperature issues. They are typically thickened with PTFE, and work with all but silicone-based rubber. Their temperature range extends from –60 to 200°C. Though silicone oils are known to migrate, the molasseslike viscosity of the silicone base oil gelled in PTFE virtually eliminates migration issues.
Damping greases are not for every application. They are successfully used in many low-torque devices but flea-power devices couldn't overcome even the lightest damping grease. And there is a premium to be paid for a quality damping grease, so low cost devices may have to pass.