DECEMBER 2006motionsystemdesign.com

A Penton publication

Greaselightning

New lube formulationsmeet need for speed,

page 22

Limit switches get withthe program, page 16

Reducing friction the“hard” way, page 28

THE ENGINEER’S GUIDE TO DRIVE, CONTROL, AND SENSING TECHNOLOGY

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Need for speed

Taking the heatProper lubrication and sealing systems arecritical in maintaining the right temperaturelevels in high-speed applications.

High speeds generate heat in motionsystems. Choosing the right lubri-cants — oils, grease, and other fluids

— and installing proper sealing systems helpsa system achieve optimal temperature opera-tion. Read on to learn more from our experts.

What lubrication and sealingattributes are most tightlylinked to speed, and how dothey affect it?

Steve • Kluber: Several lubricant charac-teristics must be considered to minimize heatgeneration, maintain fluid friction, and pro-vide adequate lubricant lifetime.

Whether the application is a grease-filledbearing, a linear way, or even an oil-filledgearbox, one must consider a lubricant’s opti-mal base oil viscosity based on its runningspeeds. As speed increases, the required vis-cosity of the lubricant decreases. If the viscos-ity is too high, fluid friction generates exces-sive heat and reduces lubricant lifetime. Forevery 15° C increase, the expected lubricantlife is cut in half. If viscosity is too low, bound-ary friction between the mating componentsgenerates heat — and excessive wear of thecomponents. The proper oil viscosity, on theother hand, provides a fluid film (fluid fric-tion) between mating components and stabi-

lizes at the lowest possibletemperature for extendedlubricant life.

Selecting the base oil isalso important. Sometypes are better for rollingfriction (found in rollingelement bearings and ballscrews.) Others are betterfor the sliding friction ofworm gears, linear ways,and plain bearings. Oneexample is the use of poly-alkylene glycol (PAG) oils,which have low coefficients of friction andcan reduce the operational temperature insliding friction, but do not provide protectionunder rolling friction.

Synthetic base oils typically have betterthermal stability than mineral oils, so they lastlonger as speeds and temperatures increase.

Colin • Busak+Shamban: Sealing systemsmust withstand heat. This is addressed in twoways — with materials and designs that havelow friction to generate low levels of heat, orwith materials that withstand high levels ofheat without significant loss of properties.

John • SKF: In the case of radial lip shaftseals, these function to retain lubricants andpressure and exclude contaminants from bear-ing housings. There are many different seal

Meet the expertsJohn P. CrawfordSKF Sealing Solutionswww.skfusa.com

Ron HudasThe Timken Co.www.timken.com

Colin MacqueenBusak+Shamban Americaswww.busakshamban.com

Steve MazzolaKluber Lubrication N.A.www.kluber.com

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designs and seal materials, both elastomericand PTFE based, to accommodate varying ap-plication parameters, including shaft speed.

The majority of standard small bore seals,under 8-in. shaft diameter, are rated up to3,600 ft/min., while the large diameter seals,with shaft diameters over 8-in., are rated toapproximately 5,000 ft/min..

Ron • Timken:For oils, viscosity has thebiggest influence on speed capabilities, and toa lesser extent, so does the oil type. The fasterthe bearing rotates, the lighter the viscosity ofthe oil must be. If the oil is too heavy, heat isgenerated and the coefficient of friction actu-ally goes up as speed increases — therefore,limiting the speed.

Synthetic oils like PAOs have a higher vis-cosity index than mineral oils. The higher VImeans that the oil’s viscosity changes less withan increase or decrease in temperature. Thisproperty can compensate for mismatches be-tween bearing speed and temperature. Esteroil is a friction modifier that can help withbearing break-in at ultra-high speeds, whichkeeps temperature lower.

For grease, the base oil and thickener bothinfluence attainable speed. Typically grease islimited to speeds of 2,500 ft/min. The base oilcan be treated as above. The thickener affectsspeed because different thickener types(Lithium complex, polyurea, clay) vary intheir resistance to a bearing’s rolling motion.For example, at ultra-high speeds a thickenerwith the lowest resistance to rolling motion,such as a polyurea thickener, is required. Theconsequence of using too heavy of a thickenerresults in more heat generated.

In general, as shaft speed increases, the fric-tional heat that develops at the seal/shaft in-terface also increases. Higher operatingspeeds require that both the lubricant and sealelastomer material be formulated to handlethese corresponding higher temperatures.

What are some of the limitingfactors associated with lubricantsand seals in terms of speed?

Colin • Busak+Shamban: Componentsmating to the seal — typically housings and

shafts — may have poor heat dissipation prop-erties, poor surface finishes, or tolerance vari-ations that demand higher degrees of interfer-ence and hence heat generation. Bearings maypermit eccentricity of the dynamic compo-nent, leading to excess load on one side of theseal. Too, contaminants may abrade PTFEseals — though these can be addressed by theuse of robust wiper and exclusion systems. Fi-nally, lubricant properties also affect seal per-formance. For example, high-speed sealing ofwater-based fluid is particularly challengingdue to the tendency of water to boil under thedynamic interface.

Ron • Timken: Seal design parameters(such as lip interference) and misalignmentand run-out capabilities should be tailored forhigh-speed applications. Keeping these pa-rameters at a minimum reduces the amount offrictional heat generated. Whenever possible,labyrinth or bearing isolator-type seals shouldbe used for high-speed applications. Theseseals are designed with “non-contacting”components, so there is no frictional heat de-veloped by the seal itself at any shaft speed.

John • SKF: It’s best to concurrently evalu-ate speed with other operating parameters, forthe optimal balance of capability and seal life.To illustrate: When surface speed increases,seal torque, power consumption, under-liptemperature, and the effects of dynamic run-out increase.

To combat the negative effects of highershaft speeds, reduce interference of the seal lipwith the shaft, and reduce the seal lip’s radialload with alternatives in material, spring de-sign, and seal head section design. Also,change to a sealing material that can handlehigher temperatures; change the lubricanttype or viscosity; optimize the shaft sealingsurface; and use a non-contacting seal designif possible.

Adequate cooling and circulation of the lu-bricant in a sealing system is also essential incontrolling the excess heat generated by in-creased surface speeds.

Steve • Kluber: The importance of goodshear stability increases with operational bear-ing speed. Grease structures can be damagedby high-speed shearing of thickeners. Most

Need for speed

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grease manufacturers provide limiting speedsfor their bearing greases. These “limiting speedfactors” are usually identified as nDm where n isthe speed of the bearing and Dm is the mean di-ameter of the bearing. These limiting speedsare usually established through testing, whichcan include the testing of grease in rotatingbearings on a dynamic test rig, and testing theapparent dynamic viscosity of the grease on arotary viscometer. After completion of thesetests, the grease can be analyzed in a lab, toidentify any changes in its structure.

As bearing speed increases, it can only toler-ate lower amounts of grease. Excessive greasefill quantities (especially in a sealed or shieldedbearing) generate heat and can damage thegrease structure. This is caused by the constanthigh-speed movement of the grease into andout of the ball (or roller) path. Limiting theamount of grease in the bearing keeps thisproblem at bay.

High-speed bearing operation also benefitsfrom a run-in process. Running-in the bearingallows the grease to stabilize for high-speed op-eration, and it gently pushes the grease out ofthe direct ball (or roller) path. Most greases usea soap-type thickener (lithium, calcium, alu-minum, and barium are common) which havefibrous structures. A run-in can orient this fi-brous structure in the direction of rotation andstabilize the oil release property.

Without running-in the grease, a high-speedbearing can fling the grease too far out of theroller path, and leave the bearing vulnerable tothe effects of insufficient lubrication.

What’s the fastest applicationyou’ve seen?

Ron • Timken: I’ve seen spindle ball bear-ings operating at two million Dm. Synthetic

base oils are used for this application, because ithelps keep viscosity more stable. A blend ofabout 5% ester reduces wear and reduces tem-perature during break-in. If grease is used, apolyurea thicker is chosen.

Although most standard elastomer lip sealsare designed to operate at speeds up to 3,000sfpm (surface ft/min.) there are some designsthat can go as high as 7,000 sfpm. These higherspeed capabilities are usually due to using ahigher-grade elastomer with a lower coefficientof friction.

Steve • Kluber: Historically, the highest al-lowable speed of a greased bearing is lower thanone that is lubricated with oil. Most high-speedgreases have had limiting speed factors less thanone million Dm. However, newer high-speedgreases are dynamically lighter and more stablefor high-speed shearing. The greases can actu-ally be used up to bearing speeds typically ratedfor oil only. Of course, the considerations asmentioned above (fill quantity, run-in, and soon) are critical to long-term protection. That’sbecause even the best grease for an applicationmay fail very quickly if those considerations arenot carefully met.

Colin • Busak+Shamban: We have ad-dressed high-speed sealing applications in lin-ear mechanisms such as aircraft landing gearwith speeds of 20 m/sec, hydraulic hammerswith speeds up to 12 m/sec, and relief switchesin high-voltage power systems with accelera-tion to 12 m/sec2. High-speed rotary applica-tions include turbocharger seals with surfacespeeds of 120 m/sec, oxygen compressor sealsrunning at 40,000 rpm, yielding surface speedsaround 42 m/sec, and surgical bone drills at upto 75,000 rpm. MSD

For more information, contact the editor atmgannon@penton.com.

Need for speed

Copyright © by Penton Media, Inc.

Klüber Lubrication North America L.P.

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Londonderry, NH 03053

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