For these slideway applications, lubricants are formulated with rust and corrosion inhibitors, friction modifiers and extreme pressure and antiwear additives. Tackifiers are used to help products cling to vertical surfaces.

Slideway lubricants are exceptional in their ability to control stick-slip behavior, which is vibrating or quivering of two surfaces as they slide against each other.

Stick-slip causes friction oscillations that produce trembling notes from stringed instruments, glass harmoniums and grasshoppers legs. But the phenomenon sounds far less melodious to machinists and production engineers, who must deal with chatter, screeching and incorrect positioning of machine tools and parts, inaccurate cutting and defective products. Shudder in hydraulic systems and vehicle transmissions is another consequence of stick-slip.

Slideways present a unique challenge to lubricant formulators because the motion of a carriage for a machine tool on a slideway is comparatively slow and reciprocating (back-and-forth). The carriage moves to one end of the slideway, stops, reverses direction, travels back to the opposite end, stops, reverses direction and so forth. These conditions tend to wipe and squeeze lubricant away from the slideway where it is needed. In contrast, the continuous motion of many bearings and gears tends to draw lubricant into contacts.

In metalworking, a tool pauses at the end of its slideway or any time the lubricant film is not thick enough to prevent the carriage from adhering to the slideway. Then the static coefficient of friction comes into play and keeps the carriage from moving. The applied force on the carriage increases until it overcomes the static CoF and the carriage lurches into motion again under the kinetic CoF. The greater the difference between the static and kinetic CoFs, the more pronounced the stick-slip behavior.

The fundamental mechanism or root cause of stick-slip appears to involve the elastic component of the response or relaxation of sliding surfaces on an atomic or molecular scale and is the subject of advanced study.

The 2018 global market for slideway lubricants was 1.2 million metric tons, split primarily between Asia-Pacific (41 percent), North America (22 percent), Europe (19 percent) and South America (10 percent), reported John Hogan, project manager for metalworking with Wickliffe, Ohio-based Lubrizol Corp. Demand is expected to increase at a compound annual rate of 3.7 percent between 2019 and 2024, according to an analysis by IndustryARC that Hogan cited during the annual meeting of the Society of Tribologists and Lubrication Engineers in May.

Slideway lubricant formulations are versatile, for use with sliding or rolling elements and various metals, elastomers and synthetic materials. Performance priorities include protecting machine tools from wear and corrosion, helping equipment operate efficiently and reducing maintenance needs. Compatibility with metalworking fluids is important because cutting and other fluids carry slideway lubricants into the sump where metalworking fluids collect to be skimmed off for disposal or recycling.

Modern machine tools require slideway lubricants that provide stick-slip free operation for smooth, accurate positioning of tools and work pieces and high quality surface finishes. As such, the ability to prevent stick-slip behavior is indispensable for these fluids.

Measuring Stick-slip

Despite their commercial significance, no industry standard test is currently available to measure the frictional properties of slideway lubricants.

According to Hogan, lubricant suppliers relied on the Fives Cincinnati stick-slip test to obtain original equipment manufacturer approvals for slideway lubricants. A laboratory at Fives Machining Systems, a Cincinnati-based offshoot of theMilacronMachine Group, performed this test by measuring friction versus time while a pair of lubricated test pieces underwent sliding, then determining the stick-slip ratio. For OEM approval, the stick-slip ratio could be no greater than 0.80.

The lag time of months between sample submission and test results created a bottleneck, and the repeatability, consistency and meaning of the stick-slip ratio were questionable. The test was discontinued last year when Fives closed its dedicated lubricants laboratory. Lubricant suppliers changed the wording on product data sheets from has Cincinnati Machine Fives P-47 approval to meets the requirements of P-47. Formulators found themselves in an unexpected quandary, without means to obtain approval for new slideway lubricants.

When the Fives test fell by the wayside, Lubrizol began a study of the stick-slip ratio and friction testing for slideway lubricants. A team of engineers and scientists adopted the framework of vocabulary, definitions and guidelines for tribology measurements from ASTM G40, Standard Terminology Related to Wear and Erosion, and ASTM G115-10, Standard Guide for Measuring and Reporting Friction Coefficients.

The stick-slip ratio obtained from the Fives P-47 test is not recognized formally in the lexicon of tribology, according to Hogan.

Hogan and his colleagues learned that most readily available standard tribological tests are performed under conditions, such as surface roughness, stress, speed and distance of sliding, that differ from industrial applications of slideway lubricants. While Cameron Plint TE-77 and SRV tribometers are used for many laboratory tests, the researchers felt uncertain whether these instruments produce stick-slip behavior and raw data comparable to the Fives P-47 test.

Tribology to the Rescue

The Lubrizol team built a tribological test system based on designs of equipment formerly supplied by Cincinnati Milling Machine Co. for ASTM D2877-70 (Methods of Test for Measuring Frictional Properties of Slideway Lubricants, withdrawn in 1974) and the Fives Cincinnati P-47 stick-slip test.

Their benchtop instrument held a sandwich of two horizontal metal plates. Before starting a test, the lower plate was lubricated with 10 milliliters of test fluid and covered with the second plate. A stack of weights applied a normal (perpendicular) force to the top plate, and a load cell applied and measured the force to keep the top plate in place while the lower plate oscillated. The sensitive load cell, high-speed data acquisition system and statistical data analysis were improvements over the Cincinnati system.

The researchers adapted the ASTM D2877-70 test method to the conditions of the Fives P-47 stick-slip test. They optimized procedures for initial break-in and testing-a cycle of sliding in one direction, pausing for five minutes, and then sliding in the opposite direction-to deliver repeatable, statistically meaningful test results for friction forces and CoFs.

The adapted method called for calculating ratios of mean values of CoFs obtained from kinetic friction force data and pseudo-static friction force data.

For main tier and premium slideway lubricants, relative rankings or trends were identical for Lubrizols results and those from prior Fives tests performed at the Cincinnati laboratory. The ratios were significantly higher for main tier fluids versus premium fluids with better frictional properties.

Can the new test predict results from the Fives test? Hogan was optimistic about this possibility based on the data for five fluids: two controls (slideway lubricants that had received Fives approval from Cincinnati) and three new formulations. Lubrizols ratios for all three new fluids were comparable to the two controls and less than 0.80 (the Fives standard).

Hogan also noted that the Cincinnati laboratory calculated their stick-slip ratio from two data points, one from static data and the other from kinetic data. The Lubrizol team calculated their ratio from mean values of sets of kinetic friction force data and pseudo-static friction force data. While results from both tests were comparable in this study, future work may demonstrate an advantage to using the mean because a single data point might not be representative and could introduce an error.

Hogan also reported that his team observed that resurfacing specimens between tests did not improve the repeatability for good lubricants that left undamaged surfaces, but could be significant in tests of base oils, where surfaces were damaged by testing.

Caveats

According to Hogan, the term stick-slip ratio can be misleading. Stick-slip refers to fluctuations of force and velocity between elements in sliding contact, whereas the stick-slip ratio is a calculation from kinetic friction force and pseudo-static friction force measured after a period of rest. He explained that in principle, the breakaway force corresponds to the static CoF rather than this force after a period of rest.

However, Hogan cautioned, there is a downside to evaluating lubricants on the basis of the ratio of pseudo-static friction force to kinetic friction force data. Improving the kinetic friction performance of a lubricant, which reduces the kinetic CoF, increases this ratio, making it more difficult to meet an approval criterion, such as the Fives P-47 limit of 0.8, for a given value of pseudo-static friction.

In fact, he emphasized, there is no fundamental understanding of ratios of pseudo-static (or static) and kinetic friction that would support their use as criteria for lubricant approvals or selection. He suggested that comparing the breakaway friction versus the kinetic friction would be an easier and better way to evaluate slideway lubricants.

Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at mmmoon@ix.netcom.com or 267-567-7234.