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Does R_a Tell the Truth, the Whole Truth?

by Norm Judge

Little things, like the microscopic features of a machined surface, sometimes have an enormous impact. That's why the ability to measure a bearing surface, evaluate the process used to generate it and ensure the bearing's ability to perform its intended function is essential. But doing so isn't as simple as just checking Ra (the arithmetic average of the height of peaks and depth of valleys from a mean line).

For example, when a precision-ground part is examined under a microscope, fragmented material will be seen. This amorphous, unstable, surface-material layer is part of every turned or ground part. On non-microfinished bearing journals, during normal operation, this material loosens, erodes the mating component, and breaks up the supporting film of lubricant, degrading performance and even causing bearing failure.

Some wearing surfaces may require a texture to retain lubrication. On a cylinder wall in an engine block, a plateau cross-hatch finish provides oil retention. When a pressurized lubrication system is used, however, the smoother the surface, the better. Modern engines feature a very small clearance and oil film between the mating surfaces of the bearing shell and load-bearing journal, such as on a crankshaft. Any protrusions breaking through the oil film can cause premature bearing failure.

Roughness average (Ra) surface measurements are nearly always shown on engineering drawings where surface finish is specified, but it's not the parameter that indicates a good bearing surface. When measuring the surface only for Ra values, two different surfaces can have the same Ra values, even though they appear quite different. One surface - smooth but with tiny pits left by the removal of ferrite nodules form the material - could be a bearing surface that features good oil film retention. Another, ostensibly rougher, surface with better Ra could be quite unsuitable.

Other surface finish parameters include Rp (highest peak in the measure length) Rz (five highest peaks and five lowest valleys in the a sampling length), and Rk/Rp/Rpk, a surface finish measurement parameter used to evaluate the bearing area curve, Tp (Abbott-Firestone or material ratio curve). Curve height is termed the roughness total, and its width is the percent of material at different profile amplitudes. The Rpk value measures the portion of the surface that will wear off during initial loading, while Rk describes the portion of the surface that will support most of the load on the surface.

It's essential that Rpk be reduced as far as practical to eliminate undesirable wear and potential bearing failure. But, all too often, a surface is evaluated against an Ra specification. This is almost useless in process analysis, or, more importantly, in evaluating a surface's bearing capability.

Although Ra may suffice to measure some surfaces, nodular iron's characteristics renter Ra inadequate or even misleading. This parameter cannot distinguish between very different surface profiles like homogeneous (steel) and non homogeneous (nodular iron) materials.

While grinding may improve the average roughness (Ra) of many materials, on nodular iron it exposes ferrite caps or pulls them out, leaving tiny craters in the surface. The result is a changed - actually rough - surface. Measuring the change is essential to determining whether the surface can perform its intended purpose, and not all surface-finish measurements are helpful.

For nodular iron parts, the better the finish, the worse the Ra. The Ra readings look bad because the measuring instrument's needle drops into craters during its trace, and skips when it comes out, suggesting a peak on the chart. Diameters and depths of the holes could be 50 um.

Because load-bearing journals will support a bearing shell in the engine, and are subjected to high loads, Rk and Rpk might well be used to evaluate bearing journals. In a test, the surface of a ground journal has an Ra of 0.27 and Rk of 0.75. After processing with 40 um microfinishing film, 15 um film, and then 9 um film, Ra improved to 0.15 from its ground state. More importantly, the Rk improved from 0.75 as-ground to 0.15. This created a surface that would reliably support a bearing load - which is the purpose of crankshaft bearing journals.

It's essential that manufacturers understand and use appropriate surface measurement parameters and evaluation techniques. While Ra averages the peak and valley displacement from a mean line, it provides no information about the heights of the peaks and valleys, or the ability of the material to bear a load (bearing ratio), a vital characteristic for crankshafts.

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