Wear study of structured coated belts in advanced abrasive belt finishing
Introduction
Friction is one of the major issues of mechanical engineering, especially in the automotive engines. One of the ways to reduce friction is to act on surface morphology. In practice, this is achieved either by using anti-friction coating technologies and texturing technology, or in a more traditional way by reducing surfaces roughness with one or multi-step finishing process [1], [2].
In a passenger car engine, about 30% of the total frictional loss is accounted by the bearings alone [3]. The process engineering departments working on this key organ have to maintain specific geometrical specifications and very strict surface finishes. In this context, the abrasive belt finishing is surprisingly simple and economical [4]. During the process, pressure-locked shoe-platens circumferentially press an abrasive coated belt on a rotating workpiece. Commonly, this finishing process also includes a low frequency oscillatory movement of the workpiece and belt finishing arm in a direction parallel to the rotations of the workpiece. This abrasive process is used extensively in the automotive industry to superfinish crankshaft journals and pins, which reduces surface irregularities, improves the geometrical quality, and increases wear resistance and fatigue life. However, in practice, belt finishing is a highly complex process.
The performance of this process depends on a large number of variables (grit density, shape and size, oscillations frequency, normal force, contact surface, lubrication fluid type, belts feeding, etc.). It makes its optimization particularly difficult. Moreover, the poor mastery of the active contact area and the abrasive characteristics [4], [5], [6], [7], [8] complicate the physical understanding. The crankshaft superfinishing also needs several steps of belt-finishing while successively decreasing the grit size, which involves substantial manufacturing and investments costs. Lastly, the conventional belt finishing machines used in the industry does not ensure sufficiently a good dimensional flexibility and also a change of the diameter of the workpiece necessarily implies a change of the shoes. Several technologies exist to partially solve these issues. Some of them propose to modify the shoes (geometries, material, servo pressure of the inserts), while others change the abrasive belt (grit's morphology, grit's material) or the global kinematic of the apparatus (high oscillations frequency, radial micro-vibration).
In our previous work, we studied, in the same process configuration, the link between grits' morphology, the surface finish of belt-finished workpieces, and the physical mechanisms which govern their wear performance [9]. We found that a coating with slanted grits had the advantage to ensure a good clipping of roughness profile while preserving valleys depth and thus, preserving the retention capacity of the surface roughness. The belt composed of pyramidal agglomerated grits allowed a very good reducing of the roughness amplitude while having a good wear resistance. Moreover, we saw that dense structures of grits could obstruct the chip's evacuation, generating unpredictable results in terms of roughness. The present study thus aims to extend our previous one by varying the cycle time and the rotation speed.
Section snippets
Nomenclature
- tc
cycle time (s)
- N
rotation speed (rpm)
- Nr
revolutions number of the sample
- Nrc
characteristic revolution number
- D
initial workpiece diameter (mm)
- R
initial workpiece radius (mm)
- L
belt finished width (mm)
- Rpk
reduced peak height (ISO 13565) (μm)
- Rk
core roughness depth (ISO 13565) (μm)
- Rvk
reduced valley depths (ISO 13565) (μm)
- C
circularity (μm)
- Ma
multiscale arithmetic roughness average (μm)
- MPS
Multiscale Process Signature
Experimental procedure
In this work, three abrasive structures are considered. Their behaviors during belt finishing are determined by measuring the surface roughness, the circularity gains, the consumed energy, the belts' wear and the generated chips. An extensive discussion based on physical explanations underlines the behavior of three abrasive structures when the cycle time or the rotation speed increases.
The belt finishing test rig consists of a conventional lathe (power of 9 kW) and a superfinishing apparatus
Workpiece surface finish
The relative reductions (gain ratio) of the functional roughness parameters Rpk, Rk and Rvk (ISO 13565 standard) were assessed in order to estimate the surface finishing improvements. These parameters are all based on the analysis of the Abbott–Firestone curve (see Fig. 4), which is simply a plot of the cumulative probability distribution of surface roughness height [10].
Rpk, Rk and Rvk are particularly relevant when characterizing textured surfaces since they allow to well describe the
Conclusions
This paper shows all the effects of the abrasive belts structure during a one step belt finishing. The tests conducted have demonstrated that, at the same number of revolutions, modifying cycle time or rotation speed can lead to a different surface finish, belt's wear, and consumed energy depending on the abrasive morphology considered.
With Type I and III abrasive structures, we see that the number of revolution is not sufficient to explain the roughness modifications and the results follow
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