Shot Peening in the Automotive Industry
in Vol. 10 - September Issue - Year 2009
Shot Peening Gears: The Role of Standards and Experiments
Shot peening is a widely used surface treatment in the automotive industry and is it possible to affirm that in the car of every one of us there are more than 10 peened parts. In particular, in almost every car there are many gears that are shot peened, thus permitting to improve the performances of the car without re-designing the gearbox and all the gears that form the power transmission, from the engine to the wheels.
Shot peening of gears is important since it allows us to increase the strength of the gear tooth both as regards the surface damage on the flank induced by cyclic contact and as regards bending fatigue at the gear root. In both cases the results of shot peening can be surprising. It is not rare to read of fatigue limits that are two times the one obtained without shot peening gears and of endurances that after shot peening are more than ten times the ones that is possible to get by using not peened gears.
But how can shot peening be correctly designed? How is it possible to choose the correct peening parameters, the ones that will allow us to obtain the maximum performances from shot peened gears? How can we change the way shot peening is done by considering different gears materials and geometry (different modulus)?
The first idea that probably an engineer has is to look at international standards, that for gears mainly means to look at AGMA and ISO.
Indeed, most of the standards consider only how to do shot peening and not how to get the maximum beneficial effects from this treatment. However, some data can be found.
As regards the ANSI/AGMA 2004-B89, January 1989, standard, entitled “Gear Materials and Heat Treatment Manual”, it includes a paragraph dedicated to shot peening. The standard states that bending fatigue resistance can increase about 25% due to shot peening, and consequently this surface treatment can be utilised to upgrade existing gears. Concerning surface fatigue, the standard states that resistance can be improved; but states also that quantitative data to substantiate this condition is poor. The well known ANSI/AGMA 2001- D4 Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth does not give a great importance to residual stresses; the standard only states that this kind of stresses, if properly induced (that is, if compressive), “should” increase the bending load limit of gears.
In other paragraphs of this standard, residual stresses obtained by different methods (nitriding, case-hardening) are mentioned. Finally, in the Appendix a paper of A. Datano e K. Namiki, 1992, is quoted, concerning the effects of shot peening on automotive gears; a Wöhler (S/N) diagram is reported, taken from this paper, that shows an improvement of bending fatigue resistance (or perhaps of fatigue limit) of about 30%.
If the ISO Standards are considered, it can be noted that they too give little evidence and data concerning shot peening as a method to improve the fatigue resistance of gears; quantitative data is poor. The ISO 6336-3 (1996) Calculation of load capacity of spur and helical gears, part 3, Calculation of tooth bending strength, states that the factor YRrelT, concerning surface roughness effects at tooth root, can have a different value for shot peened gears; but no data of this improvement is given. The ISO 6336-3 del 1996 Calculation of load capacity of spur and helical gears, part 5, Strength and quality of materials, states that, in general, the allowable limits depend on residual stresses. In the tables that give the materials quality grades (ML, MQ, ME), it is stated that for specific materials, in agreement with the customer, a savage of existing gears can be obtained by shot peening. The paragraph 6.7 is dedicated to shot peening, and some data on the beneficial effects is given. For case hardened gears the beneficial effects on bending fatigue are zero for ML grade, +10% for MQ grade and +5% for ME grade. No details are given to support this data.
What can we learn from these data? As a general rule they underline, once more, that standards are conservative. Consequently, they do not emphasize the effects of specific surface treatments as shot peening, whose beneficial effects are well known but cannot easily be generalized and quantified, because it is highly dependent on the way the treatment has been done. In particular, it’s difficult to quantify the beneficial effects concerning gear rating. Bearing this fact in mind it is better to experiment new shot peening solutions when something changes in the design of the power transmission of the car.
Author: Mario Guagliano