PowderMet 2013: Innovative solutions to gearing challenges using Powder Metallurgy
Capstan Atlantic is prominent among the PM companies who have succeeded in developing a range of PM solutions to gear applications. Although the major target of manual gear-box transmission gears remains elusive, the company has an impressive track record in penetrating other gear markets.
This track record was described in the presentation which was structured around a number of case studies of established applications, some of them past award-winning parts, and the technical solutions that have supported them. The featured case studies included:-
Crankshaft gear for a lawn & garden application
The technical requirements included sufficient elongation to allow a press fit on to a keyless mating shaft, good gear tooth surface finish to satisfy NVH issues, tooth wear resistance and durability and low involute profile lead error to minimise gear whine.
The solution involved the use of copper infiltration and surface rolling densification of a gear shape with customised tooth relief (Fig. 1).
Fig. 1 Crankshaft gear for lawn & garden application [1]
Transfer case sun gear for a heavy duty off-road application
The technical requirements were for high density (for strength), high surface hardness (for wear resistance), good core ductility (for impact resistance) and precise tolerance of the involute profile and helix (for NVH performance).
The solution comprised the use of a modified (low core carbon) FLN2-4405 material at a minimum density of 7.5 g/cm³, providing a UTS > 1550 MPa. The parts are carburised (for wear resistance), shot peened (for surface finish improvement) and subjected to 100% NDT (for structural integrity validation) (Fig. 2).
Fig. 2 Transfer case sun gears [1]
“High dense” transfer case sprockets
These sprockets replaced AISI 8620 forged sprockets and required a high UTS, extreme wear resistance and excellent NVH properties.
The solution involved the use of a low alloy, highly compressible raw material, processed to a high density in single compaction and with a carbonitrided case for wear resistance.
The gear design included a customised involute tooth shape for smooth chain roll-off (Fig. 3).
Fig. 3 “High Dense” transfer case sprockets [1]
Bellville cup for a torque compensator in an all-terrain vehicle
The challenge in this application was to provide a wear resistant inner wall with precise spline and strong tabs for load resistance.
The tooling concept for this part included a lower punch action, designed for densification of the three outer tabs to 7.1 g/cm³, and powder transfer to the spline/flange region to achieve high density. The key part characteristics are spline size, counter-bore diameter, flange parallelism and run-out. These are tightly controlled dimensions.
The part is then subjected to a fixtured sintering treatment and a specially designed low distortion heat treatment, to maintain tolerance control.
The part is made from a modified FL-4405 material, through hardened to achieve a minimum UTS of 160,000 psi, a yield strength of 150,000 psi and a particle hardness of HRC 60 (Fig. 4).
Fig. 4 Belville cup for torque compensator in all-terrain vehicles [1]
Crowned spur pinion
The challenge here was to modify the crowning on the 15-tooth transfix compound gear to reduce contact stress and increase gear life. System tolerances yield mesh angle errors across the gear flanks that result in high edge contact stresses (point loading) that exceed the 1200 MPa design specification, leading to early fatigue failure (Fig. 5).
Fig. 5 Picture of gear mesh – a 15T spur gear Fig. 6 Lead crown implemented to reduce edge
meshes with an effective 189T sector gear [1] contact stress due to gear misalignment [1]
The solution was to implement a lead crown (Fig. 6) to reduce edge contact stress due to gear misalignment. The incorporation of a 0.01 mm crown to compensate for a 0.1 radian misalignment was shown to reduce stress to 922 MPa, well below the 1200 MPa design specification (Fig. 7).
Fig. 7 Contact stress distribution with crown to compensate for misalignment [1]
References
[1] R H Slattery, Innovative Solutions to Gearing: Challenges Using Powder Metallurgy, as presented at PowderMet2013, MPIF, USA
Author
Dr David Whittaker is a consultant to the Powder Metallurgy and associated industries. Contact +44 1902 338498 email: [email protected]
PowderMet2013 was organised by the Metal Powder Industries Federation. For more information please visit the MPIF website: www.mpif.org
