Improvements on a Hydraulic Impact Piston for Percussive Rock Drilling

Abstract: This Master Thesis was done at Atlas Copco Rock Drills AB in Örebro. It treats how the geometry of impact pistons in hydraulic percussive rock drill hammers can be improved. The material in these pistons is highly loaded during each impact and there are cases where fatigue failure has occurred. One critical location for fatigue damage is the fillet between the boom and the tail of the piston. This is because the stress pulse from the impact is amplified in this area.
By developing a full transient analysis for a piston in ANSYS, and by using design optimization, a new type of fillet was constructed from a cubic b-spline. This new fillet has the same length as the old fillet, a double radius fillet, on the piston tail, but it has 12‒16% lower maximum stress range, depending on the stress measure. A special feature with this fillet is that it is cut into the material of the boom. In that way it could be longer than the double radius fillet which enables a larger radius of curvature and a longer distance to “lead” in the stress pulse into the tail. This results in lower stress ranges in the fillet.
Several multiaxial stress fatigue methods were used to analyze the fatigue damage in the old double radius fillet and in the new undercut fillet. It was found that the double radius fillet has a safety factor of 0.98‒1.08 to infinite fatigue life and the undercut fillet has a safety factor of 1.10‒1.23 to infinite fatigue life, depending on the fatigue method. Because infinite fatigue life normally corresponds to 106 ‒107 equivalent stress cycles, and the piston is desired to withstand close to one billon impacts, the old double radius fillet does not leave any margins to effects which can occur in the giga-cycle regime. The undercut fillet on the other hand leaves some margins to these kinds of effects. Unfortunately, no fatigue parameters were known for the material in the considered piston, so they were estimated from empirical relations.
During the work with the undercut fillet it was realized that the weight of the impact piston can be reduced by minimizing the weight of the piston tail without affecting its performance. By using design optimization with the constraint that the maximum stress range in the piston tail should not be larger than the maximum stress range in the double radius fillet, its weight was reduced by 38%. With this change in mass, less energy is needed to accelerate the piston and less reflection energy is needed to be damped out.