Grease-lubricated wear of aluminum bronze for jackscrew application

Abstract

The failure of a grease-lubricated jackscrew, following an order-of-magnitude transition in wear rate above that normally experienced in service by aluminum bronze (C95500) threads of the nut sliding against those of the steel (AISI 4140) screw, contributed to a recent accident involving a commercial passenger airliner. With maintenance records of the development of endplay and knowledge of typical loading and sliding profiles jackscrews experienced per flight, a wear rate of ∼10 −5 mm 3/N m is thought to exist under normal service with traditional grease lubrication. However, in this case, the grease type had recently been switched in the years immediately preceding the accident. Hypotheses put forth for this unexpected transition to a more rapid bronze wear rate have included a lesser lubricating capability of the newly introduced grease relative to that previously employed, or incompatibility between the greases that may have occurred upon mixing during introduction of the new grease. The overall goal of this study, to determine if any difference in relative performance of these greases existed, was pursued by testing primarily by the block-on-ring method, in which aluminum bronze blocks were slid against nitrided and bead-blasted steel rings. Tests of the two greases were conducted at an average oscillatory speed of 0.08 m/s, characteristic of jackscrew thread sliding, under loads of 250, 510, 920 and 1600 N with wear recorded as a function of time to enable characterization of steady-state wear volume. Steady-state wear behaviors were characterized at a wear scar width of 6.35 mm, though those wear rates were generally found to be adopted earlier, for smaller scar widths. Thus, steady-state wear rates were characterized from 5.5 to 55 MPa, covering a wide range of potential jackscrew thread pressures. The wear factors measured in the presence of the original grease were in the (5–8)×10 −6 mm 3/N m range, with the value increasing towards 20×10 −6 mm 3/N m under the lowest load. Steady-state wear factors in the presence of the replacement grease were in every instance lower than for the original grease, ranging over (2–5)×10 −6 mm 3/N m for the same loads. Wear factors in the presence of a 50/50 grease mixture were generally intermediate to those for each of the greases separately. These lower wear factors for the replacement grease were unaffected by potential contaminants such as water, de-icing fluid, or salt-water, whether pre-mixed (5% by volume) into the grease or dripped into the steady-state contact. However, in each of the three repeat tests conducted at 920 N without any grease lubricant, dry sliding caused the wear factor of aluminum bronze to transition to higher values of (40–70)×10 −6 mm 3/N m. In thrust-washer tests, conducted at 1.5 MPa contact pressure, wear factors were similarly high under dry sliding, to values in excess of 100×10 −6 mm 3/N m, whereas upon application of either grease or their mixture wear factor became immeasurably small.