ABSTRACT
Large diameter metal-on-metal bearings (MOM) are becoming increasingly popular, addressing the needs of young and more active patients. Clinical data has shown excellent short-to-mid-term results, though incidences of transient squeaking have been noted between implantation and up to 2 years post-operative. Geometric design features, such as clearance, have been significant in influencing the performance of the bearings. Sets of MOM bearings with different clearances were investigated in this study using a hip friction simulator to examine the influence of clearance on friction, lubrication and squeaking. The friction factor was found to be highest in the largest clearance bearings under all test conditions. The incidence of squeaking was also highest in the large clearance bearings, with all bearings in this group squeaking throughout the study. A very low incidence of squeaking was observed in the other two clearance groups. The measured lubricating film was found to be lowest in the large clearance bearings. This study suggests that increasing the bearing clearance results in reduced lubricant film thickness, increased friction and an increased incidence of squeaking.
Subject(s)
Arthroplasty, Replacement, Hip , Metals/chemistry , Biocompatible Materials/chemistry , Equipment Design , Equipment Failure Analysis , Friction , Hip Joint , Hip Prosthesis , Humans , Lubrication , Materials Testing , Prosthesis Design , Rheology , Surface Properties , Time FactorsABSTRACT
The performance of ultrasonic oil-film thickness measurement in a ball bearing is quantified. A range of different viscosity oils (Shell T68, VG15, and VG5) are used to explore the lowest reflection coefficient and hence the thinnest oil-film thickness that the system can measure. The results show a minimum reflection coefficient of 0.07 for both oil VG15 and VG5 and 0.09 for oil T68 at 50 MHz. This corresponds to an oil-film thickness of 0.4 microm for T68 oil. An angular spectrum (or Fourier decomposition) approach is used to analyze the performance of this configuration. This models the interaction of component plane waves with the measurement system and quantifies the effect of the key parameters (transducer aperture, focal length, and center frequency). The simulation shows that for a focused transducer the reflection coefficient tends to a limiting value at small oil-film thickness. For the transducer used in this paper it is shown that the limiting reflection coefficient is 0.05 and the oil-film measurement errors increase as the reflection coefficient approaches this value. The implications for improved measurement systems are then discussed.
