Manufacturing tolerances and deviations from design parameters are inevitable in real hydrodynamic journal bearing systems. Normally, the stability of the system is assessed on the design parameters alone; the effects of deviations are assumed to be negligible and only the effects of gross changes are considered. In isolation, a single deviation may not demonstrate a significant effect, but in combination with other deviations, the effect may be large. This study presents a 2 k factorial analysis of the effects of manufacturing tolerances on system stability. It is further used to determine which tolerance, or combination of tolerances, is the most significant. The results show that manufacturing tolerances, and combinations thereafter, do have a significant effect on system stability, and they influence the system stability in different ways. The study also identified a region where system stability is insensitive to deviations. To enable visualization of the effects, a modified Sommerfeld number and new stability map are presented. Furthermore, the overall philosophy of using a 'design of experiments' approach to the determination of system stability has been vindicated. A later paper will compare the results from this study to those from experimental results.

This paper presents a new design for a device to monitor the motion of fracture fragments in diaphyseal tibial fractures. The device measures the motion that occurs at the fracture site when loaded by gait or by manipulation. It has undergone rigorous calibration and acceptance trials. The device has been used in ethically approved research clinics held at the North Staffordshire Hospital (40 patients). The paper presents a selection of results obtained using the new device. The results demonstrate several new ways of assessing fracture healing by examining fracture site motion. The following conclusions were drawn: 1. If fracture monitoring devices are to be attached to bone screws, it is essential to minimize bone screw errors. To do this, each patient must have similar bone screw lengths, orientations, alignment and siting. This is only achievable using a peroperative reduction device. 2. If fracture stiffness is to be used as a measure of fracture healing, load rate should be controlled; at the very least strain rate should be controlled. 3. It is imperative that fracture stiffness be measured in more than one plane by a biplanar device so that asymmetry may be accommodated. 4. Fracture stiffness, on its own, is probably not a sufficiently rigorous measure of healing end-point. The quantifiably viscoelastic properties of healing callus should be taken into account.

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A widely used method of treatment for unstable tibial shaft fractures is unilateral external fixation. The majority of fixators act as three distinct devices: an intra-operative reduction device, a device to maintain fracture alignment during healing and an aid to healing by allowing movement at the fracture site. Conventional operative techniques require the surgeon to manipulate a number of degrees of freedom at once, making reduction of the fracture difficult, and results in the fixator being out of alignment with the long axis of the bone. An operative method has been developed that separates reduction and fixation. A dedicated device has been designed to improve the per-operative control of fracture fragments during fracture reduction. The device has been used in clinical trials for the reduction of 22 diaphyseal tibial fractures. Compared with previous operative techniques there has been a saving of 53 per cent in fracture reduction time and an overall saving of 10 per cent in operating time. Fracture alignment has been improved compared with reductions achieved with a fixator which potentially improves healing and lowers the rate of malunion. In each case the fixator has been applied in alignment with the bone, improving dynamization and reducing the likelihood of malunion due to fixator cam slippage.

The importance of gross dimensions on the design and operation of hydrodynamic journal bearing system are widely understood, however, the effects of manufacturing tolerances have only been macroscopically analysed. Building on previous theoretical analyses, an experimental investigation on the effect of dimensional manufacturing tolerances is presented. Importantly, the study looks at individual effects and combined effects. The experimental data suggested that the error created by manufacturing tolerances in estimating the onset of oil film instability can be as much as AE20 per cent, depending on the steady state operating position. This finding shows that in order to minimize the effect of manufacturing tolerances on system stability, the hydrodynamic journal bearing system should be designed to operate at a modified Sommerfeld number greater than 0.1, where the effects of manufacturing tolerances on system stability are negligible.