Condition monitoring through the use of vibration analysis is an established and effective technique for detecting the loss of mechanical integrity of a wide range and classification of rotating machinery. Equipment rotating at low rotational speeds presents an increased difficulty to the diagnostician [1-4, 23], since conventional vibration measuring equipment is not capable of measuring the fundamental frequency of operation. Also, component distress at low operational speeds does not necessarily show an obvious change in vibration signature.This paper presents a study of high-frequency stress wave analysis as a means of detecting the early stages of the loss of mechanical integrity in low-speed machinery. Investigations were centred on the rotating biological contactor (RBC) which is used for sewage treatment in small communities and rotates between 0.6 and 1 r/min (0.0167 Hz). The mechanism of stress wave generation was the relative movement between mating components experiencing loss of mechanical integrity, e.g. the loss of tightening torque between clamped components. PROBLEMS WITH VIBRATION ANALYSIS OF LOW-SPEED ROTATING MACHINERYThere is no universally accepted criterion at which machines are classified as of a low-speed type. However, it is generally accepted that 600 r/min is the minimum speed for intermediate-speed classification [1] and any speed below this could be classified as a low speed. Monitoring low-speed machines makes great demands on both the analyst and vibration diagnostic instrumentation. This is primarily because standard predictive maintenance measuring instruments are inappropriate for low speeds. Typically, low-speed machines are massive in size and consequently, when mechanical defects begin to occur, the resulting vibration is often very low and serious faults can go undetected. The main problems with vibration analysis of low-speed machinery can be divided into two categories:1. The optimum vibration parameter for low-frequency measurements must be selected. The most widely used parameter for measuring vibration is acceleration. However, acceleration decreases with reduction in rotational speed. Therefore, the best parameter for measuring vibration at low rotational speeds (less than 600 r/min) is displacement [2]. 2. There are instrument limitations for low-frequency analysis. In order to combat the inherent low-frequency instrument noise problem, many data collectors, spectrum analysers and sensors are fitted with high-pass roll-off filters at approximately 5 Hz. Unfortunately, the once-per-revolution features associated with many fault conditions go undetected within the 0-5 Hz frequency range.There have been a few attempts to develop systems for monitoring bearings at speeds between 1 and 10 r/min, although with limited success [1, 3, 4].

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Synopsis-Condition monitoring of rolling element bearings through the use of vibration analysis is an established technique for detecting early stages of component degradation. However, this success has not mirrored at rotational speeds below 16 rpm. At such speeds the energy generated from bearing defects might not show as an obvious change in signature and thus become undetectable using conventional vibration measuring equipment. This paper presents an investigation into the applicability of stress wave analysis for detecting early stages of bearing damage at a rotational speed of 1.12 rpm (0.0187 Hz). Furthermore, it reviews work undertaken in monitoring bearings rotating at speeds below 16 rpm.

The mechanical integrity of rotating biological contactors (RBCs) and their bearings are vital to maintaining uninterrupted operation. Part 1 of this work presented a study of the highfrequency stress wave (SW) technique as a means of monitoring low-speed RBCs (1 r/min). While the present authors were involved with the development of a monitoring system for RBCs, they also used the opportunity to assess the viability of the application of SWs to monitoring low-speed bearings having bore diameters ranging from 80 to 125 mm. It is concluded that the mechanism of SW generation was the relative movement between mating components experiencing a loss of mechanical integrity, e.g. the loss of tightening torque between the clamped components of RBCs.

Design charts are presented whereby the practicing design engineer can determine the equivalent flexural stiffness of flanged or curvic (toothed) couplings. Results are obtained by computing the centre line slope generated by an applied couple using a Program for Automatic Finite Element Calculations (PAFEC 75). Graphical data is provided allowing the equivalent second moment of area to be determined for flanged couplings or stress contour lines describing the effective load carrying material for curvic coupled disc assemblies.