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Reducing fatigue: Dirk van Dyk* takes a look at the problem of operational gear mesh misalignment and discusses a new technique that adds to the repertoire of predictive maintenance techniques for low speed, high torque applications.

The large gears employed in the metals processing, cement production, minerals processing and power generation industries operate under challenging and complex operating conditions. As a result, failures can occur which even specialist engineers struggle to explain. Large gears are the most critical components of many systems because of their cost and the lead-time involved in replacement. Adequate maintenance is therefore essential for economical operation over an extended period.

Common diagnostic techniques include: vibration analysis on the mill pinion bearings, main reduction gear box and main motor; oil analysis of the main reduction gear box; spray pattern analysis on the gear; stroboscope inspection of the gear teeth during operation and temperature measurement across the face width of the gear. Annual shutdowns are also planned where non-destructive testing techniques such as magnetic particle inspection are performed on the gear teeth to identify possible cracks. The gear teeth are also visually inspected for abnormal and distinct wear patterns. Each technique has its strengths and weaknesses. Vibrational analysis, for instance, despite its many attractions, is unable to identify pitting of gear teeth in low speed applications at a sufficiently early stage.

Pitting

Over many years of on site gear inspections, it has been observed that surface distress or "pitting" of gear teeth is the most predominant failure mode. The pitting is biased to the one side of the gear--an indication of misalignment between the gear teeth of the mill pinion and gear. Worryingly, in most instances of failure, all of the above maintenance techniques had been adhered to, and therefore the failure mode could not be explained in this way.

In order to maintain alignment of the gear and mill pinion, it is common practice in the industry to measure the temperature difference across the face width of the gear. One standard is a maximum temperature difference (DeltaT) of 6[degrees]C- and in the absence of any better method to calculate misalignment, many engineers have employed this standard.

Temperature difference

However it has been observed by some engineers that the temperature difference method has a fundamental error because one end of the mill pinion is free and therefore subject to convection cooling, while the drive end is connected to the main gear box and therefore heated by conduction and radiation from the main drive. For a typical mill application, the soaking time of the mill system is significant, due to the weight of the pinion of the gear, which can therefore lead to "false" temperature gradients across the face width of the gear. A further disadvantage is that this method is also time consuming because of the heat inertia of the gear and pinion and the time it takes to detect a temperature difference after an adjustment has been made.

The first of a new generation of misalignment calculation techniques looks set to address the problems outlined above however. Described as a robust gear alignment and torque monitoring system that can be used with "in service" gear drives it comprises a continuous electronic monitoring system that checks gear alignment and also detects in service gear loading. Initial applications indicate the suitability of this approach for low speed, high torque applications with variable loading--conditions that are typical of many heavy process industry environments.

The benefits of such and approach are two fold: firstly it enables the correction of gear mesh misalignment to reduce face load distribution issues and prevent surface distress and face pitting of the gear teeth, secondly it detects excess loading or overload situations during regular gear operation to enable corrective operational action to be made before any damage occurs.

A recent application of this technology was on a coal pulverizing horizontal ball mill at a coal-fired power station in South Africa. Here the measurement system was placed on the mill pinion, which meshes with the mill gear. Analysing the output a number of characteristics of the gear were observed during operation. Fig. 1 is a root-gauge-trace over two full revolutions of the shaft and the resulting peaks and troughs show the variation in stress as the gear turns. By use of an iterative process, an alignment will be achieved which displays minimum amplitude of the stress peaks. The data collected from the coal pulverising mill demonstrates a high level of sensitivity and interestingly the detail is such that it demonstrates the misalignment as well as positive features of the gear design itself. Fig. 2 shows both a misalignment issue and a feature of the gear design. The face load distribution over the width of the gear face can clearly be seen: the double apex in the strain gauge readings reflect the Y-section construction of the gear rim, the most durable design for ensuring even loading across the gear face when correctly aligned. It also shows that one apex is higher than the other--a sure sign of misalignment.

[FIGURES 1-2 OMITTED]

Finally figs. 3 and 4 show readily understandable results from the same installation before and after realignment. On the right the strain gauge peaks have effectively been removed and the loading across the face width has been balanced: gear alignment has been successfully optimised.

[FIGURES 3-4 OMITTED]

Given the advantages of this system over other gear alignment techniques and the importance in economic terms of optimising alignment of gears, a good deal of interest is being expressed by specialists and customers in a number of industries. This technology, although demonstrated here on one specific piece of plant, is readily applicable in any application requiring a large, low speed, high torque gearing solution, where typically traditional forms of condition monitoring, such as vibration analysis, often do not provide sufficient warning of an issue arising. It not only provides a visual representation of misalignment but also identifies operational loading issues. This enables operations to be modified to optimise equipment productivity, and with the ability to connect the output to GPRS or wireless technology, it also offers a practical solution to monitoring and managing a number of assets on site or even remotely.

* Mr. Dirk van Dyk, Pr. Eng, is engineering manager for David Brown Gear Industries (Pty) Ltd., South Africa.

* For further information please visit: www.davidbrown.com
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Title Annotation:MAINTENANCE MATTERS: FOCUS ON: CONDITION MONITORING
Author:Dyk, Dirk van
Publication:Plant & Works Engineering
Geographic Code:4EUUK
Date:Dec 1, 2006
Words:1025
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