1 in 1 0 taperenables an axial expansion of Another technique utilised in measuring differentialexpansion is to use tapered rather than flat collars.The use of tapered collars fitted to the turbine shaft enables longer linear ranges to be obtained.A 1 0 times the normal rangeof the probe to be measured.A problem arises however if there is any radial movementeg if the shaft moves 1 00 micrometers within thebearings,this is incorrectly seen as 1 0 x 1 00 micrometers(ie 1 mm) of differential expansion.To overcome this,twoeddy probes are fitted.Thus two unknowns can be easilysolved by two simultaneous equations through software manipulation.
Two probes monitoring expansion by observing a tapered collar Four probes monitoring expansion by observing a tapered collar Travel = Normalised Range x T3+T4+ OffsetT1+T2+T3+ T4‘Normalised range’is the total travel range divided by the pulsewidth ratio range determined from each travel extreme. A further complication arises when the casing holding the eddy probes is subjected to twisting as can happen if slides start to stick (see below).A further two eddy current probes are then required to give a correct reading of differential expansion. It is also possible to measure differential expansion oraxial movement with a small range probe using a mark space technique.This principle operates on detecting movement in special plates attached to the turbine shaft. The shaft target pattern consists of a number of pairs of ‘teeth’ and ‘slots’ surrounding the shaft and rotating with it.Each pair of teeth are tapered axially such that alternate teeth taper in the opposite directions,thenarrow parallel slot between the teeth being at an angleto the shaft axis.There is a wider parallel slot between each pair of teeth to allow the system to identify eachpair.When the shaft rotates,the voltage pulses produced bythe proximity probe and driver,have a tooth to slot pulse width ratio dependant upon the axial relationship between the shaft pattern and the probe position.The probe is mounted on a fixed part of the machine so variations in pulse width ratio are a measure of shaft axial position.The shaft pattern is illustrated below. The Sensonics Sentry machine protection MO8612module is suitable for this type of monitoring.The module exhibits a self-tracking threshold level,which ensures that the width of the signal pulses are measured at the optimum position within the pulse height.The unit is pre-programmed with specific plate patterns that can be selected to suit applications.The number of plates on the mark-space wheel is also an important parameter; when correctly set up this enables the module to minimise ‘plate wobble’ through the implementation of averaging algorithms.Customised patterns can also be entered into the module.Since this technique measures axial movement basedupon the ratio between detected pulses,it is immune to shaft movement in any other direction.This is a distinct advantage over the other techniques detailed in this section.A large expansion range can also be measured with a low cost probe through the fitting of the appropriate plate pattern,several centimetres if necessary,which would be impossible to achieve with a shaft collar.

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