Grinding machines for crankshafts
Cinetic Landis Ltd has increased its product range to include the supply of large grinding machines capable of finishing the main bearing journals and pin diameters of crankshafts for the marine diesel and static engine businesses (Fig. 1). The crankshafts, which typically weigh as much as 6.5 tonnes, range from four to eight metres in length and have both pins and journals up to 350 mm in diameter.As well as assisting with bed design and FEA analysis, three major elements of the machine, the wheelspindle, the headstock and the tailstock have been designed by Cranfield Precision. All three axes contain hydrostatic bearings capable of supporting the crankshafts and grinding process loads.
A large proportion of residual geometric errors in the finished part can be related back to the error motion of the spindles. It is therefore important to design and manufacture the bearings to minimise such problems before they arise. Once built, there is nothing to be done short of starting again to remove any geometry related errors.
The synchronous radial error motion of the spindles is principally determined by the roundness error in the manufacture of the journal diameters of the spindle. Axial error motion is caused by lack of perpendicularity of the thrust faces. Both errors contribute directly to the systematic errors in the part. Asynchronous error is less likely to cause surface finish defects in a grinding process that uses a large contact area between the wheel and the part, but its effect cannot be completely ignored. The accuracy in manufacture of the matched size and roundness of the housings control the fluid flow and bearing stability. The design process confirmed that Cranfield Precision's five pocket hydrostatic bearing could be scaled three times from its current application in diamond turning. The LT3 crankshaft grinding machines built to date have used matched bearing parts that have proven to work well. Even so, significant improvements in performance are expected when the latest Horizontal Precision Boring Machine, HPB43, designed and built by Cranfield Precision enters production of the spindle housings at the Cinetic Landis machine shop this Autumn (Fig. 2).
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Cranfield Precision, a division of Cinetic Landis Ltd, is based in Kempston, Bedford, UK. www.cranfieldprecision.com
