Dowel pins in machinery have at least 3 functions. One function is to restrict relative movement and aid in the placement of two mating parts such as seen in bearing housing caps. Another function is to prevent improper orientation of components. Lastly, dowel pins are used to keep machine components in alignment. Our discussion will investigate this last function. Dowel pins are commonly seen on larger machinery. The dowels are usually placed in machine mounting feet or bearing housings. The intent is to prevent movement between these components and the base plate or foundation, thus keeping the machine train aligned. People rely on the dowel pins to maintain machine alignment and assume that if the dowel pins are in place the machine is aligned. During alignment checks I have never found a doweled machine to meet the tolerances of a precision alignment. I recall one instance where a machine was precision aligned and after the alignment technician left the worksite the maintenance leader insisted on re-installing the dowel pins. Re-installing the dowel pins forced the machine back into a misaligned condition, so the previous efforts and expense were a waste.
How should we approach the alignment of machines held in place with dowel pins? First, we should realize that proper machine mounting bolts when properly torqued are sufficient to hold machinery in place. The dowels should not be required if proper mounting fasteners and proper fastener techniques are used. When installing a machine with pre-existing dowels set the machine in place and re-install the dowels if you like. This will provide a starting point equivalent to a “rough alignment”. Now remove all the dowels except from the machine component that you would designate as the stationary machine, remembering that this dowel may also have to be removed if, for some reason, the alignment process requires that this component needs to be moved. Proceed as with any other machine alignment. The component with the dowel left in place doesn’t really need the dowel and the dowel becomes only a point of reference.
Related article: Alternatives to using dowel pins in machine feet
Alignment by Bill Hillman CMRP