What is shaft alignment of industrial rotating machines?

The main result of implementing an industrial rotating machine shaft alignment system for your plant is fewer headaches. When you are able to reduce the maintainability of machines and increase their productivity, you are assured of having a machine running in optimum condition, with the wear parts being subjected to less stress, thus increasing the life cycle of the machine and decreasing the likelihood of failure and breakdown.

In this article we want to give you some key concepts about what shaft alignment of industrial rotating machines really means.

Definition of shaft alignment of industrial rotating machinery

The shaft alignment of industrial rotating machines is the process by which one or two machines (usually a motor or a pump) are positioned in such a way that the point of energy is transferred from one axis to the other, causing the axes of rotation of both parts to be co-aligned when the machine is operating under normal conditions.

As with any standard definition there are exceptions. Some types of couplings, for example gear couplings or cardan shafts, require a certain misalignment to ensure correct lubrication when in operation.

Important points to note in the above definition are:

• At the point of energy transfer....

  All axles have some kind of catenary due to their own weight, so the axles are not straight, and therefore the location where the alignment of the two axles can be compared is only at the point of power transmission from one axle to the other.

• ...the axes of rotation...

  Alignment of machine shafts" should not be confused with "alignment of couplings". Coupling surfaces should not be used to measure alignment conditions, as they do not represent the rotational axes of the shafts themselves. To save on maintenance costs, coupling surfaces are treated only superficially or not at all.

• The accuracy of the fit of the coupling on the shaft is unknown.

  Rotating only one axis and using dial gauges to measure the opposite mating surface does not determine the axis of rotation of the two parts.

• ...under normal operating conditions

  The alignment condition can change when the machine is running. This can occur for a number of reasons including thermal rise, pipe pressure, machine rotation, foundation movement and bearing clearance. As shaft alignment is normally measured with cold machines, the alignment condition being measured is not necessarily the zero alignment condition of the machines.

  Alignment conditions should be measured while rotating the shafts in the normal direction of rotation. Most pumps, fans, motors, etc. have arrows on the end housing showing the direction of rotation.

• Machinery catenary

  The amount of deflection of a machine depends on several factors such as the stiffness of the shafts, the amount of weight between the hangers, the design of the support and the distance between the supports.

  The following picture shows the natural bending of the axles under their own weight.

  

  For the vast majority of closed paired rotating machines this catenary arc is negligible, and can therefore be ignored for practical purposes. In machines with long transmissions, e.g. generating turbines in power plants, the catenary curve must be taken into account.

If you are interested in learning more about the key concepts of shaft alignment of industrial rotating machines and how to apply them in your day-to-day work, we invite you to find out about shaft alignment parameters of rotating machinesas well as to subscribe to our newsletter.