The question of why it is necessary to precisely align machines that are connected with flexible couplings has often been asked.
On the face of it, this is a very sensible question since the manufacturers of flexible shaft couplings specify that they are capable of accepting larger alignment errors than those specified in the laser shaft alignment standards used throughout the industry. Furthermore, the process of undertaking precision shaft alignment, even though it is greatly simplified by high performance laser alignment systems, is an additional step in the process of installing a machine which requires precision equipment, additional time and additional costs.
The short answer to this question is that there are actually four important reasons for aligning all rotating machinery to the tight tolerances that are specified by the industry standards.
Firstly, even though a coupling may be able to withstand a large misalignment without failure, distortion of the flexible coupling elements creates increased oscillating forces, vibration and heat, and this affects the performance and operating life of other machine components, particularly the shaft, bearings and seals of both the motor and the driven machine, as well as the flexible elements themselves. Failure of any of these components creates unscheduled down-time and if not found and promptly rectified may cause a more extensive machine failure and shut-down losses.
Secondly, heat generated by a misaligned coupling flexing rapidly in each direction during each revolution also represents lost energy and may considerably reduce machine operating efficiency. Continuously operating plants have calculated large cost reductions from the power savings made by investing in a laser alignment program to ensure all their machines are running efficiently.
Thirdly, the misalignment of a machine that occurs under dynamic operating conditions and over an extended period of service is also a critical consideration. The drive forces and pipe strain applied to a machine at full load are not easily measured but are substantial in higher power machines, and the total misalignment that is created by distortion of machine feet and the machine base needs to be accommodated by the flexible coupling. For this reason, some high end coupling manufacturers specify setting tolerances which are often only 10% of the coupling operating tolerances so that the couplings misalignment range is available to protect the machine under load and is not used up by initial misalignment errors.
Finally, an important aspect of precision machine alignment is eliminating soft-foot errors. One reason to always control soft-foot is that it makes good machine alignments easier to achieve, but soft-foot also creates distortion in the motor and driven machine housings when they are tightened down on a base that is not flat. Large soft-foot errors can directly cause machine failures, such as bearing damage or cracked feet, but even small amounts of soft-foot error may producing an uneven rotor clearance in electrical motors resulting in motor vibration, reduced efficiency and possible failure. Diesel engines may incur main bearing damage or even crankshaft failure and gearboxes may suffer bearing misalignment or uneven tooth wear.
In summary, there are four very important reasons to precisely align all rotating machinery: increased machine life, improved efficiency, providing for dynamic misalignment and the elimination of soft-foot problems. The fastest and simplest way to achieve accurate alignments is to use a professional quality laser alignment system designed to deliver outstanding functionality and accuracy, such as one of the Hamar X-Series laser alignment systems. Call ISTecnik to get additional information on the X-Series lasers, or just to chat with us about any aspects of precision machine alignment.