During our Best Alignment Practices Training classes our staff of VibrAlign Trainers stresses the importance of rough aligning the machines as part of the pre-alignment steps. The main reason to do so is to minimize the coupling influences on the movable and stationary machine’s rotational shaft center-lines so the final alignment can be completed with a few moves as possible using the VibrAlign Verti-Zontal Compound Move.
When two machines are grossly misaligned, even flexible couplings can influence the alignment readings whether you are using dial indicators or a laser shaft alignment system. The question that does come up in class is “how much can the coupling influence the alignment readings or results?” The answer(s) are as varied as there are coupling types and machine designs.
As Stan stated in his recent January 10th blog post “Flexible Couplings and Flexible Shafts” this problem “in no way lays blame on the coupling or machine design” it is simply a fact of life an aligner needs to be aware of and deal with accordingly.
So how much can a coupling influence the alignment? Good question! I recently had the opportunity to experiment with a 30 HP pump and motor set with a “tire” style flexible coupling. All shims were removed from the under the motor feet and 3 sets of measurements were taken. 1st with the coupling assembled, 2nd with one half of the outer cover removed and 3rd completely uncoupled. The differences between the 3 measurements are very enlightening!
Results with coupling outer covers in place.
Results with one half of the out cover removed.
When comparing the “coupled” measurement to the “uncoupled” measurement it is obvious the coupled set isn’t close to the true vertical position of the motor to the pump as indicated by the uncoupled set. In this case the flexible coupling and gross misalignment, together, are influencing the Vertical Angle by 1.5mils/1”, the Vertical Offset by 17 mils, the front foot by 31 mils, and the rear foot by 47 mils!
If your 1st set of vertical and horizontal corrections doesn’t give you the expected results, by a large disparity, take a small step back and rough in the movable machine to the stationary machine, uncoupled if necessary. The few minutes you take to do so can save you a lot of time and frustration during the final precision alignment process.