The Process of Shaft Spine Alignment
Every shaft is not perfectly straight, nor perfectly round, and the wall thicknesses are perfectly uniform. This condition creates what is called a spine or hard side. To find the hard side the shaft is place inside a barrel containing a sealed bearing on either end. Using a third sealed bearing, load is placed on the tip of end of the shaft. While under load the shaft is then slowly turned. The hard side is found when the shaft will not sit in a particular position and wants to turn out of position. Notice the tape on the shaft in the left picture and without touching it, the shaft turned 90 degrees in the right picture.