During failure investigations we often see misalignment on vertical motors as the failure mode and when we ask for alignment readings at time of installation, we often hear that the “motor and the base have a register fit and alignment isn’t required”. This is far from the truth.
Let’s look at why alignment should at least be checked. There are three mechanical fits that come into play when considering vertical motor alignment. Those are face runout, register fit runout and shaft runout. One of the major manufacturers of vertical motors gives these tolerances for allowable runout for a 447-frame motor as: face runout of .007”, register fit runout of .007” and maximum shaft runout of .002”. Now you may think that this really isn’t a lot and it won’t matter. But let’s consider the part that the motor sits on. The pump mounting flange has a machining tolerance too. Assuming the manufacturer builds their pump base to the same allowed tolerances as the motor manufacturer, .002” on the face and .007 on the register fit. If the pump base has a double register fit as shown below you can actually double all readings if the allowable runout for both sides are not concentric with each other.
1) Face runout .002 (max tolerance)
2) Register fit. 007 (max tolerance)
3) Shaft runout .0015 (Max tolerance)
Runouts based on a 24" diameter flange
If the above maximum allowable tolerances weren’t bad enough, consider those runouts on a 3600-rpm application. In instances where the pump configuration utilizes an upper steady bearing and the steady bearing is actually in the center of the pump base then you could be preloading that upper bearing with a .015” side preload.
Our preferred method to alleviate this problem is to open up the register fit on the bottom flange by .020” and then have the motor properly aligned. Also, it is best to check all runout on the motor flange and shaft runout before attempting the alignment.
Author: Bob Bolhuis @ BBolhois@hecoinc.com
Bob is the Director of Equipment Management at HECO