Misalignment can damage flange couplings for fire pump motors and diesel engines, resulting in excess vibration and wear of the coupling inserts. Proper alignment is required by NFPA installation and inspection standards to prevent failures during a fire when the pump output is critical. While rubber inserts are forgiving of some misalignment, greater accuracy is needed for couplings on commercial and industrial pumps. Checking for misalignment should be done during regular inspections and installation.

1. MISALIGNMENT
This was originally written for fire pumps. As a result of flange coupling insert failures over the years,
currently only jaw and grid couplings are UL listed for electric motors and drive shafts for diesel engine
fire pumps. Misalignment can result in excess axial vibration, hammering the coupling hubs and flanges
apart, increasing the spacing between them and reducing the contact area on the inserts like the photos.
An un-level skid, “soft feet” and/or pipe strain may result in a line bore problem even if the coupling is
lined up. Checking fire pump alignment is an NFPA 20 installation requirement (see drawing) and NFPA
25 inspection requirement. NOTE: Always observe Lockout and Tagout (LOTO) procedures.
Misalignment and coupling problems, hidden by the coupling guard, are not so easily-identified. If an
insert fails during a fire, that’s the end of the pump’s output. If this is a sole supply pump, the
consequences could be disastrous. These photos are so obvious that you don’t need a laser to see it. A
good straight edge would have been better than the condition these were left in. A small magnetic
torpedo level is great to keep on hand since it frees up both hands while maneuvering a driver in place. It
will not work on the better grades (non-ferritic) of stainless steel. A stainless steel (good 300 series)
feeler gauge will fit in-between the level and hubs without hanging up on the magnet. Removing any
“visible daylight” is usually more than good enough for most rubber inserts on older units. The newer
Listed “Steel-flex” couplings are much more forgiving on misalignment than the old rubber inserts units
are on existing installations. That’s still no excuse to walk off and leave one misaligned like some of
these in the photos.
While these tips are sufficient for rubber inserts and the grid series couplings, the flexible couplings used
on commercial and industrial pumps require greater accuracy.

2. MISALIGNMENT - FLANGE COUPLING
Damaged coupling guard from previous failure - no insert, no coupling.

3. MISALIGNMENT & INSERT
DAMAGE - JAW COUPLING
Misalignment, excess hub space and missing
rubber tooth on spider. At least with an insert
failure, you still have a "coupling" of sorts.

4. MISALIGNMENT - JAW COUPLING
Coupling misalignment, over-greased pump inboard bearing and non-listed replacement fire pump motor.

5. ALIGNMENT DETAILS
2010 NFPA 20, Figures A.6.5(a) and A.6.5(b)

6. NFPA 25
Maintenance and
inspection intervals
for fire pumps.

7. NFPA 25
Maintenance and
inspection intervals
for fire pumps.