If a pump's flexible coupling is very noisy at startup, which issue could be the cause?

• A. Misalignment
• B. Improper clearance
• C. Improper adjustment
• D. Damaged shaft

Answer: (A)

Misalignment between the motor and pump shafts is what usually causes a loud startup in a pump with a flexible coupling. Flexible couplings are built to accommodate small deviations, but when shafts aren’t in proper line, the coupling has to bend as torque is applied. That sudden bending and the resulting uneven loading on the shaft and bearings can cause the coupling elements to rub, bind, or strike as it engages, producing a noticeable clatter or squeal at start.

Improper clearance would show up as too much or too little space in the connection, leading to either play or binding effects that aren’t specifically tied to startup noise. Improper adjustment can create pre-load issues, but the characteristic startup noise tied to torque engagement is more a sign of misalignment. A damaged shaft would typically bring more obvious symptoms like excessive vibration, wobble, or failure signs beyond just noisy startup.