What causes centrifugal pump shafts to snap or break?

It’s more common than you’d think. Pump shafts are designed to bend under radial hydraulic forces when the pump operates away from the best efficiency point. But if the shaft breaks, the user is quick to blame the manufacturer or think that they need a pump with a stronger, more durable shaft. But the “stronger is better” mentality isn’t always the right route. A stronger shaft may mean that the centrifugal pump shaft won’t snap as easily, but the reason it snapped in the first place remains.

Here’s some of the main reasons that a pump shaft could break:

Flaws in the metal or manufacturing process

Pump shafts can fail due to flaws in the metallurgical and manufacturing process, but this is surprisingly rare. These could be due to porosity in the base stock, incorrect process treatments or annealing – which means the result is not soft enough to be formable. Others may fail because the dimensions are wrong, or when the shaft has been badly grinded or polished. A few pump shafts can fail because design margins haven’t compensated for operating conditions such as torque, fatigue, or corrosion. In fact, fatigue failure (caused by excessive rotary bending) is the most widespread cause of pump shaft fractures/failures (more on this in the second series on shaft failure).

Vibration

Poor alignment between the motor and pump shaft can cause misalignment. Other problems such as cavitation, and operating outside the pump’s best efficiency point, can stress the pump shaft, causing it to move laterally as the bearings start to wear. When exposed to these conditions, the shaft will begin to flex and, eventually, break.

Imbalance

While the centrifugal pump is running under normal operating conditions, an imbalance can cause vibration. This will reduce the lifespan of seals and bearings, and the pump itself. Interestingly, if you stop the pump, the shaft might still measure straight, even if not correctly balanced. But a wise pump user will be able to identify an imbalance issue by the sound that the pump makes when an imbalance exists. It’s also crucial that a new impeller is corrrectly balanced and trimmed.

Other things that the maintenance team can check are:

• The vanes, to identify any foreign object or sign that the vanes are out of plane or bent
• Balance holes, to see if plugged
• The impeller, to ensure placement and balancing are correct, or check for any signs of product buildup

There are a few other factors that can contribute to pump breakage, such as fluid properties that are set incorrectly, hydraulic shock, or when check valves upstream aren’t working or correctly plugged.

For a closer look at shaft failures and what to do about them, read the next article in the series here.

If your pump is a regular shaft buster, it’s time to call an expert on 1-800-367-4180 (toll-free). Our team is on hand to help you choose, install and maintain a variety of equipment and answer questions about things you’ve previously tried gone wrong.