A closer look at shaft failures and what to do about them

Learning what causes a centrifugal shaft to break will help you prevent this from happening in the first place, or again! In our previous article, we discussed the most common reasons for a shaft to break; manufacturing or material flaws, vibration, and imbalance. In this, the second article of the series, we’ll get into some more detail on shaft failures and what to do about them.

When trying to address why a centrifugal pump shaft keeps breaking, consider the number of times you can bend it on a cycle before you exceed the material’s limits.

Let’s delve into this.

The most commonly used shaft materials can vary widely in hardness, strength and resistance to corrosion. Yet, they fit within Young’s Modulus for Materials which measures a material’s elasticity, how formable or shape-able it is under tensile or compression. The shaft must bend on each cycle under normal operating conditions, but the real question is how far can it bend before it breaks?

Since most shaft materials are made from stainless steel or an alloy, they all have similar Young’s Modulus. This means that changing the material of the shaft you choose won’t actually address the problem causing the shaft to break in the first place. The strength, toughness, or hardness of the material itself isn’t usually the issue. The fatigue limit of a common stainless steel bar shaft is exactly the same as that of the most expensive shaft money can buy.

Pump users must address other operational factors.

As we mentioned, one of the most cited causes of shaft breakage is bending fatigue. For any material, the number of cycles the shaft must go through gives a clue as to how long the shaft will stay intact. Other factors affecting the bending cycle are the frequency and the distance of this cycle.

The shaft may start to wear and snap at the weakest point. It can also snap at the bending moment.

A wise engineer or maintenance team will check the shaft for signs of fracture. In most common materials, this usually occurs at right angles to the centreline of the shaft itself. Unfortunately, if you can see this, it’s already too late, but knowing the cause can prevent the same failure from happening to the next shaft. These fractures might look like the shaft has been snapped or cut off from that failure point.

To see if fatigue failure was the cause of a snapped centrifugal shaft, see if the fracture occurred at a 45-degree angle to the shaft centreline. Here’s some answers to the most frequently asked questions we deal with when it comes to centrifugal shafts:

Can I bend the shaft till it breaks?

Yes, though we can’t imagine how you’d do this inside any centrifugal pump on the market today. There just isn’t enough space inside the pump to bend it very much. However, by misaligning the pump and driver, the pump shaft can break near the bearings. Usually, misalignment would cause the bearings to fail before the shaft does, but it is possible to break the shaft due to misalignment.

Can stress cause the shaft to break?

Shafts are designed to accommodate stress according to an estimated fatigue limit. If the stress is low, it can be repeated on a daily basis without doing any damage.

For example, a shaft can be designed to bend at the center by .005 inches, nearly 4000 times a minute without breaking. By increasing its bend to .010 inches, the shaft may break in 7 days or less. This is because you’ve gone over the shaft’s fatigue limit.

So, what factors affect the shaft’s ability to circumvent fatigue failure?

• Size of the shafts. The stiffness of the shaft is predicated more by the distance between the bearings than the diameter of the shaft
• Surface and stress concentration. Shafts should be surface ground, with threads undercut, and with rounded bottoms on keyways.
• Operating conditions. Pump shafts that allow liquid to reach stress concentration points fail faster than ones that are dry throughout.

What happens if there is cavitation?

At first glance, it doesn’t seem like a huge issue if some air bubbles pass through the pump. The impeller contains liquid. Surely it can manage the addition of some air, should there be high discharge pressure (which makes it difficult for the liquid to be discharged from the pump).

But what we often forget to consider is that engineers, maintenance teams or anyone operating the pump can’t calculate how much imbalance this air might cause, because we just don’t know how these air bubbles will be distributed around the impeller. They can create a high vacuum and form bubbles that implode, creating shock waves that remove material from the housing and impeller and can even cause the shaft to break entirely. For your shaft to have a long life, make sure that you catch cavitation early.

If your pump shaft keeps breaking, it’s time to call in an expert. 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. Give us a call on 1-800-367-4180 (toll-free) today!