What Are Vertical Cantilever Pumps?

“Vertical cantilever pumps”, as the name implies, are pumps designed to operate with the shaft running in a vertical plane with some sort of cantilever component in their design. The shaft running in a vertical plane is self-explanatory, but the term “cantilever component” may require a quick explanation.

Impellers between bearings ​

​Rotating shafts/assemblies are normally supported by two bearing points. Some pumps have an impeller mounted between the bearing points, while some impellers are mounted outboard of the bearing points.

Cantilever overhung design ​

When the impeller is mounted outboard of the bearings but in close proximity to them, the design is referred to as an“overhung” design.

If the impeller is purposely mounted with a significant overhung length (distance away from the bearing points), it is still an overhung design but is generally referred to as a cantilever design. 

So why build a vertical pump with the impeller mounted so far from the support points? Surely it increases all the loads within the pump. 

Standard overhung design ​

Well, it does, but it also distances the bearings from the pumpage and therefore allows the elimination of the seals in the wet end, which is a significant source of failure for standard overhung pumps. This is particularly true with slurry pump,s where the abrasive nature of some slurries can lead to extremely short seal life.

Loads

In the previous paragraph, I mentioned increased loads. Well, yes, the further a force is from the support the greater the mechanical advantage, and therefore the greater the forces on the entire structure. In a rotating assembly, vibration is a killer and often a result of shaft deflection, causing an imbalance or just plain balance issues. The long overhang associated with vertical cantilever pumps makes this design of pump very susceptible to shaft deflection issues, and engineers pay particular attention to this aspect of design. To quantify this, the load increases to the fourth power as the overhang increases. That means if you double the overhung distance, you increase the deflection by a factor of 16 and very quickly create a destructive imbalance.​​

Defection​

Engineers control the deflection and vibration in a cantilever shaft by adjusting the distance between the bearing points, increasing the diameter of the shaft, altering the material the shaft is made of, and, of course, specifying operational parameters.​

The selection of material is pretty easy. You generally avoid materials that stress relieve over time or have a low modulus of elasticity (bend easily). This means opting for materials like 4140 for general applications or 174ph for some more corrosive applications is usually a good choice.​

Increasing the diameter of a shaft to control defection is the simplest solution, but you can only go so far before problems arise. With deflection increasing by the fourth power, you quickly require a very thick and heavy shaft. As the diameter increases, it becomes impossible to find bearings that can support a very large shaft and sustain any decent operating RPM. In short, the peripheral speed of the bearings becomes too fast for the rating of any bearing strong enough to handle the load.

Shortening the distance between the bearings can minimize deflection between the bearings and therefore greatly alter the deflection at the impeller. However, as the simplified load sketch below illustrates, halving the distance between the bearings increases the radial load on both bearings. These same bearings may also be under duress from the heavy weight of the shaft and the high peripheral speeds discussed in the previous paragraph.​

​Last but not least, the design engineer can control deflection and vibration by restricting operational parameters so as to avoid critical speeds and any overspeed condition.​

​How Deep Can You Go??

The elimination of submerged seals and the overall simplicity of the design make a cantilever pump ideal for sump applications, but how deep can you go? Generally speaking, the deflection issue discussed above limits setting lengths (distance from the base plate down to the pump end) to around 7 ft for agitator pumps. For non-agitator pumps, a suction tail pipe can be added to increase the “pump down distance”. When adding tail pipes, engineers must also consider any structural limitations of the support assembly and the NPSA limitations of the wet end. Tail pipe applications will, of course, require the sump level at startup to be high enough to ensure a flooded wet end and thus proper pump priming.​

On that note, it seems like today’s discussion is exposing a need for more information on cantilever applications and the many advantages of this type of pump. Stay tuned, we will get to those points next time!​​

​Cheers

RJ