Cutless Bearing Vertical Pumps

If you have a pump application in a sump that is deeper than 7 or 8 feet, you probably cannot use a cantilever pump due to shaft deflection issues, and using a vertical column pump or a submersible may be your only option. Yes, tail pipes and limiting pump down levels can provide a workaround, but if you need to use the full depth of a sump and/or be able to start the pumps at any fluid level, cantilevers are out of the question. Similarly, if you require mechanical agitation by the pump near the sump floor, column pumps or submersibles are the only options. The question then becomes which is best, a long vertical column pump or a submersible pump?

Submersible Pumps

Submersibles are portable and can be easily moved from one sump to another often providing for some level of standardization.  Lead times can be short as these pumps are sometimes available as off the shelf or stocked items by suppliers. Submersibles are often installed utilizing a flexible hose and electrical plug connectors making them simple to install and disconnect as required. The biggest concern with a submersible however is the issue of submerged bearings, seals and electrical windings. A seal failure can result in a need to rewind the stator. The motor in any submersible pump is integral to the pump so when a motor winding failure occurs it must be repaired by a shop with specialized equipment and skills. This is often not available within the owners maintenance facility, necessitating off site repairs and exposing the user to extended down times.

Vertical Column Pumps

The advantages of a vertical column pump are the inverse of the weaknesses of the submersible. Having just detailed the issues with the submersible in the paragraph above, I will simply say that the motor on the vertical pump is mounted high and dry above the baseplate, and the ability to use standard NEMA framed motors is a significant advantage. Furthermore, fixed rigid piping is normally associated with vertical column pumps and, when used, eliminates messy flexible hoses, making for a cleaner installation.

One disadvantage of a column pump is its shear length. This can often make the installation or removal procedure for maintenance an issue. In some cases, headroom above the sump may be a limiting factor.  Many column pumps, like submersibles, also have seals to protect submerged bearings. As such, a seal failure doesn’t damage the motor, but it will damage the lower bearings, resulting in unwanted repairs.  There is, however, a style of vertical column pump that eliminates the need for submerged seals. This statement leads me back to the subject title for today, “Cutless  Bearing Pumps.”

Cutless Bearings

The first Cutless bearing is said to have been made by a mining engineer in the 1920s. The story goes that a pump bearing failed, and the resourceful engineer pressed a piece of steam hose inside a sleeve to get them by. It worked well, and the Cutless bearing was created. Today, it is most commonly used to stabilize propeller shafts in boats, but it still has a home in the pump industry.

Basic Cutless Bearing

A Cutless bearing is a water-lubricated sleeve bearing used to support the rotating shaft. In a vertical column pump, it is located at the bottom end of the column just above the pump’s wet end. It is there to radially support the shaft, minimizing deflection and any resulting vibration. These bearings typically consist of a rigid outer shell lined with a grooved rubber insert.

Lubrication to prevent friction between the shaft and the rubber insert while being used in clear liquid applications is generally provided by the pumpage. In slurry applications, lubrication is provided by an external liquid source, in most cases, just clean water. Whether external or provided by the pumpage, the lubrication acts to minimize wear on both the bushing and the shaft.  The internal grooves aid this by forming a thin hydrodynamic film, physically separating the shaft from the bearing surface. In addition, the grooves provide a pathway to expel any solids that may find there way into the bearing.

The Cutless bearings primary function of supporting the shaft against radial loads is augmented by its ability to dampen any deflection that may be periodically induced. The resilient rubber compound absorbs shock and dampens vibrations, contributing to quieter operation and a longer lifespan for the pump’s structural components.

Common materials used for the outer sleeve include stainless steel, brass, and composite/phenolic, while the bushing is usually nitrile rubber or polyurethane. As mentioned, grooves in the bushing help to expel contaminants. Some manufacturers shape the grooves in a spiral configuration to aid in this process. Some suppliers can also offer split-shell designs that eliminate the need to disassemble the pump’s lower end when replacing the bearing.

Axial and Radial Bearings

A Cutless pump, like any piece of rotating equipment, has both axial and radial bearings. The difference in a vertical Cutless pump is that the conventional bearings are all above the base plate and so are kept far from the liquid that would destroy oil- or grease-lubricated bearings. Back-to-back ball bearings are commonly used in the top end and provide the radial support for the top end as well as securing the shaft axially. The Cutless bearing provides the radial support required at the wet end and accommodates any axial shaft growth due to heat expansion. This ability to absorb axial growth is a key advantage over conventional submerged bearing pumps that often have to incorporate expensive features to compensate for this issue.

As a side note: Using process liquid or clean water as a lubrication medium negates the need for grease or oil lubrication inside the sump and eliminates bearing lubrication as a potential source of contamination of the pumpage. 

Repair Shops​

​As soon as the motor is separated from a cantilever, it becomes a purely mechanical device. As such, a standard maintenance department has no issues accepting it for repair. Submersibles, on the other hand, are often thought of as an electric motor and sent to the electric shop. The electric shop, however, sees it as a pump and refuses to work on it. This problem is generally resolved by sending the submersible off-site for repairs, both costly and consuming. Another point for cantilevers!​

​Cutless Pump Construction

As discussed above, a conventional bearing housing utilizes two radial bearing locations, which is not required in a Cutless pump. As such theres often just a” bearing hat” containing the top conventional bearings and the discharge piping visible above the base plate. With most Cutless pumps being direct driven, even the bearing hat is hidden by the motor stool.

The Cutless bearing is not usually deemed as suitable for applications with high radial loads. As such, impeller/casing designs that provide for balanced hydraulic loads are most commonly used for cutless pumps.

Dual volute/discharge or recessed impeller designs would be prime examples of hydraulically balanced wet ends.

The question of “what is the practical pump setting length for this design?” is a key one. In answer to that, the simplicity of cantilever pumps often makes Cutlass pumps unnecessary in settings less than 6 ft.  How long you can go is application-dependent, but lengths around 15 to 20 ft are not uncommon.

Summary

Cutless bearing pumps can provide a very attractive alternative to conventional bearing vertical pumps. Hevvy/Toyo does manufacture several product lines utilizing this technology. For additional information or help with a specific project, please feel free to contact the application department at head office or any of our sales team.

Cheers,

RJ