Quick Overview
If you’ve ever stared at a pump datasheet and seen “OH2” or “BB1” and had no idea what it meant, you’re not alone. These codes come from API 610, the standard most industrial buyers lean on when specifying centrifugal pumps for petroleum, chemical, and general process work. The letters aren’t random. They describe how the impeller sits relative to its bearings, and that one design choice affects everything from shaft life to how much pressure the pump can handle. We work with these pump families every day at AMED-US, so we wanted to lay out what actually separates overhung and between-bearings designs, and why the choice matters more than most spec sheets let on.
Key Takeaways
- Overhung (OH) pumps support the impeller on one end of the shaft, outside the bearings. Between-bearings (BB) pumps support the impeller on a shaft held by bearings on both sides.
- Overhung designs are simpler, cheaper, and easier to maintain, but they’re limited on pressure and flow.
- Between-bearings pumps handle higher pressures, higher flows, and multistage service, at a higher upfront cost and more complex maintenance.
- Shaft deflection, not just cost, is the real engineering reason one type gets picked over the other.
- API 610 splits pumps into three groups: OH, BB, and VS (vertically suspended), each with sub-types numbered 1 through 6 or more.
- The right choice depends on your fluid, pressure, flow rate, and how much downtime you can tolerate.
What “Overhung” and “Between Bearings” Actually Mean
Picture a fishing rod with a weight tied to the tip. Hold the handle and that weight hangs off the end, unsupported except by your grip. That’s roughly how an overhung pump works. The impeller sits at the end of the shaft, cantilevered past the bearing housing, with nothing on the far side to help carry the load.
A between-bearings pump flips that setup. The shaft runs through the impeller with a bearing supporting each end, similar to how a barbell is balanced between two hands instead of held out at arm’s length. That single design difference changes almost everything else about the pump.
Overhung (OH) Pumps
Overhung pumps carry a single bearing set on one side of the shaft. API 610 breaks this category into sub-types, OH1 through OH6, based on how the pump mounts and couples to the driver. Foot-mounted, centerline-mounted, and close-coupled versions all fall under this umbrella. Because the design is simpler, these pumps tend to cost less, install faster, and take up a smaller footprint. That’s a big part of why they show up so often in general industrial service, water transfer, and lighter chemical duty.
Between-Bearings (BB) Pumps
Between-bearings pumps use two bearing sets, one on each side of the impeller (or impellers, in multistage builds). This category covers axially split, radially split, and multistage designs, labeled BB1 through BB5. Because the rotor gets support from both directions, these pumps handle much higher pressures and flow rates than an overhung design ever could. You’ll see them in boiler feed service, high-pressure water injection, and large-scale process transfer where a single-bearing design just wouldn’t hold up.
The Real Difference: Shaft Deflection, Not Just Cost
Most articles on this topic stop at “overhung is cheap, between-bearings is strong” and move on. That’s true, but it skips the actual engineering reason behind it.
An overhung shaft acts like a cantilever beam. The farther the impeller sits from its bearing support, and the thinner the shaft, the more that shaft flexes under hydraulic and rotor-dynamic load. Engineers sometimes use a ratio called L3/D4 (shaft overhang length cubed, divided by shaft diameter to the fourth power) as a rough gauge of rotor stiffness. Lower numbers mean less deflection and a more forgiving design. Higher numbers mean the shaft bends more under load, which raises the risk of seal failure, bearing wear, and impeller contact with the casing over time.
That’s the practical reason overhung pumps get capped at moderate pressures and flows. Push an overhung design past its comfort zone, and the shaft deflection problem gets worse, not better. Between-bearings pumps sidestep that limitation because the shaft is supported on both ends, which keeps deflection low even under heavy hydraulic loads. It’s the same reason a bookshelf sags less when it’s braced on both sides instead of just one.
When an Overhung Pump Is the Right Call
For moderate flow, moderate pressure applications, overhung is usually the more sensible choice. A few situations where it tends to make sense:
- General water transfer, cooling loops, or booster service where pressures stay reasonable
- Budget-conscious projects where a simpler pump means lower capital cost and easier spare parts management
- Tight installation footprints, since vertical inline OH3 and OH4 types take up minimal floor space
- Applications where routine maintenance access matters more than raw pressure capability
When a Between-Bearings Pump Makes More Sense
Between-bearings pumps earn their keep in demanding, high-stakes service. Consider this design when:
- Discharge pressures or flow rates exceed what a single-bearing shaft can support without excessive deflection
- The application runs continuously and can’t tolerate frequent unplanned downtime
- Multistage pressure boosting is required, such as boiler feed or high-head water systems
- The fluid or process conditions demand tighter tolerances and rotor stability over the long haul
Common Applications We See in the Field
At AMED-US, our team supports industrial pump distribution for clients across asphalt production, water and wastewater treatment, construction, and general manufacturing. We see both pump families in daily use, and the choice usually comes down to a straightforward question: how much pressure and flow does this application actually demand?
Asphalt plants, for instance, often rely on gear and rotary vane pumps for handling viscous liquid asphalt, but the centrifugal side of the operation, cooling water, transfer lines, and general utility pumping, frequently uses overhung designs because the pressures involved don’t call for anything heavier. Water and wastewater operations swing both ways depending on whether you’re looking at a simple lift station or a high-pressure injection system. Fire protection systems, meanwhile, often use end-suction or vertical overhung configurations, since fire pump codes prioritize reliability and simplicity over raw pressure capacity in most building applications.
We’re an authorized distributor of Ruhrpumpen equipment, and their lineup is a good example of how both families coexist under one manufacturer. Ruhrpumpen builds overhung process pumps for standard industrial service alongside between-bearings horizontal multistage and double suction pumps for higher-pressure demands. If you’re weighing which family fits your project, our Ruhrpumpen pump distribution page breaks down both product lines side by side.
Overhung vs Between Bearings: A Quick Comparison
| Factor | Overhung (OH) | Between Bearings (BB) |
| Bearing support | One side of shaft | Both sides of shaft |
| Pressure capability | Low to moderate | Moderate to high |
| Flow capability | Low to moderate | Moderate to very high |
| Shaft deflection risk | Higher at extremes | Lower, more stable |
| Upfront cost | Lower | Higher |
| Maintenance complexity | Simpler | More involved |
| Typical use | General service, utility pumping | High-pressure, continuous-duty process service |
Not sure which column your application falls into? That’s a fair place to get stuck. A lot of buyers assume bigger pump means better pump, but oversizing a between-bearings unit for a job an overhung pump could’ve handled just adds cost and maintenance overhead you don’t need.
Get Expert Help Choosing the Right Pump
Picking between an overhung and a between-bearings pump isn’t something you should guess at, especially when the wrong call means premature seal failure or a pump that can’t keep up with your process. Our team reviews your flow rate, discharge pressure, fluid characteristics, and duty cycle before recommending a specific pump type or model. We work with industrial pump suppliers and manufacturers across North and South America, and we back every recommendation with pump repair and maintenance support once the equipment is installed.
Browse our full range of overhung and between-bearings pump types to see what’s available, or reach out directly so we can match the right equipment to your application.
Talk to AMED-US About Your Pump Selection
Whether you’re specifying a new pump or troubleshooting a failing one, our engineers can help you sort through the options and avoid an expensive mismatch. Contact AMED-US today to get a recommendation based on your actual operating conditions, not just a datasheet.
Frequently Asked Questions
What’s the main difference between overhung and between-bearings pumps?
Overhung pumps support the impeller on one end of the shaft, outside the bearing housing. Between-bearings pumps support the shaft with bearings on both sides of the impeller, which allows them to handle higher pressures and flows with less shaft deflection.
Are between-bearings pumps more reliable than overhung pumps?
Not automatically. Between-bearings pumps handle high-pressure, continuous-duty service more reliably because of their dual bearing support. But for moderate-duty applications, a properly sized overhung pump can run just as reliably, generally speaking, and at lower cost.
What do OH and BB mean on a pump datasheet?
OH stands for overhung, and BB stands for between-bearings. Both are classification codes from the API 610 standard, followed by a number (like OH2 or BB1) that identifies the specific mounting and coupling configuration.
Can overhung pumps handle high-pressure applications?
In most cases, no, not at the upper end of industrial pressure demands. Overhung pumps are generally limited to low and moderate pressure service because the cantilevered shaft design is more prone to deflection under heavy hydraulic load.
Which pump type costs less to maintain over time?
Overhung pumps are usually simpler and cheaper to maintain because of their single-bearing design and smaller parts count. Between-bearings pumps cost more to maintain but often make up for it through longer service intervals in demanding applications where an overhung pump would wear out faster.
Do fire pumps use overhung or between-bearings design?
Fire pumps commonly use end-suction or vertical overhung configurations, since fire protection codes prioritize simplicity and dependable startup over the high-pressure capability that between-bearings designs are built for.
How do I know which pump type my application needs?
The right choice depends on your required flow rate, discharge pressure, fluid type, and how continuously the pump runs. A pump specialist can review your operating conditions and recommend the correct API 610 type instead of leaving it to guesswork.
