TL;DR
Conrad and Max ball bearings share the same external dimensions, but they are not interchangeable in every application. Conrad (non-filling-slot) bearings carry both radial and axial loads and account for roughly 90–95% of ball-bearing unit sales. Max — also called full-complement or filling-slot — bearings pack in more balls for higher radial capacity, but they have almost no axial capacity. Substituting a Conrad for a Max in pump service is usually safe. Going the other way is not.
Two Designs, One Envelope
Conrad (non-filling-slot)
To assemble a Conrad bearing, the inner ring is placed in an eccentric position relative to the outer ring. That creates a gap opposite the point of contact, allowing the balls to be inserted. The balls are distributed evenly around the assembly, the rings are aligned concentrically, and a cage is fitted to hold the balls in position.
Max (filling-slot / full-complement)
Max bearings carry a single filling slot in both the inner and outer ring, which allows a higher ball count. Because of the increased ball complement, filling-slot bearings carry a higher radial load than bearings without slots — but their axial load capacity is reduced. They suit applications with heavy radial loads, or a combination of radial and thrust loads where the radial load is predominant. They are not recommended where thrust loads alone are present.
Where Each Design Shows Up
| Design | Share of ball-bearing sales | Typical applications | Load character |
|---|---|---|---|
| Conrad / non-filling-slot | 90–95% | Electric motors, fans, general rotating equipment | Radial + axial |
| Max / full-complement / filling-slot | 5–10% | Some pumps and high-radial-load applications | Radial predominant; little or no thrust |
The practical consequence: Conrad bearings are broadly available off the shelf. Max bearings, because they are a small slice of the market, are frequently not — and the less common sizes are often obsolete or long-lead.
The Substitution Question
The Max type, with its loading notch, was created to gain extra radial capacity. It works well when the radial load is predominant and there is little or no axial loading. If a substantial axial load is present, the balls are pushed to the side of the race groove and can strike the groove as the bearing rotates — ruining the bearing in a short period.
When a pump is designed, care is taken to supply the correct bearing. But the same pump is sometimes repurposed for a different product and used differently than intended, which changes the loading and therefore the bearing requirement. Several years ago, numerous pump manufacturers published repair instructions recommending Conrad bearings in their pumps for all applications — eliminating the risk that an early failure could result from a Max bearing seeing too much axial load.
Because Conrad and Max share external dimensions, the two designs are dimensionally interchangeable. That is what makes the substitution question possible in the first place — and it is also why it must be answered on load, not on dimensions.
Comparing a Conrad 6308 and a Max 308
| Bearing | Dynamic load rating | Limiting speed |
|---|---|---|
| 6308 (Conrad) | 9,500 lb | 11,000 rpm |
| 308 (Max) | 10,200 lb | 9,000 rpm |
The gap is modest: roughly 7% more dynamic capacity for the Max, offset by a lower limiting speed. A well-designed application with an adequate service factor will usually run either design successfully.
When to Substitute — and When Not To
- Conrad in place of Max: generally acceptable when the application has meaningful axial load, when the radial load is within the Conrad rating, and when the substitution is made deliberately. This is the direction pump manufacturers themselves endorsed.
- Max in place of Conrad: risky. The Max bearing's reduced thrust capacity is the failure mode — if the application carries axial load, the substitute can fail early.
- Never substitute on dimensions alone. Verify radial load, axial load, speed, and the original design intent.
How RBC Handles It
When a Max bearing is obsolete or unavailable on an urgent need, our first step is to confirm the external dimensions match, then evaluate the actual axial and radial loading before recommending a Conrad equivalent. That is why we stock Conrad design bearings for pump and rotating-equipment service — they cover the majority of applications and remain broadly available.
Key Takeaway
Conrad and Max bearings look interchangeable on the drawing. They are not interchangeable under load. Match the dimensions, check the axial load, and choose deliberately.
Need a bearing matched to your application? Send us the part number, the equipment, and the failure history — our team will confirm the right design. Call RBC Industrial at (915) 845-8188 or request a quote at rbc-industrial.com.