Bearing Selection Mistakes Procurement Can Catch Before the Order: A Pre-RFQ Checklist

Start with load direction and magnitude, not the incumbent part number

The first and most expensive mistake is a bearing type chosen without the load case being written down. Multiple independent sources treat load direction and magnitude as the primary input that narrows type before any other factor is considered^7,9,16,18.

Direction splits radial from thrust. A load perpendicular to the shaft points to a radial bearing, and a load parallel to the shaft points to a thrust bearing^7,9.

Magnitude then splits ball from roller, with light loads favoring ball and heavy loads favoring roller^7,16. Combined radial and axial duty is the case that most often gets mis-typed.

It may need an angular contact or tapered roller bearing, or an arrangement built for combined loads rather than a single bearing^9,18. When you review a quote, ask what load case the quoted type was selected against.

If the answer is a part number carried over from the last order, the type has not been selected at all — it has been copied. A cross-reference replacement carries the same risk, since the load case of the original may never have been documented³².

No source in this set establishes a numeric load threshold at which a ball bearing must give way to a roller bearing for a specific application. Treat the ball/roller split as a direction of travel rather than a cut-off you can compute.

Check that clearance and fit were chosen as one decision

Clearance and fit are usually quoted as two separate line items, and that is where the second common mistake sits. A heavy shaft fit consumes internal clearance, and speed-generated heat consumes more.

A Normal-clearance bearing under a heavy fit can end up too tight²⁵. The sources set the direction of travel.

A heavy shaft fit on ISO deep groove ball, cylindrical roller or spherical roller bearings points to C3 rather than Normal. A light-to-medium fit running at high speed that generates heat also points to C3.

A heavy fit combined with high speed typically points to C4²⁵. C3 is described as the most popular stocked clearance.

And as the standard recommendation for most electric motor bearings and for compressors, pumps, wheel hubs, alternators and transmissions, where heat and thermal expansion are present^23,25. That does not make C3 a default you can apply blind.

A hot-running bearing has many causes, and insufficient clearance is only one of them. Too tight a fit, poor lubrication, overload or misalignment can all raise temperature¹⁵.

If you are specifying clearance for a hot application, ask the supplier to state which of those causes the clearance change is meant to address. No source provides a calculation method to derive the required operating clearance from fit, temperature and speed for a specific bearing.

The clearance code on the quote should be justified in words, not computed by you²⁵.

Reject a lubrication line that says only "grease" or "oil"

A lubrication specification without a fill quantity and an operating point is not a specification. Grease fill has to fall as speed rises, because the faster a bearing runs the more resistance the grease presents to motion.

Excess grease in a high-speed bearing raises energy consumption and operating temperature⁴. The commonly quoted one-third free-space fill is described as a good approximation only for medium speeds.

High-speed bearings are suggested at 10-15% of free space, while low-speed bearings can be packed because churning losses are negligible⁴. Lubricant form follows the same logic.

Grease suits sealed bearings and heavy loads. Oil gives lower friction and better heat dissipation for high speed.

Solid lubricants cover aerospace, marine and extreme high-temperature environments²⁷. Temperature sets a practical ceiling: most users prefer to keep housings below 60 C (140 F).

Above that, the source calls for specialized oil, constant level checks and much shorter re-greasing intervals where grease is used¹². Operating speed within 80% of the limiting speed is flagged as a case to scrutinize further¹².

Both over-greasing and under-greasing are named as common causes of overheating, wear and early failure. Grease compatibility has to be confirmed before products are mixed²⁶.

When you review a quote, ask for the fill quantity, the base oil viscosity and the speed and temperature the lubricant was chosen for. No source in this set gives a re-lubrication interval or a specific grease grade for a named bearing or application.

Those remain questions for the supplier rather than values you can fix in the RFQ.

Make documentation and physical inspection a condition of acceptance

Documentation is described as the clause buyers most often forget, and it is the clause that leaves origin and quality unverifiable when it is missing³³. Counterfeit bearings show up as physical anomalies — poor rolling-element and raceway finish, non-genuine cage construction, and inappropriate packaging, pallet configuration or country-of-shipment mismatch.

A goods-in check is the counterpart to the paperwork⁶. The sources converge on requiring verifiable certification, batch traceability and inspection documentation, but they do not agree on a single mandatory certification set^6,32–34.

• ✓Require ISO 9001 certification and material test reports confirming steel cleanliness and alloy composition.
• ✓Add ATEX or RoHS documentation where the operating environment requires it.
• ✓Ask for batch-specific traceability: material origin, heat treatment logs and inspection reports.
• ✓For critical applications, ask what dimensional and material verification the supplier performs, rather than accepting random sampling.
• ✓Inspect rolling-element and raceway finish on receipt for rough or abnormal appearance.
• ✓Compare cage construction against the specified design — a two-piece riveted cage where a machined brass cage was specified is a flag.
• ✓Check packaging quality, pallet configuration and whether manufacturer country matches shipment country.

Compare unit price only at the same order quantity

Unit prices from suppliers quoting different quantities are not comparable, and treating them as if they were is a quiet way to overpay. MOQ varies by product type, specification and whether customization is needed.

The supplier needs the model, clearance code, quantity and application before it can quote at all¹⁵. More generally, higher-cost products usually carry lower MOQs while low-ticket products may require higher MOQs.

MOQs differ between suppliers because each sets them against its own cost structure³⁷. That means the order quantity is a decision, not a given.

An EOQ-style formula balances annual demand, ordering cost, unit cost and holding cost, then rounds up to standard packaging quantities³⁶. The holding-cost input in that formula is typically taken as 20-30% of inventory value, a figure from a general inventory source rather than a bearing-specific one³⁶ (supplier-reported figure).

Treat it as a starting assumption to check against your own carrying cost³⁶. Before you compare two quotes, put them on the same quantity and the same clearance code.

No source in this set provides bearing-specific MOQ values, price-break volume thresholds or lead times, so those have to come from the suppliers themselves.

Match the seal and material to the real environment

Sealing is often specified after the type is frozen, which is the wrong order when the environment is aggressive. Seal choice trades friction and speed capability against contamination protection.

A non-contact labyrinth seal has no friction, no speed limit set by material and unlimited life, while a lip seal is limited by wear and by its lip material³⁹. The trade is not free in either direction.

A labyrinth seal can only reliably seal granular contamination when it is applied to a horizontal shaft, so a vertical-shaft application with grit present needs a different answer³⁹. Temperature range follows the seal material rather than the bearing.

Plastic labyrinth components are quoted from -40 F to 284 F (-40 C to 140 C), and steel or aluminium components from -40 F to 340 F (-40 C to 171 C)³⁹. Lip seals require lubrication at the sealing location and, at higher shaft speeds, a hardened and ground mating surface³⁹.

When you review the quote, ask which seal was selected and against what contamination, moisture and speed condition. No source in this set gives numeric contamination, vibration or shock limits for sealing or material selection.

The environment has to be described in words on the RFQ rather than reduced to a threshold.

Check whether a short-run or custom bearing is worth pursuing at all

For standard production the cage is a detail settled after type and size are chosen, usually influenced by speed and lubrication method⁸. Short-run custom bearings invert that order.

The cage can become the deciding factor on bearing type, because stamped and moulded cages of unique pitch diameters, rolling element complements and configurations are often unrealistic due to high tooling cost, regardless of how well they suit the application⁸.

So a technically correct custom type can be commercially unavailable at your volume, and the constraint is tooling rather than bearing performance. If your order is a short run or a one-off, ask the supplier early whether the cage design you have in mind requires tooling.

Ask whether a standard cage configuration would change the type recommendation. That question is cheaper to ask before the drawing is fixed than after.

Where the sources disagree on lubrication

The lubrication figures below come from different sources under different stated conditions. The disagreement is about whether a fill quantity can be treated as a general rule at all.

Keep both sides visible when you write the specification.

Before you request a quote

• The evidence leaves this open — put it to the supplier: no source provides numeric bore, OD or width tolerance tables, or a rule mapping an application to a required tolerance class.
• The evidence leaves this open — put it to the supplier: no source gives a method to calculate operating clearance from fit, temperature and speed, or any preload values.
• The evidence leaves this open — put it to the supplier: no source gives re-lubrication intervals or speed limits for named bearing types and greases.
• Ask for relevant export destinations, installed-base references and comparable customer contacts.
• The evidence leaves this open — put it to the supplier: no source establishes which certification is mandatory for a given application or region.
• Request a written quote with price basis, MOQ, tooling, payment terms and production lead time.
• The evidence leaves this open — put it to the supplier: no source gives numeric contamination, vibration or shock limits for sealing and material selection.
• The evidence leaves this open — put it to the supplier: no source addresses the bearing selection checklist question as a whole; the checklist elements here are assembled from separate selection guidance.