Bearing Specification for Dusty Conveyor Pulleys and Idlers: Seal, Rating Basis, Fits and Clearance

Fix the seal first, and rule out open or shielded bearings

Fine dust is the failure driver here, and the seal is the component that decides whether the lubricant stays clean. A seal's job is to keep lubricant in and dirt and water out, and bearing life is closely tied to how well it does that¹.

A contact rubber seal (2RS, RSH/RS1) puts a lip against the rotating ring and blocks fine dust and water better than a non-contact metal shield (ZZ). Which only keeps a gap and stops larger particles⁹.

That protection is paid for in speed and heat. Contact friction lowers the maximum speed by roughly 40% against an open or shielded bearing of the same size⁷ (supplier-reported figure).

And standard NBR seals are limited to about 100°C, with special materials such as Viton needed above 100–120°C at added cost^7,9. On a low-speed heavy conveyor the speed penalty rarely binds, but the temperature ceiling does, because tight fits already push running temperature up³.

So write the seal into the specification per position and treat a quote that offers only open or shielded bearings for fine-dust duty as non-responsive.

If the supplier proposes a shield, ask what dust exclusion it claims and on what test, because no source in this set gives an IP rating or a quantified dust-exclusion result.

Contact seal or labyrinth seal, position by position

Where the ~100°C ceiling or the speed penalty rules out a contact seal, a non-contact labyrinth seal is the alternative. On a horizontal shaft, a labyrinth seals fine and coarse granular contamination reliably, has no speed limit from the seal itself, and needs no lubrication at the sealing location¹⁵.

The same comparison table states that lip seals are not suitable for granular contamination, because the particles wear the lips rapidly¹⁵. That gives you a clean split for the RFQ.

For pulley and idler positions on horizontal shafts carrying granular material, ask for a labyrinth seal where the duty runs hot or fast enough that a rubber lip is marginal.

Where the position is wet as well as dusty, note that labyrinth performance with water depends on rust-proof materials and a water level below the sealing diameter¹⁵. One supplier states its labyrinth seals have unlimited life and no speed limit from the seal¹⁵.

Treat that as the supplier's own claim and verify it against your duty rather than as a specification value. The sources do not state how a labyrinth performs on vertical or inclined shafts, so if your layout has any, put that question to the supplier directly.

Size on C0 or C, and make the supplier state both in kN

The rating basis changes with speed. For bearings rotating below 10 r/min, oscillating very slowly, or standing still under load for long periods, size on the basic static load rating C0 rather than the dynamic rating C².

Many conveyor pulleys and idlers sit in exactly that band, so the C0 basis is the one to write into the specification for those positions. For the type, spherical roller bearings are the standard choice for conveyor pulleys, carrying heavy radial load while tolerating shaft deflection and misalignment over long spans^6,19.

Their barrel rollers self-align, which is what makes them suit shock and deflection in bulk material handling¹⁹. Two things must come back from the supplier.

First, the actual C and C0 values in kN for the offered bearing, because no source in this set gives them for conveyor pulleys. Second, the safety factor the supplier applied, since the requisite ratings are calculated from the load and a temperature factor rather than read off a catalogue².

Note also that the L10 life equation stops being valid when the equivalent load exceeds C0 or 0.5C, so ask the supplier to confirm the calculation sits inside that limit.

Set the housing fit and verify running clearance

Shock and a rotating outer ring both point to the same housing fit. A rotating outer ring load, with conveyor rollers named as the example, maps to an M7 housing bore, and high impact load maps to M7 as well²¹.

A separate fit table maps heavy shock loads to M7 too⁵. Write M7 into the specification for shock-loaded pulley and roller positions, and ask the supplier to confirm it against your housing type.

Fits are not the end of the clearance question. A fit that is too tight reduces internal clearance, which raises operating temperature and accelerates lubricant deterioration³.

The clearance printed in the catalogue is therefore not the clearance the bearing runs with, so require the supplier to state the running clearance after fits and operating temperature are accounted for.

One condition changes the housing class: on large bearings above 250 mm where the outer ring and housing differ by more than 10°C, G7 is used instead of H7⁵. If your pulley bearings are that size and run warm, raise it before the order.

The sources give no shaft fit table specific to conveyor pulleys, so the shaft side has to be confirmed with the supplier.

Pin the clearance grade and the grease regime

Clearance grade and grease are the two lines most often left to the supplier's default, and both should be stated. Plummer-block housings typically use spherical roller bearings with C3 or C0 clearance⁴.

A separate clearance guide maps C4 to high-temperature, high-speed, heavy-impact duty such as crushers and coal-preparation equipment¹⁸. Those are different application contexts rather than a settled rule for low-speed conveyor duty, so name the grade per position and ask the supplier to justify it against your speed, temperature and fit.

On lubrication, two rules are firm enough to write in. Overfilling the bearing cavity with grease raises temperature and can cause failure, so leave room for grease to escape during start-up¹⁴.

And when you change grease, check base-oil compatibility first, because not all base oils mix¹⁴. Lubricant choice itself depends on temperature, speed and load, and heavy-load service needs higher pressure resistance to keep an oil film¹⁶.

Ask the supplier for the grease's base-oil type and its pressure resistance, and for the relubrication interval for your duty, since no source in this set gives a grease life figure for dusty low-speed operation.

Documentation and supplier screening for the RFQ

Ask for these with the quotation, not after the order. Quality paperwork is what lets you reject a bearing before it reaches the pulley, and the commercial thresholds below are one source's suggested screening values rather than industry standards.

• ✓ISO 9001 manufacturing certification, with material test reports confirming steel cleanliness and alloy composition
• ✓ATEX or RoHS documentation where the conveyor runs in an explosive dust atmosphere
• ✓Stated C and C0 values in kN, plus the safety factor used in the rating calculation
• ✓Seal type named per position, with the dust-exclusion basis the supplier claims
• ✓Running clearance after fits and temperature, not catalogue clearance
• ✓Grease base-oil type, pressure resistance and relubrication interval for your duty
• ✓Supplier evaluation matrix covering certifications, manufacturing capability, material traceability and commercial terms

Where the sources disagree on clearance grade

The clearance grade is the one specification item where the evidence points two ways, so settle it with the supplier before you sign the specification.

Before you request a quote

• The evidence leaves this open — put it to the supplier: no IP rating or quantified dust-exclusion test result for conveyor bearings — ask the supplier what dust test backs its seal claim.
• The evidence leaves this open — put it to the supplier: no actual C or C0 values in kN and no numeric C/P safety factor for conveyor pulleys — require both from the supplier.
• The evidence leaves this open — put it to the supplier: no clearance grade recommendation specific to low-speed heavy conveyor duty — the C3/C0 and C4 positions cover different applications.
• The evidence leaves this open — put it to the supplier: no grease life or relubrication interval figures for dusty low-speed operation — ask the supplier for an interval for your duty.
• The evidence leaves this open — put it to the supplier: no shaft fit table specific to conveyor pulleys — only the M7 housing fit is supported.
• The evidence leaves this open — put it to the supplier: no unit cost or total-cost-of-ownership comparison across bearing types or sealing options.
• Confirm named service coverage, response expectations and warranty terms for your destination.
• The evidence leaves this open — put it to the supplier: no evidence on multi-source interchangeability of parts.