Calculator

Idler Load and Bearing Life Calculator

How much load does one idler station actually carry, and how long will the bearing in it last? Four numbers answer both — the rest are refinements you can leave alone. Everything runs in your browser: nothing is sent anywhere, and there is nothing to sign up for.

What this does not do: it does not hand you a CEMA class. Published CEMA load limits contradict each other across sources, so we do not publish a class table. What you get instead is the load, the speed and the bearing life — the three numbers a class is chosen from.

1 · The duty

Throughput (t/h)

Belt speed (m/s)

Carry-side idler spacing (m)

Roll diameter (mm)

2 · Refinements — leave at the defaults if you do not know

Belt weight (kg/m)Off the belt data sheet. 0 means belt weight is ignored.

Rotating parts per set (kg)The rolls themselves. 0 means ignored.

Service factor1.0 for steady, even feed. Raise it for impact at the loading point and uneven feed.

Share of station load on the roll being checked (%)33% = the three rolls share the load equally. The centre roll of a troughing set usually carries more than a third — set 60–70% to check it on its own.

3 · The bearing

Bearing basic dynamic load rating C (kN)From your bearing supplier’s data sheet. Reference values for common sizes are listed below.

Target L10 life (hours)What you want the bearing to reach, not what it is guaranteed to reach.

Result

—Material load per metre of belt

—Static load on one idler station

—Design load on one roll (with service factor)

—Roll speed

—L10 bearing life at that load

—C needed to reach your target life

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How it works

Every Formula Used, and What It Leaves Out

You should be able to check any of this by hand. Nothing below is a rule of thumb dressed up as an answer.

Step	Formula	Notes
Material load per metre	w_m = Q / (3.6 × v)	Q in t/h, v in m/s. Straight unit conversion: t/h to kg per metre of belt.
Static load on one station	W_i = (w_B + w_m) × S_i + w_R	One station carries the material and belt over one spacing, plus the weight of its own rolls. This is the form used in CEMA idler selection.
Roll speed	n = 60 000 × v / (π × D)	D in mm. Belt surface speed divided by roll circumference.
Bearing life	L_10h = 16 667 / n × (C / P)³	ISO 281 basic rating life. Exponent 3 because idler bearings are radial ball bearings; roller bearings use 10/3.
C needed for your target	C = P × (L_10h × n / 16 667)^1/3	The same expression solved the other way round.

What it leaves out — read this before you buy on it

L10 is a fatigue figure, not a promise. It is the life 90% of a population of identical bearings is expected to reach or exceed under laboratory conditions.
No life adjustment factors are applied. ISO 281 provides a_ISO to correct for lubrication, contamination and load relative to the fatigue limit. It is not applied here, because it needs data this page does not have.
Most idlers do not die of fatigue. They die of seal failure, contamination and grease loss. A long L10 on a badly sealed roll means nothing — which is why the sealing duty matters more than the bearing rating on a wet or dusty line.
The centre roll carries more than a third. Left at 100% the load is shared equally, which understates the centre roll. Set the share to 60–70% to check it on its own.
This is a selection aid, not a design. Have your engineer or your conveyor supplier confirm the result before it goes into a specification.

Reference

Bearing C Values for Common Sizes

Taken from SKF product pages in October 2026. Values move between catalogue editions and differ between makers — the number on your bearing supplier’s data sheet is the one to use.

Bearing	Bore × OD × width (mm)	Basic dynamic load rating C (kN)
6204	20 × 47 × 14	12.7
6205	25 × 52 × 15	14.8
6305	25 × 62 × 17	23.4
6306	30 × 72 × 19	29.6

If the C you need is above the C of the bearing that fits your roll, the options are a larger bearing, a shorter idler spacing, or a heavier roll class — and that is the conversation worth having before the order, not after the first failure.

How CEMA, DIN and GB/T differ on idler classes → · Dimension table and enquiry template → · Glossary of idler terms →

Have the Numbers but Not the Supplier?

Send the output above with your duty and we will tell you what we would build, and what we would change.

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