A conveyor idler (roller) is a cylindrical part with bearings inside that supports and guides the belt on a belt conveyor. A conveyor may have hundreds of idlers, and even a single seized idler can cut the belt and increase friction and energy consumption. The most common problem when ordering replacement idlers is the wrong size: if the roller length, shaft end or diameter does not match, the idler will not fit the frame.
In this guide we explain how conveyor idler sizes are determined, the roller length chart by belt width, standard roller diameters, shaft and shaft end sizes and the information needed for a correct order. For idler types and failure symptoms see our conveyor idlers guide.
Dimensions of a Conveyor Idler
| Dimension | Definition | How to Measure |
|---|---|---|
| Roller diameter (D) | Tube outside diameter | Caliper, or tape circumference / π |
| Roller length (L) | Tube (shell) length | Between tube ends |
| Overall shaft length | Length between shaft ends | From shaft end to shaft end |
| Shaft diameter (d) | Diameter of the shaft through the bearings | Caliper |
| Shaft end type | Spanner flats, drilled, threaded, etc. | Visual and caliper |
| Spanner flat size | Distance across flats and flat length | Caliper |
| Tube wall thickness | Tube wall thickness | Caliper (at the end) |
Roller Length Chart by Belt Width
The values below are common roller lengths according to the DIN 15207 series. For three-roll (troughed) carrying sets, the length of each roller is given; for single-roll flat return sets, the return roller length:
| Belt Width | 3-Roll Carrying Roller Length | Flat Return Roller Length |
|---|---|---|
| 500 mm | 200 mm | 600 mm |
| 650 mm | 250 mm | 750 mm |
| 800 mm | 315 mm | 950 mm |
| 1000 mm | 380 mm | 1150 mm |
| 1200 mm | 465 mm | 1400 mm |
| 1400 mm | 530 mm | 1600 mm |
| 1600 mm | 600 mm | 1800 mm |
Different standards (e.g. CEMA) and manufacturers may use different lengths; lengths differ for two-roll V return sets and special frames. Order by measuring the existing roller or taking frame measurements.
Standard Roller Diameters
Common conveyor roller diameters are: 63.5 – 76 – 89 – 102 – 108 – 114 – 127 – 133 – 140 – 152 – 159 – 168 – 194 mm. Diameter selection depends on belt width, belt speed and load:
| Application | Common Diameter Range | Note |
|---|---|---|
| Light conveyors, narrow belts | 63.5–89 mm | Low speed and load |
| Crushing plant conveyors (500–1000 mm belt) | 89–133 mm | The most common group |
| Wide and fast belts, heavy duty | 133–194 mm | Lower speed, longer bearing life |
| Impact rollers | Rubber-ringed, larger outside diameter | Loading point |
At the same belt speed, a larger diameter roller turns at a lower speed, which extends bearing life and reduces noise. At high belt speeds, small diameter rollers can overheat.
Shaft Diameter and Shaft End Sizes
| Shaft Diameter | Common Spanner Flat (across flats) | Typical Bearing | Use |
|---|---|---|---|
| 20 mm | 14 mm | 6204 | Light and medium duty |
| 25 mm | 18 mm | 6205 / 6305 | Medium and heavy duty |
| 30 mm | 22 mm | 6206 / 6306 | Heavy duty, wide belts |
| 40 mm | By manufacturer | 6308, etc. | Very heavy duty |
The spanner flat size and flat length are critical for the roller to seat in the frame slot. For bearing numbers and sizes see our bearing size chart guide.
Idler Types
- Carrying (troughed) idlers: Support the loaded belt; usually 3-roll sets, 20–35° troughing angle.
- Return idlers: Support the empty return belt; flat or V-type.
- Impact idlers: Absorb impact at the loading point with rubber rings.
- Disc return idlers: Reduce build-up of sticky material on the return side.
- Self-aligning idlers: Correct belt mistracking.
- Rubber-lagged or polyurethane idlers: Abrasive or corrosive environments.
Idler Spacing
| Zone | Common Spacing |
|---|---|
| Carrying (load) side | About 1.0–1.5 m (depending on load) |
| Loading point | About 0.3–0.5 m (impact idlers) or an impact bed |
| Return side | About 2.5–3 m |
| Pulley transitions | Special arrangement according to belt transition distance |
Spacing is set by belt sag, load and belt tension. For capacity and power calculations see our belt conveyor calculation guide.
Choosing the Right Idler: Sealing and Quality
- Labyrinth sealing: In dusty, wet crushing plant environments, the single most important feature for bearing life.
- Tube quality and runout: An idler that does not run true causes vibration and belt wear.
- Bearing quality: Good-quality, lubricated-for-life bearings should be preferred.
- Weld and end cap quality: End caps properly welded to the tube.
A cheap idler with poor sealing can seize quickly and cut the belt; the right basis for comparison is idler life and downtime cost, not unit price.
Information Needed for an Order
- Belt width and set type (3-roll carrying, flat return, V return, impact),
- Roller diameter and tube length,
- Shaft diameter, overall shaft length, shaft end type and spanner flat size,
- Bearing type (if known),
- Lagging requirement (rubber, polyurethane),
- Quantity and operating conditions (dust, water, temperature).
Common Mistakes
- Confusing roller length (tube length) with overall shaft length.
- Not checking the spanner flat size.
- Using small diameter rollers at high belt speeds.
- Not using impact idlers or an impact bed at the loading point.
- Not replacing seized idlers in time.
Frequently Asked Questions
What is the roller length for an 800 mm belt?
According to DIN 15207, each roller in a 3-roll carrying set is 315 mm and the flat return roller is 950 mm.
What is the most common conveyor roller diameter?
89, 108 and 133 mm diameters are common in crushing and screening plants.
How is the roller shaft diameter chosen?
By roller load, length and belt width; 25–30 mm shafts are common in heavy duty.
Why does a conveyor idler stop turning?
Bearing failure, dust and water ingress, material jamming or a bent shaft are the main causes.
Steps for Measuring an Existing Idler
- Remove the idler from the frame and clean it.
- Measure the tube outside diameter with a caliper; if the caliper is too small, measure the circumference with a tape and divide by π.
- Measure the tube (shell) length from end cap to end cap.
- Measure the overall shaft length from shaft end to shaft end.
- Measure the shaft diameter, the across-flats size and the flat length.
- Photograph the shaft end type (spanner flats, drilled, threaded, circlip groove).
- Note any label and bearing number on the idler.
Sending photos of the idler and frame along with the measurements prevents incorrect orders.
Field Example
At one plant, newly purchased carrying idlers do not fit the frame. Investigation shows the tube length is correct but the overall shaft length and spanner flat size differ from the old idlers. The order had simply said “idlers for 1000 mm belt”. When the order is placed again with the shaft end dimensions, the problem is solved. This example shows that belt width alone is not enough information.
Factors Affecting Idler Life
| Factor | Effect |
|---|---|
| Sealing quality | Dust and water ingress determine bearing life |
| Roller speed | Small diameter and high belt speed increase speed and heating |
| Load | Overload and impact cause bearing and shaft fatigue |
| Material build-up | Material stuck on the roller causes runout and belt mistracking |
| Alignment | Skewed idlers cause belt mistracking and edge wear |
Glossary
- Carrying set: The roller group and frame supporting the loaded belt.
- Spanner flats: Two flat faces on the shaft end that stop the idler turning in the frame.
- Labyrinth seal: A non-contact seal that stops dust and water before they reach the bearing.
- Runout: Deviation of the roller from its axis as it turns.
- Impact bed: A slider-type belt support at the loading point.
More Questions
What is the roller length for a 1000 mm belt?
According to DIN 15207, each roller in a 3-roll carrying set is 380 mm and the flat return roller is 1150 mm.
When are rubber-lagged idlers used?
With abrasive or sticky material, in corrosive environments and on the return side to improve belt cleaning.
Should idlers be replaced as a set?
Failed idlers can be replaced individually, but the rollers in the same set must have the same diameter and dimensions.
CSP Mühendislik Support
CSP Mühendislik supplies carrying, return, impact and special conveyor idlers to size and supports conveyor maintenance. For idlers and other conveyor parts see our belt conveyor spare parts page and conveyor spare parts guide, and for conveyor structure our belt conveyor guide.
