Overview
ISO 3684:1990 - Conveyor belts - Determination of minimum pulley diameters - specifies a method to calculate minimum pulley (drum) diameters for conveyor belts made of rubber or plastics with textile or metal carcasses. This second edition (1990) replaces the 1976 edition and gives guidance to avoid premature belt damage caused by pulleys that are too small. It is limited to ordinary-service belts (not heat-resistant belts for products >100 °C or belts used below −40 °C) and does not apply where the carcass thickness exceeds 20 mm or where intermediate rubber/plastic layers between plies exceed 0.8 mm.
Key topics and technical requirements
- Formula-based minimum diameter: Minimum drum diameter D is calculated from the belt carcass thickness (e) multiplied by a material factor (C). The result is then rounded up to the next standardized drum diameter (series R10 or R20 per ISO 3).
- Material factor (C): Values depend on carcass material (examples given in the standard):
- Cotton: C = 80
- Polyamide / Cotton-Polyamide: C = 90
- Polyester: C = 108
- Rayon: C = 118
- Metal carcass: C = 145
- Drum types and tolerances: Pulleys are grouped by function - Type A (drive/high-tension drums), Type B (return/low-tension drums), and Type C (take-up or angle-change drums). The standard permits selecting smaller standardized diameters depending on drum type, but with limits (e.g., B one step down, C up to two steps down in R10 series, subject to additional constraints).
- Influence of belt tension: Allowed diameters can be reduced according to the belt’s operating tension expressed as a percentage of the Recommended Maximum Belt Tension (RMBT). Tables in the standard show combinations of carcass thickness, material, drum type and tension usage.
- Rounding and normal sizes: Calculated diameters must be rounded up to the immediately higher normalized diameter (ISO preferred numbers).
Applications and users
ISO 3684 is practical for:
- Conveyor design engineers selecting pulley sizes to prevent carcass fatigue and failure
- Belt manufacturers specifying minimum diameters for product datasheets
- OEMs and integrators in mining, bulk handling, ports, quarrying and material handling systems
- Maintenance and reliability teams verifying retrofit drum sizes for existing conveyors
- Compliance and safety professionals ensuring equipment meets international belt-handling standards
Keywords: conveyor belts, minimum pulley diameters, pulley diameters, conveyor drum diameters, belt carcass, RMBT, ISO 3684
Related standards
- ISO 3 - Preferred numbers (R10/R20 series)
- ISO 583 - Conveyor belts - Thickness definitions and tolerances
- ISO 7590 - Steel-cord conveyor belts - Measurement of cover thickness
- ISO 1536 - Belt conveyors for bulk materials (relevant values and dimensions)
Use ISO 3684 as the technical baseline for selecting safe, standardized pulley diameters that maximize belt life and system reliability.
