What Is System Turn Ratio?
The System Turn Ratio is the geometric relationship between the spiral system radius and the conveyor belt width. It defines how tightly the belt must turn around the spiral.
The System Radius is one-half of the drum diameter. Because the radius and belt width are expressed in the same unit, the resulting turn ratio is dimensionless.
For a spiral with a 2200 mm drum diameter and a 1000 mm wide belt, the system radius is 1100 mm and the System Turn Ratio is 1.10.
Why Turn Ratio Matters
Every spiral conveyor belt has a minimum allowable turn ratio defined by its construction. A belt designed for a certain minimum turn ratio cannot operate correctly on a spiral whose System Turn Ratio is lower.
When the system radius is too small for the selected belt, the inside edge is forced to compress beyond its intended geometry. This may prevent the belt from collapsing correctly and may create excessive internal forces, instability and permanent deformation.
Difference Between System Turn Ratio and Belt Turn Ratio
System Turn Ratio
A property of the spiral conveyor system. It is determined by the drum diameter and belt width.
Belt Turn Ratio
A property of the belt design. It represents the minimum turn ratio the belt can negotiate while maintaining its intended geometry.
The belt turn ratio is mainly determined by the relationship between the expanded pitch and the compressed pitch of the belt.
- Expanded pitch
- Compressed pitch
Common Minimum Turn Ratios
| Minimum Turn Ratio | Common Belt Classification |
|---|---|
| 2.2 | Standard radius spiral belts |
| 2.0 | Standard and medium-radius spiral belts |
| 1.7 | Compact-radius spiral belts |
| 1.6 | Reduced Radius belts |
| 1.0 | Small Radius belts |
| 0.8 | Super Small Radius belts |
As the minimum turn ratio decreases, the belt must accommodate a tighter curve. This requires a belt geometry specifically designed to achieve greater inside-edge compression.
Consequences of Using an Incorrect Turn Ratio
The inside edge may not collapse correctly around the drum.
Links may be forced beyond their intended compressed position.
Incorrect geometry may contribute to poor tracking and unstable belt movement.
Repeated operation under excessive geometric restriction may reduce belt service life.
Excessive internal forces may increase wear or deformation at the belt connections.
Even when the belt continues to run, incorrect turn-ratio compatibility may shorten its expected operating life.
Engineering Recommendation
- Determine the System Turn Ratio from the spiral geometry.
- Confirm the minimum allowable turn ratio of the proposed belt.
- Only use a belt whose minimum turn ratio is equal to or lower than the System Turn Ratio.
- Also evaluate product support, belt weight, open area, fatigue performance, sanitation and operating conditions.
BELTPRO Engineering Calculators
The BELTPRO website includes dedicated calculators for System Turn Ratio and Belt Turn Ratio. These tools can assist with preliminary geometry verification and belt compatibility checks.
This article is intended for general technical guidance and preliminary engineering reference only. Belt selection must be verified against the actual system geometry, operating conditions and the belt manufacturer's specifications.