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ENGINEERING TOOLS

CONVEYOR BELT SAG CALCULATOR

Belt sag and sag percentage between idlers

Calculate the maximum sag of a loaded conveyor belt between two carrying idlers, plus the sag-to-spacing percentage that CEMA and DIN 22101 use as the design check. Compare the result against industry-standard limits with a built-in PASS / CAUTION / EXCESSIVE verdict.

Units

Material flow

Belt geometry

Sag check
Belt sag
16.0
mm
0.63 in
Sag percentage
1.33 %
Verdict: CAUTION

Above 1 % — acceptable for short / lightly loaded belts but review at loading zones and skirts.

Formulas
  • Wm = Q × 1000 / 3600 / v (material mass per length, kg/m)
  • s = (Wb + Wm) · g · a² / (8 · T)
  • Sag % = s / a × 100

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How belt sag is computed

A loaded belt suspended between two carrying idlers behaves like a uniformly loaded cable. The maximum deflection at the midpoint depends on the load (belt mass + material mass), the idler spacing, and the belt tension at that location. CEMA and DIN 22101 both use the textbook formula s = w·a² / (8·T), where w is the running load per metre and T is the local belt tension.

Engineers express the result as a percentage of the idler spacing — sag % = s/a × 100 — because the absolute sag value scales with the conveyor design. CEMA recommends keeping sag under 1 % for general operation and under 0.5 % at loading zones to prevent material spillage and skirt-seal failures.

Belt tension varies along the belt. Use the lowest tension on the carry side (typically T2 just downstream of the drive) for a worst-case check. For inclined conveyors the local tension also drops with height, so check sag at the highest point of the carry side.

Recommended maximum sag by application

Engineering practice limits vary with conveyor type, material, and operating speed. Use these as starting points and tighten where the consequence of spillage is high.

ApplicationMax sag %Notes
General-purpose conveyor1.0 %CEMA / DIN baseline limit
Loading zone / impact area0.5 %Reduce material disturbance and skirt-seal failure
High-speed conveyor (v > 4 m/s)0.75 %Avoids material chatter and dust generation
Fragile / sensitive material0.5 %Grain, coke, friable ores
Short or lightly loaded conveyor2.0 %Acceptable when spillage risk is low
Inclined conveyor (> 15°)1.0 %Standard limit; verify against rollback
Belt with continuous skirts0.75 %Maintain skirt seal at the loading area

Common pitfalls

  • Checking sag only at full design load. Sag is worst at low speed / partial load combined with high spacing, because Wm shrinks but Wb stays the same and T2 may drop on a stopped belt — design for both states.
  • Using T1 (tight side) instead of T2 in the formula. T1 occurs at the drive head and is the highest tension; sag is set by the LOWEST tension on the carry side, typically T2.
  • Forgetting to tighten idler spacing in the loading zone. Even a well-designed belt sags excessively under impact loading if carrying idlers are spaced as for the rest of the conveyor.
  • Increasing belt tension to fix sag without checking belt rating. Higher T raises wear, drum stress, and belt class requirement — sometimes adding an extra idler is cheaper.
  • Mixing tension units. T must be in newtons when Wb, Wm in kg/m, a in m, and g in m/s²; double-check before reading the sag value.

When to consult an engineer

This calculator gives a steady-state geometric sag check. Real installations also need to consider startup transients, partial-load conditions, return-side sag (lighter belt, higher idler spacing), and combined sag at vertical curves. For new conveyor design, idler-spacing optimisation, or sag complaints on existing belts, talk to a BisonConvey engineer for a full review.

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