The distance between idlers drives belt sag, sealing quality, power draw, and belt life. Pick it well, and you reduce spillage and downtime; pick it poorly, and you chase dust clouds, edge wear, and mis-tracking. This guide summarizes practical, CEMA‑aligned recommendations for conveyor idler spacing, why they work, and how to justify your choices with a simple sag‑based method. It’s written for maintenance, reliability, and design engineers in bulk‑material plants.
Standards and sources used here reference CEMA practice via accessible materials from Martin Engineering’s Foundations knowledge base and PPI manufacturer guides. Where exact CEMA tables are proprietary, we cite public, standards‑aligned guidance so you can defend decisions internally.
Quick Reference — Idler Spacing Chart by Zone (mm/in)
Below are widely used bands consistent with CEMA‑aligned practice. Choose within these ranges using the principles in the next section.
| Conveyor section | Typical spacing band |
|---|---|
| Carrying/troughing (outside loading) | 900–1,200 mm (36–48 in); tighten to 600–900 mm (24–36 in) for stricter sag/sealing control |
| Impact/loading (under chute) | ~300 mm (12 in) centers for heavy impact and sealing; some designs use 450–750 mm (18–30 in) beyond primary impact |
| Graduated spacing under skirts | Start ~300 mm (12 in), then 450–750 mm (18–30 in), transitioning to 900–1,200 mm (36–48 in) after skirts |
| Return run | ~2.4–3.0 m (8–10 ft) |
| Transition approach to pulley | Closer spacing near pulleys and first full‑trough idler; commonly 300–600 mm (12–24 in) increasing outward |
| Training/belt trainers | Spaced ~31–46 m (100–150 ft); keep ≥15 m (50 ft) from terminal/bend pulleys |
Guidance sources include Martin Foundations’ overview of belt conveyor idler spacing and PPI’s Idler Selection Guide, which note common values like 4 ft carrying spacing, 1 ft impact spacing, and 8–10 ft return spacing. See: Martin’s Belt Conveyor Idler Spacing and PPI’s Idler Selection Guide (IDL‑012).
Principles Behind Spacing — Sag, Trough Angle, Load, and Sealing
Idler spacing is set to keep belt sag within limits so material stays inside the belt edges and skirt seals remain tight. Think of sag as the “smile” the belt makes between idlers: too much smile, and your edges leak.
-
Allowable sag ratios: For carrying runs outside loading, many designers work toward ~1–2% of span. Under skirts and heavy impact, aim at ~≤1% of span, assisted by impact beds or slider cradles where needed. Martin Foundations summarizes these sag targets in its idler spacing overview.
-
Trough angle and belt width: A practical rule for maximum spacing ties to belt width and trough angle. Manufacturer instructions aligned with CEMA note examples such as 20° trough → max spacing ≈ belt width; 35° → ≈1.5× belt width; 45° → ≈2× belt width. PPI documents this relationship in Belt Conveyor Idler Instructions.
-
Load and tension: Heavier material and lower belt tension increase sag; tighten spacing or raise tension (within belt limits). Confirm idler class ratings against calculated loads; PPI’s Idler Selection Guide provides formulas to check carrying idler load versus class.
-
Speed, lump size, and sealing: Higher speeds and large lumps in the load zone warrant closer spacing for control and sealing quality. Martin Engineering’s dust/spillage guidance points to favoring tighter pitches under skirts; see What is the Low‑Hanging Fruit of Reducing Dust and Spillage.
Practical Guidance by Section of the Conveyor
-
Carrying/troughing run: Start at 900–1,200 mm (36–48 in) for many plant conveyors and tighten toward 600–900 mm (24–36 in) when sag or sealing standards demand. Verify using your sag target and idler load check.
-
Impact and skirted loading zone: Use ~300 mm (12 in) impact idler spacing under the chute for heavy impact and seal support. Then graduate spacing: 450–750 mm (18–30 in) further under the skirts, easing to 900–1,200 mm (36–48 in) after skirts end. Where sealing is critical, near‑continuous support can be achieved by positioning roll faces so adjacent ends are within ~25 mm (1 in) in the skirt area, as noted by Martin Foundations.
-
Return run: Space idlers at ~3.0 m (10 ft) where practical; use ~2.4 m (8 ft) if belt is light, speed high, or devices (trainers, cleaners) require more support.
-
Transition areas and trainers: Keep loading off transition idlers; begin material loading only at the first full‑trough idler to avoid edge stress. Place self‑aligning trainers no closer than ~15 m (50 ft) from terminal or bend pulleys, and avoid transition/high‑tension zones. PPI’s Pro Tracker instructions summarize these placements.
Neutral micro‑example (impact zone): Disclosure: BisonConvey is our product. On a quarry conveyor with early dust issues, tightening the impact zone pitch from 450 mm (18 in) to 300 mm (12 in) and using heavy‑duty impact idlers (e.g., ceramic‑lagged or UHMWPE‑lined roll faces) improved skirt sealing and reduced carryback under the chute. The decision followed the sag target ≤1% and stayed within idler class ratings; spacing then graduated to 600 mm (24 in) under the remaining skirts and 1,000 mm (39 in) beyond.
Worked Example — From Inputs to a Defensible Spacing Choice
Let’s walk a representative case and make choices you can defend in a design review.
Inputs:
- Belt width B = 1,200 mm (47.2 in); trough angle = 35°; belt speed v = 3 m/s; design capacity Q = 800 t/h of ore (ρ ≈ 1.6 t/m³); belt weight Wb and load distribution factor K1 from manufacturer tables; target sag ratio δ/S: carrying 1.5%, skirted 1%.
Steps:
- Confirm belt width and trough angle support capacity. Using CEMA capacity methodology (often applying ~85% of theoretical cross‑section for surge allowance), a 1,200 mm belt at 35° readily carries 800 t/h at 3 m/s; see Martin’s design discussion in World Cement, 2023%20March,%202023.01.pdf).
- Pick target sag ratios. Carrying run: ~1.5% of span. Skirted load zone: ≤1%.
- Select carrying spacing from band and check against width/angle rule. For 35°, a practical upper bound is ~1.5×B → ~1,800 mm (71 in). To keep sag modest and alignment robust, select initial S = 1,000 mm (39 in) in the main carrying run.
- Compute idler load check using manufacturer formula (per PPI): CIL ≈ 1.5 × ((Wb + Wm × K1) × S). Insert Wb (belt N/m), Wm (material N/m), K1 from tables, and S in ft; compare CIL to CEMA class rating for your idler. If CIL approaches rating, reduce S or select a higher class. See PPI’s Idler Selection Guide.
- Set impact zone pitch to meet skirted sag target ≤1%. Choose 300 mm (12 in) under the chute for heavy impact and sealing. Beyond primary impact, graduate to 600 mm (24 in) under remaining skirts.
- Transition out of skirts to standard spacing. After skirts end, increase to S = 1,000 mm (39 in). Maintain consistent alignment and verify sag under normal load.
- Return run spacing. Select 2.4–3.0 m (8–10 ft). For this belt, use 3.0 m (10 ft) to simplify maintenance, unless belt flutter suggests tighter spacing.
- Finalize and document. Record selections: impact 300 mm (12 in); skirted 600 mm (24 in); carrying 1,000 mm (39 in); return 3,000 mm (118 in). Attach calculations, idler class checks, and a sketch of graduated spacing for review.
Troubleshooting — Symptoms That Point to Spacing Issues
- Excessive belt sag between carrying idlers: tighten spacing toward 600–900 mm (24–36 in), raise belt tension within limits, or add slider/impact support under skirts; recheck idler class ratings.
- Edge spillage or poor sealing under skirts: confirm skirtboard distance vs. troughed belt width, align idlers, and use 300–600 mm (12–24 in) pitches in the sealed area before graduating outward.
- Tracking instability downstream: verify idler alignment, clean frozen rolls, add training idlers at 31–46 m (100–150 ft) intervals, and keep ≥15 m (50 ft) from pulleys; avoid installing trainers in transitions.
- High power draw or unusual noise: excessive sag can increase drag; re‑evaluate spacing and tension, check for damaged or undersized idler classes.
Site Survey — What to Capture Before You Change Spacing
- Belt width and trough angle; belt speed; design capacity; material bulk density and dominant lump size.
- Existing idler spacing by zone; measured belt sag under typical load; idler class ratings and condition.
- Loading zone geometry: chute drop height, skirtboard distance, sealing system, and the first full‑trough idler location.
- Transition lengths; presence and placement of trainers; photos of alignment and any dust/spillage patterns.
- Maintenance constraints: access points, isolation procedures, and safe work areas.
References and Further Reading
- Martin Foundations — overview of carrying vs. return spacing, sag targets, and load‑zone practices: Belt Conveyor Idler Spacing (2024–2025 updates).
- PPI — common spacing values, idler load calculation, and installation notes: Idler Selection Guide (IDL‑012) and Belt Conveyor Idler Instructions.
- Martin Engineering — practical context on transitions and loading discipline: Ten Common Mistakes in Conveyor Specification & Design and Conveyor Belt Transition Distance.
- Return trainer placement detail: PPI Pro Tracker Instructions.
By: Senior Conveyor Application Engineer
