Quick answer: belt pull builds along a conveyor from the weight of the product plus the weight of the belt, multiplied by the friction factor between belt and wearstrip, plus an extra term for any lift. Work it out in newtons per metre of belt width and compare it directly against the belt strength figure, for example 11,800 N/m for 5935 flat top in PP or 40,300 N/m for 900 flat top in POM. If your calculated pull is above roughly two thirds of the quoted strength, change something.
Why belt strength is quoted per metre of width
A modular belt is a row of identical modules side by side. Double the width and you double the number of hinge sections carrying the load, so the belt carries twice the total pull. Quoting strength per metre of width lets one number describe every width of that series.
There is a trap in that, and it catches people regularly. Widening a belt does not automatically fix an overload. If your product loading is expressed per square metre of belt surface, a wider belt carries proportionally more product, the pull per metre of width stays the same, and the utilisation does not move at all. Widening only helps when the total throughput is fixed and you are spreading the same mass over more area.
The formula
Belt pull, in newtons per metre of belt width:
F = g × [ f × L × (Wp + 2 × Wb) + H × Wp ]
| Variable | Meaning | Units |
|---|---|---|
| F | Belt pull to compare against belt strength | N/m of belt width |
| g | Gravity, 9.81 | m/s² |
| f | Friction factor between belt and wearstrip | dimensionless |
| L | Conveyor length between shaft centres | m |
| Wp | Product load on the belt | kg per m² of belt surface |
| Wb | Belt weight, confirm for the series and material | kg per m² |
| H | Vertical lift on an inclined section | m |
Two details in that formula are worth explaining. The belt weight appears as 2 × Wb because the belt slides on the carry side and again on the return side, so its own weight is dragged twice. Product weight appears once, since product only rides the carry side. In the lift term the belt weight cancels: what goes up the incline comes back down the return, so only the product is genuinely being raised.
The friction factor is not a constant
This is the variable people want a single number for, and it is the one number nobody should hand out blind. It depends on the pairing of belt material and wearstrip material, and it changes with the running condition. A dry clean run, a wet run, a run with fat or product residue on the wearstrip, and a run with abrasive debris in the same conveyor can all give different values, and wet does not reliably mean lower.
Get the figure from the wearstrip data for your actual belt material and wearstrip material combination, in the condition the line really runs in. If you cannot get it before you need an answer, calculate with a deliberately pessimistic value, size the belt on that, and confirm the number afterwards. The examples below use 0.35 and 0.25 as illustration values only; they are not Veybelt specifications for any pairing.
Worked example
A washdown line, 25 m between shaft centres, with a 3 m lift over that length. Product loading 60 kg/m². Belt weight taken as 8 kg/m² for this example, which must be confirmed for the series and material actually chosen. Friction factor assumed at 0.35 for a wet run.
- Sliding term: 0.35 × 25 × (60 + 2 × 8) = 0.35 × 25 × 76 = 665 kg/m
- Lift term: 3 × 60 = 180 kg/m
- Total: 845 kg/m × 9.81 = 8,290 N/m
On a 900 mm wide belt that is about 7,460 N of total pull at the drive shaft. Now compare it against real belt strengths:
| Candidate belt | Belt strength | Utilisation at 8,290 N/m | Verdict |
|---|---|---|---|
| 5935 flat top, PP | 11,800 N/m | 70% | Too close, redesign |
| 900 flat top, PP | 17,500 N/m | 47% | Workable |
| 986 flat top, PP | 18,056 N/m | 46% | Workable |
| 4809 raised rib, PP | 38,900 N/m | 21% | Comfortable |
| 7705 flat top, POM | 52,800 N/m | 16% | Comfortable |
Our position on where to stop: if the calculated pull sits above about two thirds of the quoted strength, redesign rather than accept it. That is a judgement, not a manufacturer rating. It exists because the calculation is only as good as its inputs, and because start-up, a jam or a partially frozen return path all produce peaks the steady-state formula never sees.
Four ways to fix a number that is too high
| Fix | Effect on the example | Cost of the fix |
|---|---|---|
| Same series in POM instead of PP | 5935 goes from 11,800 to 20,700 N/m, utilisation 70% to 40% | Material cost, and acetal dislikes acidic cleaning |
| Move to a stronger series | 5997 flush grid at 47,500 N/m in PP takes it to 17% | Coarser 57.15 mm pitch, new sprockets and frame changes |
| Split into two conveyors with a second drive | Halving L and H gives 4,145 N/m, 35% on the original 5935 PP belt | A second drive, a transfer point, more floor space |
| Lower friction wearstrip pairing | f from 0.35 to 0.25 gives 6,426 N/m, 54% on 5935 PP | Wearstrip cost, and the figure must be confirmed for the pairing |
Reducing the incline is the fifth option and often the cheapest, since an incline carried on flights and side guards is what puts the lift term into the sum. In the example the lift contributes 180 of the 845 kg/m, so taking the belt flat and lifting the product elsewhere removes 21% of the pull before you have bought anything.
Belt strength by series and material
| Series | Pitch | PP | POM |
|---|---|---|---|
| 5997 flush grid / raised rib | 57.15 mm | 47,500 N/m | Not quoted |
| 400 flush grid / raised rib | 50.8 mm | 39,200 N/m | Not quoted |
| 4809 raised rib | 57.15 mm | 38,900 N/m | Not quoted |
| 936 flush grid / nub top | 38.1 mm | 33,000 N/m | Not quoted |
| 956 | 50.8 mm | 27,000 N/m | Not quoted |
| 966 | 25.4 mm | Not quoted | 26,800 N/m |
| 7705 flat top | 25.4 mm | 23,300 N/m | 52,800 N/m |
| 986 | 25.4 mm | 18,056 N/m | 30,851 N/m |
| OPB | 50.8 mm | 17,600 N/m | Not quoted |
| 900 | 27.2 mm | 17,500 N/m | 40,300 N/m |
| 1000 | 25.4 mm | 16,700 N/m | 39,300 N/m |
| 976 | 25.4 mm | 14,650 N/m | Not quoted |
| 4802 raised rib | 38.1 mm | 13,900 N/m | Not quoted |
| 1100 | 15.24 mm | 12,300 N/m | 31,100 N/m |
| 5935 | 19.05 mm | 11,800 N/m | 20,700 N/m |
| 926 | 12.7 mm | 9,500 N/m | 21,200 N/m |
| 7100 radius | 25.4 mm | 24,200 N/m | 30,900 N/m |
| 300 radius | 46 mm | 18,200 N/m | Not quoted |
| 2400 radius | 25.4 mm | 17,000 N/m | 31,800 N/m |
| 916 radius | 25.4 mm | 14,200 N/m | 14,700 N/m |
Read that table with the pitch column in view. Fine-pitch belts carry less, which is the price of a smooth transfer, and coarse-pitch belts like the 57.15 mm 5997 sit at the top of the range. Radius belts sit lower than their straight-running equivalents because the slots that let the module turn also remove the material that would have carried the pull. Full listings are on the straight running belt page, and the series index lines the specifications up side by side.
Flat top chains do not use this number
A chain is not a belt. Slat and flat top chains are rated as a chain plate load in newtons for the whole chain, not per metre of width: 4,300 N on the 820 straight running chain and 4,420 N on the 880 side flexing chain. Comparing a chain plate load against a belt N/m figure is meaningless. Run the pull calculation the same way, then convert to a total force in newtons and compare against the plate rating. The flat top chain range shows which plate widths pair with which rating.
One thing the formula does not include
Temperature. The quoted strength is a room-temperature figure for the resin, and a belt running hot has less of it. Thermal movement also changes how tightly the belt sits against the frame, and a belt binding on a frame adds drag the calculation never predicted, which surfaces later as tracking and wear complaints. The thermal expansion guide covers that side.
What to send with the enquiry
- Conveyor length between shaft centres, and any lift in metres
- Belt width and line speed
- Product loading, either in kg per m² or as throughput in kg per hour with the product size
- Wearstrip material, and whether the line runs wet, dry or greasy
- Running temperature and cleaning temperature
Send those to the contact form or by WhatsApp and you will get the calculation back with two series that clear it, and what each one gives up. If the belt style is still open, the selection guide comes first.
