A belt scale weighs material while it is still moving. Nothing stops, nothing is batched into a hopper — the system measures what is on the belt and how fast the belt runs, and multiplies the two. Get either measurement wrong and the error goes straight into the recipe.
What it is for
A weigh belt feeder proportions the ingredients going into a process so the mix matches the recipe. Consistent product quality follows from that, and so does lower production cost.
Mechanically it resembles an ordinary conveyor scale. The difference is the drive: the motor speed is variable, so the system can change how much material it delivers. That also raises the accuracy and stability demanded of it — a large error here damages both product quality and machine throughput.
Accuracy you can expect
- Weigh belt feeder / belt scale: ±0.25% to ±0.5%, depending on the model and what the plant requires
- Apron feeder: ±1%
What the system is made of
Indicator, load cells, a variable frequency drive for motor speed, a speed sensor, a belt-misalignment sensor, the motor and the conveyor structure itself.
The control loop
You give the system a set point. It compares the measured rate — the actual value — against that figure. Reading low, it speeds the motor up until the set point is reached; reading high, it slows down.
Because the accuracy demand is high, these systems normally carry two load cells, one each side.
Measuring the belt load
Material passes over a weighing platform set beneath the belt and bounded by two idlers, with one or more of those idlers carrying the load cells. The platform transfers a force into the cells; their output is proportional to the belt load, and that millivolt signal is amplified and digitised in the indicator.
Effective length
On a platform with a single weighing idler, only half the material weight actually bears on it — the white triangle in the diagram shows how the load is distributed. Converting that distributed load into a length is standard practice in weighing:
Leff = Lg ÷ 2
where Leff is the effective length over the weighing platform and Lg the platform length.
Where the platform carries several weighing idlers, the divisor is a coefficient other than ½.
Belt speed
The second measurement needed for throughput is belt speed v. It is taken either directly with a belt speed sensor, or from motor speed and then converted.
Where the belt load is constant and high accuracy is not required, speed measurement can be omitted altogether.
Throughput
Belt load in kgf per metre:
Q = QB ÷ Leff
Throughput follows from load and speed together:
I = Q × v = (QB × v) ÷ Leff
| Symbol | Quantity | Unit |
| I | Throughput | kgf/s |
| Q | Belt load | kgf/m |
| QB | Load on the weighing platform | kgf |
| v | Belt speed | m/s |
| Leff | Effective length | m |
| Lg | Weighing platform length | m |
For throughput in kgf per hour, multiply by 3600: I (kgf/h) = Q × v × 3600.
Where belt scales lose accuracy
Belt tension, material build-up on the idlers and belt misalignment all change the force reaching the cells without any change in the material being carried. That is why the misalignment sensor is part of the system rather than an accessory, and why regular zero checks on an empty running belt matter more here than on a static scale.
Application note
KALA Technical Notes
The accuracy a belt scale can hold depends as much on the conveyor structure as on the cells — idler alignment, belt tension and the length of straight run either side of the weighing platform. Send us the conveyor drawing and the throughput you need, and we will say what accuracy is realistic on it.
● Belt scales, weigh feeders and in-motion checkweighing systems.