A band brake is an engineering device for loading a rotating shaft and measuring the torque applied to it. It is mainly used to determine brake horsepower. In its simplest form an engine is connected to a rotating drum by means of an output shaft. A friction band is wrapped around half the drum's circumference and each end attached to a separate spring balance. A substantial pre-load is then applied to the ends of the band, so that each spring balance has an initial and identical reading.
When the engine is starting the frictional force between the drum and the band will increase the force reading on one balance and decrease it on the other. The difference between the two readings is used to calculate torque, because the radius of the driven drum is known. If the engine speed is measured with a tachometer, the brake horsepower is easily calculated.
The experiment must be performed for many engine speeds to obtain power and torque curves for the engine, since there is a non-linear relationship between torque and engine speed for most types of engine.