Prony brake

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.

If the engine speed is measured with a tachometer, the brake horsepower is easily calculated.

The torque is then related to the lever length, shaft diameter and measured force.