The Modern Brake Dynamometer
A modern brake test Dynamometer uses an absorption unit, in this case a brake disk and calliper assembly, to dissipate or convert the power or kinetic energy into heat or thermal energy. The power used to do this is calculated as follows:
Horsepower = Torque x RPM / 5252
Remember that RPM is Revolutions per Minute and the unit for Torque in this instance is measured in foot/pounds. 5252 is a constant and is also derived from Watt’s observation.
The above imperial system for units of measure is used to provide the Horsepower formula as it describes the origins of the measure of power best. The SI system or metric system uses, amongst other units, kilowatts (Kw) in place of horsepower and Newton-meter (Nm) for torque.
With a modern dynamometer, such as is used by ECE-90 Brake Testing (Pty) Ltd, the Dynamometer Test Report uses Newton Meter (Nm) and the Friction-coefficient (Mu) as a unit and ratio of measure respectively.
The dynamometer used to generate the test report used in this article is constructed as shown below.
A Dynamometer consists of the following main elements. (See Fig.1 above)
- The drive-train (revolving assembly) consists of the following elements: Motor (1), Interchangeable flywheels (2) and Brake Disk (3). The flywheels and brake disk is matched to the part number to be tested.
- The test bed (mounted in bearings aligned with the drive-train but held in position with the load arm. It has therefor the potential to rotate but is retained by the load arm) The test bed consist of the following elements: Calliper & Adapter (4), Power transfer axle (5), Load Bearing Arm (6) and Loadcell.
- The motor (1) is engaged and accelerated to the required speed and then disengaged to allow the drive-train assembly to run free as a result of the inertia generated by the flywheel (2). It follows that the Brake Disk (3) is spinning with this assembly.
- The brake is then applied, using a brake booster similar as is fitted in road vehicles, which in turn activates the piston in the Calliper (4), causing the brake pads that are being tested to clamp onto the Brake Disk (3). (The brake pressure is measured during this operation in Bar)
- This causes the drive-train to be stopped against the inertia generated by the flywheel (2) The kinetic energy generated by the flywheel is matched to the energy that would be generated by a road vehicle of the brake pad reference being tested. The formula used for determining the kinetic energy is: E = ½mv²
- During the braking operation the Calliper assembly (4), which has the potential to rotate about the same axes as the drive-train, is forced in the same turning direction but is constrained by the Load Arm(5). The Loadcell (6) act as an anchor or pedestal between the Load Arm (5) and the chassis of the dynamometer.
- As a result of the action described in 4 above, the Loadcell is placed under strain which in turn is designed to gauge/measure this strain it is subjected to. The Loadcell then transmits this information, in real time, to the computer that controls the Dynamometer. (This is how torque is measured in Nm)
- Further to the above a rubbing thermocouple is placed against the Brake Disk (3) which measures the disk temperature during the test program. (Measurement is in °C)