BIZOMAT MAKEOS · Mechanics
Prony Brake and Absorption Dynamometer Power
How much power is the machine delivering according to the brake arm, scale reading and speed?
Torque and power absorbed by a Prony brake or water brake from the brake arm length, the scale reading under load, the zero reading and the shaft speed, with the heat that must be carried away.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Brake arm length | m | Horizontal distance from the shaft axis to the knife edge on the scale. |
| Scale reading with the brake absorbing power | N | Scale reading while the brake absorbs power. |
| Zero reading (frictionless) | N | Scale reading with friction removed (average of slow forward and backward readings). |
| Shaft speed | rpm | Speed of the braked pulley. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Brake arm length
- What it is
- The horizontal distance from the shaft axis to the point where the brake arm presses on the scale (the knife edge), measured at right angles to the scale force.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 100 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque and power rise in proportion for the same scale reading.
What this means: Scale reading with the brake absorbing power
- What it is
- The force shown on the scale while the brake is loaded and absorbing power at the test speed, before the zero reading is taken off.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 100000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 250 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Power rises in proportion to this reading minus the zero reading.
- Get it right
- Take the reading only once the speed and the scale have settled.
What this means: Zero reading (frictionless)
- What it is
- The scale reading from the weight of the arm and brake alone with no friction acting, found by averaging readings taken while the pulley is turned slowly forward and backward.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 100000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be lower than the loaded reading; the difference is the net brake force.
What this means: Shaft speed
- What it is
- The rotational speed of the braked pulley or shaft while the scale reading is taken.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 1000000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Power rises in proportion; the torque does not depend on speed.
Method and formulas
- Brake reading W = tare − zero reading.
- Torque = W L; power = 2π L n W ÷ 60 (the handbook’s H.P. = 2π L N W ÷ 33,000 in feet, rpm and pounds).
- All absorbed power becomes heat in the brake.
- Example: 200 N net on a 1 m arm at 600 rpm is 12.566 kW.
Limitations
- Steady running; readings fluctuate with friction changes.
- Water brakes need continuous water flow to remove the heat.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Measure the output power of a motor or engine on test
- Check a gearbox’s delivered torque
- Size cooling for a brake test rig
Who it is for
- Test engineers
- Maintenance engineers
- Engineering students
What you get
- Brake reading and torque
- Power absorbed
- Heat to remove
Interpreting the result
- Take readings at several loads to plot the machine’s torque and power curve.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2998–2999. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Friction linings and measuring torque
In Clutches and Brakes by the Numbers
What it covers: How friction linings behave, why a design uses a low friction coefficient for a brake and a high one for the parts behind a clutch, and how to measure torque with a brake arm.
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Open the Prony Brake and Absorption Dynamometer Power calculator