BIZOMAT MAKEOS · Mechanics
Brake Dynamometer Test and Cooling Plan (Power, Water Flow, Water Use)
How much power is my prony brake or absorption dynamometer taking, how much cooling water does it need for the temperature rise I allow, and do my brake rating and water supply cover it?
Plan for a power test with a prony brake or absorption dynamometer: net scale force, torque and power absorbed from the arm length, scale readings and speed (as the prony brake tool), the share of the power carried away by the cooling water, the water flow for your allowed temperature rise (as the coolant flow for a heat load tool), the water used over the test, and checks against the brake rating and the water supply.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Brake arm length | m | Distance from the shaft centre to the scale load point. |
| Scale reading with the brake absorbing power | N | Scale reading while the brake absorbs power. |
| Zero reading (frictionless) | N | Scale reading with the brake slack (the arm’s own weight). |
| Shaft speed | rpm | Shaft speed during the reading. |
| Cooling | ||
| Share of the power removed by the water | % | Share of the absorbed power taken away by the cooling water (the rest goes to air and the structure). |
| Allowed water temperature rise | K | Largest rise in water temperature through the brake. |
| Water specific heat | kJ/(kg·K) | Specific heat of the water. |
| Water density | kg/m³ | Density of the water. |
| Test duration | min | Length of the test run. |
| Limits | ||
| Brake power rating | kW | Largest power the brake can absorb continuously. |
| Water supply available | L/min | Water flow the supply can give. |
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 length of the brake arm: the horizontal distance from the shaft centre to the point where the arm presses on the scale.
- 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 to the arm length.
- Get it right
- Measure square to the line of the scale force, from the shaft centre, not from the brake drum.
What this means: Scale reading with the brake absorbing power
- What it is
- The scale reading while the brake is loaded and the engine or motor is running at the test speed and steady.
- 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
- Net force is this reading minus the zero reading.
- Get it right
- Wait for a steady reading; it must be larger than the zero reading.
What this means: Zero reading (frictionless)
- What it is
- The scale reading with the brake slack and the shaft free, which is the weight of the arm resting on the scale.
- 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
- Take it with the brake released, not with the scale empty.
What this means: Shaft speed
- What it is
- The shaft speed at the moment of the scale reading, from a tachometer.
- 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 is proportional to speed for a given torque.
Cooling
What this means: Share of the power removed by the water
- What it is
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Water flow needed rises in proportion to it.
- Get it right
- Ask the brake maker or estimate from a measured water flow and temperature rise.
What this means: Allowed water temperature rise
- What it is
- The largest rise you allow in water temperature between the brake inlet and outlet.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts 0.000001 K to 100 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Doubling the allowed rise halves the water flow needed.
- Get it right
- A temperature difference, not the outlet temperature.
What this means: Water specific heat
- What it is
- The specific heat capacity of the cooling water, the energy needed to warm one kilogram by one kelvin.
- Units and format
- Enter a number in kJ/(kg·K): kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/(kg·K) to 10 kJ/(kg·K). Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4.18 kJ/(kg·K) as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Water flow needed is inversely proportional to it.
- Get it right
- This is in kilojoules, not joules, per kilogram per kelvin.
What this means: Water density
- What it is
- The density of the cooling water, used to turn the mass flow of water into litres per minute.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 2000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 998 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Test duration
- What it is
- The total length of the test run during which the brake absorbs power and uses water.
- Units and format
- Enter a number in min: minutes. The form accepts 0.000001 min to 100000 min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Water used is the flow times this duration.
- Get it right
- Enter the duration in minutes, not hours or seconds: 1.5 hours is 90 minutes.
Limits
What this means: Brake power rating
- What it is
- The largest power the brake can absorb continuously, from its maker’s rating.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0.000001 kW to 1000000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the continuous rating at the test speed if the maker gives one.
What this means: Water supply available
- What it is
- The water flow the supply can deliver to the brake at the test site, measured with a flow meter or bucket and stopwatch.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 1000000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Net force = reading − zero; torque = force × arm; power = 2π n T ÷ 60 (as the prony brake tool).
- Heat to water = power × share; flow = heat ÷ (ρ c_p ΔT) (as the coolant flow for a heat load tool).
- Water used = flow × test duration.
- Example: the brake absorbs 12.566 kW at 200 N·m; it needs 16.267 L/min of water and 488 L for the test.
Limitations
- Steady readings; scale damping is needed for a stable reading.
- Water share depends on the brake design; air-cooled brakes need a different heat path.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Plan an engine or motor power test
- Size the water supply for a water-cooled brake
- Check a dynamometer is big enough
Who it is for
- Test engineers
- Maintenance engineers
- Technical college staff
What you get
- Torque and power absorbed
- Cooling water flow and test water use
- Checks against rating and supply
Interpreting the result
- If the supply is short, allow a larger temperature rise within the brake maker’s limit or recirculate through a cooler.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2998–2999, 371–374. Topic reference; no source tables or recommended settings are reproduced.
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Open the Brake Dynamometer Test and Cooling Plan (Power, Water Flow, Water Use) calculator