BIZOMAT MAKEOS · Mechanics
Punch and Shear Flywheel Sizing
What motor power and flywheel rim does this punch or shear need for its energy per stroke?
Simplified flywheel design for punches and shears: average motor power from energy per stroke and stroking rate, energy the flywheel must give up during the cut, rim mass for the permitted speed reduction, rim section for a depth-to-width ratio, and the rim hoop stress from its speed.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Energy per stroke from | choice | Whether to work out the energy from force and travel, or enter it. |
| Peak force | kN | Peak force during the cut. |
| Working travel (e.g. plate thickness) | mm | Distance over which the force acts. |
| Energy per stroke (entered) | kJ | Energy needed per stroke. |
| Strokes per minute | Strokes per minute. | |
| Time of the actual cut | s | Duration of the working part of the stroke. |
| Mean rim diameter | m | Mean diameter of the flywheel rim. |
| Flywheel speed | rpm | Flywheel speed before the cut. |
| Speed reduction during the cut | % | Permitted fall in speed during the cut. |
| Rim material density | kg/m³ | Density of the rim material (cast iron about 7200 kg/m³). |
| Rim depth ÷ width | Ratio of radial depth to axial width of the rim section. |
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: Energy per stroke from
- What it is
- Whether the energy per stroke is worked out from the peak force and working travel, or entered directly from your own figures.
- Units and format
- Choose one of 2 options from the list: Peak force × working travel; Entered energy.
- Example
- The form opens with “Peak force × working travel” selected as a worked example. Choose the option that matches your case.
- Get it right
- Force and travel use the force-times-distance estimate; enter energy if you have a measured or supplier figure.
What this means: Peak force
- What it is
- The peak force the punch or shear exerts during the cut, used with the working travel to estimate the energy per stroke.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1601.4 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Energy per stroke, motor power and rim mass are all proportional to it.
- Get it right
- Use the force at the tool, not the press’s rated capacity. Not used when energy is entered.
What this means: Working travel (e.g. plate thickness)
- What it is
- The distance over which the peak force acts during the cut, such as the plate thickness for shearing or punching.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Energy per stroke, motor power and rim mass are all proportional to it.
- Get it right
- Use the distance under load, not the full stroke of the ram.
What this means: Energy per stroke (entered)
- What it is
- The energy needed for one working stroke, used when you choose to enter it rather than derive it from force and travel.
- Units and format
- Enter a number in kJ: kilojoules. The form accepts 0 kJ to 1000000 kJ. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 kJ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Motor power and rim mass are directly proportional to it.
- Get it right
- This is in kilojoules; ignored when energy comes from force and travel.
What this means: Strokes per minute
- What it is
- The number of working strokes the machine makes each minute, which sets the stroke time and the average motor power.
- Units and format
- Enter a plain number with no unit. The form accepts 0.000001 to 1000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More strokes per minute raise the average motor power and shorten the stroke time.
- Get it right
- Use continuous running rate; single-stroke work gives more recovery time.
What this means: Time of the actual cut
- What it is
- The time taken by the working part of the stroke, while the tool is actually cutting. The flywheel supplies the energy the motor cannot deliver in this time.
- Units and format
- Enter a number in s: seconds. The form accepts 0 s to 600 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.75 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A shorter cut time makes the flywheel supply more of the energy, so the rim gets heavier.
- Get it right
- This is in seconds and must be shorter than one full stroke.
What this means: Mean rim diameter
- What it is
- The mean diameter of the flywheel rim, measured to the middle of the rim section. It sets the rim speed.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 20 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.9812 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger diameter raises rim speed, so the rim mass falls with roughly the square of the diameter.
- Get it right
- Use the mean rim diameter, not the outside diameter of the flywheel; it is in metres.
What this means: Flywheel speed
- What it is
- The speed of the flywheel just before the cut starts, at full running speed.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 10000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher speed lowers the rim mass with the square of speed but raises rim hoop stress with it.
- Get it right
- Use the flywheel shaft speed, not the motor or crank speed if they differ.
What this means: Speed reduction during the cut
- What it is
- The permitted fall in flywheel speed during the cut, as a share of the running speed, which sets how much energy the rim gives up.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.5% to 50%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger permitted slowdown lets the rim be lighter.
- Get it right
- Larger slowdowns need slip in the motor or a slip coupling; choose with the motor supplier.
What this means: Rim material density
- What it is
- The density of the flywheel rim material, used to turn the rim mass into a section size and to find the rim hoop stress.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 20000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7200 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Rim hoop stress is directly proportional to density.
- Get it right
- Take it from the material supplier; it does not change the rim mass, only its size and stress.
What this means: Rim depth ÷ width
- What it is
- The shape of the rim cross-section: its radial depth divided by its axial width. The tool uses it to split the rim area into a width and depth.
- Units and format
- Enter a plain number with no unit. The form accepts 0.2 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.22 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Type it as a ratio of depth to width; it changes only the section dimensions, not the mass.
Method and formulas
- Energy per stroke E = F × travel; average power = E × strokes per minute ÷ 60.
- Energy from the flywheel E₁ = E (1 − T₁ ÷ T), with T the stroke time.
- Rim mass m = 2 E₁ ÷ (v₁² − v₂²), with v the rim speed at the mean diameter and v₂ = v₁ (1 − reduction) — the physics behind the handbook constants C.
- Rim hoop stress σ = ρ v² (the handbook’s s = V² ÷ 10 for cast iron in ft/s and psi).
- Example: the handbook hot-slab shear (120,000 ft-lb per stroke at 20 strokes per minute, 6 ft 6 in. rim at 200 rpm) needs about 2,984.1 kg of rim (handbook 6,570 lb).
Limitations
- Rim only; arms and hub add some inertia (a margin).
- Hoop stress ignores rim bending between arms, which can be considerably higher.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a flywheel for a mechanical press, punch or shear
- Check motor power for a stroking rate
- Check flywheel rim speed and stress
Who it is for
- Machine designers
- Press maintenance engineers
- Mechanical engineers
What you get
- Average motor power
- Energy from the flywheel and rim mass
- Rim section, rim speed and hoop stress
Interpreting the result
- Allowing a larger speed reduction shrinks the flywheel but slows the stroke and loads the motor harder.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3004–3006. 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
Flywheel energy and slowdown
In Power Presses by the Numbers
What it covers: Energy per stroke for MP-40 and AB-75, the flywheel's stored energy and slowdown, why a variable-speed press has far less energy at low speed, motor power, and a press that stalled on the bottom.
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