BIZOMAT MAKEOS · Fabrication
Mechanical (Crank) Press Capacity Above Bottom
How much force can this crank press deliver at the height where my operation happens, and is it enough?
Force available from a crank press at any height above bottom dead centre: the slider-crank geometry gives the slide position against crank angle, the rated crankshaft torque follows from the rated force at the rating point, and the available force at the working height is that torque divided by the slide’s mechanical advantage there (capped at the frame rating), with the energy of the working stroke.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Rated force | kN | Nominal tonnage of the press. |
| Rating point above bottom | mm | Height above bottom dead centre at which the nominal tonnage is rated (often 3–13 mm). |
| Stroke | mm | Slide stroke (twice the crank radius). |
| Connecting rod length | mm | Centre distance of the connecting rod. |
| Force needed by the operation | kN | Force the die needs. |
| Height above bottom where the force starts | mm | Height above bottom where the die starts working (for blanking, the material thickness above bottom; for drawing, the draw depth). |
| Working distance (until bottom) | mm | Distance over which the force acts. |
| Flywheel energy available per stroke | kJ | Energy the flywheel can give per stroke without slowing too much (from the press maker). |
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: Rated force
- What it is
- Nominal capacity of the press as a force. The press can deliver it only at or below the rating point above bottom dead centre.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Kilonewtons, not tonnes: 1 tonne-force ≈ 9.81 kN.
What this means: Rating point above bottom
- What it is
- Height of the slide above bottom dead centre at which the maker rates the nominal force, from the press data.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Stroke
- What it is
- Total travel of the slide from top to bottom dead centre, which is twice the crank radius.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Connecting rod length
- What it is
- Centre distance between the crank pin and the wrist pin of the connecting rod (pitman).
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be longer than half the stroke.
What this means: Force needed by the operation
- What it is
- Force the die operation needs, such as the blanking, piercing or forming force.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 800 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Height above bottom where the force starts
- What it is
- Height of the slide above bottom dead centre at the moment the die starts working.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from bottom dead centre, not from the top of the stroke.
What this means: Working distance (until bottom)
- What it is
- Distance the slide travels under load, from the start of work down to bottom, used for the working energy.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Working energy is force × this distance.
What this means: Flywheel energy available per stroke
- What it is
- Energy the flywheel can give up per stroke without slowing too much, from the press maker.
- Units and format
- Enter a number in kJ: kilojoules. The form accepts 0 kJ to 100000 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.
- Get it right
- 0 = energy check skipped.
Method and formulas
- Slide height above bottom h(θ) = r (1 − cos θ) + L − √(L² − r² sin² θ), with r = stroke ÷ 2 and θ from bottom.
- dh/dθ = r sin θ [1 + r cos θ ÷ √(L² − r² sin² θ)]; rated torque T = F_rated × dh/dθ at the rating point.
- Force available at height h = T ÷ dh/dθ there, capped at the rated force.
- Working energy ≈ force × working distance.
- Example: a 1000 kN press rated 6 mm above bottom gives only 581.61 kN at 20 mm.
Limitations
- Crank (eccentric) drives; link and knuckle presses have different curves.
- Press makers publish the official capacity curve; use it for final selection.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check whether a press can blank or draw high in the stroke
- Choose between presses with different rating points
- Explain why a press stalls on deep draws
Who it is for
- Press shop engineers
- Toolmakers
- Manufacturing engineers
What you get
- Crank angle at the working height
- Force available there
- Working energy against flywheel energy
Interpreting the result
- Blanking happens near the bottom; drawing starts high in the stroke where capacity falls away quickly.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1386–1392, 1342. 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
The crank press and its force curve
In Power Presses by the Numbers
What it covers: Why a crank press loses force with height above the bottom of the stroke, the rating point, Press 4's curve from 400 kN at 3 mm to 184 kN at 25 mm, and a pad it could not carry.
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Open the Mechanical (Crank) Press Capacity Above Bottom calculator