Why Press Selection Starts With a Calculation, Not a Catalogue
Many factory owners make the same mistake when sourcing a hydraulic press from China: they ask the supplier for "a 100-ton press" before calculating the force their part actually requires. The outcome is one of two costly scenarios. Either the press is undersized and cannot complete the stroke, leaving distorted edges and dies that wear out prematurely, or it is far larger than needed, inflating the purchase price, freight cost and power consumption with no gain in output. Required force is not a guess — it is a number you can calculate with reasonable accuracy before you send the first request for quotation.
Calculating Required Force by Operation Type
Shearing and Punching
For shearing and punching, force is derived from the cut perimeter, sheet thickness and the shear strength of the metal:
- Force (tons) = cut perimeter (mm) × thickness (mm) × shear strength (kg/mm²) ÷ 1000
- Approximate shear strength values: mild steel 35, stainless steel 55, aluminium 14, copper 22 (kg/mm²).
- Worked example: punching an 80 mm diameter hole in 3 mm mild steel. Perimeter ≈ 251 mm, so force = 251 × 3 × 35 ÷ 1000 ≈ 26.4 tons.
Add a safety margin of at least 25% on top of the result, because die sharpness degrades with use and raises the force needed, and because actual metal batches often differ in hardness from the paper certificate.
Deep Drawing and Forming
Deep drawing is more demanding because it requires both drawing force and blank-holding force. The practical rule is that blank-holder force equals 30% to 40% of the drawing force, and the press must deliver it through an independent hydraulic cushion beneath the bolster. State the cushion force and cushion stroke explicitly in your enquiry — many cheap quotations omit this point, and buyers only discover later that no cushion was ever fitted.
Pressing, Assembly and Compaction
For bearing and bushing insertion or powder compaction the force is usually lower, but position and pressure control matter far more. Here the control system outweighs raw tonnage: you need a pressure sensor and a position sensor with a recordable force/distance curve to guarantee quality.
Hydraulic, Mechanical or Servo?
| Criterion | Hydraulic | Mechanical (crank) | Servo |
|---|---|---|---|
| Full force available | Throughout the stroke | Only near bottom dead centre | Available and programmable |
| Strokes per minute | Low to medium | High | Medium to high |
| Speed control | Variable | Fixed | Fully programmable |
| Energy consumption | Relatively high | Medium | Lowest |
| Best suited to | Deep drawing and heavy forming | High-volume simple parts | Precision parts and mixed products |
The practical takeaway: if your product needs a long stroke and constant force, hydraulic is the answer. If the goal is thousands of simple parts per day at the shortest cycle time, a mechanical press is the more economical fit. A servo press combines the advantages but raises the initial investment by roughly 40% to 70%, and is only justified by a varied product mix or a high electricity bill.
Specifications You Must Not Leave Out of the Enquiry
- Full stroke and daylight between bolster and ram — together they cap the maximum die height you can install.
- Actual bolster dimensions and T-slot pattern, checked against your existing dies.
- Approach, working and return speeds in mm/second, since these — not tonnage — determine real cycle time.
- System working pressure in bar, pump rating in kW, and the recommended hydraulic oil grade.
- Safety package: light curtain, two-hand control, dual safety valve, and CE compliance if your market requires it.
- Supply voltage and frequency matched to your plant grid before shipment, not after.
Common and Expensive Mistakes
- Trusting the tonnage in the model name without reading the force curve along the stroke.
- Overlooking die height when calculating daylight, so the press arrives and the die will not fit inside it.
- Skipping a factory acceptance test (FAT) run on a real part from your own production, in your own metal.
- Ignoring oil cooling — oil temperature above 60°C reduces efficiency and accelerates seal failure.
- Ordering the minimum spare parts to save a small amount, then halting production for weeks waiting on a seal or a valve.
The golden rule: calculate the force required for the hardest part on your line, add a 25% safety margin, then buy the press that matches that number — not the larger one the supplier suggests, nor the smaller one that saves on price.
The Next Step Before You Buy
Before releasing any down payment, prepare a technical drawing of your most demanding part, specify the metal, its thickness and your target parts per hour, then ask the supplier to state the force calculation in writing within the quotation. A serious supplier documents that calculation and accepts tying it to a contract clause; an unserious one offers a generic line about "high quality". If you want a press specification reviewed before signing, or quotations from several Chinese manufacturers compared side by side, contact our team for an independent technical assessment of your project.