Machine Design Case Study: the 80 kN Press Frame
Frame stress and deflection, weld fatigue life, safety factors, standards and a design checklist for a complete machine.
The brief
Machine design joins every element of this guide, and the rest of the site, into one object that has to work, last, be safe and be made at a cost. This page designs the frame of the 80 kN hydraulic press that has run through the guides (the cylinder and circuit of Hydraulics and Pneumatics, the motor of Electrical Power, the rod and weld of Materials and Processes). The requirement: a C-frame press that pushes 80 kN with a 400 mm stroke, throat depth 300 mm, working 360 strokes an hour for 4 000 hours a year for 15 years (about 22 million cycles), in a workshop, to be built from S355 welded tube.
Loads and the load path
Start by tracing how the force travels: the cylinder pushes down on the workpiece and, by reaction, up on the cylinder foot, which is attached to the upper arm of the frame; the force goes round the C through the back (the throat) into the lower arm and the bed. The frame is therefore loaded in bending and tension at the throat, and the bending moment is the force times the distance from the load line to the throat section (the throat depth plus half the section): M = F a = 80.4 kN × 400 mm = 32.2 kN·m. Loads to consider: the working force, an overload (a stuck die or a blocked relief valve: the relief valve limits it to 160 bar plus the override), the weight of the machine, handling and transport, side loads from off-center work (assume 10 percent of the force at 100 mm offset) and impact when the die closes.
Stress in the frame
Worked example: a 200 × 100 × 10 mm tube
- Section area A = 5 600 mm²; second moment I = (100 × 200³ − 80 × 180³) ÷ 12 = 27.8 × 10⁶ mm⁴; section modulus Z = I ÷ 100 = 278 × 10³ mm³ (see Structures and Strength of Materials).
- Bending stress σb = M c ÷ I = 32 169 920 × 100 ÷ 27 786 667 = 116 MPa; direct tension F ÷ A = 14 MPa; maximum σ = 130 MPa at the inner fiber (tension) and 101 MPa compression at the outer fiber.
- Static safety factor against yield of S355 (355 MPa): 355 ÷ 130 = 2.7: adequate for a well-defined load, but this is not what governs (see fatigue).
- At the 160 bar relief setting with the override the force may reach 9 percent more: the stress becomes 142 MPa; a blocked relief valve with the pump at full pressure is limited by the valve, but a wrong setting by a technician is a real risk, so the frame must survive the relief valve set at its maximum.
- Local stresses: the corners, the cylinder foot and any hole or notch concentrate the stress by 2 to 3 times (use fillets, doublers, avoid cut-outs near the throat); a finite-element analysis (FEA) of the frame confirms the hot spots.
Deflection and stiffness
A press frame must also be stiff: it springs open under load, so the die gap and the stroke change, and the work alignment suffers. Treating each arm as a cantilever of length a, the deflection at the load line is δ = F a³ ÷ (3 E I) = 80 425 × 400³ ÷ (3 × 210 000 × 27 786 667) = 0.29 mm per arm, so the throat opens by about 0.59 mm (and a C-frame also rotates, so the die faces tilt: allow more). The frame stiffness is 137 kN/mm: the die-set parallelism must tolerate a tilt of about 0.3 mm per 300 mm, or the dies must be designed for it (a guided ram and a stiff die set). If it is too much: a deeper section (stiffness ∝ I), a shorter throat (a³) or a closed (H or four-column) frame. A frame is also a spring that stores energy: ½ F δ = 24 J in this case, released violently if a die breaks.
Fatigue life of the weld
The press cycles from zero to full force 360 times an hour: 1 440 000 cycles a year, 22 million over 15 years. Welded details fatigue at stress ranges well below yield. A transverse fillet weld has a fatigue detail category around 36 to 80 N/mm² (EN 1993-1-9; the number is the stress range at 2 million cycles), and the S–N curve has a slope of m = 3: N = 2 × 10⁶ × (Δσc ÷ Δσ)³.
Worked example: where to put the weld
| Design | Stress range at the weld | Cycles to failure | Life at 1.44 million cycles a year |
|---|---|---|---|
| A. Weld at the high-stress inner corner of the throat (category 80) | 130 MPa | 464 628 | 3.9 months |
| B. The weld moved to the low-stress zone near the neutral axis of the arm | 45 MPa | 11 237 311 | 7.8 years |
| C. Section enlarged so that the stress is 40 percent lower at the same weld | 78 MPa | 2 151 055 | 1.5 years |
- Design A passes the static check with a factor of 2.7, yet fails in about 4 months: fatigue, not static strength, governs. The frame must be designed so that its welds lie in low-stress regions, or it must be thicker where the weld sits.
- Design B (the weld near the neutral axis, at a stress range of 45 MPa) lasts 7.8 years against the required 15: a factor of 0.5 short. Design C (the section’s Z raised by 67 percent) gives 1.5 years. Either needs a combination: thicker wall, welds in a low-stress area, ground weld toes (which raises the category by one class) and a stress-relieving heat treatment.
- Other fatigue details: bolt threads (rolled, preloaded), the cylinder foot lugs, any drilled hole, and the cylinder rod end (see Fatigue and Stress Concentration). Preload bolts so that the cyclic load is a small fraction of the bolt load, as in the next section.
Connections and mountings
Worked example: the cylinder foot flange bolts
- The cylinder is held to the upper arm by 4 M20 bolts class 8.8 (stress area 245 mm², proof 580 MPa). Preload at 70 percent of proof: 99 kN each, 398 kN in all, 4.9 times the separating force of 80.4 kN: the joint never opens, and the bolts feel only a fraction of the cycle (about 10 to 20 percent for a stiff joint): fatigue is acceptable. See Bolted and Welded Joints.
- The press is bolted to the floor on the base plate; the working force has no net reaction on the floor (the frame carries its own load), so the mounting sees only the weight and the vibration (the isolators of Vibration, Isolation and Noise apply).
- Frame mass: 44.0 kg per meter of tube × 3.5 m = 154 kg of S355 (about 185 of material at 1.2 per kg), before the cylinder, the guards and the power unit.
Safety factors
| Case | Safety factor (typical) |
|---|---|
| Static, well-known loads, ductile steel (yield) | 1.5 to 2.0 |
| Static, uncertain loads or material | 2.0 to 3.0 |
| Fatigue (against the endurance limit or S–N curve) | 1.5 to 2.5 |
| Welds (fillet, design stress) | by the standard (γM2 = 1.25) |
| Buckling of long compression members | 3 to 5 |
| Lifting gear and slings | 4 to 5 (by the code) |
| Brittle materials (cast iron, ceramics, glass) | 3 to 10 |
| Pressure vessels | by the code (ASME, PD 5500, EN 13445) |
A safety factor covers ignorance: of the load, the material, the model and the quality of manufacture. The more you know (a measured load, a certified material, a tested analysis), the lower it can be. It does not replace a calculation of the right failure mode: the frame above had a static factor of 2.7 and a fatigue life of a few months.
Standards and safety
A machine sold or used at work must be safe by law (the EU Machinery Directive, the US OSHA rules and national equivalents) and the way to show it is to follow the harmonized standards:
| Standard | Covers |
|---|---|
| EN ISO 12100 | Safety of machinery: general principles for design, risk assessment and risk reduction |
| EN 693 | Hydraulic presses: safety requirements |
| ISO 4413 | Hydraulic fluid power: general rules and safety requirements |
| EN ISO 14120 and ISO 13857 | Guards, and safety distances to prevent reach into danger zones |
| EN ISO 13849-1 / EN 62061 | Safety-related parts of control systems (performance level and SIL) |
| EN 60204-1 / NFPA 79 | Electrical equipment of machines |
| EN ISO 13850 | Emergency stop function |
| EN 1993 / AISC 360 | Design of steel structures; EN ISO 5817 for weld quality |
| EN ISO 3744 and ISO 11201 | Noise emission of machinery |
| Machinery Directive 2006/42/EC (CE) and OSHA 1910.217 (mechanical power presses) | legal framework in the EU and the US |
- Risk assessment first: identify the hazards (crushing at the die, stored energy in accumulators and the frame, hot oil, noise, electrical), estimate the risk and reduce it in this order: eliminate by design, guard, then warn and train.
- Press-specific: two-hand control or a light curtain at a safe distance (see Robot and Machine Safety and the stopping-distance formula D = K T + C), a fixed guard on the sides and back, a mechanical stop or block for die changes, a dump valve to release the accumulator and a lockable isolator.
- Documents: drawings, calculations, a risk assessment, a technical file, instructions for use and maintenance, test records and a declaration of conformity.
The design checklist
- Requirements: loads, speed, life, environment, standards, cost and quantity.
- Concept and layout: the load path, the main dimensions and the choice of frame, drive and control.
- Strength and stiffness: static, fatigue, buckling, deflection of every major part (this guide and Structures).
- Materials and processes: grades, heat treatment, welding procedure, corrosion protection (Materials and Processes).
- Machine elements: springs, seals, screws, bearings, couplings, brakes, lubrication.
- Dynamics: natural frequencies and resonance, balance, isolation, noise (Vibration, Isolation and Noise).
- Power and control: motor, drive, protection, hydraulics, sensors, safety circuits.
- Tolerances and fits: the functional dimensions, GD&T and the fits (Fits and Tolerances, GD&T).
- Manufacture and assembly: can it be made, handled, assembled, inspected and serviced with the facilities and skills available?
- Safety, testing and documentation: guards, labels, a proof test at 125 percent of the working load, a record and the manual.
Related
- Previous: Flywheels, Balancing and Rotors. Back to the overview: Machine Design.
- Structures and Strength of Materials and Materials and Processes.
- Hydraulics and Pneumatics.
FAQ
Why did the frame fail the fatigue check with a safety factor of 2.7?
Because the stress range at the weld is large for a weld that lasts 22 million cycles; fatigue limits the stress to a small fraction of the yield. Static safety factors tell you nothing about it.
How do I choose between a C-frame and a four-column press?
A C-frame gives open access on three sides but opens up (and tilts) under load; a four-column frame is much stiffer and symmetrical but blocks access. Use the stiffness requirement and the access needed.
When do I need FEA?
For complex shapes, local stress concentrations and weight-critical designs. Check it with a hand calculation of the main section (as on this page) and compare: a factor of 2 difference means a mistake in one of them.
What comes after the calculations?
A prototype and a proof test, with strain gauges on the critical places and a run at the rated load, then the documentation and the safety validation.
References
- Shigley, J. E., Mischke, C. R., Budynas, R. G., Mechanical Engineering Design, McGraw-Hill; Norton, R. L., Machine Design: An Integrated Approach, Pearson; Juvinall, R. C., Marshek, K. M., Fundamentals of Machine Component Design, Wiley.
- EN 1993-1-9 (fatigue of steel structures), EN ISO 12100 and the machine standards of the table; Hobbacher, A., Recommendations for Fatigue Design of Welded Joints and Components, IIW.
- Machinery’s Handbook, Industrial Press; Pahl, G., Beitz, W., Engineering Design: A Systematic Approach, Springer.
Last reviewed: October 2026. Detail categories and factors are typical; follow the standard that applies to your machine.