A hydraulic shop press is a ram in a rigid frame, and its force is the relief pressure times the ram area: a 100 mm ram held to 20 MPa presses with 157 kN, which is 16 tonnes-force, not the 20 the carrier reaches only by setting the relief at 25 MPa. A hand pump with an 8.7 mm piston on an 8:1 lever makes 20 MPa from 15 kgf of handle force, but moves the ram only 10 mm in 43 strokes, so the working stroke must be short.
Part of Hydraulic Systems
A hydraulic press squeezes work between a ram and a fixed bed: pressing bearings, straightening shafts, bending plate and forming metal.
Force Is Set by the Relief Valve and the Ram Area
The carrier describes a 10-20 tonne press: a heavy H-frame, a vertical cylinder of 80-100 mm bore and 200-300 mm stroke, a hand pump on the frame, a relief valve, a directional or release valve and a gauge.
| Ram bore | Relief setting | Push | Tonnes-force |
|---|---|---|---|
| 80 mm | 20 MPa | 100.5 kN | 10.2 |
| 100 mm | 20 MPa | 157.1 kN | 16.0 |
| 100 mm | 25 MPa | 196.3 kN | 20.0 |
| 112 mm | 20 MPa | about 197 kN | 20.1 |
Check the 100 mm rows: area 7.854 x 10⁻³ m²; 20,000,000 x 7.854 x 10⁻³ = 157,080 N (157 kN), which is 16.0 tonnes-force (about 16 tonnes) at 9,810 N each; at 25 MPa it is 196,350 N. The carrier sets the relief at 25 MPa but states its cylinder wall rule only up to 20 MPa (Hydraulic Cylinders), so the 20-tonne figure on a 100 mm ram has no stated margin. To reach 20 tonnes-force without exceeding 20 MPa, use a larger bore: √(4 x 196,200 / (π x 20,000,000)) = 0.1118 m, 112 mm. Whatever the choice, the relief setting comes from the weakest part’s own rating (Pressure Relief Valves).
The Frame Carries the Same Force
Every newton the ram pushes down, the frame pulls back up through its columns. For an 80 mm ram at 20 MPa (100.5 kN) and a two-column frame, each column carries about 50.3 kN in tension. Using the corpus’s mild-steel tensile strength of 50,000 psi (345 MPa) and a safety factor of 6 from the 4-8 band for machine parts (Tensile Strength, Safety Factors):
- Allowable stress = 345 / 6 = 57.5 MPa
- Column area = 50,265 / 57,500,000 = 8.74 x 10⁻⁴ m², 874 mm² per column (for example a 50 mm x 18 mm bar)
The crosshead or bed is a beam in bending, and that is usually the weak point; this article does not compute it, so work it out with the corpus’s beam methods, weld or bolt it for the full force, and proof-test the finished frame before trusting it. Cast iron must not be used for the frame, as it is brittle.
Pumping a Press by Hand
The pump piston area for a pressure P with hand force F and lever ratio R is A = F x R / P. With 15 kgf (147.2 N), an 8:1 lever and 20 MPa:
- Force on the pump piston = 147.2 x 8 = 1,177 N
- Area = 1,177 / 20,000,000 = 5.89 x 10⁻⁵ m² = 0.589 cm², a diameter of 8.66 mm, about 8.7 mm
- Oil per 20 mm stroke = 0.589 x 2 = 1.18 mL
- An 80 mm ram (50.27 cm²) moving 10 mm needs 50.27 x 1 = 50.3 mL, so 50.3 / 1.18 = 43 strokes
- Moving the full 250 mm of stroke would take about 1,067 strokes
The power at one stroke a second is 20,000,000 Pa x 1.18 x 10⁻⁶ m³/s = 23.5 W, within a person’s reach; compare 126 W that a 6.3 mL pump at the same pressure and rate would demand (Single-Acting Pump). So the design needs a fast approach: bring the ram to the work with spacers, a screw stop, or a second large-piston low-pressure pump, and use the pressure pump only for the last few millimetres.
Method: Build and Prove the Press
Step 1: Weld and bolt the frame with full-penetration welds on the highest-loaded joints.
Step 2: Mount the cylinder vertically with the line of force through the centre of the frame.
Step 3: Fit the pump, relief valve, gauge and release valve close to the cylinder with short, high-rated lines; see Practical Systems.
Step 4: Set the relief from the weakest rating, and mark the frame with the maximum tonnes-force.
Step 5: Proof-test with a solid test block, raising the pressure in steps and watching the frame for any movement.
Common Mistakes
- Pressing off-centre. The ram and frame twist and the work shoots out.
- Raising the relief setting to get more tonnes. The weakest part is then overloaded.
- Pressing a hardened or cast part cold. It can shatter and throw fragments.
- No mark for the safe load. The next user trusts the gauge or the claim.
Safety
A press stores force in the frame and throws what breaks
A part pressed past its strength can shatter, shoot sideways or spring out of the press with the energy of the whole load. Put a guard between you and the work, stand to one side, wear eye and face protection, and never stand in line with the ram. A burst line or a pinhole in a fitting at 20 MPa injects oil through skin; never search for a leak with a hand, use card or wood, and treat any suspected injection as a surgical emergency. Relieve pressure before adjusting the work.
What’s Next
- Learn next: Hydraulic Jack — the same ram and pump built for lifting
- Combine with: Forging — one of the jobs a press does