Power Press Working Principle, Parts and Maintenance
How a power press works
A mechanical power press turns the steady rotation of an electric motor into one short, heavy, straight-line blow. The motor spins a flywheel. The flywheel stores energy. When the operator presses the pedal, a clutch connects the flywheel to the crankshaft, the crankshaft turns once, and the connecting rod pushes the ram down onto the die. The stored energy does the cutting or forming, and the motor tops the flywheel back up before the next stroke.
That is the whole principle. Everything else on the machine exists to do this accurately, thousands of times a shift.
The working cycle, step by step
Motor runs the flywheel. A belt drive keeps the flywheel turning continuously, whether or not the press is stroking.
Clutch engages. The pedal trips the clutch and the flywheel is locked to the crankshaft.
Crankshaft converts the motion. The offset crank pin turns rotation into up-and-down travel. The distance the ram travels is the stroke.
Ram comes down. The connecting rod and its adjusting screw drive the ram down in its guides. The punch meets the sheet a few millimetres above the bottom of the stroke.
Work is done. The punch shears or forms the metal against the die. The flywheel slows slightly as it gives up energy.
Ram returns. The crank carries the ram back to the top, the clutch disengages and the brake holds the ram at top dead centre.
Main parts and what we make them from
Part | Job | How we build it |
Frame | Carries the full load of every stroke | All-welded steel plate, C-frame or H-frame |
Crankshaft | Converts rotation to ram travel | Alloy steel |
Bushes | Carry the crankshaft and connecting rod | Gunmetal or bronze |
Ram and table | Hold the punch and the die | Graded cast iron |
Flywheel | Stores the energy for the stroke | Cast iron |
Clutch | Connects flywheel to crankshaft | Rolling key as standard, pneumatic as an option |
Lubrication | Feeds oil to bushes and guides | Centralised hand pump |
At the same rated tonnage, our frames are heavier than most presses built in Ludhiana. Frame weight matters because a frame that stretches under load opens the die clearance on every stroke, and that shows up as burrs and short die life.
Rolling key clutch or pneumatic clutch
The clutch decides how the press can be used, and it is the part buyers most often get wrong.
Rolling key clutch (our standard fitment). A hardened key rolls into a slot and locks the flywheel to the crankshaft. Once it engages, the press must complete the full stroke. It cannot be stopped halfway. It is simple, low cost and easy to maintain, and it suits hand-fed blanking and piercing work.
Pneumatic clutch and brake (optional). Air pressure clamps friction plates together to drive the crankshaft, and a spring-applied brake stops it. The ram can be stopped at any point of the down-stroke and inched for die setting. Choose it for progressive dies, continuous running, coil feeding, and any job where a two-hand control or light curtain is required.
Rated tonnage is only available near the bottom of the stroke
A 100 ton press does not deliver 100 tonnes through its whole stroke. Because of the crank geometry, full rated force is available only in the last few millimetres above bottom dead centre. Higher up the stroke, the available force is much lower.
This is why a deep drawing job, where the punch meets the sheet well above the bottom, needs a larger press than a blanking job with the same calculated force.
How to size a press: the 70 to 80 percent rule
We advise customers to choose a press so that the job needs only 70 to 80 percent of its rated tonnage. The margin covers blunt tooling, harder batches of material and off-centre loading.
Worked example. Blanking a 100 mm diameter disc from 2 mm mild steel, shear strength 400 N/mm²:
Cut length = 3.1416 × 100 = 314 mm
Force = 314 × 2 × 400 = 251,300 N, which is about 25.6 tonnes
At 80 percent loading: 25.6 ÷ 0.8 = 32 tonnes
At 70 percent loading: 25.6 ÷ 0.7 = 36.6 tonnes
Choose a press rated between 32 and 37 tonnes or the next standard size above.
Use the free press tonnage calculator
What breaks first when a press is overloaded
Overloading is the fault we see most often on presses in customers' shops. On our presses the connecting screw or its ball seat gives way first, before the crankshaft or the frame. It is the cheapest part in the load path to replace, but the press is down until it is.
Three warning signs come before the failure. Stop and check the job if you see any of them:
The ram sticks at the bottom. The press jams at bottom dead centre and has to be backed off.
A loud knock at the bottom of the stroke. Louder and duller than the normal snap of the punch breaking through.
The connecting screw keeps working loose. The lock nut on the ram adjustment will not stay tight.
The usual causes are a die set too low, a double blank, material thicker or harder than the die was designed for, or a job simply too big for the press.
Every shift
Work the centralised hand pump before starting, and again through the shift. [CONFIRM: strokes and how often]
Use Servo System 68 oil, or an equivalent ISO VG 68 oil, in the pump reservoir.
Check the oil level in the reservoir.
Listen for a change in sound at the bottom of the stroke.
Check that the ram stops at top dead centre and does not repeat.
Every week
Check the ram adjustment lock nut and the die clamping bolts.
Check belt tension and condition.
Check the clutch key and the brake for wear. A ram that over-runs the top position is the first sign.
Clean scrap and slugs from the table and bolster.
Every month
Check ram guide clearance and adjust the gibs. [CONFIRM: clearance figure]
Check foundation bolts.
Inspect oil lines from the pump for blockage or leaks at each point.
Check electrical connections and the motor.
How we check a press before dispatch
Every press is assembled, aligned and trial-run at our works in Sahnewal, Ludhiana. The customer or their inspector is invited to witness the trial before the machine is dispatched. [CONFIRM: what is checked at the trial, and for how long the press is run]
What our customers make on these presses
Auto parts: brackets, washers and clamps for auto ancillary units. [CONFIRM: one real part, material, thickness, press tonnage]
Cycle parts: components for Ludhiana's bicycle industry. [CONFIRM: one real part and press tonnage]
Utensils: deep drawn kitchenware on H-frame presses. [CONFIRM: one real part and press tonnage]
Hot forging: hand tools, fasteners and fittings on friction screw and forging screw presses. [CONFIRM: one real part and press tonnage]
Frequently asked questions
What is the working principle of a power press? An electric motor drives a flywheel that stores energy. A clutch connects the flywheel to a crankshaft, which converts rotation into the up-and-down travel of the ram. The ram drives a punch into a die to cut or form sheet metal.
What is the difference between a rolling key clutch and a pneumatic clutch? A rolling key clutch must complete the full stroke once engaged. A pneumatic clutch and brake can stop the ram at any point of the down-stroke and can be inched for die setting.
Why does a power press jam at the bottom of the stroke? The job needs more force than the press can deliver at that point, usually because the die is set too low, two blanks were fed, or the material is thicker or harder than planned.
How much load should a power press run at? Size the press so the job needs 70 to 80 percent of its rated tonnage.
Which oil is used in a power press? We specify Servo System 68 or an equivalent ISO VG 68 oil for the centralised lubrication pump.
Call to action
Send us your part drawing, material and thickness. We will tell you the tonnage, frame type and clutch you need. Phone or WhatsApp: +91 98147 15935 · info@birsonforgings.com
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