A double-screw toggle press is a mechanical press that combines two opposing screws driving a toggle (knuckle-joint) linkage to multiply force at the bottom of the stroke. Schuler and Weingarten coining presses use this exact arrangement to deliver 600+ tonnes of strike force from a modest flywheel input. The twin-screw drive feeds the toggle symmetrically, eliminating side thrust on the ram, while the toggle geometry approaches infinite mechanical advantage as the links straighten near dead centre — perfect for coining, embossing, and cold forging where peak force matters more than ram speed.
Double-screw Toggle Press Interactive Calculator
Vary the toggle angles and screw mechanical advantage to see how near-dead-centre geometry multiplies ram force.
Equation Used
The toggle multiplier is modeled as 1/sin(theta), so force rises sharply as theta approaches the straight dead-centre position. The total stacked multiplier is the toggle multiplier times the screw mechanical advantage.
- theta is measured from the straight dead-centre position.
- Ideal symmetric twin-screw loading with side thrust cancelled.
- Friction, bushing clearance, elastic deflection, and screw efficiency losses are ignored.
- Screw mechanical advantage is entered as a separate multiplier.
How the Double-screw Toggle Press Works
The mechanism stacks two force multipliers in series. The two screws — one left-hand, one right-hand, or two right-hand fed from opposite sides — convert flywheel rotation into linear pull on the upper toggle pin. That pull straightens a pair of toggle links arranged in a knuckle joint, and as the links approach the straight (dead-centre) position, the vertical force on the ram climbs sharply. At 10° from dead centre you get roughly 6× force multiplication from the toggle alone. At 2° you get over 28×. Stack that on top of the screw's own ~20:1 mechanical advantage and you understand why a 5 kW motor can coin a brass blank at 400 tonnes.
The symmetry is the whole point of using two screws instead of one. A single-screw toggle press loads the ram guide with significant side thrust because the screw nut wants to walk sideways under load. Run two screws of opposite hand, fed by a common gear or belt, and the lateral forces cancel. The ram tracks straight down the gibs with no parasitic friction, and ram-die alignment stays within 0.05 mm even at full tonnage — a hard requirement for coin-grade detail.
Tolerances bite hard near dead centre. If the toggle pin centres are off by more than 0.1 mm relative to the ram axis, the force vector tilts and you get one-sided die wear within a few thousand strikes. If the screw pitches differ by even half a thread (the classic mistake when sourcing replacement screws separately), the toggle reaches dead centre crooked and the ram bottoms out before peak force develops. Common failure modes are toggle-pin bushing wear (shows up as a knocking sound 5° before bottom), screw-thread galling from inadequate lubrication of the bronze nuts, and flywheel-key shear when an operator double-strikes a workpiece that didn't eject.
Key Components
- Flywheel and clutch: Stores rotational energy and releases it on demand to the screw drive. A typical coining press flywheel stores 40-80 kJ at 300 RPM, enough for one full-tonnage strike before the motor recharges it over 2-3 seconds.
- Twin screws (opposed hand): Convert flywheel rotation into symmetrical linear pull on the toggle assembly. Pitch must match between the two screws within ±0.02 mm per revolution or the toggle reaches dead centre skewed. Bronze nuts run in an oil bath at 40-60 °C.
- Toggle (knuckle-joint) linkage: Two links pivoting on a central knuckle pin that connects screw motion to ram motion. Mechanical advantage approaches infinity as links straighten — practical designs hit peak force at 2-5° before true dead centre to keep the system controllable.
- Ram and gibs: Carries the upper die down through a precision-guided stroke. Gib clearance held to 0.03-0.05 mm on coining presses; loosen to 0.1 mm and you'll see double-strike marks on the workpiece.
- Bolster and lower die: Anchors the lower tool. Bolster deflection under peak load must stay below 0.02 mm/m or the coined detail loses sharpness at the edges of the blank.
- Overload protection (shear pin or hydraulic bolster): Releases or yields if the press tries to bottom out on a stuck workpiece. On older Schuler presses, a 12 mm bronze shear pin in the flywheel hub fails first and costs 20 minutes to replace — far cheaper than a cracked frame.
Where the Double-screw Toggle Press Is Used
You see the double-screw toggle press wherever peak force matters more than cycle rate, and where ram alignment has to stay perfect under that peak force. Coining is the textbook case — striking a blank between two engraved dies and demanding the metal flow into every detail of the relief — but the same mechanism shows up in cold forging, medal striking, precision embossing of cutlery, and even some heavy-duty assembly presses where bearing races get pushed onto shafts.
- Mints and coinage: Schuler MRH coining presses at the Royal Canadian Mint and U.S. Mint Philadelphia facility — strike forces of 160 to 1,200 tonnes for circulating coins and bullion.
- Cold forging: Weingarten PSR knuckle-joint presses used for cold-forged automotive fasteners and bearing cages — typical 400-1,000 tonne range at 30-60 strokes per minute.
- Cutlery and tableware embossing: Heavy embossing of stainless flatware patterns at Robbe & Berking and Christofle — the toggle's dwell at bottom dead centre transfers fine pattern detail without ram bounce.
- Precision medal striking: Commemorative medal production at the Paris Mint (Monnaie de Paris) — multi-strike sequences using the controlled dwell of a toggle press to fully form deep relief.
- Powder metallurgy: Densification of pre-formed sintered metal parts where final stroke compaction force needs to peak precisely at bottom — common on Dorst TPA toggle-style presses.
- Watch and jewellery: Case-back and bezel coining at Rolex and Patek Philippe manufacturing — small-tonnage (40-80 t) toggle presses delivering hairline-sharp engraved relief.
The Formula Behind the Double-screw Toggle Press
The headline number for a toggle press is the force multiplication ratio at the ram as a function of toggle angle. This is what tells you whether your press will actually reach the rated tonnage on your specific workpiece thickness. At the high end of the stroke (toggle angle 30-45° from straight) the ram moves fast but force multiplication is modest — 2× to 4× from the toggle. At the design strike point (typically 2-5° from dead centre) you hit the sweet spot where force has climbed to 15-30× and ram speed has dropped to a controllable creep. Push the design angle below 1° and you gain force on paper but lose it in real life because pin and link deflection eats the gain.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Fram | Force delivered at the ram (the strike force on the workpiece) | N | lbf |
| Fscrew | Combined linear pull from both screws acting on the toggle knuckle pin | N | lbf |
| θ | Toggle angle measured from the straight (dead-centre) position of the links | rad or ° | rad or ° |
| ηtoggle | Toggle efficiency accounting for pin friction and link deflection (typically 0.85-0.92) | dimensionless | dimensionless |
Worked Example: Double-screw Toggle Press in a 250-tonne medal-striking press
You are sizing the toggle geometry for a 250-tonne double-screw medal-striking press intended for 50 mm diameter bronze commemorative medals at a regional mint. The two screws together deliver 90 kN of pull on the toggle knuckle pin at full flywheel discharge. Toggle efficiency η is 0.90. You need to confirm the strike force at three toggle angles to set the ram-stop position correctly.
Given
- Fscrew = 90,000 N
- ηtoggle = 0.90°
- θnominal = 3 °
- θlow-end = 8 °
- θhigh-end = 1 °
Solution
Step 1 — at the nominal design angle of 3° from dead centre, compute the toggle multiplier:
Step 2 — apply the screw force and efficiency to get the nominal ram force:
That is short of 250 tonnes, which tells you the design strike angle needs to sit closer to dead centre than 3°. Now check the low-force end of the operating range. At 8° from dead centre — typical for the start of the work-doing part of the stroke when the upper die first contacts the blank:
59 tonnes at first contact is plenty to start plastic flow on a 50 mm bronze blank — you do not need full tonnage until the metal is filling the deepest engraved detail. Now the high end — push the strike angle in to 1°:
On paper that obliterates the 250-tonne rating, but in practice you will not get 473 tonnes. Below about 1.5°, link deflection and toggle-pin clearance steal 30-40% of the predicted multiplication, the ram bottoms before peak force, and you risk frame yield. The realistic sweet spot for a 250-tonne strike is θ ≈ 2°, giving roughly 232 tonnes of usable force with margin against deflection losses.
Result
Set the ram stop so the toggle reaches θ ≈ 2° at full die contact, which gives a usable strike force of approximately 232 tonnes — within the 250-tonne frame rating with sensible margin. At 8° (early contact) you have 59 tonnes for initial metal flow, at 3° you have 157 tonnes for relief filling, and pushing below 1.5° is a trap because deflection eats most of the theoretical gain. If your measured strike force comes in 15-25% below predicted, the most common causes are: (1) screw-nut bronze wear above 0.3 mm radial clearance, which delays the toggle reaching the design angle, (2) flywheel speed dropping below rated RPM because the clutch slips on engagement, robbing the screws of input torque, or (3) frame stretch above 0.4 mm under load on older cast-iron frames, which effectively shifts dead centre away from the ram's mechanical stop.
Choosing the Double-screw Toggle Press: Pros and Cons
The double-screw toggle press is one option among several force-multiplying press architectures. Pick the wrong one and you either overpay for tonnage you cannot use, or you cycle so slowly the part cost kills the job. Compare on tonnage, stroke profile, cycle rate, and capital cost.
| Property | Double-screw toggle press | Hydraulic press | Mechanical eccentric (crank) press |
|---|---|---|---|
| Peak force capacity | 100-2,500 tonnes | 50-50,000 tonnes | 50-4,000 tonnes |
| Force profile near bottom | Sharp peak with brief dwell, ideal for coining | Flat — full force anywhere in stroke | Smooth peak, no dwell |
| Cycle rate | 30-90 strokes/min | 5-20 strokes/min | 60-200 strokes/min |
| Ram positional accuracy at bottom | ±0.02 mm (mechanical stop defines it) | ±0.1 mm (depends on servo control) | ±0.05 mm |
| Capital cost (250-tonne class) | $180k-$300k | $90k-$160k | $120k-$200k |
| Energy per stroke at rated tonnage | 1.5-3 kJ | 8-15 kJ | 2-5 kJ |
| Best application fit | Coining, embossing, cold forging | Deep drawing, variable stroke | Blanking, shallow forming, high-volume stamping |
| Maintenance interval (full overhaul) | ~10,000 hours | ~6,000 hours (seal packs) | ~8,000 hours |
Frequently Asked Questions About Double-screw Toggle Press
The press is making the force, but the force is arriving at the wrong moment in the stroke. Toggle presses produce a sharp force peak with very brief dwell — typically 8-15 ms at greater than 90% of peak — and if your blank thickness is even 0.1 mm under spec, the toggle reaches dead centre before the dies fully close. You hit peak force on air, the ram bounces back, and the workpiece sees a flatter pressure pulse than the gauge suggests.
Measure your actual blank thickness with a micrometer across 10 samples. If the standard deviation is above 0.03 mm, that's your problem. Either tighten blank tolerance upstream or shim the lower die to bring the work into the toggle's force peak.
Above roughly 250 tonnes, the double-screw arrangement pays for itself within 12-18 months. The single-screw version puts asymmetric load into the ram guide, and at 400 tonnes that asymmetry shows up as gib wear of 0.05 mm per 100,000 strikes — meaning a re-shim every 6 months and a full ram regrind annually. The double-screw cancels the side thrust at the source.
If your tonnage is 150 or below, single-screw is fine and saves you 25-30% on capital cost. The crossover point is where ram alignment under load starts dominating tooling cost.
Below about 1.5° from dead centre, the rigid-body formula breaks down. Real toggle links bend elastically under load, the knuckle pin and bushings have 0.05-0.15 mm of clearance that closes up under force, and the press frame itself stretches. All of these effectively rotate the toggle backward away from true dead centre during the strike, so the ram never sees the angle the formula assumes.
Rule of thumb: discount the theoretical multiplier by 30% below 1.5°, by 15% between 1.5° and 3°, and by 5% above 3°. If you need accurate force at sub-1° angles, you need a finite element model of the linkage and frame, not the closed-form equation.
Work backward from the energy per strike, not the peak force. A toggle press only needs the flywheel to deliver one strike's worth of work — the motor recharges between strikes. For a 250-tonne press striking a typical bronze medal, the actual plastic deformation work is around 1.8-2.5 kJ. Add 30% for screw-nut friction and bearing losses, and you need a flywheel storing 3-3.5 kJ usable, which usually means 8-10 kJ total at rated speed (you only ever use the top 30% of stored energy before the speed drop becomes unacceptable).
Motor power then sizes off cycle rate: 3.5 kJ × 60 strokes/min ÷ 60 s = 3.5 kW continuous. A 5.5 kW motor with a soft-start gives sensible margin and lets you run double-strike sequences without bogging.
Two screws fed in opposition must reach dead centre at the same instant, which means their effective lead must match within roughly ±0.02 mm per full revolution. Two screws of the same nominal pitch from different production batches can differ by 0.05 mm per turn after thread-grinding tolerance stack-up — enough that one side of the toggle straightens 1-2° before the other.
The fix is to buy screws as a matched pair from the same grinding setup, or to shim the bronze nut on the leading side to delay its engagement. Schuler ships replacement screws in matched sets specifically because of this — if you sourced yours individually, that's the root cause.
No, and trying it will damage the press. Deep drawing needs sustained force across a long stroke — 100-300 mm of ram travel under near-constant load. The toggle press delivers force only in the last 5-10 mm before dead centre. Anywhere else in the stroke, the mechanical advantage is too low to draw thick stock, and the ram speed is too high to control material flow into the die.
For deep drawing you want a hydraulic press or a link-drive mechanical press with an extended dwell. Use the toggle press for what it's good at: short-stroke, high-peak-force operations where the work happens in the final few millimetres.
References & Further Reading
- Wikipedia contributors. Screw press. Wikipedia
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