A Double Toggle-joint Screw Press is a mechanical press that drives a vertical ram through two stacked toggle linkages powered by a flywheel-driven screw, producing very high force over a short stroke at bottom-dead-centre. A typical industrial unit delivers 200 to 2,500 tonnes peak force at the bottom 3 mm of stroke. The double toggle multiplies force geometrically as the joints straighten, which is why coining mints, refractory brick plants and briquetting lines pick this design over a plain screw press or a hydraulic ram.
Double Toggle-joint Screw Press Interactive Calculator
Vary the starting and working toggle angles to see how the press mechanical advantage rises as the toggles straighten toward bottom dead centre.
Equation Used
The calculator uses the article relationship MA = 1 / sin(theta). As the toggle angle theta becomes small near bottom dead centre, the sine term shrinks and the theoretical force multiplier rises quickly.
- Toggle angle theta is measured from the straight bottom-dead-centre line.
- Ideal geometry is used; friction, pin deflection, screw losses, and frame stretch are not included.
- The result is a force multiplier, not a full press tonnage rating.
Inside the Double Toggle-joint Screw Press
The press has a flywheel mounted on a vertical screw at the top of the frame. A friction wheel or electric drive spins the flywheel up to working speed — typically 60 to 180 RPM at the screw — and the operator trips the clutch. The screw drives downward, and instead of pushing the ram directly it pushes the centre pivot of an upper toggle pair. That upper pair drives the lower toggle pair, and the lower pair drives the ram. As both knuckle joints approach full extension the mechanical advantage climbs steeply, so the ram moves slowly but pushes hard. At roughly 5° from straight, force at the ram is about 5× the screw thrust. At 1°, it is closer to 30×. At true bottom-dead-centre the linkage is theoretically infinite — in practice the frame stretches and the linkage pins deflect, so you cap the design well short of straight.
Geometry decides everything. The link-length ratio between upper and lower toggles, the pin centre distances, and the angle at the moment of work contact set both the available tonnage and the stroke profile. If your link lengths are off by even 0.5 mm on a 400 mm toggle, the ram bottoms out before the knuckle reaches its design angle and you lose 20% of rated tonnage. The pins must be ground to H7/g6 fit — slop in the pivots shows up as a visible bounce on the ram at the moment of impact, and you will see witness marks doubled up on coined parts.
Failures cluster around three points. The screw thread galls if the flywheel is over-spun and the operator dwells too long at BDC — the screw is meant to deliver energy in a flash, not push static load. The toggle pins seize if grease runs dry, because the loading is high and oscillating in a tight angle range. And the frame cracks at the throat if the press is run beyond its rated tonnage — toggle presses don't blow a relief valve like hydraulics, they just keep multiplying force until something yields.
Key Components
- Flywheel and Screw: The flywheel stores rotational energy — typically 40 to 200 kJ on a mid-size press — and the screw converts that energy to linear thrust. Screw lead is usually 30 to 60 mm per revolution so the energy releases over a short axial travel. The screw nut is bronze and is a wear part.
- Upper Toggle Pair: Two links pinned at a centre knuckle that the screw drives downward. The upper pair handles the higher-speed, lower-force phase of the stroke. Pin diameters run 60 to 120 mm on industrial sizes and must be hardened to 55 HRC minimum.
- Lower Toggle Pair: Two links between the upper-pair output and the ram. The lower pair handles the high-force, low-speed phase at the bottom of the stroke. The angle at work contact is typically set to 3° to 6° from straight.
- Ram: The vertical slide that carries the upper die. Stroke is short — 25 to 80 mm total — and the working stroke at high force is only the last 3 to 8 mm. The ram is guided in long box ways to keep it square to the bolster within 0.05 mm over the full stroke.
- Frame: Either a solid forged C-frame or a tied four-post frame. Stiffness is critical because frame stretch directly subtracts from coining depth. A 1,000 tonne frame will stretch about 0.4 to 0.8 mm at full load, and that number is part of the die-height calculation.
- Clutch and Brake: Couples the spinning flywheel to the screw on demand and stops the screw at top-dead-centre between strokes. Pneumatic clutches are standard. Engagement time below 80 ms is required to hit a 30 strokes-per-minute production rate.
Industries That Rely on the Double Toggle-joint Screw Press
Double toggle-joint screw presses earn their place wherever you need a clean, repeatable, very high force over a short distance — and where the workpiece responds to a sudden energy release rather than a slow squeeze. That rules them in for coining, embossing, refractory brick pressing, hot forging of small parts and briquetting. It rules them out for deep drawing or anything needing controlled velocity through a long stroke.
- Coinage and Medallions: Schuler MRH-series coining presses at the U.S. Mint and the Royal Canadian Mint use toggle-amplified screw drives to strike circulation and proof coins at 100 to 750 tonnes per blow.
- Refractory Bricks: Lais and Sacmi-style toggle screw presses at refractory plants like Saint-Gobain in Worcester press magnesia-carbon and high-alumina brick at 1,200 to 2,500 tonnes.
- Hot Forging of Fasteners: Vaccari and Weingarten double-toggle screw presses produce hex-head bolts and special fasteners in batches of 30 to 60 strokes per minute at 400 to 1,000 tonnes.
- Coal and Biomass Briquetting: Komarek and Sahut-Conreur briquetting presses use toggle-amplified linkages to compact charcoal fines and biomass at 80 to 200 MPa peak die pressure.
- Silverware and Cutlery Embossing: Sheffield-based cutlery makers running restored Bliss and Clearing-style toggle screw presses for pattern embossing on stainless flatware blanks.
- Powder Metal Compaction: Dorst and Osterwalder toggle-screw compaction presses for sintered bearing blanks and small gear preforms in the 50 to 250 tonne range.
The Formula Behind the Double Toggle-joint Screw Press
The number you actually care about is ram force as a function of toggle angle, because that tells you where in the stroke your tonnage shows up. At the start of the stroke the toggles are bent — say 30° from straight — and you barely have any mechanical advantage. As the joints straighten, force climbs hyperbolically. The sweet spot for design is 3° to 6° from straight at the work-contact point. Below 3° you are riding too close to the singularity, and small wear in pins or screw lead error throws your tonnage all over the place. Above 8° you are leaving force on the table and stressing the screw harder than you need to. The formula below treats both toggle pairs as identical for simplicity — most production presses are designed that way.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Fram | Force delivered at the ram face | kN | tonf |
| Fscrew | Axial thrust delivered by the screw at the upper toggle centre pin | kN | tonf |
| θ | Angle of each toggle link from the straight (fully-extended) position at work contact | rad or ° | ° |
Worked Example: Double Toggle-joint Screw Press in a refractory brick press at a magnesia-carbon plant
A refractory brick plant in Visakhapatnam is sizing a double toggle-joint screw press to compact 250 mm × 150 mm × 100 mm magnesia-carbon bricks at a target die pressure of 130 MPa. The screw delivers 120 kN of axial thrust at the moment of work contact. The plant engineer needs to know the ram force at the design angle, and how that force varies if the toggles are set up a few degrees out of spec.
Given
- Fscrew = 120 kN
- θnominal = 5 °
- θlow-range = 3 °
- θhigh-range = 8 °
- Brick face area = 0.0375 m²
Solution
Step 1°— at the nominal design angle of 5°, calculate tan θ and apply the toggle force-multiplication formula:
Fram,nom = 120 × (1 / (2 × 0.0875))2 = 120 × 32.7 = 3,920 kN ≈ 400 tonnes
That 400 tonnes spread over the 0.0375 m² brick face gives about 105 MPa — a touch under target. Acceptable, but the press is sized close. Now look at what happens if the toggles end up shimmed too tight and sit at 3° at work contact instead of 5°:
Fram,low = 120 × (1 / (2 × 0.0524))2 = 120 × 91.0 = 10,920 kN ≈ 1,113 tonnes
That is nearly 3× the nominal tonnage on the same screw thrust — and it is exactly how toggle presses crack frames. The brick will overpress, the die will spring, and on the next stroke the frame throat takes the punishment. Now the high end of the range, 8°, where the press is set up loose:
Fram,high = 120 × (1 / (2 × 0.1405))2 = 120 × 12.7 = 1,520 kN ≈ 155 tonnes
At 155 tonnes on the same brick face you only see 41 MPa — the brick comes out underpressed, soft, with visible lamination cracks after firing. So the practical operating window between under-pressed garbage and frame-cracking overload is barely 5° wide on the toggle angle.
Result
At the nominal 5° toggle angle the press delivers 3,920 kN — about 400 tonnes — at the ram, giving 105 MPa on the brick face. That is the right ballpark for a magnesia-carbon brick: hard enough that the green brick rings when you tap it, not so hard the binder squeezes out. The range tells the real story: dropping the angle to 3° spikes ram force to 1,113 tonnes (frame-cracking territory) and opening to 8° drops it to 155 tonnes (soft, laminated bricks). If you measure your delivered tonnage with a load cell and it lands well below predicted, three suspects: (1) screw lead error stacking up on the bronze nut so the screw bottoms before the toggle reaches design angle, (2) shim pack under the lower toggle anchor losing thickness from creep — check with a feeler at TDC, and (3) flywheel speed sagging because the friction drive belt is glazed, which costs you stored energy and lets the press stall before BDC.
Choosing the Double Toggle-joint Screw Press: Pros and Cons
The choice is almost always between a Double Toggle-joint Screw Press, a plain flywheel screw press without toggles, and a hydraulic press. Each picks a different point on the speed-vs-force-vs-stroke triangle.
| Property | Double Toggle-joint Screw Press | Plain Flywheel Screw Press | Hydraulic Press |
|---|---|---|---|
| Peak force per kW of motor input | Very high — 30× thrust at 1° toggle | Moderate — direct screw thrust only | Limited by pump pressure and cylinder area |
| Strokes per minute (production rate) | 20 to 60 SPM | 30 to 80 SPM | 5 to 20 SPM |
| Working stroke at full force | 3 to 8 mm at BDC only | Full screw stroke | Full cylinder stroke (controllable) |
| Tonnage controllability mid-stroke | Poor — force is geometric, not commanded | Poor — energy-limited | Excellent — pressure is commanded directly |
| Capital cost (250 tonne class) | High — complex linkage and frame | Moderate | Moderate to high — depends on pump system |
| Overload protection | None inherent — frame is the fuse | Slip clutch limits energy | Relief valve caps pressure |
| Best application fit | Coining, brick, briquetting, embossing | Hot forging, upsetting | Deep draw, trim, blanking, assembly |
| Frame stretch sensitivity | Critical — directly affects coining depth | Moderate | Low — pressure-controlled |
Frequently Asked Questions About Double Toggle-joint Screw Press
That is almost always toggle-pin clearance, not tonnage. When the ram hits the workpiece the linkage tries to keep going, then the frame springs back and the slack in the pin bores lets the ram bounce against the workpiece a second time before the brake catches. Measure the pin-to-bore clearance cold — anything over 0.05 mm on a 100 mm pin will produce a visible double strike on a 25 mm coin face.
Fix is to ream the bores and fit oversized pins. Don't shim — the load cycles will pound the shims flat in a few thousand strokes.
Work backwards from the energy you need to deliver, not from peak force. Calculate the work the part needs (force × deformation distance), then size the flywheel to store roughly 1.5× that energy. Once flywheel and screw are sized, the toggle angle at work contact gets chosen so peak ram force lands just above your required tonnage — typically 3° to 6° from straight.
Going tighter than 3° is a trap. The force multiplier is so steep there that a 0.2 mm change in die height (from thermal growth or die wear) can swing tonnage by 30%. The press becomes uncontrollable. 5° is the industry default for a reason.
Three suspects in order of likelihood. First, frame stretch — a C-frame at full load can stretch 0.5 to 1.0 mm, and on a press with only 5 mm of high-force working stroke that stretch eats most of your useful travel. Put a dial indicator between bolster and ram cap and measure stretch under load.
Second, the bronze screw nut is worn and the screw is climbing inside the nut instead of pushing down — typical after 2 to 5 million strokes. Third, flywheel energy is down because the friction drive is slipping, so the press stalls just before BDC. Tach the flywheel before and after a stroke; it should drop no more than 15 to 20% of working RPM.
Depends on the part geometry and your tolerance budget. Toggle screw presses dominate small, simple geometries like sintered bushings and plain bearings because the energy release is fast, the part density is uniform, and SPM is high. The mechanical sweet spot of the toggle delivers very repeatable density part-to-part, often within ±0.5%.
Hydraulic wins for multi-level parts, parts with cores or steps, and anything that needs withdraw-pressing or controlled platen velocity. If the press tooling has ejector pins that must move on a programmed timing curve, you cannot do that with a toggle.
Stalling before BDC is an energy problem, not a force problem. The flywheel did not have enough stored kinetic energy to push the work through. Common cause is the friction drive wheel glazing or the belt slipping during spin-up, so the flywheel reaches the trip RPM but with less energy than the dial reads.
Verify with a tachometer at the moment of clutch engagement. If the press is running cold-start work (first part of the shift) and the parts are out of spec, the flywheel is undersized for the duty cycle and the spin-up time between strokes is too short. Either lower your SPM or raise the flywheel inertia.
It absolutely does need overload protection — the design just handles it differently and most of the time, badly. Toggle presses are energy-limited, not force-limited. You put a fixed amount of kinetic energy into the flywheel, and that energy gets dissipated somewhere. If the workpiece is too hard or the die-height is too short, the energy goes into stretching the frame and yielding the linkage instead of deforming the part.
Modern designs use an oil-filled hydraulic overload puck under the bolster — it collapses at a set pressure and absorbs the excess energy. If your press is older than about 1970 and has no such device, the frame itself is the fuse. That is why a single mis-set die can crack a 50-year-old toggle press in one stroke.
References & Further Reading
- Wikipedia contributors. Screw press. Wikipedia
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