The Buchanan Rock Crusher is a heavy-duty jaw-style ore crusher that breaks rock between a fixed jaw and a swinging jaw driven by an eccentric shaft. James Buchanan patented the design in 1878 in St. Louis, Missouri, refining the earlier Blake jaw-crusher arrangement. The swinging jaw oscillates 20-40 mm at the bottom, compressing rock against the fixed plate until fragments fall through the discharge gap. The result — a primary crusher that reduces 600 mm boulders down to 100-150 mm feed in a single pass.
Inside the Buchanan Rock Crusher
The Buchanan Rock Crusher works on brute-force compression. Rock drops into a wedge-shaped chamber formed by a fixed jaw plate and a movable jaw plate hinged at the top. An eccentric shaft turns at 250-350 RPM, and a pitman arm hanging off that shaft drives the movable jaw forward and back. Each forward stroke crushes whatever rock is sitting in the chamber. Each return stroke lets gravity pull the broken material lower until it exits the discharge gap at the bottom. Simple in principle, but the geometry has to be right or the machine eats itself.
The critical dimensions are the closed-side setting (CSS) and the open-side setting (OSS) — the gap at the bottom of the chamber when the jaw is fully closed and fully open. CSS sets your product size. If the CSS is 75 mm, almost nothing larger than 100 mm leaves the chamber. The toggle plate at the back of the pitman is a sacrificial element — it's designed to snap if a piece of tramp steel or an oversized boulder jams the chamber, protecting the eccentric shaft and main bearings. You would be amazed how often a worn toggle seat causes phantom failures that look like bearing problems.
If you run the eccentric too fast, the rock doesn't have time to fall between strokes and you choke the chamber. Too slow, and throughput collapses. Tolerances on the eccentric bearings are tight — typical clearance is 0.15-0.25 mm on a 200 mm shaft, and anything beyond 0.4 mm starts hammering the housing. Common failure modes are pitman cracking at the top eye, jaw plate bolt shear from missed re-torquing, and toggle plate fatigue from running with rock packed in the discharge.
Key Components
- Fixed Jaw Plate: The stationary crushing surface bolted to the front frame. Made from manganese steel (typically 14% Mn, Hadfield grade) with a corrugated face. Wears 6-12 mm per 100,000 tonnes of hard ore and gets flipped end-for-end at half life to even out wear.
- Movable (Swing) Jaw Plate: Mirrors the fixed jaw and bolts to the pitman. Travels 20-40 mm at the discharge end on each cycle. Bolt torque must hit spec — typically 800-1200 Nm on M30 hardware — or the plate walks during operation and shears the bolts.
- Eccentric Shaft: The heart of the machine. A forged steel shaft with an offset of 15-25 mm that converts rotation into the jaw's swinging motion. Runs in spherical roller bearings rated for 100,000 hours under design load. Bearing temperature should stay below 70°C — anything higher means oil starvation or misalignment.
- Pitman Arm: The connecting rod between the eccentric and the movable jaw. Cast or fabricated steel, oil-lubricated at the top eye. Cracks usually start at the bolt holes for the jaw plate, so weekly dye-penetrant inspection on high-tonnage units is normal practice.
- Toggle Plate: A sacrificial flat bar between the bottom of the pitman and the back frame. Designed to fail in compression if the chamber jams. Replacement toggles cost a fraction of an eccentric shaft — that's the whole point. Length sets the CSS, so changing toggle length is how you adjust product size on older Buchanan-style units.
- Tension Rod & Spring: Holds the toggle in compression and pulls the swing jaw back against the eccentric on each return stroke. Spring preload is typically 8-15 kN. Lose the spring and the jaw chatters, hammering the toggle seats.
- Flywheels: Two large flywheels on the eccentric shaft store kinetic energy between crushing strokes. Mass is typically 1500-3000 kg per side on a medium crusher. Without them, the motor would stall every time a hard rock entered the chamber.
Real-World Applications of the Buchanan Rock Crusher
The Buchanan design and its descendants run in primary crushing roles across mining, quarrying, and demolition recycling. Anywhere you need to take run-of-mine rock and reduce it to something a secondary cone or impact crusher can handle, a jaw crusher of this lineage is doing the work. The mechanism scales from 50 tph mobile units up to 2000 tph stationary plants.
- Hard Rock Mining: Primary crushing of gold ore at the Newmont Carlin operation in Nevada — feed up to 900 mm reduced to 150 mm CSS before the SAG mill circuit.
- Aggregate Quarrying: Metso Nordberg C160 jaw crushers (direct mechanical descendants of the Buchanan/Blake layout) running granite and basalt at 600-900 tph in European quarries.
- Construction & Demolition Recycling: Mobile tracked units like the Sandvik QJ341 process reinforced concrete on demolition sites, pulling rebar with overband magnets after the crusher.
- Iron Ore Processing: Vale's Carajás mine in Brazil uses primary jaw crushers in this lineage to handle hematite at 60% Fe before transfer to the secondary circuit.
- Cement Plant Feed Prep: Limestone primary crushing at Lafarge plants — typical reduction from 1000 mm pit feed down to 200 mm for the raw mill.
- Coal Preparation: Run-of-mine coal sizing at Appalachian operations, where the jaw crusher reduces top size before the breaker and screen circuit.
The Formula Behind the Buchanan Rock Crusher
The throughput of a Buchanan-style jaw crusher comes from a classic empirical relation tying chamber geometry, eccentric stroke, and rotation speed to volumetric capacity. At the low end of the operating range — say 180 RPM — you starve the machine and waste installed power. At the nominal 280 RPM, the chamber empties cleanly between strokes and you hit rated capacity. Push past 350 RPM and centrifugal effects throw rock back upward instead of letting it fall, so capacity actually drops. The sweet spot sits where stroke time matches the free-fall time of broken rock through the chamber height.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Throughput capacity | tonnes/hour | tons/hour |
| N | Eccentric shaft rotation speed | RPM | RPM |
| L | Length of jaw opening (along shaft axis) | m | ft |
| S | Stroke at discharge (eccentric throw) | m | in |
| Goss | Open-side setting (discharge gap) | m | in |
| ρ | Bulk density of crushed material | kg/m³ | lb/ft³ |
| k | Material flow factor (typically 0.4-0.65) | dimensionless | dimensionless |
Buchanan Rock Crusher Interactive Calculator
Vary feed size, CSS, jaw stroke, and eccentric speed to see product size, reduction ratio, jaw opening, and crushing cycles update.
Equation Used
This calculator uses the article example that a 75 mm closed-side setting gives about a 100 mm maximum product size. It also estimates open-side setting as CSS plus jaw stroke and treats each eccentric shaft revolution as one crushing cycle.
- Product top size follows the article example: 75 mm CSS gives about 100 mm maximum product.
- Open-side setting is approximated as CSS plus bottom jaw stroke.
- One jaw cycle occurs per eccentric shaft revolution.
- Ore hardness, liner wear, chamber fill, and fines generation are not modeled.
Worked Example: Buchanan Rock Crusher in a granite quarry primary crusher
You're sizing a Buchanan-lineage jaw crusher for a granite quarry primary stage. The crusher has a 1200 mm × 900 mm feed opening, eccentric stroke of 30 mm at the discharge, OSS of 150 mm, and granite bulk density of 1600 kg/m³. Material flow factor k = 0.5 for hard granite. You need to estimate throughput across the operating speed range to size the downstream conveyor.
Given
- L = 0.9 m
- S = 0.030 m
- Goss = 0.150 m
- ρ = 1600 kg/m³
- k = 0.5 dimensionless
- N = 180 / 280 / 350 RPM
Solution
Step 1 — compute the geometric volume per stroke. The discharge cross-section opens by S each cycle over the jaw length L:
Step 2 — at nominal 280 RPM, plug into the throughput equation with the correction term for chamber emptying:
That number is realistic for a 1200 × 900 jaw at this CSS — Metso publishes 550-700 t/h for the C120 in granite at similar settings, so we're in the right neighbourhood.
Step 3 — at the low end of the typical operating range, 180 RPM:
At 180 RPM the machine is loafing — you have spare power but you're undersizing your tonnage. The chamber sits half-full between strokes and the operator will see uneven discharge belt loading. Push the eccentric to the high end of the range, 350 RPM:
But in practice you won't see 840 t/h. Above roughly 320 RPM on a 30 mm-stroke machine, broken rock can't free-fall fast enough to clear the chamber between strokes. Capacity flattens then drops, the motor amperage spikes, and you start packing the discharge — known on site as choke-feeding the wrong way. Real-world peak on this geometry sits around 720 t/h at 310 RPM.
Result
Nominal throughput is approximately 672 t/h at 280 RPM with a 150 mm OSS. That's enough to keep a 1200 mm primary belt loaded at roughly 70% cross-section, which is right where you want it for surge tolerance. The full operating curve runs from 432 t/h at 180 RPM up to a real-world ceiling around 720 t/h near 310 RPM — the sweet spot sits at 280-300 RPM where chamber emptying time matches stroke period. If your measured throughput comes in 15-25% below this prediction, the most common causes are: (1) wet or sticky feed packing the lower chamber and reducing effective stroke volume, (2) jaw plate wear that has opened the CSS by 20+ mm without re-shimming the toggle, and (3) feed segregation putting fines straight to the discharge instead of bridging across the chamber so they're never properly nipped.
When to Use a Buchanan Rock Crusher and When Not To
The Buchanan-style jaw crusher isn't the only way to break rock at the primary stage. Gyratory crushers and impact crushers compete for the same job, and each wins on different ground. Here's how they stack up on the dimensions that actually drive selection.
| Property | Buchanan Jaw Crusher | Gyratory Crusher | Horizontal Shaft Impactor |
|---|---|---|---|
| Feed top size | Up to 1000 mm | Up to 1500 mm | Up to 600 mm |
| Throughput range | 50-2000 t/h | 1000-10000 t/h | 100-1500 t/h |
| Reduction ratio (single pass) | 4:1 to 6:1 | 5:1 to 7:1 | 10:1 to 20:1 |
| Capital cost (relative) | Low (1.0×) | High (3-4×) | Medium (1.5×) |
| Wear part life on hard granite | 6-12 weeks per jaw plate | 12-24 weeks per liner set | 1-3 weeks per blow bar |
| Best application fit | Hard, abrasive ore at moderate tonnage | Very high tonnage hard rock | Soft to medium rock, cubical product |
| Footprint and headroom | Compact, low headroom | Tall, requires deep installation | Compact, low headroom |
| Tolerance to tramp metal | Good — toggle plate sacrifices | Fair — relief springs | Poor — destroys blow bars |
Frequently Asked Questions About Buchanan Rock Crusher
CSS sets the closed-side gap, but rock leaves the chamber during the open phase too — and the OSS is always larger by the eccentric stroke. So if your CSS is 100 mm and your stroke is 30 mm, oversize up to 130 mm regularly slips through during the open stroke. That's normal physics, not a fault.
What inflates the spread further is jaw plate wear. As the corrugations flatten, the effective nip angle increases and rock slips upward instead of getting nipped, exiting only after multiple weak strokes that produce flaky particles. If your top size is creeping up over a campaign, measure your actual CSS with a lead ball test — you'll usually find it has opened 15-30 mm from the setpoint.
Double-toggle units move the swing jaw in nearly pure horizontal motion — there's almost no vertical rubbing component. That makes them the right pick for highly abrasive ore like silica-rich quartzite, where vertical rubbing on a single-toggle jaw chews through manganese plates fast.
Single-toggle machines are simpler, cheaper, and have higher throughput per unit mass because the swing jaw also moves vertically and pulls rock downward. The trade is jaw plate life — expect 30-50% shorter wear life on the same ore. As a rule of thumb, if your abrasion index (Bond Ai) exceeds 0.4, the double-toggle pays back through wear-part savings within 18 months.
Amperage swing on a stable feed almost always means the chamber is bridging and clearing intermittently. A slab of flat rock can wedge across the upper chamber, hold for several strokes while material accumulates above it, then collapse all at once — that collapse spikes the motor.
Check three things: feed segregation (too many flat slabs reaching the crusher because the grizzly is missing bars), choke-feed level (you should be running with the feed visible 200-300 mm below the top of the jaws, not lower), and flywheel keyway condition. A slipping flywheel key gives identical symptoms because the energy buffer between strokes vanishes.
The flow factor k in the throughput equation is the most-fudged number in jaw-crusher engineering. Textbook k = 0.5-0.65 assumes free-flowing dry hard rock. If your feed has more than about 4% surface moisture or more than 8% fines below 10 mm, real k drops to 0.3-0.4 and your throughput is exactly where physics says it should be — the prediction was wrong, not the machine.
Second-most common cause is feed presentation. If material drops onto one end of the jaw instead of distributing across the full length L, you're effectively running a shorter crusher. Centre-feed it through a properly sized feeder and you'll often recover 15-20% throughput overnight.
The crossover sits around 1200 t/h sustained. Below that, a jaw crusher costs less to buy, less to install (no deep concrete sub-structure), and is faster to repair. Above 1500 t/h sustained, a gyratory wins on throughput-per-dollar and on continuous discharge — a jaw crusher's reciprocating action pulses the discharge belt while a gyratory feeds it smoothly.
The other deciding factor is feed shape. Gyratory crushers handle slabby feed poorly because slabs sit flat across the mantle gap. If your blast pattern produces lots of flats, a jaw crusher actually outperforms a gyratory of equivalent rated capacity on real tonnage.
Repeat toggle failures usually trace back to one of three things, none of which is the toggle itself. First, worn toggle seats — the cast pockets in the pitman and back frame deform over time, and once the seat radius opens up, the toggle no longer sits flat and bends in three-point loading instead of pure compression. Replace the seats or weld them up and re-machine.
Second, tension spring fatigue letting the swing jaw slap forward at the end of each return stroke — the toggle takes a shock load it wasn't designed for. Third, running with the discharge packed because the conveyor below stops and nobody notices for 30 seconds. The packed material transmits the full crushing force into the toggle on every stroke until something gives.
References & Further Reading
- Wikipedia contributors. Crusher. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
