A Bark or Cob Mill is a heavy-duty grinding machine that reduces tree bark or dried corn cobs into coarse particles using rotating beaters, burrs, or toothed rolls inside a fixed cage. Unlike flour-grade burr mills aimed at fine powder, this mill targets a coarse 3 mm to 25 mm output for tannin leaching or livestock feed. It exists because intact bark and cobs resist water, enzymes, and digestion — breaking them open releases tannin or makes the carbohydrate available. A typical farm-scale cob mill processes 200 to 800 kg of cobs per hour at 600 to 1200 RPM.
Bark or Cob Mill Interactive Calculator
Vary rotor diameter, RPM, breaker gap, and screen size to see tip speed, gap ratio, and how the mill compares with cob and bark speed targets.
Equation Used
The main mill speed calculation is hammer-tip speed: rotor circumference times revolutions per second. Cobs normally shatter around 40-60 m/s, while fibrous bark often needs about 60-80 m/s. The gap-to-screen value shows how tightly the breaker plate is set relative to the discharge opening.
- Rotor diameter is measured at the hammer tip circle.
- Speed targets use the article ranges: cobs about 40-60 m/s and bark about 60-80 m/s.
- Gap ratio compares breaker clearance to selected screen opening.
The Bark or Cob Mill in Action
The mill drives a rotor — fitted with hammers, beater bars, or a toothed cone — inside a stationary housing lined with breaker plates or a screen. Bark strips or whole cobs drop through a hopper onto the rotor, get smashed against the housing, and exit through a screen or grate sized to the target particle. The two jobs look similar but the materials behave differently. Bark is fibrous and stringy, so the mill needs sharp leading edges and a high tip speed (around 60 to 80 m/s) to cut fibres rather than just bash them. Corn cobs are brittle and shatter cleanly at lower tip speeds (40 to 60 m/s), which is why a hammer mill set up for cobs eats bark badly and packs the screen.
Clearance between rotor tip and breaker plate matters more than people expect. On a tanbark mill running 5 mm output, the gap should sit at 3 to 4 mm — open it past 6 mm and you get ragged ribbons that clog the screen instead of passing through. Corn cob grinders run wider, typically 8 to 12 mm gap, because the cob fragments size themselves against the screen rather than the breaker plate.
The common failure modes are screen blinding (wet bark plugs the holes), hammer wear (tanbark is abrasive thanks to embedded grit and silica), and rotor imbalance after uneven hammer wear. If you notice the motor amps creeping up over a shift, the screen is loading. If you feel new vibration through the frame, weigh the hammers — a 15 g difference across opposing pairs throws a 600 mm rotor out of balance enough to chew the bearings inside a month.
Key Components
- Rotor and Hammers (or Beater Bars): The rotor carries free-swinging hammers on pins or fixed beater bars. For tanbark, hardened hammers around 6 mm thick last roughly 200 to 400 hours before re-tipping. Hammer mass typically sits at 0.8 to 1.5 kg each on a farm-scale mill — opposing pairs must match within ±5 g to keep the rotor balanced.
- Breaker Plates / Liners: Replaceable wear plates bolted to the inside of the housing. They give the material something hard to be smashed against. On a cob mill running 800 kg/hr, expect to flip the plates at 500 hours and replace them at 1000 hours. Mn-steel or AR400 are the standard materials.
- Screen or Grate: Perforated steel below the rotor sets the maximum particle size leaving the chamber. Tanbark mills typically run 5 to 10 mm round perforations; cob mills run 6 to 25 mm depending on whether the output goes to dairy, beef, or biomass. Screen open area should be 35 to 45% — below 30% the mill chokes.
- Feed Hopper and Throat: Gravity or chain-fed inlet. Bark needs a controlled feed because a single thick strip can stall a 15 kW motor. Most production tanbark mills use a magnetic separator at the throat — a single nail or staple buried in bark will destroy a hammer set in one rotation.
- Drive (Belt or Direct): V-belt drive is standard because it slips on a sudden jam and spares the gearbox. A 600 mm rotor at 1200 RPM needs roughly 11 to 22 kW for cobs at 800 kg/hr, and 18 to 37 kW for bark at the same throughput because tanbark is tougher to shear.
- Discharge Chute: Carries the milled product to a bag, conveyor, or leaching vat. On tannin mills the chute often empties straight into a soaking pit because bark oxidises quickly once exposed and loses tannin yield within hours.
Where the Bark or Cob Mill Is Used
These mills sit at the front end of two completely different industries — leather tanning and animal feed — but the machinery is so similar that 19th-century mill builders like the C. & G. Cooper Company and Williams Patent Crusher sold the same base unit with different screens for either job. Modern operators ask the same questions: what particle size, what throughput, what does the material do to the hammers. The answers depend on whether you're feeding a tannin leach or a beef herd.
- Leather tanning (vegetable tan): Hemlock and oak tanbark grinding at heritage tanneries like S.B. Foot Tanning in Red Wing, Minnesota — bark milled to 5 to 8 mm, then leached for tannic acid before going into the pit liquor.
- Livestock feed (dairy and beef): Whole-cob meal at on-farm operations using Roskamp Champion or Bliss Industries hammer mills, producing 5 to 10 mm fragments to bulk up a high-energy ration with digestible fibre.
- Specialty mushroom substrate: Hardwood bark and cob mixes ground at small substrate facilities for shiitake and oyster log production, screened to 3 to 6 mm for moisture retention.
- Biomass pelletising: Pre-grinding step at wood pellet plants — bark from sawmill debarkers is reduced to 6 to 10 mm before passing through a CPM or Andritz pellet press.
- Smoking and BBQ wood: Hickory and oak bark crushed to 6 to 12 mm chips at suppliers like Kingsford and small craft producers for cold-smoke generators.
- Horticultural mulch and bedding: Pine and fir bark milled to 10 to 25 mm for landscape mulch, often produced as a co-product at sawmills running Morbark or Vermeer-style grinders.
The Formula Behind the Bark or Cob Mill
The throughput of a Bark or Cob Mill is governed by how much energy you can put into the material per unit mass to reduce it from feed size to product size. Sizing the motor is the practical question — too small and you stall on the first thick strip of bark, too big and you waste capital and burn idle current. At the low end of the typical operating range you're grinding dry, brittle cobs that need 4 to 6 kWh per tonne. At the high end you're grinding green, fibrous tanbark that needs 15 to 25 kWh per tonne. The sweet spot for a mid-sized farm or tannery sits around 8 to 12 kWh/tonne — enough margin to absorb a wet feed without tripping the overload, without oversizing the gearbox.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| P | Required motor shaft power | kW | hp |
| ṁ | Throughput (mass flow rate of material) | kg/hr | lb/hr |
| Especific | Specific grinding energy for the material | kJ/kg (or kWh/tonne × 3.6) | BTU/lb |
| η | Drive and mill mechanical efficiency | dimensionless (0.65 to 0.85) | dimensionless (0.65 to 0.85) |
Worked Example: Bark or Cob Mill in a small Pennsylvania tannery cob and bark mill
A small Pennsylvania vegetable tannery is sizing a hammer-mill drive for hemlock tanbark, with a target throughput of 500 kg/hr through a 6 mm screen. They also want the same mill to handle dried corn cobs as a side business for a neighbouring feed lot. Specific grinding energy for hemlock bark sits around 36 kJ/kg (10 kWh/tonne), and dry cobs around 18 kJ/kg (5 kWh/tonne). Drive efficiency is 0.75 across the V-belt and rotor bearings.
Given
- ṁ = 500 kg/hr
- Especific, bark = 36 kJ/kg
- Especific, cobs = 18 kJ/kg
- η = 0.75 —
Solution
Step 1 — at the nominal operating point, 500 kg/hr of hemlock bark, compute shaft power:
Round up to a standard 7.5 kW motor and you have a sensible margin. That's what you'd specify if bark were the only material on the menu.
Step 2 — at the low end of the typical operating range, dry cobs at the same 500 kg/hr:
The motor will loaf along at less than half load on cobs. Amps will sit low, the rotor barely notices the feed, and you can push throughput up to 800 to 1000 kg/hr on cobs without breaking a sweat.
Step 3 — at the high end, wet green bark with Especific climbing to 60 kJ/kg after a rainy week:
Now the 7.5 kW motor will trip on overload within minutes. Either drop throughput to 340 kg/hr, dry the bark first, or go up to an 11 kW (15 hp) drive to keep production steady through wet stock.
Result
The nominal shaft power for 500 kg/hr of hemlock bark is 6. 67 kW, so specify a 7.5 kW (10 hp) motor. On dry cobs the same mill draws roughly 3.3 kW and could run at nearly double the throughput; on wet bark after rain the demand climbs to 11 kW and a 7.5 kW motor will overload — the sweet spot for a multi-material tannery is the 11 kW drive with the V-belt sized as the slip fuse. If you measure the motor pulling rated amps but throughput sits below the predicted 500 kg/hr, three suspects: (1) the screen is blinding because bark moisture is above 25%, (2) the hammers have worn back more than 8 mm and lost their cutting edge, or (3) the breaker-plate clearance has opened past 6 mm and bark is folding instead of shearing.
Choosing the Bark or Cob Mill: Pros and Cons
Bark or Cob Mills aren't the only way to reduce these materials. The choice between a hammer-style bark/cob mill, a roller mill, and a chipper-shredder comes down to particle shape, throughput, and what the downstream process actually needs. Here's how they line up on the engineering attributes a buyer searches.
| Property | Bark or Cob Mill (hammer/beater) | Roller Mill | Chipper-Shredder |
|---|---|---|---|
| Typical output particle size | 3 to 25 mm | 0.5 to 5 mm | 10 to 75 mm |
| Throughput at 15 kW | 400 to 1200 kg/hr | 1500 to 3000 kg/hr (grain) | 800 to 2000 kg/hr |
| Tip / surface speed | 40 to 80 m/s | 5 to 10 m/s | 30 to 50 m/s |
| Wear part life (abrasive feed) | 200 to 400 hr (hammers) | 2000+ hr (rolls, regrooving) | 100 to 250 hr (knives) |
| Tolerance to tramp metal | Poor — destroys hammer set | Catastrophic — cracks rolls | Moderate — chips knives |
| Best application fit | Coarse fibrous bark, brittle cobs | Fine-grind grain, malt | Whole branches, slash, brush |
| Capital cost (farm-scale) | Low to moderate | Moderate to high | Low |
Frequently Asked Questions About Bark or Cob Mill
Specific grinding energy nearly doubles when bark moisture climbs from 15% to 30%. Wet fibres absorb impact energy instead of fracturing — the hammers smear the bark across the screen rather than shearing it cleanly. The screen blinds, back-pressure rises, and motor amps climb until the overload pops.
Quick check: pull the screen and look at the underside. If the holes are partially plugged with stringy fibre, you've confirmed wet-stock blinding. The fix is either pre-drying the bark to under 20% moisture, opening to a larger screen perforation temporarily, or de-rating throughput by roughly 40% during wet weeks.
Partly, but the hammer geometry matters too. Tanbark wants sharp, hardened leading edges to slice fibres at high tip speed. Cobs shatter on impact, so blunt or worn hammers still process cobs acceptably. If your mill has been running cobs, the hammers are probably too dull for efficient bark work — expect throughput on bark to be 30 to 50% lower than book values until you re-tip or replace.
The other gotcha is rotor speed. Many cob mills run 1800 RPM direct-drive; tanbark prefers 2400 to 3000 RPM tip speeds for clean fibre cutting. If your drive is fixed, the mill will work but it won't be optimal for both jobs.
Smaller is not automatically better. For tannin extraction, surface area drives leach rate, but particles below 3 mm pack tightly in the pit and channel water around the bed instead of through it. The classic spec from heritage tanneries like S.B. Foot is 5 to 8 mm — enough surface to release tannin in 4 to 6 weeks, coarse enough that the leach liquor still percolates uniformly.
If your pit is a modern pumped recirculation system rather than gravity percolation, you can drop to 3 to 5 mm and accept the higher pumping head. If it's traditional pit leach, hold the line at 5 mm minimum.
Almost always it's hammer mass mismatch. New hammers off the same pallet can vary by 10 to 20 g out of 1 kg — fine individually, ruinous on a rotor that needs opposing pairs matched within 5 g. At 1800 RPM a 15 g imbalance at a 250 mm radius generates roughly 130 N of rotating force, which the bearings feel as constant hammering.
Weigh every hammer on a kitchen scale before assembly and pair them up in matched sets across the rotor. If you've already assembled and the vibration is bad, pull the rotor and re-pair — don't try to balance by adding washers, which loosen and become projectiles.
If you need fine output below 2 mm — say for tannin powder or precision feed rations — a roller mill is more energy-efficient and produces a tighter particle distribution. Hammer mills inevitably make a wide spread, with both fines and oversize on either side of the target.
If your feedstock is whole branches with bark still attached, or sawmill slash with stringy material longer than 200 mm, a chipper-shredder upstream of the mill makes more sense than trying to feed long pieces directly into a bark mill, which will jam the throat and stall the drive.
Bark from trees harvested on sandy or volcanic soils carries embedded silica that acts like grinding paste against hardened steel hammers. Above roughly 1.5% ash content by weight, hammer life drops below 150 hours regardless of hammer grade.
The cheap field test: burn a 100 g sample in a crucible and weigh the residue. Above 2 grams of ash, plan on chrome-carbide overlay hammers or accept that you're replacing the set monthly. Operators who skip this test usually figure it out after their second hammer set vanishes in three weeks.
References & Further Reading
- Wikipedia contributors. Tanbark. Wikipedia
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