The unglamorous star of a palm oil mill is a rotating steel drum that rarely tops 25 rpm. Its speed—and the way its internal baffles lift and drop bunches—decides whether mills hit ≥95% fruit detachment with minimal bruising.
Walk the line in a palm oil mill and you’ll find a giant cylinder doing quiet, relentless work. This is the thresher (also called a stripper): a large rotating drum, typically 1.2–2 m in diameter and 6–12 m long, that tumbles sterilized fresh fruit bunches (FFB; whole bunches after steam sterilization) until the fruits shear off and fall away Palm Oil Mill Handbook Vortech Global. The drum sits slightly inclined; sterilized bunches enter one end, and as the shell turns, internal lifter bars (baffles or flights) raise each bunch before gravity drops it—an impact-and-friction routine that frees the fruit. Perforations in the shell—often ~25–40 mm holes—let detached fruits fall to a cleaning conveyor, while empty fruit bunches (EFB; the stripped bunch skeleton) exit the far end for biomass use Vortech Global Palm Oil Mill Handbook.
In practice, drum diameters commonly run 1.8–2.0 m and lengths 3–5 m, with longer drums (up to ~5 m) giving a “better threshing effect” Palm Oil Mill Handbook. Designs often use spiral or radial lifter paddles welded inside the shell to lift bunches uniformly—typically 4–8 lifter bars spaced evenly around the circumference at a slight angle—so each rotation produces repeated lift-and-drop cycles that maximize agitation and fruit release. Proper clearance between lifters helps detached fruit fall cleanly while carrying EFB forward Vortech Global Vortech Global.
Drum geometry and capacities
Industry guides peg processing capacities at 5–60 t FFB/h for 1.2–2.0 m drums, driven by 15–45 kW motors, with the best designs targeting ≥95% fruit detachment and <2% bruise rate Vortech Global Vortech Global. One supplier cites well‑designed drums of roughly 1.8 m × 8–12 m running ~18–25 rpm, achieving ~95% fruit recovery with under 2% damage, slow enough to prevent bruising yet forceful enough to strip efficiently Vortech Global Vortech Global.
Rotational speed and fruit release
The drum’s rotational speed (rpm; revolutions per minute) is the make‑or‑break parameter. It must be high enough to lift and accelerate bunches, but not so high that centrifugal force pins them to the wall. Large drums (≥1.8 m diameter) operate very slowly—typically 18–25 rpm—so gravity can do the heavy lifting at the top of each turn Vortech Global Palm Oil Mill Handbook. For a 1.8 m drum, the handbook’s empirical guidance lands at ~24–25 rpm (expressed as sqrt(1800)/d ≈ 25), setting the tip speed so the bunch lifts to just before the vertical center line and then drops as weight overcomes centrifugal force—precisely when detachment is strongest Palm Oil Mill Handbook Palm Oil Mill Handbook.
Too slow, and fruits won’t shake free; too fast, and both fruits and whole bunches stick to the wall, choking off the threshing action. As the handbook puts it, the bunch should rise “just before the vertical center line… where the weight of the bunch overcomes the centrifugal force… and the bunch drops”—if bunches remain atop the drum (from too‑high speed or improper lifter elevation), “threshing action will be incomplete” and re‑stripping may be needed Palm Oil Mill Handbook.
What the data say about speed
Even small machines show the trend. In a farm‑scale stripper (0.5–0.6 m drum), raising shaft speed from 320 to 500 rpm increased output from ~59 to 249 kg/h and boosted stripping efficiency from ~16% to ~94%—dramatic gains that underline the role of rotational energy, even though small drums run at much higher rpm than mill‑scale units ResearchGate. In real mills, the equivalent tip speed is achieved at low rpm on large diameters, with a target benchmark often set at ≥95% fruit recovery and <2% bruising as KPIs (key performance indicators) Vortech Global Vortech Global.
An industry example: a vendor spec sheet shows 1.2–2.0 m drums (6–12 m long) running at 18–25 rpm with 30–45 kW drives, yielding 5–60 t FFB/h capacity—numbers consistent with the handbook’s ~24–25 rpm for a 1.8 m drum and the “just fall off” criterion at the top of the shell Vortech Global Vortech Global Palm Oil Mill Handbook Palm Oil Mill Handbook.
Internal baffles (lifters) and their geometry
The baffles—those internal steel bars or wings—are as consequential as rpm. Their job is to lift bunches and let them fall, turning rotation into fruit‑detaching impacts. Lifters are often arranged as spiral (helical) or radial arms, with spiral flights giving continuous upward motion and gentle mixing, and radial lifters releasing more abruptly on each pass. Both approaches aim to maximize kneading without over‑agitating Vortech Global.
Three design levers dominate. First, lifter height and angle: plates are set so bunches lift to near top‑dead‑center before gravity takes over, matching the handbook’s drop‑point guidance; too short and bunches won’t drop decisively; too tall or steep and bunches can stick Palm Oil Mill Handbook. Second, spacing and count: evenly spaced lifters (e.g., 30°–90° apart) tune drop frequency and height—too many gives frequent, small drops; too few yields large, infrequent drops that risk fruit shattering or trapping. Manufacturers typically use 4–12 lifter bars, adjusted for drum length and bunch size. Third, adjustability: mills equip drums with adjustable or replaceable lifter bars to fine‑tune impact frequency and intensity as fruit condition and wear evolve Vortech Global.
The results are measurable. Configurations that maximize tumbling achieve fruit release efficiencies close to theoretical (≥90–95%); in a small‑machine trial, multiple internal flights delivered an average stripping efficiency over 90% across ripe bunches, while a poorly configured drum creates dead zones where bunches slide without dropping, trapping fruit instead of freeing it ResearchGate. The handbook is explicit: if bunches stay at the top because speed is too high or lifter elevation is off, “threshing action will be incomplete” and re‑stripping can follow Palm Oil Mill Handbook.
Maintenance matters: worn or deformed lifters reduce lift, and clean perforations are essential—blocked holes can raise internal pressure and prevent smooth outflow of fruit, changing the way bunches tumble and undermining efficiency Palm Oil Mill Handbook.
Performance metrics and losses
The operational targets are clear. Fruit removal efficiency should be ≥95% in well‑designed stripper drums, with remaining fruit typically handled downstream by secondary equipment; bruising should stay under 2% Vortech Global. Throughput ranges widely: large drums (≥1.5 m) at around 30 rpm can process roughly 30 t/h of FFB, and vendor specs bracket 5–60 t/h for 1.2–2.0 m drums; in small‑scale tests, a 500 rpm unit handled ~249 kg/h with <5% unstripped fruit Vortech Global ResearchGate.
Specific energy depends on motor size (often 15–45 kW for large drums) and feed rate; good practice is to run as slow as possible for the required throughput. Incomplete stripping shows up as oil loss in EFB. Milling texts note that up to ~30% of oil loss can originate from poor sterilization/threshing, whereas well‑optimized drums can reduce this to ~1% loss at the threshing stage Palm Oil Mill Handbook Palm Oil Mill Handbook Palm Oil Mill Handbook.
Material condition and control
Material condition compounds machine settings. In one development study, increasing shaft speed (320→500 rpm) and ripeness raised output from ~60 to 249 kg/h and boosted stripping efficiency from ≈16% to ≈94% ResearchGate. A more recent Nigerian design report found threshing efficiency rose from 87% to 93% as FFB storage time increased from 1 to 3 days (fruits softened), with throughput gains from 258→468 kg/h over the same interval ResearchGate.
The takeaway: small changes in speed or lifter setup can have large effects. Managers track fruit recovery, the weight of fruit remaining on EFB (incomplete stripping), and fruit damage to set rpm. Automation or feedback control can help—torque/load monitoring can cue adjustments to speed or feed rate to hold stripping near the ≥95% target Vortech Global.
Design rules of thumb
Across handbooks and vendor specs, the recipe is consistent: match rotational speed to drum size so bunches lift to near the top and drop under gravity; configure spiral or radial steel lifter bars—typically 4–8, adjustable for height and angle—to maximize repeated lift/drop cycles; keep perforations clean and maintain clearances. Benchmarks: ~20–25 rpm for a ~1.8 m drum, ≥95% fruit recovery, and <2% bruising, with 5–60 t/h capacities on 1.2–2.0 m drums powered by 15–45 kW motors Palm Oil Mill Handbook Palm Oil Mill Handbook Vortech Global Vortech Global.
