Design and Fabrication of a Pedal-Operated Twin-Roller Coconut Dehusking Machine

This project designs, fabricates and tests a pedal-operated coconut dehusking machine for small coconut farmers and copra processors in the Pollachi–Udumalpet belt of Tamil Nadu. Dehusking is still done largely by hand on a fixed pointed spike, which is tiring, needs skilled labour that is increasingly hard to find in the harvest season, and causes frequent hand injuries. Motorised dehuskers exist, but they cost more than a small grower can justify and depend on reliable power at the farm shed.

Our machine uses two counter-rotating spiked rollers driven by a bicycle-style pedal crank and chain drive, with a pair of equal spur gears to reverse the direction of the second roller. The seated operator pedals while pressing the nut into the nip between the rollers from a feed tray; the spikes pierce and peel the husk away in strips. No electricity is required.

The work covers the full engineering cycle expected in a final-year mechanical project: problem study at a farm, design calculations (dehusking force, torque, chain ratio, shaft diameter using the ASME code and maximum-shear-stress theory, bearing selection), 3-D modelling in SolidWorks and 2-D drawings in AutoCAD (ME3681 CAD/CAM Lab), optional FEA of the shaft and spikes in ANSYS, fabrication with a process sheet, a cost estimate in ₹, and field testing of dehusking time, husk-removal percentage and operator effort. SolidWorks and ANSYS are industry-standard tools used alongside the syllabus labs.

Abstract

Coconut dehusking in small farms of Coimbatore and Tiruppur districts is a manual, skill-dependent operation performed on a fixed spike. It is slow, fatiguing and a common cause of hand injuries, and labour for it is scarce in peak season. This project presents the design and fabrication of a low-cost, pedal-operated twin-roller dehusking machine. Pedal power is transmitted through a chain drive to a spiked roller, and a spur-gear pair drives a second roller in the opposite direction. The machine is designed using standard machine-design procedures, modelled in SolidWorks, fabricated from mild steel and EN8 shafting, and evaluated on dehusking time per nut, husk-removal percentage, kernel damage and operator effort.

Introduction

Tamil Nadu is one of India's major coconut-producing states, and Pollachi is known for its coconut trade, copra drying and coir units. Dehusking is the first post-harvest step before copra making, tender-nut trade or sale to processors. A skilled worker on a fixed spike can process many nuts per hour but tires quickly, and the operation needs strength and practice that younger workers often lack.

Existing Methods and Literature Gap

• Manual fixed-spike method: cheap but skill-dependent, fatiguing and unsafe.
• Hand-lever tools (developed by agricultural universities and research institutes): safer, but still slow and rely on arm strength.
• Motorised roller dehuskers: high output, but cost, power supply and maintenance are barriers for a smallholder.
• Gap: few designs use the leg muscles, which are far stronger than arm muscles for sustained work, to drive a continuous roller mechanism at low cost with no electricity.

Proposed System

• Seated pedalling at a comfortable cadence, reduced through a chain drive to drive the spiked rollers at low speed and high torque.
• Twin counter-rotating spiked rollers with adjustable centre distance for different nut sizes.
• Mild-steel welded frame, pillow-block bearings, guard over the chain and nip, and a feed tray at elbow height.

Feasibility

• Technical: uses standard sections, bearings, bicycle chain parts and workshop processes available in any college lab.
• Economic: estimated material and fabrication cost below ₹20,000, with no running energy cost.
• Operational: one operator, minimal training, easily repaired by a local welding shop.

Small coconut growers and copra processors in Tamil Nadu depend on manual dehusking with a fixed spike. The method is slow, physically exhausting, needs skilled labour that is scarce and increasingly expensive during harvest, and causes frequent hand and wrist injuries. Motorised dehusking machines are available but are beyond the budget of a farmer with a few acres, need a reliable power supply at the farm shed, and require maintenance skills that are not available locally.

There is a need for a low-cost, human-powered dehusking machine that uses the operator's leg power through a pedal and chain drive, dehusks nuts of varying size with little kernel damage, is safe for an unskilled operator, can be fabricated and repaired in a rural workshop, and whose performance — dehusking time per nut, husk-removal percentage and operator effort — is measured and compared with the traditional method.

In scope

• Mature (dry) coconuts of the local tall and hybrid varieties commonly harvested around Pollachi.
• One seated operator; pedal-driven twin spiked rollers; adjustable roller gap.
• Design calculations, CAD models and drawings, optional FEA, fabrication, cost estimate and field testing.
• Safety features: chain guard, nip guard, and a stop brake on the pedal shaft.

Out of scope

• Tender (green) coconuts, which need a different cutting mechanism.
• Motorised or hybrid drives (discussed only as future work).
• Shell breaking, copra drying or coir-fibre processing.
• Long-term durability testing beyond the project duration.

The project follows a design–fabricate–test methodology in the 300-period ME3811 slot:

Worked design calculations (replace assumed values with your measurements)

• Human power: sustained seated pedalling ≈ 75 W at 50 rpm crank speed (standard ergonomics figure; confirm with your operator).
• Chain ratio: 18T pedal sprocket → 36T roller sprocket, i = 2, roller speed = 25 rpm.
• Average roller torque: T = P/ω = 75 / (2π × 25/60) ≈ 28.6 N·m.
• Peak torque: pedal force 250 N × crank 0.17 m = 42.5 N·m; at roller = 42.5 × 2 × 0.95 ≈ 81 N·m.
• Dehusking force check: with spike tip radius 60 mm, peak tangential force ≈ 81 / 0.06 ≈ 1,350 N; this must exceed the tearing force from your pilot test (spring-balance or load-cell pull on a spike driven into the husk).
• Shaft (ASME code): M = F L / 4 = 800 N × 0.4 m / 4 = 80 N·m (assumed radial load at mid-span); Kb = 1.5, Kt = 1.0; C45/EN8 with τ_allow = min(0.3 Syt, 0.18 Sut) reduced 25% for keyway ≈ 81 MPa; d³ = (16/πτ)√((KbM)² + (KtT)²) gives d ≈ 21 mm → select 25 mm.
• Maximum-shear-stress check: τ_max = (16/πd³)√(M² + T²) ≈ 37 MPa at d = 25 mm, below Syt/(2 × FOS 2.5) ≈ 76 MPa.
• Bearings: UCP205 pillow blocks (25 mm bore, 6205-class insert). Equivalent load ≈ 750 N with service factor; L10 life far exceeds requirement, so the bore size, not life, governs selection.