Case study
Low-Cost Root Crop Washer for Ghana
A hand-cranked, electricity-free root crop washer for smallholder farmers in Ghana, specified with QFD and evaluated with mathematical models and a design FMEA. CAD is in progress; prototype build and testing are upcoming.
- Role
- Purchasing and supplier engineering project lead
- Affiliation
- Iowa State ME 2700, team of five
- Tools
- SolidWorks, QFD, Design FMEA
- Started
- Aug 2026
- Status
- Status: In progress
Key results
- Batch capacity (predicted)
- 5 kg
- From models, not tested
- Crank torque (predicted)
- 20 N·m
- 100 N at 0.20 m radius
- Cycle time (predicted)
- 6.3 min
- Target 5 min or less
- Throughput (predicted)
- 47.7 kg/hr
- From models, not tested
Problem and constraints
This is a team design project for ME 2700 (Introduction to Mechanical Engineering Design) at Iowa State, run with a Design for Six Sigma framework. We are designing a low-cost, manually operated washer for smallholder farmers in Ghana who process cassava, yams, and sweet potatoes. It has to work without electricity. We identified two customer groups: farmers, and local entrepreneurs and manufacturers.
We benchmarked manual washing, a Lee Valley gardener’s wash basket, a Univerco vegetable washer, and a FoodTechProcess rotary drum washer. Manual washing is cheap but labor-intensive. The commercial washers are high-capacity but costly and complex.
| Requirement | Target |
|---|---|
| Soil and debris removal | 80% or more |
| Crop damage | under 1% |
| Water per load | 2 L or less |
| Manual operating force | 130 N or less |
| Cycle time | 5 min or less |
| Operators | 1 to 2 |
| Power | none |
| Manufacturing cost | roughly $100 |
The targets came from Quality Function Deployment (QFD). The project charter lists me as Purchasing and Supplier Engineering Project Lead. My specific contributions: all of the engineering drawings, the base subassembly, and the handle subassembly.
Approach
We split the function into five parts: produce loading, water delivery, cleaning, drainage, and unloading. From a function tree, flowcharts, and morphological charts we generated four full concepts, each emphasizing one of portability, efficiency, reliability, or affordability. A weighted QFD decision matrix selected a hand-cranked rotating wash drum with controlled water delivery, gravity drainage, and simplified unloading. The refined design scored highest in a second-round weighted matrix.
Loading 3D model
Analysis
Preliminary mathematical models cover crank torque, water consumption, gravity drainage, and unloading. They rest on the assumptions below, and nothing has been measured yet.
| Model input | Value |
|---|---|
| Hand force on crank | 100 N at 0.20 m radius |
| Drum speed | 20 rpm (100 revolutions in 5 min) |
| Water delivery | 0.5 L/min for 4 min (2.0 L per load) |
| Loading | 0.73 min (6.8 kg/min) |
| Drain | 25 mm gravity drain, Cd 0.62 |
| Unloading | friction coefficient 0.40, hatch angle 30 degrees |
| Metric | Predicted | Target |
|---|---|---|
| Manual operating force | 100 N | 130 N or less |
| Cycle time | 6.3 min | 5 min or less |
| Water per load | 2.0 L | 2 L or less |
| Crank torque | 20 N·m | - |
| Batch capacity | 5 kg | - |
| Throughput | 47.7 kg/hr | - |
The predicted 6.3 min cycle time exceeds the 5 min target.
A Design FMEA ranked insufficient water flow (RPN 96), drain blockage (RPN 84), and crank slipping or breaking (RPN 64) as the top failure modes, with mitigations for the highest-risk ones.
Test plan
The planned protocol has three parts, with three repetitions per condition: water flow versus reservoir head, drainage and clogging with debris, and cleaning effectiveness versus load mass. None of these tests have been run.
Status
The CAD assembly is in progress. The BOM, manufacturing documentation, prototype build, testing, and Design Expo are upcoming. There are no measured results yet.