The Practical Utility Platform
The PUP is an example of ‘form follows function’; it is designed for low-cost, reliable, PRACTICAL applications. It can be summarized as a three wheel tractor, truck, and taxi whose primary power source (typically a small 6-7kW single cylinder diesel engine) can also be used to power a variety of implements like water pumps, generators, and maize grinders.
The Miriam-Webster dictionary defines UTILITY as “something useful or designed for use”; and the freedictionary.com defines UTILITY as “designed for various often heavy-duty practical uses”. Combining these definitions as ‘something designed for heavy duty practical uses’ provides a good description of the PUP project.
The PUP is a flexible PLATFORM for transporting heavy loads over rough terrain, and for powering a variety of labor-saving implements. It is designed for low cost operation and expandability. The term PLATFORM captures the project aspect of being a support for multiple activities, and a flexible launching pad for new opportunities.
Designed as a low-cost mobile utility platform suitable for off-road conditions, able to carry a payload of 700-900 kg (1,500-2,000 lbs), pull implements, and to power attachments with an on-board engine, the PUP is targeted towards entrepreneurs and small-holder farmers. The Purdue team is focused on provided a design that is locally owned, manufactured, and maintained for Sub-Saharan Africa (SSA) and similar regions. The PUP is designed to help communities meet the challenges of transportation, water scarcity, crop production, harvest, energy needs and more.
Weighing less than 500 kg, and able to carry a payload of nearly double that (tested up to 900kg), the base version of the PUP has a belt clutch transmission, 4-8 kw engine and a top speed (configurable) of 30-50km/hr. The lightweight truss frame is made entirely from 30x30mm angle iron, all locally sourced. No custom tooling is required as it is constructed simply by cutting and welding angle iron. Each wheel has a hydraulic break and coil spring suspension for safe and comfortable operation. A torsion bar has been incorporated into the design to improve the vehicle stability (increased roll stiffness), an important addition since the single front wheel does not contribute to roll stability and the roll stiffness without a torsion bar is approximately one-half that of a 4-wheel vehicle.
The three wheel design has several advantages. Cost is reduced since there are fewer components and the steering mechanism is a simple mechanical tiller. The frame design is also simpler since the frame experiences very little twisting, even under heavy loads, as opposed to 4-wheel vehicles where one wheel can lose contact with the ground on uneven road or trails. Performance off-road is improved for 2WD vehicles as well, since the rear traction forces are nearly constant, a result of the single front tire not contributing to the roll stiffness of the suspension. As with all designs there are trade offs, and in this case the primary disadvantage of a three wheel vehicle is that the roll (tipping) axis is a line between the front and rear tires and high speed sharp turns can lead to roll-over tendencies. With reasonable speeds (the PUP is limited to 50km/hr, and typically set up for 30km/hr max), maneuvers, and load placements this risk is avoidable.