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DRIVE MECHANISM COMPARISON

It is mentioned above that the trolley pump was chosen in preference to the 2-man pedal pump. The advantages of the trolley pump are:

• the pump is simpler to construct – it requires less man-hours, less skill, and less steel. It is therefore cheaper to produce
• two rubber-sealed ball bearings replace effectively the 2 wooden rotating bearings, the one wooden oscillating bearing, and the one wooden sliding bearing of the pedal pump, thus increasing efficiency and reducing maintenance. However, by redesigning the pedal pump, ball bearings could be used instead of 3 of the above 4 bearings.
• because no flywheel is required, mass is greatly reduced. In addition the trolley system is dimensionally more compact.

However, the pedal pump did have also some advantages:

• the flexibility to adjust to different pumping heads and to operators of different physical abilities is largely lost – this was possible with the pedal pump by a simple adjustment of the crank throw. With the trolley pump however, some flexibility can be provided by varying cycle time and crank stroke. Also during manufacture it is possible to choose between several cylinder diameters.

Ergonomically it would appear that pedalling is superior to the action required for this pump, which is similar to rowing. However Wilkie’s (1960) objection about energy wasteage due to the acceleration and deceleration of the whole body is probably not as valid as it is for rowing, since that energy can be absorbed by the pump suction or delivery actions.

Nevertheless, it is true that the effort exerted by each operator is concentrated over 25% of each pump cycle – during that period the operator not only provides power for the suction or delivery of water, he also accelerates the combined mass of both operators, trolley and pushrod. Towards the end of his half of the pumping cycle, however, the deceleration of this combined mass provides stored energy for the suction or delivery of water.

As an example, it requires an additional 34 watts of power to accelerate a mass of 140 kg (2 adults?) from rest through a distance of 350mm in 1 second. The required acceleration is fairly low, at 0.07 g . These figures are probably typical for a cycle time of 3 seconds, and do not appear excessive.

More troublesome, speaking from experience, is the learning curve of 2 operators learning to push at times which suit each other. For operators’ comfort, adjustable footrests would seem to be important.

No village trials have yet been conducted at Morogoro, and no news has filtered back from the prototype sold to UNICEF for the project in Mali, but people’s reaction to the drive mechanism should be interesting.

Summing up, the writers’ prejudices would suggest that the technical simplicity of the trolley pump outweighs any advantages when compared with the previous design of pedal pump, but this is not yet borne out in practice.

PRODUCTION COSTS........................................................................Pump130.htm

OPERATIONAL COSTINGS...............................................................Pump140.htm

SUMMARY AND CONCLUSIONS....................................................Pump150.htm

REFERENCES......................................................................................Pump160.htm

APPENDIX........................................................................................... Pump170.htm

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