Index.htm Thr070.htm

LITERATURE REVIEW

Kline (1969) stated that pedal-operated or hand-operated drum threshers, common in Asian rice-growing countries, were rarely used in Equatorial Africa (it is almost certain that by 'pedal-operated' he meant in fact 'treadle-operated'). A few models had been brought in for trial by various governments, but up till then they had not been actively promoted by the agricultural extension services. Kline further stated that due to the increase in rice-growing in several African countries, there was a place for a small hand-operated grain thresher which could be adapted to engine power later. However, he did add that most treadle-operated and hand-operated threshers were very tiring to operate.

According to Hopfen (1960), treadle-driven drum threshers were satisfactory only when the grain was easily detachable from the stalks, as with rice, but not with wheat or barley.

Haynes (1964) stated that outputs of 7 - 8 kg per woman per hour were observed for hand threshing of rice in Northern Nigeria.

Islam (1977) stated that hand beating of rice produced 18 kg per man per hour, and that the output from treadle threshing is about 42 kg per man per hour.

Johnson (1966) , from tests with a treadle thresher at IRRI, on rice at 25% moisture content, quotes average figures of 20 kg per woman per hour for threshing and bagging; and 12 kg per woman per hour for gathering, threshing and bagging.

Sutton (1969), suing a drum thresher driven by a 1.7 kilowatt petrol engine, quoted outputs of 230 - 690 kg per hour for rice. The machine was of rasp-bar type, drum size 300mm diameter x 300 mm width, drum speed 1500 rev/minute for rice and 860 rev/minute for beans, sorghum and other crops.

Arnold (1964) , from instrumented experiments, gave the specific threshing energy requirement for wheat to be in the range of 0.77 kilowatt-hour per tonne.

VITA (1977) listed 15 manufacturers who produce small-scale threshers for rice and other crops, but without detailing which threshers were engine-driven and which were man-powered.

The Alvan Blanch company (UK) (1976) listed specifications of their 3 tractor-powered and engine-driven threshers.

CADU (1976) gave details of their experimental non-cleaning thresher powered by a 6 kilowatt engine.

The John Darbyshire company (UK) (1972) exhibited a one-man pedal-driven rasp-bar thresher with belt drive, mounted on skids for field transport.

The Indian Standards Institute (1965) issued a specification for treadle-driven rice threshers: the drum to have wooden slats with wire hoops, drum 400 - 430 mm diameter x 450 - 700 mm width. Drum to be gear driven from the treadle at 400 rev/minute through a gear ratio of greater than 3.5 : 1. Total machine weight to be 30-40 kg.

ITDG (1974) published constructional details of a hand-powered guinea-corn (sorghum) thresher developed in West Africa.

Malaya College of Agriculture (1967) issued a report and drawings of a peanut thresher, of mainly timber construction, using a drum with 4 wooden beaters. Bicycle hubs were used as bearings, and power was provided by one person through either a bicycle chain treadle drive or a flat belt pedal drive.

VITA (1972) circulated plans for a treadle-driven rice thresher of wooden construction, using wire hoops for threshing, with a rope drive system.

Jamanre (1970) produced plans for a wire hoop treadle thresher of steel construction with a 400mm diameter drum and a 3.3 gear ratio chain drive.

Suggs (1970) produced plans for a one-man pedal thresher for soya beans, using flat belt drive of gear ratio 3. Mainly wooden construction. Drum 500mm diameter x 800mm width. 20 flat wooden beaters.

The following table by Krendel (1963) gives formulae for endurance curves describing the maximum working performance of 'average; European male laborers, assuming 20% conversion efficiency in the muscles of food energy to mechanical work.

Where t is in minutes, and t ranges from 4 minutes to 480 minutes

For pedalling, presumably by well-trained young athletes, Krendel (1963) gives the following endurance curve:

P = 395 - (42.1 x ln(t)) watts where t is 1 through 100 minutes

Krendel (1963) also stated that optimal human muscle energy conversion efficiency is 25% and that this maximum efficiency occurs when the force exerted by the muscle is about 50% of maximum and the speed of muscle movement is about 25% of maximum. He further stated that optimal conversion efficiency and maximum power output do not occur together.

Based on German, French and Italian experimentation with man-assisted gliders in the 1920s, Krendel (1975) gave the following endurance curve formulae:

For a strong, but not highly trained cyclist:

P = 670 x (t) exp (-0.39) watts

Where t = 5 to 60 minutes

For a renowned racing cyclist;

P = 1490 x (t) exp (-0.42) watts

Where t = 5 to 60 minutes

Krendel (1963) stated that for steady-state activity, mechanical power production depends on the oxygen supply and the efficiency with which oxygenated blood can be transported to the muscles.

VITA (1975) stated that the muscle-mass of the legs is more than large enough to utiulise all the oxygen that can be absorbed, and that overall efficiencies of around 25% have been achieved for bicycle-type mechanisms. However, the muscles of the arms alone are not of sufficient mass to utilise completely the available oxygen, and overall efficiencies for arms alone are only about 16%. The maximum efficiencies which have been measured in practice have been for cycling mechanisms.

Wilkie (1960) stated that rowing is a relatively uneconomical method of producing external mechanical work because of the disproportionate wastage of energy due to the acceleration and deceleration of the whole body.

Sutton (1974) , in an experimental study of various types of man-powered winnowing devices, found by the use of respirometers that pedalling had 14 - 34 % ergonomic advantage over hand-cranking when gross power expenditure was greater than 350 watts; however pedalling was found to have 7 - 103% disadvantage when gross human power expenditure was less than 175 watts.

Addendum 1999 - Weir - Note that average male food consumption per day is 3,100 kilocalories, which is equivalent to about 3.5 kilowatt hours of energy. Therefore one can reasonably assume that 1 kilowatt-hour of mechanical energy per day per person is a probable upper limit for man-powered equipment, regardless of over how many hours or minutes that energy is produced and by what mechanism.

For Milling Grain, Omar (1977) showed that the traditional practice of pounding appeared to require 1.3 to 1.7 times the specific energy required by a plate mill utilising roller bearings. He also determined that the 'Hunts Minimill' (UK) used required a torque input of 26 Newton-metres when milling low moisture content maize to a fineness modulus of 2.4 .

The writer (1976) in unpublished tests, found that a 'home-made' mill with roller bearings throughout required only 80% of the specific energy required by a commercial mill, using the same plates, but with metal-to-metal journal bearings throughout.

Standard textbooks on milling, when comparing plate milling with hammer milling, state that plate milling is more efficient for coarse flour (e.g. for chicken feed), whereas hammer milling is more efficient for fine flour (by a factor of approximately 1.4 for maize of fineness modulus 2.4).

The UK Tropical Products Institute (1977) exhibited a one-man-powered bicycle-mounted hammer mill, driven at high speed by a roller rubbing on the rear tyre of a bicycle mounted on its stand.