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CLOSE THIS BOOKIntroduction of Animal Powered Cereal Mills (GTZ, 1996, 70 p.)
Part II: Construction of the Animal-Powered Mill
VIEW THE DOCUMENT2. How Efficient is a Draft Animal and how much Power is Required for Driving a Mill?
VIEW THE DOCUMENT3. The Frictional Wheel -Principle of the Animal-Powered Mill
VIEW THE DOCUMENT4. The Idea and its Realization
VIEW THE DOCUMENT5. Functional Structure of the Animal-Powered Mill
VIEW THE DOCUMENT6. Construction of the Functional Units
VIEW THE DOCUMENT7. Dear Reader...

Introduction of Animal Powered Cereal Mills (GTZ, 1996, 70 p.)

Part II: Construction of the Animal-Powered Mill

1. Preface

The second part of this guide is intended for the technically proficient reader who considers the animal-powered mill a practicable alternative to both manual labor and motor-driven mills in his field of activities, and who wants to test, construct and possibly propagate it. This construction guide should not be used as a conventional manual. We take the view that instructions which precisely dictate the arrangement, dimensions and materials to the reader are not in line with the concept of appropriate technology. Moreover, they make it impossible for the reader and his co-workers to contribute their own ideas and experiences and thus help to improve the device. The development of the animal-powered mill is yet to be completed. There remains a lot of work to be done before the animal-powered mill, including all its components, can be constructed in countries which are short in material supplies, and before it fully satisfies the needs of its users. We would like to invite the readers of this guide to help us with this work. We do not intend to offer universal solutions but rather construction examples. The main emphasis will be put on the explanation of the reasons why we opted for certain solutions, why other solutions turned out to be less suitable, and why some components involve specific problems.

This guide is not designed to take away the role of the reader in constructing his own animal-powered mill. On the contrary' it is our prime intention to give the reader an incentive to think over his own needs and then design his own adaptations to specific problems of application. Another purpose of this guide is to help the reader avoid experiencing difficulties which others have already encountered.

2. How Efficient is a Draft Animal and how much Power is Required for Driving a Mill?

Table I surveys the performance of different draft animals. In view of the fact that the average performance of draft animals can be compared with the efficiency of electric kitchen appliances, it may seem hardly understandable that the mastery of the animal tractive power can pose actual problems for the mechanical engineer. It is, however, necessary to bear in mind that the maximum tractive power of the animals, which can be maintained for a very limited time, can be ten times the normal optimum level. Although their maximum performance is relatively low, they can develop enormous tractive power because they move very slowly.

The dimensioning of the driving gear of the power gear depends on the maximum torque the draft animal can exert on the power gear. If the machine of a traditional toothed power gear without predetermined breaking points blocks, when driven by an ox of 500 kg, the maximum torque would be approximately 15 000 Nm, thus corresponding to 15 times the maximum motor torque of a Peugeot 504 Diesel.

For this reason it is necessary to limit the torque that can act upon the power gear by means of predetermined breaking points as e.g. shearing pins or by means of torque limiters such as belt drives or frictional wheels.

On the other hand, the machine - in this case the grinding unit - must be designed for the average performance of draft animals i.e. for 200 to 500 W because it has to be operated for several hours a day.

In spite of the low efficiency, the daily continuous - operation requires a solid construction of the grinding unit. During the initial stages of the project, relatively small grinding units were used which had been designed for the actuation by means of small motors (3 to 5 cont.hp.) The power required for driving these mills was reduced by operating them with a lower number of revolutions (60 to 150 rpm) than originally planned (400 to 600 rpm).

Tab. 1: Tractive power of different draft animals in developing countries


Average weight of animal (kg)

Approx. draft (continuous) (kg)

Average speed (m/s)

Power developed (W)

Light horse













0.8 -0.9










0.9- 1







At the same time, however, the development of a grinding unit adapted to the power gear began; a grinding unit which, moreover, could be constructed by local craftsmen.

The construction is determined by the power to be transmitted and by the ratio of transmission. For the time being, these parameters can only be calculated roughly. Their actual values differ very much from region to region depending on the condition of the draft animals, the cereals, and the desired fineness of the flour. When a power gear is tested in a certain region for the first time it is always likely that further modifications will be necessary after the installation.

The power to be transmitted corresponds to the average performance of the draft animals available in the respective region (see Table 1).

Experience shows that the required speed of the mill varies between 60 and 150 rpm. For donkeys, a lower value can be assessed, for oxen a higher one, and for horses a medium value.

3. The Frictional Wheel -Principle of the Animal-Powered Mill

At first sight the technician might consider a number of solutions more appropriate than the somewhat unconventional construction principle of the animal-powered mill (for a more detailed description see Chapter 3.1 of the first part of this guide). Toothed power gears, rope power gears, chain power gears (Fig. 1-3) are only a few of the numerous constructions known in the history of technology.

All the solutions have two things in common:

- due to the high torque exerted by the animals the machine parts of the first gear stage have to be extremely large and are therefore expensive.
- the gearing has to be connected by means of underground shafts and cardan joints.


Fig. 1: A 19th century German toothed power gear
Fig. 2: A rope power gear patented in Germany in 1882

Fig. 3: A "chain power gear" patented in the German Reich in 1879

Since high-quality and expensive machine parts must be used, small farmers in Third World countries cannot afford these animal-powered mills. Today, a power gear like the one shown in Fig. I would cost as much as a small car. We take the view that a modern-day comeback of traditional animal powered mills as shown for instance in Fig. 1 will only make sense if they are already used widely in the respective area, and if the stone required for the grinding unit and the necessary know-how for the production of the stones are available locally. Even in this case it is doubtful whether such a mill would be cheaper than a mill with small, high- speed grinding stones and a gearing.

In comparison with conventional power gears, the principle of the "runner wheel power gear" has the following main technical advantages:

- for the often problematic first gear stage of the power gear, a concrete path that can be easily constructed locally and a universally available car wheel are used
- since the grinding unit runs round in a circle with the animal, long subterranean shafts and cardan joints can be dispensed with
- the frictional wheel principle effectively protects the power gear as well as the machine against overload.

Fig. 4: Sketch of a traditional animal-powered mill from Hammamet, Tunisia

4. The Idea and its Realization

Before going into technical details, we want to give the reader a first insight into the construction of the animal powered mill and its specific problems by presenting the "ancestral portrait gallery" of the power gear.

The idea in the history of technology

Fig. 5: A 19th century runner wheel power gear patented in Germany in 1888. The wheel was made of steel and ran on a steel rail. Due to the low friction coefficient of the friction pairing steel-steel the wheel had to be extremely heavy and was therefore expensive. It may be presumed that its high price was the reason why this power gear did not become generally accepted.
in technical cooperation

Further development

Fig. 10: A staff member of the American organization ATI and Cheikh Gueye have worked out a number of additional simplifications. which - if they prove their worth in practical operation - could result in a further considerable reduction of the production costs. The semicircular harness has been dispensed with. This implies certain disadvantages (less ideal power transmission more difficult guidance of the animal). On the other hand it is now possible to replace the rigid frame construction by a simple pipe (3") since the bending moment acting upon the axle is lower. Instead of the chain drive a friction roller made of cast iron which is directly driven by the car wheel is used as a second gear stage. The construction is presently being tested in Senegal. If it proves its worth in practical operation, a more ' detailed description will be given in a supplementary sheet to this guide.

5. Functional Structure of the Animal-Powered Mill

In order to tackle the construction in a structured manner, a number of conditions must be established. These conditions must be considered when dividing the overall function into several partial functions which are then assigned functional units (Fig. 12, p. 34). Most of the conditions were already defined in the previous chapters.

The precondition that the animal goes round in a circle might be considered banal. One could, however, conceive a device whose wheel runs on a straight path (i.e. a kind of cart whose wheels drive the mill). The most important advantage of the circular movement is that the animal moves almost without guidance once it gets used to it. Another advantage of the circular movement is that there is no loss of time because the animals must not turn round as is the case with a straight path.

Before a functional principle can be established, first decisions with regard to the basic construction of the individual functional units must be made:

Drive mechanism: The car wheel which runs along on a circular path is the heart of the drive mechanism. The transmission obtained in this way is, however, not sufficient for driving the mill. A second transmission by means of a chain, a belt drive, or similar' is essential. The wheel is connected to the second gear stage by a shaft.

Central axle, frame: The wheel must be guided in a circle. For this purpose the drive unit is connected to a bearing unit at the central point of the path by means of a frame, or similar.
Grinding unit: A high-speed, small-size mill with a grinding mechanism made of stone disks, similar to the one used with small-power motors, serves as a grinding unit.

The combination of these functional units results in the functional principle shown in Fig. 11.

Fig. 11: The functional principle of the animal powered mill

FIGURE 12: Functional diagram of the animal powered mill.

6. Construction of the Functional Units

6.1 Harnessing

The harnessing is designed to transmit the tractive power of the animal with the least possible loss to the power gear. The slightest loss is attained when the power transmission is

- as tangential as possible and
- as horizontal as possible.

Tangential power transmission

It is easier for the draft animal to move along a circular track if the front and hind legs move on the same radius.

Therefore, power transmission will always be subject to losses of power when transmitted to the power gear behind the animal, since in this case power transmission at a right angle is impossible (Fig. 14).

For this reason it is advisable to transmit the force to the power gear between the front and hind legs of the animal (Fig. 15)

Fig. 13: Harnessing

Fig. 14: Power losses occur because the draft animal is not harnessed at a right angle.

Horizontal power transmission

When a draft animal draws a load under a vertical angle, this load can be divided into a horizontal tractive load component and a vertical carrying load component (Fig. 16). When the power gear is in operation the carrying load component results in a loss in efficiency. After all, the draft animal is supposed to pull the power gear and not to lift it. The ideal solution would be to harness the draft animal horizontally to the power gear by means of a pull rope or a pull chain. This is only possible with horses and donkeys, because these animals draw mainly with their breasts and shoulders, whereas oxen and cows draw with their necks. For pressing the yoke onto the necks of the cows a slightly angular guide of the pull rope or the pull chain is required. With regard to the size of the angle no definite statement can be made. It should, however, be as wide as necessary and as small as possible.

Fig. 15:Power losses are avoided by harnessing the draft animal to a semi-circular harness]

Fig. 16: Vertical and horizontal components of the tractive power; no power losses occur when the pull chain is in a horizontal position.

Fig. 17: The semi-circular harness

Construction examples

Most of the power gears that have been constructed so far have a semi-circular harnessing consisting of two curved pipes (Fig. 17).

The two pipes are welded together in such a way that from above they look like a trapezoid.
On the outer side of the semi-circular harness there are hooks at various heights to which the animal's outer pull chain can be fastened.

It is advisable to construct the harnessing in such a way that it can be removed after use of the mill. This construction helps to prevent children playing with the mill and furthermore makes its unauthorized use impossible. For example, the semi-circular harness may be inserted into two pipes that are welded to the frame. In this case limit stops must be provided for at the two inner pipes.

It must be decided from case to case whether such a relatively costly harness should be used or whether the disadvantages of a more simple harness, as shown in Fig. 10, p. 32, can be accepted for the sake of cost saving.

Dimensioning proposal


pipe d = 40 mm, s = 3 mm

Harnessing hooks:

round steel d = 10 - 12 mm

Limit stops: round steel

d = 10-12 mm

Fastening of the harness to the framepipe

d = 50 mm, s = 3 mrn

- The height of the semi-circular harness depends on the height of the wall, the diameter of the wheel and the height of the draft animals.
- The width of the semi-circular harness depends on the breadth of the draft animals.

Construction tips

- To bend the pipes, they are first filled with sand, then heated and finally curved.

6.2 The path

The main function of the path is to ensure a good power transmission between the ground and the car wheel. Concrete has proved to be a good material for this purpose, since the coefficient of friction between concrete and the car wheel is between 0.85 and 1. For two reasons the path should be slightly raised:

- to prevent the path from filling with sand and dirt
- to facilitate a horizontal power transmission

If possible, the power should be transmitted at the height of the contact surface of the wheel and the path. Otherwise the torque acting upon the frame could result in a major distortion of the frame.

The height of the path is limited by its costs. The paths of most animal powered mills constructed so far consist of ringshaped walls made of two rows of concrete blocks. This corresponds to a height of 40 to 50 cm. Attempts have been made to cast the entire wall in one concrete block. The construction of the shuttering, however, turned out to be much more costly and time-consuming than the construction of the path walls.

The capacity of resistance to wear of conventional concrete blocks is not sufficient to resist the continual stress caused by the circulating wheel. Therefore, the wall must be reinforced by a more solid concrete covering of 5 - 10 cm thickness which must contain more cement and be mixed with small laterite stones. The laterite stones help considerably to reduce abrasion. Moreover, the covering should be fortified with concrete reinforcing round steel.

The construction of the wall is difficult because

a) it has to be almost perfectly round
b) it has to be almost perfectly level

Fig. 18: The path

Dimensioning proposal

Average diameter of the path:

6 m

(see also Chapter 6.3)

Width of the path:

at least 25 cm

Height of the path (from ground).

40 - 50 cm

Height of the foundation:


Height of the covering:


Ratio of components:

foundation: 150 kg/m³

concrete blocks: 110 kg/m³

covering: 350 kg/m³

Construction tips

- Before beginning with the construction, the central point of the ring should be marked with the help of a concrete reinforcing round steel of 1 m length. A rope which should be as inflexible as possible serves as a compass. The excavation required for the foundation is drawn on the ground with the help of the compass. Later, the rope compass is used for controlling the position of the concrete blocks.
- A level foundation of the wall is obtained by driving a concrete reinforcing round steel into the ground to be excavated at its highest point. 5 cm of the reinforcing round steel must jut out of the ground. Further reinforcing bars must now be driven into the ground at 1 m intervals. Each reinforcing bar has to be adjusted to the previous one with the help of a water balance. The concrete for the foundation is then filled up to the very top of the concrete reinforcing round steel.
- An easily removable shuttering for the concrete covering can be produced with plywood strips 5 - 8 mm thick and 20 cm wide. The plywood strips are pressed against the wall from the inside as well as from the outside by means of simple clamps (consisting of concrete reinforcing round steel of 12 - 14 mm thickness. The covering should, however, not be higher than 5 - 8 cm, because otherwise the shuttering could collapse (Fig. 19).

Potential problems

The concrete path is one of the most expensive component parts of the animal powered mill.
A reliable but cheaper solution could certainly be considered as a major contribution to making the device more economical.

Fig. 19: Structure of the path

6.3 The drive unit

The drive unit is designed to transmit the force required for driving the mill from the draft animal to the mill and at the same time increase the "number of revolutions" of the draft animal.
The drive mechanism consists of the following components - a car wheel - a second gear stage (chain, belt drive, etc.) - a shaft - bearings

Fig. 20: The drive unit

The car wheel

The force that can be transmitted by the car wheel depends on the coefficient friction between path and wheel and on the load on the wheel. The maximum peripheral force which can be transmitted by the wheel is

F max = µ x Fg
Fg: weight
µ: coefficient of friction

(for rubber tires running on concrete µ is 0.85 - 1. On a dry path a higher coefficient of friction can be achieved with bald tires than with new ones).

The maximum power that can be transmitted is

Pmax =µ x Fg x d x pi x n/60 x e
e= efficiency of the power transmission
d = diameter of the wheel (m)
n = revolutions of the wheel (1/min)
Fg = weight (N)

Losses occur in the first gear stage. Primarily, these losses are a result of boring friction and rolling resistance.

Boring friction occurs when a cylindrical body (an automobile tire is a cylindrical body) runs on a circular path. In principle, the tire has the tendency to run straight ahead. When forced to move on a circular path the tire runs too slowly on the outer radius of the circular path and too fast on the inner radius. This relative motion is the reason for increased wear and power loss.

The wider the tire becomes in relation to the path, the higher the boring friction. The diameter of the path should therefore be as large as possible, whereas the width of the tires should be as small as possible.

The rolling resistance is a result of the elastic deformation of the tires and of the path. For a car wheel loaded with 200 kg which has the proper tire pressure, the rolling resistance is 30 - 40 N (3-4 kg). By increasing the tire pressure the rolling resistance can be reduced.

Dimensioning proposal

Size of tire 155 - 13. These tires can be obtained almost everywhere. Their outside diameter is approximately 60 cm. A transmission ratio of 1:10 can be achieved with a path diameter of approximately 6 m.

The second gear stage

Assuming that

- a transmission ratio of 1:10 is achieved in the first gear stage,
- the draft animal walks 2-3 rounds per minute,
- a speed of 60 - 140 rpm is required to drive the mill, a transmission ratio of 1:3 - 1:7 is needed in the second gear stage.

The structural components of the second gear stage should not demand a very precise mounting. Taking this into account

- a roller chain gearing - a V-belt drive gearing - a flat belt drive gearing - or a friction gear in which the car wheel serves as a driving pulley (see also p. 32, Fig. 10) could be used.

Dimensioning examples:
The power to be transmitted should on average be 1 kW, the transmission ratio 1:5, and the speed of the driving gear should be 20 rpm.

Chain drive:


10 B. 5/8"


76 teeth

d = 391 mm


15 teeth

d = 83 mm

Flat belt drive:

driving disk:

d = 500 mm

driven disk:

d = 100 mm

leather belt:

belt width approx.200 mm, thickness of belt: 3 mm

synthetic belt:

belt width approx. 60 mm, thickness of belt: 1.4 mm

V-belt drive:

driving disk:

500 mm

driven disk:

100 mm standard V-belt: width 17 mm

Due to the large belt width which would otherwise be necessary, a flat belt drive can only be used if high-quality belts are available. With narrow axle bases and a vertical installation, the required high initial tension of the belt causes problems. The most important advantage of flat belt drives is the fact that the pulleys can be produced locally. To prevent wear of the belt the bearing surface of at least one pulley must have a crowned surface.

The biggest disadvantage of V-belts is the high price of the pulleys, which can hardly be produced locally (the lathe must have a turning diameter of 500 mm!). The required V-belts are difficult to obtain in developing countries.

Roller chains can be bought in almost all developing countries. Even though the chain wheels can only be produced by extremely gifted craftsmen or with the help of expensive machine tools, they are cheaper than industrially manufactured V-belts or flat belt pulleys.

The length of the chain can easily be varied by removing or adding chain links. Tensioning devices are unnecessary.

For these reasons chain drives were used for the second gear stage of all animal powered mills that have been installed so far.

Dimensioning proposal

Roller chain 10 B 5/8", wheel 76 teeth If it is the first installation in a region, it is advisable to keep a number of pinions ready (13, 17, 21, 25 teeth), for adapting the transmission to the performance of the draft animals and to the required flour fineness.

Bearings and shafts

Fig. 21 shows appropriate and less appropriate arrangements of the wheel, the bearings and the chain wheel.

The shaft must be capable of resisting and/or transmitting the following forces and torques:

- the weight of the frame
- the longitudinal force of the tires - the chain and/or belt forces
- the torque necessary for driving the mill

The shaft material of most of the so far installed animal powered mills was bright drawn round steel with a diameter of 40 mm. Bright drawn material fits if it has a slightly smaller dimension than specified (h 11 - h 8). It need not be turned to size, but it is quite expensive.

The necessity of using bright drawn shafts is primarily a consequence of the use of ball bearings for the animal powered mills already installed.

In most cases Y-bearing units have been used so far, They have a crowned outer ring and can, if necessary, be adjusted in the bearing housing for compensating alignment errors.
The inner ring has an eccentric ring, headless pins or an adapter sleeve which serves for pressing the bearing to the shaft. Self-aligning ball bearings are also available with such mounting devices.

Fig. 21: The most suitable arrangement of the chain wheel between two bearings

A conventional round steel shaft can of course be used, provided that a lathe for turning is available.

In this case ordinary deep groove ball bearings can also be used (press fit indispensable). With deep groove ball bearings a more rapid wear must be accepted in the case of alignment errors.

Potential problems

The rolling bearings account for a relatively high share of the total costs of the device. We favored this solution because we wanted to ensure a high reliability and a high efficiency of the power gear in the introductory phase. It should be considered whether lower costs would possibly justify a less complicated bearing construction and a lower efficiency. The use of wood bearings might, for instance, be an alternative. Wood bearings (Fig. 22) made of hardwood could easily be produced locally. In order to increase the wear resistance, the wood bearings should be soaked in hot oil for 24 hours.

Fig. 22:A simple hardwood bearing. For transmitting the axial forces the shaft must be equipped with a steel disk.

Fig. 23: Hub for mounting the wheel to the shaft

Fig. 24: The frame

Installation of the drive unit

The easiest way of installing the chain wheel on the shaft is to install it by means of a headless pin (M 8 or larger). The necessary boring inside the shaft should fit precisely because otherwise it will widen very soon. The car wheel is best mounted on the hub of a donkey or ox cart and should be fastened to the shaft with a bolt. In case there is no hub, it can be constructed by using sheet steel and a suitable pipe (Fig. 23). The drive unit and the bearings are fastened to two crossheads of the frame (Fig. 28).

6.4 The frame

The frame has several functions:

- to fasten the drive unit, the grinding unit and the harnessing
- to carry the seat for the user, so that the user need not go round in a circle behind the power gear
- to connect the wheel and the central axle so that the central axle can keep the wheel on its circular path.

The frame must resist a vertical bending moment which is primarily produced by the weight of the user. A horizontal bending moment results from the tractive power of the animal which must be supported by the central axle. It is, however, the torsional moment that acts upon the with latticework reinforcement frame via the lever arm of the frame (Fig. 25) which is of crucial importance for the considerations regarding the stability of the frame.

Fig. 25: Action of the torsional moment on the frame

Fig. 26: Power gear frame made of channel steel

For this reason the torsional resistance of the frame must be as high as possible.

Construction examples

Channel steel frame with latticework reinforcement

The frame of the first animal powered mills was made of channel steel. Since channel steel has a relatively low section modulus of torsion, the frame was rein forced by a latticework construction (Fig. 26)

Frame made from pipes

Later, water pipes were used for the frame construction. Due to the higher polar section modulus of pipes in comparison with channel steel, the latticework construction could be dispensed with (Fig. 27). The water pipes can be replaced by a square tube of a similar dimension. In this way the drive unit, the grinding unit etc. can be mounted even more easily.

In the final analysis, however, the choice of the materials will be determined by their costs.

Fig. 27: Pipe construction of the frame

Arrangement of the grinding unit, the drive unit, the seat and the holding device for the calabash (Fig. 28)

The height of the table on which the grinding unit is mounted depends primarily on the required axle base between the two chain wheels. The axle base should be at least 400 mm.

The table should be very rigid' because the grinding unit must not vibrate under the influence of the chain forces. Of particular importance is the reinforcement of the table around the mounting holes for the grinding unit. The table is only necessary if a separately bought grinding unit is used. If the grinding unit is produced locally, the table can be integrated into the grinding unit. It should be made sure that the crossheads for mounting the drive unit are connected as securely as possible to the frame. The arrangement of the seat and of the holding device for the calabash should be determined by ergonomic considerations.

Dimensioning proposal


-pipe (round) 60 x 3 or 48 x 4
-square tube 50 x 4
-channel steel U 60 x 6

Latticework reinforcement: round steel (concrete reinforcing round steel) 12 mm Holding device for the calabash: round steel (concrete reinforcing round steel) 10 or 12 mm

Crossheads for mounting the bearings: channel steel U 60 x 6

Table for mounting the grinding unit: angle steel L 40 x 3, sheet metal 3 mm

Seat. angle steel L 40 x 3, sheet metal 2 mm

Fig. 28: Arrangement of the components on the frame

6.5 The central axle

In order to guide the wheel precisely' the axle must be exactly central and must be absolutely vertical.


The central axle must be connected to a concrete foundation in the center of the concrete path.

Dimensioning proposal

For saving concrete, a cross-shaped construction of the foundation has proved to be advantageous. The dimensions are shown in Fig. 30. Ratio of mixture: 200 kg/m³. The foundation should be fortified by concrete reinforcing round steel.

Fig. 29: The central axle

Fig. 30: Shuttering of the foundation for the central axle with tins for fastening the anchorages

Construction tip

It has turned out to be difficult to embed the bolts for the attachment of the central axle in the foundation in such an exact manner that they are all level with the mounting holes in the central axle.

It is therefore advisable to reserve holes for subsequent filling out. At the bottom of these holes rod irons are anchored in the concrete. These holes can be produced with the help of tins, polystyrene blocks or sand-filled plastic bags. Subsequently, the bolts with a hook at one end are hung in the rod irons (Fig. 30). In this way, the bolts have enough play for adjusting their position to the mounting holes in the central axle and thus make a precise alignment of the axle possible. It is advisable to complete the entire metal structure before completing the foundation. The height of the foundation can then be adjusted to the dimensions of the animal powered mill (Fig. 31).

Fig. 31: Determination of the foundation height

Metal construction

The design of the central axle of the animal powered mills installed so far has often been- modified.

The factors that determine the construction are

- the rigidity of the construction (Considerable power peaks may occur. The resulting vibrations of the animal powered mill have a negative effect on the grinding process.)
- saving of material (Due to the use of roller bearings - as in the case of the drive unit - expensive bright drawn shaft has up to now been used for the central axle. As a consequence, the axle must be as short as possible).

Fig. 32: Different constructions of the central axle

Construction examples

Fig. 32 shows several construction examples

Dimensioning proposals

Cross-shaped foundation: channel steel 60 x 6
Axle: bright drawn shaft d = 40 mm
Reinforcements: sheet metal 3 mm
Attachment of the frame: channel steel 100 x 6
Hub: channel steel 60 x 6 or pipe 80 x 5 and channel steel 80 x 6

In order to compensate possible inaccuracies of the path and the central axle, the frame of the animal powered mills constructed so far was mounted to the central axle in a vertically flexible installation. Initially, wood bearings were used for this purpose. Practice has shown, however, that the excursions are so negligible that it is sufficient mount the frame direct on rubber plates or rubber strips (Fig. 33).

Fig. 33: Coupling of the frame to the central axle

Potential problems

Up to now, rolling bearings have been used for the central axle and driving shaft. As to the possibility of replacing the rolling bearings with wood bearings, see also Chapter 5. Due to the lower forces and the lower speed of the central axle, it is less difficult to replace the rolling bearings by wood bearings in this case than in the case of the driving shaft. The bright drawn shaft can therefore be dispensed with. As regards the coupling of the frame to the central axle, it should be considered whether inaccuracies of the path could not be compensated by the elasticity of the frame and whether in this way a flexible fastening could be unnecessary.

6.6 The grinding unit

Initially, an industrially manufactured grinding unit of the French manufacturer Moulis was used for the animal powered mill. This device came off best in several tests even though it has a number of major deficiencies. For this reason and also for saving costs, the development of a grinding unit that can be constructed by African craftsmen began. The construction of a grinding unit requires, however, a great deal of experience and precision. A technician should therefore only undertake the construction of a grinding unit, if he has already had practical experience with the animal powered mill. For the first installation of an animal powered mill, we recommend the use of a Moulis grinding unit. Another possibility is to order via Gate a grinding unit constructed by one of the craftsmen who cooperate with the project. The Moulis grinding unit and the grinding unit built by craftsmen have the same functional principle. In the course of the development of the locally constructible grinding unit other construction principles were tried. Eventually it turned out that the traditional construction was the most appropriate one. It consists of one static and one rotating and axially adjustable grinding disk made of stone. The grain flows through a hole in the static grinding disk into the space between the disks. Due to the structure of the grinding stones, the grain moves on a spiral between the disks, leaving the grinding space at the outer side of the spiral as flour (Fig. 35).

Fig. 34: The grinding unit

Fig. 35: Grinding principle of the Moulis grinding unit and of the locally constructible grinding unit

When driven by a power gear, the use of the Moulis mill poses the following problems:

1. The case and the outlet opening are too small. Flour sticking to the inner walls of the case causes blockages that result in a reduction of the grinding capacity.
2. The case is manufactured inaccurately. The supports of the grinding unit and of the shaft are not parallel. As a result, it is difficult to mount the shaft of the grinding unit and the shaft of the power gear so that they are parallel.
3. The grinding disks are not parallel. As a consequence, the clearance between the grinding disks differs at least during the feeding phase.
4. The cover of the case does not fit properly. Since the second bearing of the shaft is inside the cover, the shaft has too much play so that the clearance between the grinding disks changes continually.
5. Inside the case cover, between the shaft and the adjusting screw, there are a small bearing and some other components which fall off whenever the cover is removed, e.g. for cleaning the grinding unit.

One of the purposes of the construction of special grinding units for animal powered mills was to avoid such deficiencies. The construction of this grinding unit will be described later. First, however, we want to describe the installation of the Moulis grinding unit.

Installation of the Moulis grinding unit

At its feeding opening the Moulis grinding unit has a vibration mechanism which ensures a steady feeding of the grain. This mechanism is not only very fragile but also unnecessary when driven by a power gear, since the vibrations of the power gear are sufficient for guaranteeing a steady feeding. The vibration mechanism should therefore be dismantled. Once this has been done, the funnel provided for by the manufacturer, can no longer be installed. This does not matter because it is too small anyway. As a consequence, a new funnel must be constructed (see also Fig. 36).

This funnel should be equipped with a watertight cover since grinding particles, which deteriorate as a result of humidity in the grinding unit, can have a negative impact on the quality of the freshly ground flour. The grinding unit is fastened to the grinding table by means of two screws. In order to achieve a parallel position of the mill shaft and the driving shaft, inaccuracies of the support of the grinding unit have to be compensated by welding sheet metals onto it. The table must have a hole at the outlet openings of the grinding unit. A pipe (which must not be too thin) or a sheet metal guide directs the flour into the calabash and prevents it being blown away by the wind.

Fig. 36: Components of the locally constructible grinding unit

Construction of a locally constructible grinding unit

Fig. 36 shows the design of the grinding unit. It is constructed in such a way that it can, in principle, be constructed by craftsmen with simple tools. The craftsmen should, however, be able to work with the utmost accuracy. A lathe is required for the construction of some of the components. For the construction of the grinding unit the following factors are of decisive importance.

1. A parallel position of the two grinding disks

It is quite obvious that an optimum grinding result will only be achieved if the clearance between the grinding disks does not differ at any point. The carrier disk of the rotating grinding stone must therefore be constructed with the help of a lathe. First, a disk (140 mm) consisting of 8 mm sheet metal is cut to size. A hole (40 mm) must be drilled in its middle.

The disk is then welded onto a piece of shaft (40 mm). On the lathe the shaft is first drilled in the middle (25 mm) and, when this is done, the disk is faced. Since the static grinding disk is fastened to the case, the bearings (i.e. the bearing housings) must be welded to the housing with the shaft installed and the grinding disks pressed together. The two halves of the housing must fit precisely so that they cannot move.

2. Rigidity of the housing

In order to guarantee that the grinding disks are parallel at each stage of the grinding process, the housing must be resistant to torsion. For this reason it should be made of sheet metal of at least 6 mm thickness.

3. Sufficient clearance between the grinding disks and the housing; large outlet openings

The grinding disks have a diameter of 180 mm, the inside diameter of the housing of the locally constructible mill is 250 mm. The housing of the Moulis mill has a diameter of 210 mm. The Moulis mill has only a small outlet opening whereas the entire lower side of the locally constructible mill is left open in order to guarantee that the flour can flow off without difficulties.

The grinding stones, the stone guard springs and the conveyor spiral (which also presses the grinding stones apart) are identical to those of the Moulis mill. The stone guard springs protect the grinding disks against stones and other hard objects that may be contained in the grain. Two valve springs of a Honda motorbike could also be used for this purpose. The friction bearing fastened to the housing cover is made of cast iron turned on a lathe. The second bearing is a deep groove ball bearing. The outer ring of the bearing is axially fixed inside a pipe which must be opened on a lathe. There is a fixed clearance fit between the shaft and the inner ring. For adjusting the grinding disks it is essential that the shaft can be moved axially within the inner ring of the bearing. This is necessary despite the fact that it also constitutes a disadvantage because the shaft moves on the inner ring and, as a consequence, wears out. The wear- which in practice is almost negligible - does not justify the considerably higher construction costs which a driving fit between the shaft and the bearing would imply.

The shaft of the Moulis grinding unit is borne against the adjusting screw with a small thrust ball bearing. The disadvantages of this construction have been pointed out already.

The bearing between the shaft and the adjusting screw of the locally constructible grinding unit only consists of one single bearing ball (d = 15 mm). The ball is held on the shaft by a sheet metal ring which is welded onto the shaft, and in which it can rotate.

The part of the housing to which the static grinding disk is fastened is welded to a crosshead made of channel steel 60. The crosshead is fastened to the pipe frame by means of clips which can be made either of pipe or of square steel.

6.7 Protection devices

The protection devices (Fig. 38) serve primarily to prevent accidents which can happen e.g., when someone gets trapped between the wheel and the frame or gets with his hands or clothing caught in the chain. They are also designed to protect the chain against dirt and rain water.

It is advisable to weld the protection devices to the frame. Experience has shown that dismountable protection devices are often not returned to their appropriate place after removal. Hinges allow access to the covered components.

Dimensioning proposals


sheet metal 2 mm


flat steel 20 x 2, round steel: 8 mm

Construction tips

The flap of the hood should slightly underlap the static part in order to prevent rain water from seeping into the mill. For the same reason, seals made from the inner tube of a car wheel should be affixed to the edges of the flap.

Fig. 37: Protection devices

Fig 38: Arrangement of the protective coverings on the frame

7. Dear Reader...

This guide was not designed as a "construction manual". We know from experience that such manuals often cause trouble and annoyance because very often they do not explain why certain solutions are favored, because the recommended materials and tools are not available, or because passages can be misinterpreted. The latter cannot be avoided altogether, and we are sure that some readers will have the same problems; with this guide.

Nevertheless we hope that our description of the construction is precise enough that you will be able to overcome the difficulties not mentioned. Although the power gear has proved its worth in practical operation, its construction can certainly be further improved.

On request we will offer updated information if improvements are found to be effective in practical operation or if the power gear is used for new purposes (e.g. for driving rice hullers, oil presses, etc.).

We will be very grateful for your critical remarks concerning this guide and will do our best to consider your comments and suggestions in further publications of this kind.

In particular, we would like to ask you to keep us informed of your experience with the power gear and improvements which you have tested yourself; and we would also appreciate accounts of failures and problems. They will help others to learn from your experience.