PART III. THE HULLING OF PADDY

PART III. THE HULLING OF PADDY The objective of a hulling machine is to remove the husk from the paddy grain with a minimum of damage to the bran layer and, if possible, without breaking the brown rice grain. Because the structure of the paddy grain makes it necessary to apply friction to the grain surface to remove the husk, a certain percentage of brokens cannot be avoided; however, much can be done to reduce this breakage. In this respect the construction of the machine, its precision, its maintenance, its adjustment, and the way it is operated can secure optimum performance in huller efficiency and head rice production. However, if the grain is damaged in the field as a result of moisture cracking (suncracking), breakage during the milling process cannot be avoided. Because the adjustment of the huller depends on the variety of paddy to be processed, uniformity of the grain is essential for optimum performance. However, under most circumstances, pure varieties are not available and a mixture of varieties can be responsible for a less efficient hulling process. The negative effect on hulling of mixed varieties can be avoided by the grading of paddy prior to hulling or can be reduced by the rearrangement of the grain flow in a rice mill. The most common machines used in hulling paddy are the under-runner disc huller, which is the most commonly used machine, and the rubber roll huller, which was introduced after World War II and is slowly gaining popularity. A rubber band huller developed in Europe in 1947-48 disappeared from the market shortly afterwards and the small centrifugal hullers are still in the experimental stage. Hulling machines are known by different names, such as shelters, hullers, dehuskers, huskers, and hulling mills. Most commonly these machines are called "hullers". A huller has an efficiency that does not refer to its capacity but to its performance as a huller. The huller efficiency is the percentage of grain actually hulled with a minimum of breakage. 1. THE UNDER-RUNNER DISC HULLER, OR DISC HULLER (Fig. 107) The under-runner disc huller consists mainly of two horizontal cast-iron discs, partly coated with an abrasive layer. The top disc is fixed in the frame housing; the bottom disc rotates. The rotating disc is vertically adjustable (2) so the clearance between the abrasive coating of the disc can be adjusted (3). This adjustment depends on the variety of paddy, the condition of the grain, and the wear of the coating. The paddy is fed into the centre of the machine through a small hopper. A vertically adjustable cylindrical sleeve (1) regulates the capacity and equal distribution of the paddy over the entire surface of the rotating disc. By centrifugal force the paddy is forced between the two discs and under pressure and friction most of the grain is dehusked. The adjustment of the clearance between the discs is rather critical and requires continuous rechecking to avoid excessive breakage or insufficient huller efficiency. Quite often, the effective width of the abrasive coating is made too large, causing unnecessary breakage of the grain. Empirically, it has been proven that this width should not be more than 1/6 to 1/7 of the stone diameter. The peripheral speed of the disc should be about 14 m/s, making the speed a function of the diameter of the stone. The larger the diameter, the lower the revolutions per minute of the shaft (Fig.108). Example: D = stone diameter = 700 mm (0.7 m); and V peripheral speed = 14 m/s. V = x D x n (m/s) 60 n = 60 x V = 60 x 14 = 380 rpm. x D 3.14 x 0.7 1

Fig. 107. Particulars of the under-runner disc huller: V = peripheral speed (recommended 14 m/s); W = width of coating; D = stone diameter; and W/D = 1/6 or 1/7. Fig. 108. Under-runner disc huller peripheral speed-curve for V = 14 m/s. The wear of the abrasive coating of the disc is not equal over the full surface of the coating. Hulling is concentrated in the centre part of the coated surface and, consequently, wear in that section of the coating is greater than at the outer or inner rings. Slowly a small ridge is made at the outer ring of the coating (Fig. 109, B), which is responsible for an excessive buildup of pressure on the grain between the stones. Consequently, the rice is broken not only because of this excessive pressure but also because of the presence of the ridge, which the hulled rice has to pass when leaving the machine. When these ridges are produced, the surface of the stones must be redressed (corrected), and the life of the coating is unnecessarily reduced (Fig. 109, C). It is more effective with respect to the efficiency and the life of the coating to redress the stone surfaces more often so ridges will not be formed. 2

Fig. 109. Under-runner disc huller maintenance: (A) original condition of stone coating; (B) uneven wear developed at centre of coating; and (C) redressed surface of abrasive coating. Vertical adjustment of the rotating disc is accomplished by moving the entire shaft-disc assembly. The housing of the shaft-end bearing is moved by a steel beam controlled by a handwheel adjustment (Fig. 110). The bar may be moved by two handwheels, or the supporting bar can be hinged on the base of the huller frame and controlled by a single wheel. Vibration in the machine components is to be avoided because this breaks the rice. In this respect proper bearing of the shaft and maintenance of these bearings are very important. The shaft-end bearing absorbs radial and axial forces; therefore, a double bearing assembly is recommended. For this purpose a combination of a single or double ball bearing and a taper roller bearing can be used (Fig. 110). These bearings must be maintained and greased properly. If the roller bearing is not greased the ball grooves will be damaged, resulting in excessive tolerance and vibration, which can only be corrected by replacing the bearing. In a locally made huller (and also in a whitening cone), very often only a roller bearing is used for radial forces and a simple steel ball absorbs the axial forces (Fig. 111). In this case the end of the shaft is rounded. With this construction the actual contact area between shaft and ball is very small and, when not sufficiently greased, there is immediate damage to the shaft and the ball causing vibrations. Fig. 110. Under-runner disc huller bearings and adjustment details. 3

Fig. 111. An end-bearing assembly of the under-runner disc huller containing a roller bearing and steel ball (Philippines). Fig. 112. Details of the bronze bush bearing used in under-runner disc huller : (A) poor lubrication; and (B) additional hole drilled providing proper lubrication. The upper part of the shaft is normally guided by a bronze bush bearing, lubricated by grease (Fig. 112). The grease is pressed between the bush and shaft; however, quite often only the lower portion of the bearing is lubricated. Friction between the upper portion of the bush bearing and the shaft then becomes excessive, causing heat and wear, and leads to vibrations in the upper portion of the shaft assembly (Fig. 112, wrong). This problem can be overcome by drilling an extra lubrication hole in the bronze bush bearing, allowing grease to be distributed over the full working surface of the bearing (Fig. 112, right). The housing for this bearing is normally bolted to the huller frame with bolts and nuts. When no spring washers are used the nuts may loosen and the upper part of the shaft will vibrate. Both surfaces of the discs should be fully level and remain level, otherwise the huller will be transformed into a perfect rice breaker. Consequently, continuous maintenance remains essential. The huller shaft is normally driven by a flatbelt or a V-belt transmission. When lifting the shaft assembly for disc adjustment, the driven pulley on this shaft will also be lifted. For a flat-belt pulley-drive the transmission will remain unaffected by this adjustment since the extra width of the flat-belt pulley will allow free movement of the belt over the pulley (Fig. 113, A). For a full V-belt transmission (two V-belt pulleys with V-belts), this belt correction is not possible and, consequently, the belt will be out of line when an adjustment is made (Fig. 113, B). This causes extra friction on the belt, resulting in heat and excessive wear. Nevertheless, this method is used quite often in the Philippine-made rice mills. The problem can be overcome if the large V-belt on the 4

huller shaft is replaced by a flat-belt pulley (Fig. 113, C). However, this reduces the grip of the belts on the driven pulley and more belts are required. The position of the driving pulley may be changed after adjustment of the huller shaft (Fig. 113, D). This is done in electric motor-driven machines by lifting the entire motor support assembly. Fig. 113. Methods for transmission-drive: (A) flat belt pulley-drive; (B) full V- belt transmission; (C) V-belt on transmission shaft replaced by flat pulley, and (D) position of driving pulley changed. Fig. 114. Position of the drive-pulley in under-runner disc huller. A mistake often made, when driving a huller by flat-belt transmission, is not making the pulling part of the belt perpendicular to the main transmission shaft. This is essential for an optimum transmission of force from transmission shaft to machine (Fig. 114). In many rice mills, the abrasive coating is applied to the stone in a rather primitive way making it practically impossible to secure a uniform, fully level, working surface. Fortunately, very simple and cheap tools can be made that provide a perfectly level surface of both the fixed and rotating discs, and any corrections on the surface of the abrasive coating can be done by simple redressing tools (Fig. 115). 5

The use of these tools is indispensable for smooth and efficient operation of the huller. In general, in the Philippines, a pure emery composition for the coating for the discs is used. Normally this composition consists of 100 weight units of emery grit 14, 20 weight units of magnesite, and magnesium chloride brine (30 Baume). This composition became outdated many years ago and with the newly recommended composition much better huller capacity and huller efficiency, and longer life of the stone are obtained. The secret of the new composition is a new component silicium carbide (carborundum), whose hardness is about two and one-half times higher than that of emery. The recommended composition of the new coating is grit (50% by weight emery grit 14, 16 2/a % by weight emery grit 16, and 331/s % by weight silicium carbide grit 16), 20 % by weight magnesite, and 20 % by weight chloride brine (29 Baume). In the Philippines, the under-runner disc hullers are normally mounted on elevated concrete foundations so that the actual hulling section will be at working level on the first floor (Fig. 116 A). The disc adjustment must be done on the ground floor level under less attractive working conditions. In some rice mills abroad, this adjustment is made from the working level on the first floor by simplification of the construction of the huller frame and by extension of the handwheel controlled adjustment device (Fig. 116 B). In practically all horizontally projected rice mills in other countries, the huller is mounted on the ground floor level (Fig. 116 C). Fig. 115. Special tools used for resurfacing the abrasive surface. Fig. 116. Different setups for the under-runner disc huller: (A) mounted on elevated concrete foundation; (B) modified huller frame construction; and (C) mounted on ground floor. 6

Because in the Philippines the paddy supplied to rice mills in general does not represent a pure variety and as paddy grading has not been implemented, quite often, because of the mixture of varieties, huller efficiency cannot be kept at a high level without breaking the brown rice. This is caused mainly by the varying thickness of the paddy grains. The mill operator wants to dehusk as many grains as possible and, therefore, reduces the clearance between the discs. By doing so he is actually cracking the thicker grains while dehusking the thinner ones. When he sees this, he readjusts the huller and consequently reduces the huller efficiency. The paddy not hulled during the first pass through the huller is then separated by the paddy separator and, in most of the rice mills in the Philippines, returned to the paddy bin feeding the huller. However, when samples of the initial paddy and the paddy rejected by the paddy separator are analyzed carefully, it is seen that for the same volume, the weight of the initial paddy is more than 10 % higher; whereas, the number of grains of the rejected paddy is about 10% higher. Consequently, the average size of the rejected paddy grain is smaller than the average size of the initial paddy grain. This was the main reason why these grains were not hulled and, therefore, it is not logical to return these grains to the same huller. It is recommended that the rejected paddy be returned to a separate bin for independent hulling in a smaller huller that can be adjusted to the smaller size of the rejected paddy (Fig. 117). This allows the main huller to be adjusted to avoid excessive breakage of brown rice, and the total dehusking capacity of the huller section is increased by at least 20%. The recommended rearrangement of the grain flow in the huller section is advantageous in rice mills with disc hullers as well as those with -rubber roll hullers. However, urgent implementation is more important for disc hullers than for rubber roll hullers because of the more flexible surface of the rubber rollers. Fig. 117. Methods of return of rejected paddy: (top) grain flow without separate return hulling, not recommended; and (bottom) grain flow with separate return hulling, recommended. A = precleaned and returned paddy, B = huller, C = husk aspirator, D = separator, E = precleaned paddy, F = returned paddy, G = first huller, and H = return huller. 7

2. RUBBER ROLL HULLER In principle the rubber roll huller consists of two rubber rolls. One has a fixed position, the other is adjustable to obtain the desired clearance between the two rolls. The rolls are driven mechanically and rotate in opposite directions, the adjustable roll normally running about 25% slower than the fixed one. Both rolls have the same diameter, varying between 150 and 250 mm depending on the planned capacity, and the same width, about 60 to 250 mm. When the rolls are new, their peripheral speed is about 14 m/s so that a smaller roll runs faster than a larger one (see table below). Roll diameter Roll width High speed Low speed (mm) (inches) (mm) (inches) (rpm) (rpm) 150 6 64 2.5 1320 900 220 8.5 76 3 1200 900 250 10 250 10 1000 740 The clearance between the two rolls is smaller than the thickness of the paddy grains, and because the rolls run at different speeds, their peripheral speeds differ. When the paddy is fed between the two rolls, the grains are caught under pressure by the rubber and, because of the difference in speed, the husk is stripped off (Fig. 118 A). Wear on the rubber is considerable and, with reduction of the roll diameter, capacity is also reduced. The main reason for this capacity reduction is the decrease in the relative speed of the two rolls, and it is this relative speed that actually strips the husk from the paddy grain. Fig. 118. Dehusking principle of rubber roll huller. Example (Fig. 119) When the roll diameter is reduced from 254 to 216 mm (less 15%), the peripheral speed also drops by 15%. Because the speed of rotation of both rolls does not change, their relative speed, which is the difference in the peripheral speeds of both rolls, will also drop by 15%. At that point the hulling capacity of the machine has been reduced by about 15%. In general the faster running unadjustable rubber roll wears out quicker than the adjustable roll. Because the rolls have been made interchangeable, it is recommended they be transferred from time to time to ensure even wear of the set of rolls. To obtain optimum hulling performance, the grain should be evenly distributed over the full width of the rolls. However, quite often the grain distribution device does not function correctly, and the roll surface wears out unevenly, which badly disturbs efficiency and capacity (Fig. 120). The roll surfaces are corrected by removing part of the rubber; therefore, the life of this set of expensive rolls is reduced considerably. The efficiency of rubber rolls in tropical countries is still unfavourable and is slowing down their introduction. There are many reasons for this: higher temperature, higher humidity of the air, the structure of the husk, and especially the fact that in 8

Japan short grain varieties are hulled; whereas, in tropical countries mainly long grain or medium grain varieties are hulled. Fig. 119. Relation between diameter of rolls and capacity of huller: W1 - W2 = 19 mm; Dl = 254 mm; W1 = 25.4 mm; N1 = 1000 rpm; N1' = 740 rpm; V1 = 13.30 m/s; V1 = 3.46 m/s; D2 = 216 mm; W2 = 6.4 mm; N2 = N1 = 1000 rpm; N2' = N1' = 740 rpm; V2 = 11.30 m/s; and V1 - V2 = 0.52 m/s = -15%. Fig. 120. Unevenly worn and damaged surface of rubber rolls as a result of unequal distribution of the paddy. Fig. 121. The relation between paddy variety and hulling capacity. 9

When hulling long grain varieties, the contact area of the grain with the rubber, the moment it is stripped off its husk, is much larger compared with short grain varieties. Consequently, the wear is higher and the life of the rubber is considerably shorter. Whereas 200 t of paddy can be hulled in Japan, the same quality of rubber will only hull about 100 t in tropical countries (Fig. 121). Better quality substitutes for the rubber exist; however, the increased durability in terms of tonnes of paddy hulled does not cover the much higher price of these substitutes. The dehusking performance of this type of huller is superior to the conventional disc huller, because production of unbroken brown rice and huller efficiency are higher. However, this does not mean the head rice yield of the mill is higher. Because the silver skin of the paddy is undamaged by the rubber roll huller, cracked grains appear as unbroken brown rice, but during the first pass of the whitening process these grains will break. Rubber rolls can only be stored for a limited period because their durability slowly deteriorates (Fig. 122). Therefore, a continuous supply of rolls should be guaranteed in order to utilize them at optimum efficiency in the interest of total mill efficiency. Fig. 122. Relation between durability of rubber rolls and age. The recommended period for utilization of rubber rolls for optimum efficiency is between 3 and 6 months after production. The fact that in a rubber roll huller one roll runs in a fixed position and the second roll is adjustable; that one roll runs clockwise while one roll runs counterclockwise; and that both rolls are driven, requires special provisions in the drive mechanism. There are three kinds of drive transmission: (1) chain transmission; (2) V- belt transmission; and (3) gearwheel transmission. Chain Transmission If both rolls always ran in a fixed position a simple endless chain over chainwheels could drive the rolls. However, one roll must be adjusted continually; therefore, chain tension must also be corrected immediately. A British-made design solved this problem by mounting the chain-wheel of the adjustable roll and two extra chain wheels for chain tensioning on one sturdy bevel plate that is tiltable over a fixed point (Fig. 123). To adjust the rubber roll clearance, the sturdy bevel plate is tilted by a handwheel adjustment, controlling both the rubber roll and the two extra chain wheels. The adjustment of these three gear wheels is not the same because their distance from the tilting point to the bevel plate differs. The chain tension, however, is not fully corrected by this method and, therefore, one of the two extra gear wheels swings around the initial turning point of the gear wheel. This proved to be sufficient for continuous adjustment purposes. The advantage of this rather complicated system is: the clearance of the rubber rolls can be adjusted when the machine is running even under full load. A simpler method for chain adjustment is a manually adjustable gear wheel for chain tensioning after adjustment of the roll clearance. However, in this case the roll clearance can only be adjusted when the huller is unloaded and not running, which is a great disadvantage. 10

Fig. 123. Method of adjusting clearance of rubber rolls and tension of chain for a chain-drive transmission. V-Belt Transmission This type of transmission is most commonly used for the rubber roll hullers. Figure 124 demonstrates the mechanical adjustment necessary to obtain the appropriate clearance between the rubber rolls. Assume the original positions of the linkages were A, B, C, and D for the V-belt pulleys driving the rubber rollers mounted at A and B. The belt tension is changed by moving the idler pulley at C, which is controlled by a screw adjustment. In this transmission system drive D is fixed, the rubber roller at A is fixed, and at B there is a spring loaded bearing mounted on a sliding plate, which allows the roller at B to move for appropriate clearance. For example, to reduce the clearance between the rollers, the handwheel screws out V-belt pulley C to position C' and the corresponding position of B' is obtained. Pulley C must be mounted like pulley B so that sliding is possible. Fig. 124. V-belt transmission for the adjustment of the clearance between the rubber rolls. 11

Gear Transmission Figure 125 illustrates the third common method adopted for clearance adjustment of the rubber rollers. In this arrangement, assume A, B, C, and D are the positions of the gears prior to adjustment. A and C are fixed assuring the desired contact; whereas, B and D are adjustable. However, BD is rigid and maintains the required contact between gears B and D. By adjusting the position of gear B with the screw at the right, the clearance is determined, and the link BD starts to push D out of mesh with gear C; however, this is corrected by the spring loaded rigid bar shown below the adjustment lever. Fig. 125. Gear transmission for clearance adjustment of rubber roll huller. 12