Technical handbook for the paddy rice postharvest industry in ...

47 CONVEYINGTypesBucket elevators are available in several designs to handle many products. They areclassified according to the type of discharge used and are identified as centrifugal,positive (gravity), and continuous (direct gravity). The three types are shown inFigure 3.2. The centrifugal discharge type is most commonly used with grains. It isdesigned and engineered to conform with general practice in handling grain. Headand boot shafts are provided with roller bearings. Takeups are generally screw-typeexcept on tall high-capacity units where gravity-type take-ups are more common.Buckets are usually made of steel or plastic and are bolted onto the belt. Casings orlegs are also made of steel, are welded or bolted together, and are dusttight. Thecurved hood is designed for proper centrifugal discharge of the paddy grain. Theboot can be loaded from the front or back or both (Fig. 3.3). In larger, high-capacityinstallations the head section is often vented and connected to an aspiration system.3.2. Elevators dischargingpaddy: A, centrifugal; B,gravity; C, direct gravity.3.3. Loading paddy intothe boot of an elevator.Bucket types and capacitiesBuckets are made of different materials and come in different shapes and sizes,depending on requirements. Figure 3.4 shows a typical bucket used with centrifugaldischarge elevators. The buckets are uniform, smooth, and proportioned for fastfilling and quick, clean discharge. Figure 3.5 shows the correct method of bolting thebucket to the belt.Dimensions and capacities of different buckets are given in Table 3.1. Thecarrying capacity is based on the angle of repose of paddy, which is normally 36° (seeline x - x in Fig. 3-4.) Because of the difficulty of loading all buckets to 100% of ratedcapacity and the desirability of having a small reserve capacity in the elevator,designers calculate carrying capacity on the basis of buckets being filled to 85 to 90%of rated capacity.Bucket spacing or the minimum vertical spacing between bolt holes of elevatorbuckets is also shown in Table 3.1. Buckets may be as far apart as the requiredcapacity permits. lnstalling buckets closer than the minimum will probably result inreduced carrying capacity because they will not fill properly at the recommended beltspeed.Elevator capacitiesThe bucket elevator's capacity in tons of paddy per hour depends on bucket size andspacing and on belt speed. Speed is the first critical factor to consider. The speed of3.4. Bucket used for centrifugaldischarge elevator.(Courtesy of ScrewConveyor Corp.)