FACTORS AFFECTING
AUGER THROUGHPUT
Pitch of flight
A pitch equal to the diameter of the flight generally gives satisfactory results for all angles of conveying. The potential throughput from an auger increases as the pitch increases, since a greater volume of grain is displaced in each revolution of the screw. This effect is accompanied, however, by an increase in the rotational motion of the grain, and this offsets any increase in throughput resulting from the greater pitch length. Since rotational motion is less at low angles of inclination, it is possible to use larger pitch lengths if the auger will only be used at low angles.
Elevation and speed
As the angle of elevation of an auger is increased the output decreases when running at the same speed.
However, the power requirements increase until about an angle of 45 degrees and then decrease. Table 1 gives the approximate variation of auger output for different angles.
Table 1. Variation of auger output at different angles:
 |
Angle of auger | 0 | 15 | 30 | 45 | 60 | 75 | 90 |
| degrees | ||||||||
|
Output | 100 | 90 | 80 | 70 | 60 | 50 | 40 |
| % of horizontal |
The output from an auger varies with auger speed for different angles of elevation. There is a limit to the speed at which an auger may be driven. For a 150 mm diameter auger, maximum output occurs at about 800 to 1000 rpm and any increase beyond this point will not result in an increase in output.
Choke length
This is the portion of the auger projecting below the casing at the intake end. It is essential that the auger be immersed deeply enough into the grain to completely cover the choke opening. A choke opening equal to about two pitch lengths is most commonly used in a single-flight standard-pitch auger. Anything less than this is only satisfactory at low speed. When less than two pitch lengths is immersed in the grain, at higher speeds the grain cannot feed into the screw quickly enough and the auger discharges at less than its capacity. On most augers the amount of choke opening can be adjusted to suit auger throughput. Auger life and reliability are increased by selecting larger-diameter augers and operating at slower speeds.
Choke design
Intake efficiency is defined as the percentage of auger throughput supplied by the choke. Variations in hopper geometry affect the intake efficiency of the auger. Experimental tests by G. N. Stevens showed considerable grain circulation in the hopper and therefore low intake efficiency. He found that increased intake efficiency would result in less power being wasted and an increase in throughput. Suggestions for increasing intake efficiency were to reduce the diameter of the auger core at intake, to increase the auger flight diameter, or to funnel the grain into the intake. An increase in the choke length immersed in the grain, or an increase in pitch of flighting inside the choke, increases intake efficiency. For vertical conveying it is possible to use a horizontal auger to force-feed the vertical auger; this is normally applicable to fixed installations.
Clearance
The clearance is the space between the inside of the casing and the outside of the flight. The clearance should be greater than the grain size or jamming of grain will occur. For practical reasons the majority of commercial augers have a fairly large clearance, up to 1/12 of the diameter. The reasons for this are the high cost of making the augerand casing to close limits, and the susceptibility of the auger casing to damage. Tests have indicated that throughput is reduced as clearance is increased. A 5 mm clearance in a 150 mm diameter casing showed a drop of 5 to 10% compared to zero clearance at reasonably high auger speed at any elevation. For a 9 mm clearance, the drop in throughput increased to 17%. It was found that with a 25 mm clearance the auger would not deliver at all when in a vertical position. Except for horizontal conveying, a clearance does not reduce the power required for a given throughput and may even increase it.
Moisture content of grain
Tests have shown that an increase in moisture content up to 20% has little effect on throughput, but moisture contents above 20% have a marked effect. A rise from 14 to 24% in moisture content resulted in a drop in throughput of 20% when conveying horizontally, and 45% vertically. This reduction is because of the different handling properties of wet grain. In addition to the loss of throughput, a 45? auger requires about twice as much power for wet as for dry grain; at lower angles of elevation several times as much power is required. The power requirements of vertical augers are much the same whether conveying dry or wet grain. Horizontal augers handling wet grain require several times the power required for dry grain.
Power requirements
The question of the correct power to drive augers is best left to the manufacturer. Friction losses and possible overloads must be considered. Motor burnout can be avoided by selecting a motor of sufficient capacity to overcome sudden overloads and starting with a loaded auger. Petrol engines will not easily respond to sudden overloads so it is important that 50% be added to the power requirements. Electric motors also need to be 30% oversized to overcome the sudden overload. An overload slip clutch or an overload cut-out should be fitted to augers.
Tests have shown that the power required increases proportionally to the auger speed. However, when measurements of power used and throughput are compared, after a certain throughput is reached, power use increases with little gain in throughput. Predicted power requirements per metre of auger length do not take into account drive losses and overload conditions. These values are for dry wheat but will also give reasonable values for most coarse grains and ground feeds. Reduce capacity by 25% and increase power by 40% for soybeans.
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DID YOU KNOW?
Auger conveyors are the most common method of handling grain on farms. Augers can be fixed or mobile, are of simple construction and are easy to operate and maintain.
Safety precautions designed to protect the user of our range of augers have been put in place on our complete range, to ensure the safety of usage.
Choke length refers to the portion of the auger projecting below the casing at the intake end. It is essential that the auger be immersed deeply enough into the grain to completely cover the choke opening.
Clearance is the space between the inside of the casing and the outside of the flight. The clearance should be greater than the grain size or hamming of the grain will occur.
