Performance efficiency refers to the quality of work done by a machine. The importance of a performance evaluation is not the same for all machines. For example, the quality of the job for primary tillage, like plowing, is not as critical to the profitability of the farming enterprise as the quality of the job for a combine. In addition, it would be very difficult to mathematically evaluate the quality of plowing.
For a harvesting machine, performance efficiency is a measure of the actual performance of the machine compared to the desired performance. For example, if the machine was a combine, we could measure the bushels of grain harvested compared to the total bushels of grain in the field. Combines also could be evaluated according to the amount of damaged grain. Other harvesting machines could be evaluated on the basis of the amount of bruising of fruit or on the number of cracked shells. An example is the small grain combine.
A combine can lose grain in three different ways: the gathering unit can shatter grain from the head or drop heads, the threshing unit can fail to remove grain from the head as it passes through the machine, and the separating and cleaning units can fail to separate the grain from the material other than grain (MOG). The losses are usually expressed as a percentage of the yield of the field.
Evaluating combine losses is a multiple-step problem. What we want to know is the amount of grain that the combine fails to put into the grain bin. The first step in evaluating the performance of a combine is to determine if a problem actually exists. This is accomplished by determining the total loss behind the machine.
To determine total loss, a known area is marked out, and grain is counted on the ground behind the combine. Losses at this point include grain on the ground before the combine started (preharvest loss) and grain loss by the combine (machine loss). For most cereal grains, losses are determined by counting the number of grains in a known area, for example, 10 or 100 square feet. This will provide data with units of seeds per unit area. The table in Appendix III can be used to convert seeds per square feet (sds/ft2) to bushels per acre (bu/ac) or seeds/m2 to kilograms/ha.
Problem: What is the performance efficiency (percent) in lost grain for the combine shown in Figure 9.1?
Solution: Figure 9.1 represents the results of measuring the losses from a combine. The first step is to determine the total losses. In the example illustrated in Figure 9.1,162 seeds of wheat were counted in a 10-square foot area behind the machine.
By referring to Appendix III, we can convert the total losses from the measured quantity of seeds per 10 square feet to bushels per acre. From Appendix III, 20 seeds per square foot equals one bushel per acre. Applying the units cancellation method:
Note: The previous equation may be easier to understand by remembering that when dividing fractions, inverting and multiplying is the same operation. The previous equation can be written as:
Is a loss of 0.85 bu/ac an acceptable level of loss for this machine? As noted earlier, the mathematical calculations do not usually answer the question; they just provide information to make a more informed decision. The two common methods used to make informed decisions are to compare the results with the historical performance of the machine and to compare the results to an accepted standard. Research has shown that under favorable conditions an expert combine operator should be able to keep losses to less than 1%, but that losses up to 3% are typical. Therefore to determine the acceptability of the combine performance, the total losses must be compared to the standard. To determine the loss as a percentage two values with the same units must be used. This is usually done with combines by comparing the loss and the crop yield (bu/ac).
For this example we will assume that the wheat is yielding 30 bu/ac. The total loss percentage (Lt%) is:
Y 30 — ac where Yt = the total yield in sd/ac, bu/ac, sd/m2, or kg/ha bu kg Yl = yield loss, — or — ac ha bu kg Yt = total yield, — or — ac ha
Based on the less than 3% standard, this machine's performance would be considered acceptable. If the 1% standard was the desired performance level, additional calculations must be completed to determine the source of the excessive grain loss.
At this point it is important to remember that total loss is a combination of machine loss and preharvest loss. An unacceptable total loss does not necessarily mean there is a problem with the combine. The source of the unacceptable total loss may be preharvest losses. The combine has no influence on these losses. Preharvest loss is measured by counting the grain on the ground in the standing crop that has not been harvested. In Figure 9.1, we note that preharvest losses are 5 seeds per 10 square feet. Therefore, the machine loss in bushels per 10 square feet is:
Machine loss (Lm) = total loss (Lt) - preharvest loss (Lp) 162 sds 5 sds
10 ft2 10 ft2 sds
Converting this value to bushels per acre:
L m = 157 ^f x_jc__ = ^ = 0.826... ^ 10 ft2 19 seeds 190 ac
Converting this to a percent:
Rounding to two significant figures, the total loss and machine loss are both 2.8%. The preharvest loss is too small of a percentage to be measured.
It can now be concluded that, if the 1% standard is used, the losses are excessive and the machine is the source of the excessive losses. When this occurs, it is necessary to determine which component of the machine is the source of the losses.
The first step is to check the gathering unit. The performance of the gathering unit (header) is checked by counting the seeds on the ground between the header and the uncut crop. 9.1 shows that the losses in this area are 12 seeds per 10.0 square feet. (Remember that this also includes the preharvest losses.) The gathering unit (Lg) losses are determined by subtracting the preharvest loss from seeds counted in the area between the header and the uncut crop.
12 seeds 5 seeds 7 seeds seeds
Converting this value to bushels per acre:
ibu bu = 0.7 seeds x-ac__ = °Z = 0.0368 or 0.04 bu ac ft2 19 seeds 19 ac ft2
or as a percent:
This shows that the gathering unit losses are 0.13%, a very small part of the total machine losses.
When the preharvest loss and the gathering unit losses are both low, the threshing and separating units must be checked.
Threshing losses are represented by heads or cobs on the ground behind the machine with grain still attached. Cleaning and separating losses are represented by grain on the ground behind the machine that is not preharvest or gathering unit loss. Cleaning and separating losses can be determined by subtraction:
L c&s = L t — L p — L g — L th where Lc&s = Cleaning and separating losses; Lt = Total losses; Lp = Preharvest losses; Lg = Gathering unit losses; Lth = Threshing losses.
L c&s = L t — L p — L g — L th sds sds sds sds = 162-t - 5-T - 7-T - 0
10 ft2 10 ft2 10 ft2 10 ft2
Note that the threshing losses are listed as zero. This is because the information is not available to determine this loss. The threshing and separating loss percentage is:
bu 150 sds bu 1 ft2 150 bu
ac 10 ft2 ac 19 sds 190 ac bu
Clearly, if the desire is to improve the performance of the combine the cleaning and separating units should be adjusted or repaired.
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