The best way to minimize tuber bruise damage caused by the potato harvester is to eliminate nearly all of the soil with the primary chain. Then keep the subsequent chains as full of tubers as possible without spill-out, rollback, or back-feeding under the previous chain. This approach results in lower tuber damage, and fewer vines and less soil going into the truck.

Use the following steps to adjust chain speeds for proper loading.

As an example, imagine that you wish to match the ground speed to a rear-cross chain that is running at 135 ft./min. Since the chain speed should be 50% to 60% of ground speed (use 55%), the ground speed should be about 135/0.55 = 245 ft./min. To get the speed in miles per hour, divide ft./min. by 88 (88 feet/min. = 1 mph), for example:

                            245 / 88 = 2.78 mph

In our example harvester, let's say that the elevator is currently going only 118 ft./min., but we want it to go about 135 ft./min., the same as the rear-cross. Count the number of teeth in the driver sprocket. Let's say it has 12 teeth. We want to speed up the chain, so make the driver sprocket bigger (Figure 1). Try a 14-tooth driver. You've increased the speed to:

               14/12 to (14/12) x 118 ft. /min. = 137.7 ft. / min.

That's close enough.

We also want the boom going about 55% of ground speed, or about 135 ft./min. If the boom is currently running at 112 ft./min. and has a 20-tooth driver, then we need to use a bigger driver. If we use a 24-tooth, the speed becomes:

             (24/20) x 112 = 134 ft./min.;

close enough.

The tuber trajectory from the secondary to the rear-cross is critical (Figure 5), as are other transfers where the flow changes direction by 90° . If the chain runs too fast, it throws tubers all the way across the next conveyor. If the chain runs too slowly, the tubers back-feed under the conveyor. Back-feeding occurs whenever these chains run slower than 95 to 100 ft./min. The secondary should go about 65% of ground speed, but never slower than 100 ft./min. Thus, ground speed should never be slower than:

             100/0.65 = 150 ft./min. (approximately),

or 1.75 mph. In our example harvester, the secondary is initially going 122 ft./min., but we want it going 0.65 x 245 = 159 ft./min. If the driver sprocket has 14 teeth, let's try one with 18. That gives:

              (18/14)x 122 = 157 ft./min.

The Deviner chain should be set to run at the same speed as the secondary.

In our examples, we've shown changing the driver sprocket, not the driven. The driver sprocket is the one on the powered shaft; so if you add teeth to it, the chain goes faster. The driven sprocket is the one driven by the chain, so if you add teeth to it, you decrease the conveyor speed. For example, if you change a driven sprocket from 32 to 36 teeth, the new speed will be 32/36 times the original speed.

For light to medium soil, the primary should run about 1.2 times ground speed, or:

           1.2 x 245 = 294 ft./min. (Table 1).

You may have to adjust primary chain speed as field moisture and soil texture change, but as long as yields are fairly uniform, the secondary and subsequent chain speeds can remain the same.

The primary chain is where nearly all of the soil should be eliminated. The rear-cross and subsequent chains should be kept full of tubers to minimize damage. The secondary is a buffer that will take care of occasional increases in soil loading. The blade should be just deep enough to get all of the tubers.

In all of the examples so far, we've used chain speed-to-ground speed ratios optimized for yields of 500 to 600 cwt. (25 to 30 tons) per acre. Table 2 gives ratios for other yields. Note that yield only affects the ratios for the rear-cross, elevator and boom. That's because those chains handle mostly potatoes. The primary chain handles mostly soil, so yield has little influence on its speed adjustment. The secondary is a transition chain that loads the rear-cross chain, so it should always run at about 65% of ground speed, but no slower than 100 ft./min.

For more than two rows going into the harvester (e.g., use of sidecasters), chain speed-to-ground speed ratios for the rear-cross, elevator and boom may need to be increased. The guiding principle is to keep chains as full of tubers as possible without rollback, spill-out, or back-feeding.

• Measure the total length of the chain in feet, preferably by counting the number of links in the conveyor chain, multiplying by the chain pitch in inches, and dividing by 12.

• With the tractor engine running at normal digging speed, measure how many seconds it takes for the chain to make 5 complete revolutions. (Use one revolution for long chains like the side elevator and boom.)

• The chain length multiplied by the number of revolutions and divided by the number of seconds required gives the chain speed in feet per second. Multiply by 60 to get feet per minute.

• Length x revs x 60 ÷ seconds = chain speed in ft./min.

• Count the number of teeth in one of the head sprockets (the ones that engage the conveyor chain directly).

• Determine the conveyor chain pitch in inches (the pitch when it was new is best).

• Use a tachometer to measure the rpm of the head shaft.

• Rpm x no. of teeth x pitch ÷ 12 = chain speed in ft./min.

• Accurately measure the distance in feet travelled in 5 complete revolutions of the harvester wheel.

• Measure the time in seconds required for the wheel to turn 5 revolutions.

• Ft. x 60 ÷ seconds = ground speed in ft./min.

 Figure 2.      Tuber damage at the top of the side elevator vs. soil and tuber loading in sandy soil.

Figure 3.       Tuber damage at the top of the side elevator vs. soil and tuber loading in silt-loam soil.

Figure 4.         Tuber trajectories for four conveyor speeds and for the head sprocket size and location shown.

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