Development of a hydraulic hole digger for spot treatment of landscapes and orchards

Applying appropriate fertilizing method has an important role in improvement of soil physical properties of trees root zone. Currently, digging few holes around the trees and filling it by manure is known to be the best way for nourishing trees. The most common type of post hole diggers are one or two-man engine augers as well as tractor types which neither are applicable in Iran's orchards conditions. In this study a hydraulic driven post hole digger which has the ability of digging 15-20 cm wide holes to a depth of 50 cm yet portable to restricted spaces was developed. In contrast to tractor and manual types that the procedure begins with a high speed of auger, in the developed machine the auger screws down at a relatively slow rotational speed to a given depth where lifting up the auger brings the compacted soil between the flights of augers out of the hole. For this purpose the power of a diesel engine is transmitted to a hydraulic system which provides the required torque for rotation of the auger. Tow hydraulic arm is also provided to push and pull the auger in and out of the hole. In order to choose the appropriate auger for this machine three types of spiral augers with conical tip (two size of flights pitch) and flange tip and a cylindrical type were evaluated. The auger's function were evaluated in terms of the required torque for rotation in the soil, penetration ability to a desirable depth, the quality of taking soil out of the hole and energy consumption for digging one cm2 of soil. The results showed that the spiral type auger with flange tip does not tend to penetrate in examined soils so that despite applying 90 kg force for 80 s on it, only 16 cm penetration was obtained. Whereas, by the same force the spiral augers with conical tip reached to a depth of 50 cm in less than 45s. In case of cylindrical auger the depth of penetration in each insertion would not exceed 22 cm which take one minute and the required torque for reaching to this depth is almost double as the spiral type with conical tip. Calculation of energy consumption revealed that the spiral auger with flange tip spends the highest amount of energy and the cylindrical type is in second place. This is due to longer time which is taken for these augers to reach to a relatively shallow depth. The spiral augers (with conical tip) need 40% of the total energy which is needed for cylindrical auger and their flight pitch has no significant effect on their energy consumption. Comparing of spiral augers with different flight pitches showed that the required torque for rotation of augers with 5 cm pitch is higher than the one with 7.5 cm pitch in both clayey and sandy soils. However the 5 cm pitch is able to evacuate 92% and 86% of the dug soil in clayey and sandy soils respectively whereas with 7.5 cm pitch these magnitude are 88% , 66% respectively. Moreover, field observations showed that in clayey soil an excessive time is spent on taking the stuck soil out of the auger flight which in turn reduces the field capacity. As flight pitch of spiral augers has no considerable effect on specific energy consumption, the augers with 5 cm and 7.5 cm pitch are respectively recommended for using in sandy and clayey soils.