Högskolan i Skövde

A conceptual design of a miniature vegetation cutter for use in minefields in southern Lebanon has been developed and the authors have evaluated its mechanical properties focusing on the stress in welded joints using the CAD software Pro/Engineer Wildfire 4.0. The conceptual design has been developed in the field in close cooperation with field staff from MAG Lebanon.

The requirements from the field specify that the cutter should have a cutting range of 80 cm, be equipped with adjustable covers, weigh less than 200 kg, be fitted to a commercial hydraulic excavator, and have the ability to cut vegetation, bushes and small trees including (olive) trees with a diameter up to 10 cm.

The miniature cutter is to be manufactured in the field and fitted to a commercial hydraulic ex­cava­tor (Caterpillar 301.6C). It consists of a rotor on which eight cutting blades, alternatively chains, are attached in a helix formation. The cutter is protected by adjustable covers. When in operation, the rotor spins at 750 revolutions per minute whereby the blades cut through the shrubs and bushes in the cutter’s path. The blades are mounted in a T-shape on arms, which are fastened to the rotor by a pin joint between two brackets (each) on the rotor. Blades and arms are to be welded together, as are the brackets to the rotor. These welded joints are the primary focus of the report. 3D CAD models have been created and analysed in PTC Pro/Engineer Wildfire 4.0 to ascertain that the stress in the joints will not exceed the yield strength of the weld consumables, which should be 500 MPa. Ideally, the stress in the joints would be half the yield strength.

Type of bearings and a hydraulic motor have been selected for the cutter. Based on the specifica­tions of the hydraulic motor an approximation of the forces acting on the weld joints in the case of an accidental stop (e.g. collision with a rock) has been calculated, and entered into the CAD software. Also, an approximation of the size (diameter) of branches the cutter would be able to tear apart in the case of branches getting stuck has been calculated and shown to be about 14.6 mm. Based on this, it is estimated that the cutter should be used only in areas where the shrubbery is of 20-30 mm in diameter, maximum. Considering this, and the relative light weight of the cutter, it is not likely that the cutter will be able to cut through the larger olive trees as requested, but it is considered that the tool still could be a valuable asset for mine clearing in Lebanon. In order to cut through thicker trees, it would be necessary to increase the power supply to the cutting system as well as the sturdiness of the cutting parts. Finding the required power and technical solutions for this demands further research which does not fit within the time frame for this report.

A preliminary weight approximation shows that the cutter will weigh roughly 170 kg, which falls below the limit of 200 kg and leaving some room for the bearings to be added.

The results from the stress analyses show that the stress in the welded joints falls well below the yield limit of 500 MPa, but not below 250 MPa. Still, the stress in all the welded joints is shown to be less than 300 MPa or at 40 % of the limit, which may still be acceptable. The end user will have to decide whether this is an acceptable safety margin before manufacturing the cutter and if it is not, measures will need to be taken to reinforce the weld joints and try to minimise the stress concentration in them.