The knowledge of physical and mechanical properties of onion bulb is important for successful design of any planter. Multiplier onion is propagated through bulbs and bulbs are planted manually. Manual planting of onion bulb is highly labour intensive (80-100 man-days ha-1) due to the close plant geometry (plant to plant and row to row spacing is 10 × 15 cm) among the vegetable crops. Engineering properties of multiplier onion were determined in order to design and develop a tractor operated raised bed onion bulb planter. In multiplier onion each onion comprises of 4-5 bulbs. These bulbs were cleaned, separated into single bulbs and divided in to 9 categories manually based on their individual weight viz., <2g, 2-3 g, 3-4 g, 4-5 g, 5-6 g, 6-7 g, 7-8 g, 8-9 g, >9 g. The onion bulb had a moisture content of 80.62±0.87 %. It was observed that the properties varied for different sizes of onion grades and the grades had highly significant effect on most of the properties. The linear dimensions of onion bulbs viz., length, width, thickness for nine grades ranged from 21.21±2.60 to 32.31±3.30 mm, 13.54±1.77 to 30.95±2.91 mm and 10.91±1.40 to 22.63±2.15 mm. The geometric mean diameter, sphericity, shape index and projected area all ranged from 28.22±2.04 to 14.54±0.96 mm, 0.87±0.06 to 0.69±0.08, 1.78±0.32 to 1.22±0.14 and 5.50±0.51 to 1.55±0.30 cm2, respectively. The one hundred onion bulb weight, bulk density, true density were in the range of 1185±19.59 to 121.6±6.30 g, 793.20± 9.45 to 480.19±13.13 kg.m-3, 1086± 205.22 to 1013.6±348.85 kg.m-3, respectively. The angle of repose decreased with increase in size of onion bulbs. The interaction between sphericity, shape index and angle of repose indicated that as the size of the onion size increase angle of repose decreased.
