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Modeling impact forces in rotary motion
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[QUOTE="jrmichler, post: 6445208, member: 638574"] Wood chipping is roughly a constant force while the blade is in the cut. Cutting force in wood is approximately constant with speed. It's difficult to calculate the cutting force because it is a function of wood density, moisture content, cutting depth, blade geometry, cutting direction relative to wood grain, and blade sharpness. Much easier to measure angular acceleration, and calculate cutting force from acceleration and inertia. From that, torque is force times radius, and acceleration is torque divided by inertia. Or measure total power, and calculate cutting force from number of cuts and length of each cut. A lawn mower blade hitting a rock is more complicated because the blade can bend or not bend, the rock can break up, go flying, or stay locked in place. Here again, the easiest solution is to measure angular acceleration and calculate force. You may be able estimate impact force by calculating the force needed to get observed blade damage. I once headed an R&D project to measure the force of a 24 inch diameter circular knife blade hitting a fixed object. The blade was moving in an orbital path at about 60 ft/sec while spinning 1500 RPM. Since the blade was mounted on a spinning assembly that weighed about 1000 lbs, it was impractical to measure the deceleration caused by the impact. The impact force was high and the impact duration was short because the blade was made from a brittle alloy, so the impact energy was low. The effect on orbital speed was minimal because of the high inertia and low impact energy. So we build a force transducer into a 2 inch diameter solid steel bar, and ran the blade into that. It was interesting watching the high speed video. There were multiple impacts with blade fragments flying all around. We were able to get good measurements of the impact forces. My recollection is that the peak impact force was about 30,000 lbs. These circular knives are used in log saws. Logs, in this context, are 8 to 10 foot long rolls of toilet paper that are cut into 4.5" long rolls at speeds in excess of 300 cuts per minute. Logs are also 8 foot long packs of interfolded facial tissue that are cut into the proper length by log saws. [/QUOTE]
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Modeling impact forces in rotary motion
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