force, torque, power

Apr 30, 2006 5 Replies
B

Hi guys,



need a bit of help with a project



i have to work out the power required for a motor in a hand blender, the hardest fruit i need to cut is an apple,



i put a long knife on the apple and a weight of 1.2kg on the knife cut the apple



using the equation for torque i get



Torque = r x F x sin(theta) r = distance from pivot of the knife to the cutting point on the apple F = force theta = angle at which force is applied perpendicular to r



so Torque = .31m x 1.2kg x 9.81m/s x sin(90) = 3.65 Nm



estimated time to cut the apple was 0.1s Power = Torque / time = 3.65 / 0.1 = 36.5 W



so I need a 36.5W motor to cut the apple,



why are the hand blenders on the market all 300-600 W ? are my calculations correct?



Thanks


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D

snip

They also have to cut carrots, tofu, and on up to ice. While most veggies might not be any harder to cut up than an apple I'll bet ice sure is.

Doug T

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D

For Power = Torque / time to be true, the time you're using must be the time it takes to turn 1 radian, not the time to cut through one apple. So you're turning your rotor assembly at 10 radians / second. That's about 95 RPM. I think most blenders turn faster than that when switched to full power.

Most blenders can cut/chop through several 'chunks' at the same time. Take two 'beaters' and assume two of the four blades on each 'beater' is slicing/cutting at the same time. So torque is four times higher. Now assume it spins at about 300 RPM (31.4 radians/sec).

That works out to 4*3.65*31.4 = 459 W

daestrom

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