Electrical Engineering - force, torque, power

Author

force, torque, power

blockhed

2006-04-30, 4:21 pm

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

rkazanjy@gmail.com

2006-05-01, 12:21 am

blockhed wrote:
> 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

[color=darkred]

because that's what their developement teams came up with
[color=darkred]

No

I was going to suggest that you investigate the power of existing
units but you have already done that

Maybe your knife is sharper than the typical hand bender blades

btw if you're going to us SI................. kg is a mass not a
wieght/force

you say Power = Torque / time

I say Power = force (lbf) x velocity (in/sec) OR power = torque
(lbf-in) x angular velocity (rad/sec)

check your units & estimate your angular velocity

I did a WAG for power via force x velocity & I got ~10 watts

cheers
Bob

Doug T

2006-05-01, 1:21 am

blockhed wrote:
> 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,
snip
> why are the hand blenders on the market all 300-600 W ?
> are my calculations correct?
>
> Thanks

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

daestrom

2006-05-03, 7:21 pm

"blockhed" <hjhkjhkjhjkh@hkjhkjhjhkhjkhk.com> wrote in message
news:QKmdnfuaBd44msjZRVnytw@bt.com...
> 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
>

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.

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

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

Phil Scott

2006-05-14, 3:21 pm

--
Phil Scott
Ideas are bullet proof.
"blockhed" <hjhkjhkjhjkh@hkjhkjhjhkhjkhk.com> wrote in message
news:QKmdnfuaBd44msjZRVnytw@bt.com...
> 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

You left speed of the total disintigration out of your
equation, and assumed that the apple masher machine used knife
blades perpendicular to the apple.. it cant of course.

>
>

Phil Scott

2006-05-14, 3:21 pm

--
Phil Scott
Ideas are bullet proof.
"Doug T" <dtreybo@peoplepc.com> wrote in message
news:jKf5g.11986$Es3.10322@newsread3.news.atl.earthlink.net...
> blockhed wrote:
> 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

there is also the issue of cost/vs reliablity and warranty
claims.. you wouldnt size the motor to the bare minimim...
that would be like sizing a car engine to do max 65 mph...

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