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levers, linkages and gearing
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Spockie-Tech
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Joined: 31 May 2004
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Location: Melbourne, Australia


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Hmm.. calculating the ratio of a lead-screw. interesting question.

I'm not *certain*, but I would think you would do it by working out the ratio of the driving gear to the driven gear..

Measuring the diameter of the driving gear will give the distance it travels per RPM. Spin it 20 or 50 times (to make a measureable movement distance), and measure how far the driven gear (shaft) has moved.

I'm not sure if you should measure the inside diameter (bottom of the tooth groove) or the outside diamter (top of the teeth). perhaps the middle ?

The diamter ("length" effectively) of the driven gear x the number of turns you did divided by the length of the driven gear should give you the gear ratio to multiply your torque figure by (ignoring drive losses)

The its just a matter of the ratio of the distance from the driven-point-to-pivot vs the end-point-to-pivot ratio to work our how much your lever increases or decreases the force.

Clear as Mud ? Anyone see anything wrong with that ?
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Post Wed Nov 02, 2005 8:08 pm 
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Totaly_Recycled
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Joined: 15 Jun 2004
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I think that it is the pitch of the thread (ie how many threads per cm or inch ) is what determined the ratio .a finethread will take many more turns to move the part a cm than a corse thread would so the fine thread would give you more torqe but slower speed a corse thread would do the oposite .

Post Wed Nov 02, 2005 9:11 pm 
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Spockie-Tech
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yes, but if you want to work out the numbers, you need some way of turning the "fine" or "coarse" threads into a travel number..
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Post Wed Nov 02, 2005 9:24 pm 
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Daniel
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Joined: 30 Aug 2005
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Jeez, talk about making things complicated.

Since we all seem to be ingnoring the huge loss due to friction:

Power in = Power out

Power in = rotating velocity (rpm/(2*pi*60)) * torque (N/m)

Power out = linear velocity ((rpm of the rod / 60) * thread pitch in meters) * Force

If the drill does 550rpm, 16Nm and the pitch of M10 threaded rod is 1.5mm or 0.0015m:

(550/2pi60) * 16 = ((550/60) * 0.0015) * Force

Force = 1.459 rad/sec * 16Nm / 0.01375 m/sec

Force = 1697.7N or 173kg


Of corse if you measure the actual force it will be a lot less then this because there is a lot of friction on a zinc coated threaded rod and nut. You will be a lot better off just trying to crush some scales as Prong pointed out.

Post Thu Nov 03, 2005 8:28 am 
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Nick
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I looked up some manufacturer's sites and they all quote around 35 to 40% efficiency for similar diameter and pitch lead screws. At 40%, 69.2Kg from a 9.6V drill is still very respectable - its like having a guy standing on your bot with a spike Smile
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Post Thu Nov 03, 2005 8:51 am 
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Nexus
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Have to agree with Daniel. In my experiences the friction loss element can be huge. Make sure you lubricate things and a good way to test efficiency is try running it at low voltage and see how it performs.
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Post Thu Nov 03, 2005 8:58 am 
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