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So, we have to build this scale in mechatronics class that weighs things as small as a dime to as large as however we want, but it has to weigh a dime. I came up with this idea that will be constructed with legos:

A pully (20 cm radius) will be fixed on a shaft with a cup kindda thing hanging on one end to put the weight in (I'll put another one on the other side to balance it). On the other end of the shaft, a small gear (1 cm radius) will be fixed and connected to a rack. The spring will be be on top of the rack so that when the rack moves, it pushes on the spring.

I chose to use a pully with a large radius and a small gear to multiply the force on the spring since a dime doesn't weigh that much at all and the smallest spring I found had a spring constant of .17 N/mm. Here's my work:

The weight pulling down on the pully will create a torque on the shaft = Fr

T = (.01 N) (.2 m) = .002 Nm. *.01 N is approximately the weight of a dime*

The gear then is subjected to the same torque and has a radius of .01 m, so following the same equation T = (F2) (r)

.002 Nm = (F2) (.01 m)

F2 = .2 N

F2 is the force from the small gear on the rack, which is the same as the force on the spring. So, by using a big pully and a small gear I multiply the force of a dime by 20. An optical encoder will be fixed on the shaft to see how much the shaft turns and calculate the weight of the dime.

Are my calculations right? Do you think the friction of legos will make this project a failure? Think oil or grease will help with the friction that much?

Here's a picture if you can't visualize it: www.geocities.com/iamjico/spring.bmp

A pully (20 cm radius) will be fixed on a shaft with a cup kindda thing hanging on one end to put the weight in (I'll put another one on the other side to balance it). On the other end of the shaft, a small gear (1 cm radius) will be fixed and connected to a rack. The spring will be be on top of the rack so that when the rack moves, it pushes on the spring.

I chose to use a pully with a large radius and a small gear to multiply the force on the spring since a dime doesn't weigh that much at all and the smallest spring I found had a spring constant of .17 N/mm. Here's my work:

The weight pulling down on the pully will create a torque on the shaft = Fr

T = (.01 N) (.2 m) = .002 Nm. *.01 N is approximately the weight of a dime*

The gear then is subjected to the same torque and has a radius of .01 m, so following the same equation T = (F2) (r)

.002 Nm = (F2) (.01 m)

F2 = .2 N

F2 is the force from the small gear on the rack, which is the same as the force on the spring. So, by using a big pully and a small gear I multiply the force of a dime by 20. An optical encoder will be fixed on the shaft to see how much the shaft turns and calculate the weight of the dime.

Are my calculations right? Do you think the friction of legos will make this project a failure? Think oil or grease will help with the friction that much?

Here's a picture if you can't visualize it: www.geocities.com/iamjico/spring.bmp

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