Gripper using an engine and four half-wheels

[marcoloca]

Hi!
I need some suggestions about the construction of a particular gripper, which uses four wheels to take a can (of a standard size), bring it up and release it when necessary.
We (my team and I) had this idea watching this video on youtube: http://www.youtube.com/watch?v=LUvBfAmU ... re=related
You can see this particular gripper at the minute 6:25 (more or less).

First of all, do you think that is a good idea?
Secondly, if yes is it difficult to build, or if not, can someone suggest something? (Remember that we only have an output, because the two others are occupied by the motors)

I thought we can build it thanks to a worm gear, but today we tried to build it and we saw that, if the worm gear is the one which moves, the gear connected to it can work normally, but if it is the gear moving, the worm seems not to move... Is it normal?

And finally, which gear would you connect to the worm gear? 8, 24 or 40 toothed one?

[Sorry if my English isn't the best you'll ever see, I'm Italian and I'm trying to make everybody undestand ]

Thank you very much to the ones who will answer me!

[bullestock]

Hi!
I thought we can build it thanks to a worm gear, but today we tried to build it and we saw that, if the worm gear is the one which moves, the gear connected to it can work normally, but if it is the gear moving, the worm seems not to move... Is it normal?

I hope I understand you correctly. If you have a worm gear and a normal gear, like this:
then you can drive the worm gear to move the normal gear, but you cannot drive the normal gear to move the worm gear; i.e. you can only reduce the speed.

[wrutiser]

Worm drives work best when the worm is powered and the worm turns the gear. When the gear is powered the friction between the worm and gear keeps both from turning. When a worm and gear are used in machines like jacks and hoists, the friction keeps the load from falling when the crank is released.

I suggest you make a simple model with just two axles, a worm, and a gear. Experiment with the force needed to turn each shaft.

The number of teeth on the gear determines the ratio of speeds of the two shafts. The larger the gear, the slower it will turn but with more force.

-- Bill

[marcoloca]

I hope I understand you correctly. If you have a worm gear and a normal gear, like this: IMAGE
then you can drive the worm gear to move the normal gear, but you cannot drive the normal gear to move the worm gear; i.e. you can only reduce the speed.

Yes it's something like this (with Lego, obviously)! Thanks for the explanation =)

Worm drives work best when the worm is powered and the worm turns the gear. When the gear is powered the friction between the worm and gear keeps both from turning. When a worm and gear are used in machines like jacks and hoists, the friction keeps the load from falling when the crank is released.

I suggest you make a simple model with just two axles, a worm, and a gear. Experiment with the force needed to turn each shaft.

The number of teeth on the gear determines the ratio of speeds of the two shafts. The larger the gear, the slower it will turn but with more force.

Ok, I understand now, but I was wondering how to calculate the ratio of reduction with the worm gear... With normal gears is simple, because you look at the number of teeth (at school, we learned so...), but with this...

So, you all think that is a good idea using the worm gear?

[wrutiser]

An ordinary worm acts like a gear with one tooth. Each turn of the worm moves the gear by one tooth.

Some worms have multiple start threads. A two-start worm acts like a two tooth gear.

To visualize: wind a piece of string onto a pencil -- that is like a one start thread. wind two parallel pieces of string and you get a two start thread.

I doubt Lego makes multiple start worms.

See:
http://en.wikipedia.org/wiki/Worm_drive
http://en.wikipedia.org/wiki/Screw_(simple_machine)

[inxt-generation]

I hope I understand you correctly. If you have a worm gear and a normal gear, like this: IMAGE
then you can drive the worm gear to move the normal gear, but you cannot drive the normal gear to move the worm gear; i.e. you can only reduce the speed.

Yes it's something like this (with Lego, obviously)! Thanks for the explanation =)

Worm drives work best when the worm is powered and the worm turns the gear. When the gear is powered the friction between the worm and gear keeps both from turning. When a worm and gear are used in machines like jacks and hoists, the friction keeps the load from falling when the crank is released.

I suggest you make a simple model with just two axles, a worm, and a gear. Experiment with the force needed to turn each shaft.

The number of teeth on the gear determines the ratio of speeds of the two shafts. The larger the gear, the slower it will turn but with more force.

Ok, I understand now, but I was wondering how to calculate the ratio of reduction with the worm gear... With normal gears is simple, because you look at the number of teeth (at school, we learned so...), but with this...

So, you all think that is a good idea using the worm gear?

Calculating a worm gear ratio is even easier than normal gears. The worm gear is like a gear with 1 tooth, so each rotation of worm gear equals 1 tooth on normal gear. e.g. worm to 40-tooth = 40-1 ratio.

If you don't mind the slow speed, a worm gear should work fine.

P.S. There's a excellent LEGO gear ratio calculator here.

[h-g-t]

Re gear ratios - one turn of the worm moves the gear round by 1 tooth so the gear ratio is the number of teeth on the gear wheel:1.

Unlike a normal gear train, a worm drive stays in the same position when the motor stops so you don't have to waste battery power when holding position.

Also, you get a much more compact assembly than a succession of gear wheels for the same ratio.

Even though there is greater friction between the worm and gear, overall it is much the same as a conventional gear train giving the same ratio.

If you want a really high ratio, put another gear on the worm axle and drive that with a second worm; two 40-tooth gears and two worms give a ratio of160:1!

[marcoloca]

Wonderful! Thank you very much, all the information and the links are really useful! I'll keep you up to date about our work (maybe we'll share some photos of our work)!

Thank you very much again to all! =)

[marcoloca]

Hi again guys!
The sacrificial-wheels I bought are just arrived =)
There are some problems:
- These wheels are empty inside --> How can I fill them?
- I don't know what kind of glue I must use
- Can I just cut the tire without losing efficiency?
- Would cause problems "cutting" also the hard-plastic part, where the tile is fitted with?

Perhaps I can use some different type of wheels, what do you think?

[spillerrec]

It seems like you got the wrong kinds of wheels, the wheels used in that video are solid. The ones in this set looks solid: http://shop.lego.com/en-DK/Mini-Tow-Truck-9390. There were some larger ones in the past, but I think bricklink is the only way to get those now.
But perhaps you can do this without cutting them in half and instead just reduce the thickness of the wheels in one side. I'm thinking something like this:
It might be strong enough to keep its position without glue this way, while still allowing it to drop them in one position.

It might be interesting to do this in a spiral like fashion so that the between the wheels are reduced the more you turn the wheels. This would widen the range of widths the object may have in order to pick it up (at the cost of reducing the height), though it is not quite useful for your situation... Just speculating about this anyway...