Pulley system for cardboard pulp shredder/mixer

[lucaszito]

Hi all,
This is my very first message on this forum.
I am building a machine for a friend who works with cardboard pulp, he is also a performance artist and would like a machine where 2 or 3 performers can activate the machine by hand with pulleys and wheels, and shred the pieces of cardboard that are soaking in a container filled with water. There is 4 pulleys in total and the last driven pulley is attached to a shaft with blades on it, this whole assembly constitutes the shredding mechanism.
Currently the artist is shredding the cardboard with a hand drill on which is attached some very small DIY blades. The drill runs for about 30 minutes at 8000 RPM and he manages to get the consistency he wants.
In my idea of his machine 2 performers would spin a horizontal wheel bar of 2 meter(6.56168 feet) diameter at about 5 to 15 RPM and that wheel bar is attached to a pulley which is driving a 1:5 pulley, that 1:5 pulley is attached to a 1:1 pulley which drives another 1:5 pulley. Since the blade that will be made for this machine is about 12 times bigger than the original blade he was using, I am not sure that we need as many RPM as the drill gives. But I am not very sure either. Anyhow, I thought that just a little less RPM than the drill would do the trick, maybe 5000 if that is even doable. All pulleys will be made of wood and the wheel bar will be in steel tubes.

However it is my very first time working with pulleys and ratio and I lack experience in safety, in feasibility and in physics.

I do not know how to obtain the RPM I want for the rotating blade while having a decent of size for the whole machine. I have attached some pictures that give you guys dimensions and other drawings to see the machine better. Obviously the pulleys will not be floating, I just havent thought about the structure yet, I wanted to have your opinion first.

I am wondering if such a system will output the results I want, if it is durable and safe.

Thank you very much for your help !

Lucas

 

[3DDave]

It may not. The hand drill may be as much as 1/8th to 1/4 hp. 1 hp = 760 Watts. An Olympic sprinter's legs can generate roughly 1 hp for about 30 seconds, so unless this is a really short demonstration, this probably won't scale up well.

You can tell roughly what energy the drill is putting in by multiplying the voltage times the amp-hours.

As an example if it's 1/8th hp then it's about 100 W * .5 hours or 50 W-hrs.

Alternatively if there is a 3 Amp-hour, 24 V battery, then that's a (3 * 24) = 72 W-hr battery source.

One area people make a mistaken assumption is on leverage. While increased leverage gives greater force (though with the speed step up you are generating decreased force to provide greater speed) the increased leverage requires greater amounts of movement and, with leverage (torque) times speed (rotation rate) is a measure of power, if the system (person) putting power into the device is limited no combination of gearing/levers will get the person to make more than that maximum.

The a parallel case can be true - people are also force limited and if the system requires too much force than leverage can decrease the demand (which is why bicycles have gears and wrenches are different lengths.) But this is only force. Power output is limited to converting long periods of output into short periods of application, which is why hammers and bats work. The peak power of slamming something with a hammer is from storing up a long period of power in raising the hammer.

Perhaps a hammer mill to pulp the cardboard? It's not a high continuous power system, and just beats the daylights out of stuff. It might be more easily powered from stationary bicycles and lower RPMs. Experimentation will be required to see what it takes to pulp the cardboard this way, but I think the original idea will not be useful and the hammer mill does have a chance to work.

 

[lucaszito]

Thanks a lot for your detailed answer !

Would you think that it is possible to keep the same system by adding pulleys or changing their sizes ? After checking the drill turns only 3000 RPM and generates 550W of power for 30 minutes.
For the performance, the artist really wants a system like this one (...for aesthetical reasons...) with 2 or 3 performers turning the big wheel bar. This video shows a bit the effect I would like to achieve, a first pulley greedy in torque that outputs a driven pulley with a multiplied speed. The goal is really to have the performers sweat while operating the device.

The running time would be less than 5 minutes as it really takes little time for the cardboard to be shredded into pulp.
My question would be, how is it possible to make the device happen while keeping a system with a horizontal big wheel bar ? What needs to change or to be ?

Also a plan B would be to use stationary bicycles with lower RPM but I don't really know how to gear all that with a low budget...

Thanks again for the help

 

[LittleInch]

I'll have to confess I'm reall not sure about some homemade contraption spinning "blades" in front of anyone and certainly not the paying public...

I can't envisage this pulping machine myself so a picture or drawing would help a bit.

Some numbers might help you.

In your diagram the big wheel at 10 rom will give you 250 rpm at the small wheel, but really not very much torque.
For handwheels and similar large wheels, you're probably looking at about 50N edge force per person once you facto in having to continually swap hands. That gives you some idea of what torque you could develop.

The often quoted output power from a cyclist over a reasonable time period is about 150W or maybe 300W for a true performance cyclist over a sustained period.

The thing to remember with belts is that power transmission and torque on each pully is very dependant on belt tension to avoid too much slippage. There is a huge amount of information on belt drives, but I would not be doing this as cantilever type axle - i.e. you want to support both sides and have room for tension adjustment.

But first work out what RPM and torque you need on the end device.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[lucaszito]

Thanks LittleInch for your detailed answer and support!

The machine is done already and the performance was a success! There was over 900 persons!

I'm so delighted to have found about en-tips because your feedbacks and advices were very helpful and valuable.

Unfortunately I cannot upload a video from my phone here so when the full reel from the performance will be published I'll post a link here.

In the end I managed to find or make the gears I needed to make this contraption work. There is 5 performers relaying on the spinning mechanism, each revolution on the pedaling system equals to 28 blade revolutions in the mixer. Indeed I must admit that it is very tireing and the performers can't really perform for more than 45 seconds, but it doesn't matter because the with the correct ratio of water and cardboard, the blades are really efficient and it only take about 3 minutes to get the pulp to the right consistency.

The performers were performing for 3 hours straight so it's more of a workout than a performance haha.

 

[LittleInch]

Gears might not look as fancy, but work a lot better than belt drives.

I'll look forward to seeing this.

Goes to show that humans don't actually work that hard compared to horses or electric motors....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.