Pulley cart 2

[sihaam]

Hello all
I am designing a system to use around 5kg pulling power to drag a cart weighing 60kg for 1/2 meter forward and then backwards. If I use the 6 pulley system I can get to pull the cart nicely forward using only 5kg power. But I need to pull it backwards again using 5 kg power. Is there any way one can design a pulley system to do both actions instead of using 2 x 6 pulley systems?

 

[FeX32]

Why are you using a pulley system? A linear actuator could do that very easily.


Fe (IronX32)

 

[sihaam]

Good question! However the reasons I am using pulleys are:
1. Since I am building around 40 carts for a kidnergarden play den, linear actuators will cost me a fortune.
2. I am using stepper motors (6nm) as the entire carts system will be pc controlled.
It must be pc controlled and linear actuators which could pull/push 60kg with stepper or servos cost at least 8 times the price of a simple 6NM stepper connected to a pulley system.

 

[drawoh]

sihaam,

Are your carts being pulled up and down, or back and forth?

You need to be disciplined about your terminology. Kilograms are not a unit of power, force or weight. You are exerting force to accelerate and decelerate a mass of 60kg. If you are lifting, then you need to account for a weight of around 600N. What will resist the motion of these things? Are they on wheels? Are they sliding? Are they running over tiny fingers and toes?

If I were moving 60kg masses in the vicinity of small children, I would be worrying about safety.


--
JHG

 

[sihaam]

Actually, the weight of 60 kg is about the weight of two small children in a simple open plan cart. Yes the cart is on wheels guided by a 1 meter rail track. So the cart moves forwards and backwards, not up and down as I made it clear in my original post. The wheels are bearing loaded so the movement will be quite smooth. Moving such a cart with even a DC motor is fine, but like I mentioned, it needs to be PC controlled.
You may ask -but why PC controlled? Simple, the cart moves rhythmically with sound and stops with sound. I can achieve this very simply using stepper motors. With DC motors I have no real control. So the problem I am being faced with is the sudden "stops" and "gos". I need precise stops and gos, again only achieved with steppers. It must be almost a dead stop and this is the biggest friction bother I have. However, the carts wont be moving that fast to give whiplash to the kiddies, nor is any aspect in the design or sequence any where near dangerous. I am very well aware of all safety measures concerning small children.

 

[MikeHalloran]

I'll just assume that you have a way to keep tiny fingers out of the pinch points between wheels and rails.

Steppers may not be your best choice. Their piecewise motion is known to excite resonances in other stuff, like buildings, when their speed is ramped, as in following music or otherwise accelerating.

I suggest you take a hard look at DC motors with encoders and closed loop controls. ... which can be driven by computers.





Mike Halloran
Pembroke Pines, FL, USA

 

[FeX32]

I see . Why not make a circular track to have the ride go in a circular motion. This effect has had much success in the amusement part business. The kids might like it more. Also, you won't have to worry about designing something complex with pulleys to accomplish the back and forth motion.


Fe (IronX32)

 

[BrianE22]

A "double drum" will both pull and "push" a load. For instance:

http://cordem.com/double-drum/

 

[sihaam]

I appreciate all the comments, but I have been using steppers with great success in the past. Kiddies motion carts do not need highly precision servos with encoders etc. Circular track as suggested by FeX32 will no be suitable. Those of you who are acquainted with 4D motion cinemas will understand why you need forward and backward movements. The kids sit in a seat (which is actually a cart) and will move according to the motion film.
There will be no tiny fingers getting into the way as the wheels are rubber and the rails nylon. Also, kids can even get hurt on a simple tricycle should they poke their fingers between the chain or spokes. Certain disciplines are usually enforced on all kids regarding any motion toy -otherwise no kid will ever enjoy any thrill.
Be as it may ,since I do not have any positive design coming forth I will stick to my system of using the two by 6 pulleys (one to move forward and to move backwards. Although slightly cumbersome, pulleys allows me to use much smaller steppers. 6NM steppers works fine and they are cheap too!

 

[MikeHalloran]

May I humbly suggest that you build just one and get it working before you buy parts for more.

Making mistakes in quantity gets expensive fast. This, I know.







Mike Halloran
Pembroke Pines, FL, USA

 

[FeX32]

I didn't know the application. Since you have now stated it I agree with Mike fully. And, the double drum suggested by Brian is a good candidate to consider back and forth motion.


Fe (IronX32)

 

[FeX32]

Although, what you really want is a full on 4DOF simulation machine like the one below. I recently took this picture at an AutoShow. Simulation was tons of fun! (I am not suggesting you do this, it's just fyi )

Cheers,


Fe (IronX32)

 

[sihaam]

Someone sent this design, instead of using two x pulley setups. This is what I needed. Thought I would share...

 

[MikeHalloran]

Now, that's funny!

Thanks for sharing.


Mike Halloran
Pembroke Pines, FL, USA