Rotary to vertical motion (Designing a vibrating platform)

[wakadarc]

Hi,

I am designing a vibrating platform that will move strictly up and down (total displacement of 1 inch). The load on the platform will vary between 0 and 160kg.

The mechanical assembly/mechanism (which is underneath the platform) is

motor>belt>flywheel>crank attached to a 1 inch shaft long that is off center of the flywheel which is then attached to the flat platform. The square platform is have 4 vertical rods through its corners with linear plain bearings.

Take a look at the slider crank
http://functionspace.com/topic/3704/Converting-Rotational-motion-to-Linear-motion-and-vice-versa

I need to find out how to spec the motor (torque,HP). Is this mechanical assembly the right application for a vibrating platform?

When the crank reaches the bottom of the circular motion (dead point) would it not have two ways of moving resulting in a potential failure?

 

[Nidum]

The whole system as described will be unstable and in any case without knowing running speeds (and hence loads) there is no way to decide on motor and other details .

Happy to help with design but need a full description of what it is that you are trying to do .

 

[wakadarc]

I am trying to simulate transportation vibration of packaged products. Strictly vertical vibration with fixed displacement.
The platform will hold different packaged boxes, one at a time. The heaviest package plus the weight of the platform will be a total of 160KG. The flywheel has a diameter of 5.75 inches. A belt will connect that with the motor.

The distance will the platform moves will always be the same. 0.5 inches up and back down 0.5 inches (1 cycle). The frequency will have to vary (since the load will vary).

So I used

I=mr^2 for the moment of inertia of the fly wheel
I=(325LB*(2.875IN)^2)

 

[Nidum]

Quite a lot to think about in this problem .

First thing to estimate is the forces generated by the oscillating weight . So choose a maximum speed and work out the inertial forces . Assume SHM .

Bear in mind that 160Kg is a significantly heavy weight and inertial forces could be quite high .

Next thing is to establish basic configuration of machine . Oscillating a large area heavy load through a short vertical distance is notoriously difficult to do well . One of the problems is just keeping the platform stable .

I suggest therefore that you design a drive mechanism that by its basic configuration keeps the platform level . Use the guide columns then just to keep the platform in place (in X-Y sense) and to take side loads from the drive .

One way of arranging drive is to have four drive cranks synchronised together so that platform is raised and lowered by forces acting at four well spaced locations .

However it is actually arranged this system is intrinsically dynamically and statically unbalanced . Worth considering at least partial compensation for the unbalanced forces by using balance weights on cranks .

An important consideration will be operator safety . Consider full guarding , safety trips within the system and several 'all stop' buttons .

Ponder the above and come back any time .

 

[Nidum]

A further consideration :

You don't want the load staying still and the machine jumping up and down (it does happen) so fairly massive base and frame construction all bolted to a solid concrete floor .

 

[wakadarc]

Thanks for your input! Attached is the cam shaft and flywheel that I was going to use. I can put a counter weight on the other side of the flywheel.

The largest package is 80 KG but the platform and safety factor consideration it is 160 KG.

The static vertical load would be (160x9.8) = 1600 N.

However it should accelerate at least 1 inch/sec so

F=160KG x 0.0254m/s^2 = 4.064N

HOWEVER, the packages need to be lifted into the air 1.5mm atleast. So I also need to calculate the impact force that the platform and package makes...

For there to be "air time" for the package, the platform itself needs to be accelerating vertical higher than 9.8 m/s^2...according to my calculations the angular acceleration (asssuming constant for now) would need to be 211rad/s^2

My plan is to ramp up the speed/frequency of the motor for when heavier packages need to obtain the 1.5mm lift off the platform.

 

[wakadarc]

Ill be using linear guide bearings (4 posts) to guide the platform so that it will stay flat at all times. Could I get a visual of the four drive cranks? I'm not sure whether i would need that here

 

[Nidum]

Two shafts geared together and each shaft with a crank at each end . Arranged so that lines of action for lifting forces are somewhere near the corners of the platform .

Basically imagine a table but with four matched expanding/contracting legs .

If cranks are identical and synchronised then the four corners all rise and fall exactly together .

Ideal arrangement has crank axles acting in opposite senses - ie one clockwise and one anticlockwise .

Arrangements of short linear bearings on widely spaced poles are not ideal for stabilizing platforms . Really need two bearings at each pole with some vertical separation between them .