circular to 1

[kinny]

Hi all

I'm from Bioengineering and unfortunately not too expert on the mechanical side. I'm making a device for which I need a mechanism to transform circular motion into oscillatory motion. I found the mechanism in the picture below:

In my device, the oscillating 'stick' would cover an angle of 170 degrees.

My questions are:
Is the oscillatory motion actually sinusoidal? (ie is the velocity/position actually sinusoidal, or are they just approximated to sinusoids due to the small angle shown in the picture?)
If it is/can be sinusoidal, is there a straight forward way to go about to find the specs (size and position of rotating wheel...) of the system?

Suggestions on different mechanisms I could use are very welcome. (I thought of using a cam but the oscillating 'stick' would in my case probably be jumpy due to the high velocity and low weight needed).

Thanks everyone!!

 

[asimpson]

Look up "four bar mechanism".

I am sure there would be calculators on-line to find relationship between input rotation and crank angle for four bar mechanisms. You can get quite a range of movements without having to resort to cams.

On the other hand cams give you more freedome to choose form of movement.

 

[chicopee]

I remember from physics that the attachment point on the disk is sinusoidal when the rotary motion is plotted on a graph. I would not be surprised if the oscillation when plotted on a graph produced a sine wave.

 

[Timelord]

Kinny,

Seems I answered this exact question in another forum (which shall remain nameless), but here goes again: The motion will not be pure sinusoidal, the small angles of the connecting rod will skew the output motion away from a pure sine wave. Why don't you graphically lay out the mechanism and plot the output motion vs. crank angle. From that you should be able to determine if the motion will still work for you. I can tell you that if you connect the con rod to the lever and the crank with scotch yokes, the result will be sinusoidal. Or, for a more rigorous analysis, write the equation of motion for the output, only a bit more complicated than a graphical analysis, but still simple geometry.

Timelord

Here is a link about a scotch yoke:

http://en.wikipedia.org/wiki/Scotch_yoke

 

[MikeHalloran]

A real manifestation of what you've shown will probably lock up on the first cycle.

170 degrees is awful close to 180 degrees, from which it is impossible to raise the 'stick' with a finite pushrod force.
At 170 degrees, the force in the pushrod will still be huge, so the stiffness of everything in the drive becomes an issue.

To get 170 degrees of rotation from a mechanical drive, you probably need to pivot the stick on a small gear, drive that with a bigger gear, and use the crank to drive the bigger gear over a smaller angle.

I'm not sure how faithful a sinusoid you need, or why, but in general that's not _quite_ what you get from slider cranks. You get a closer approximation from a Scotch Yoke, but it's not possible to build a real one without clearance that screws up the motion a little, and without accelerations and shocks that beat it to death.

You might also consider a valve positioner or other rotary mechanism that can just convert a rotary position command into an actual rotary position; then you just feed it a signal that represents whatever sinusoid or other function you want. You mentioned speed, but didn't give a number, so this may or may not be practical for you.




Mike Halloran
Pembroke Pines, FL, USA

 

[PNachtwey]

I agree with Mike about having an extreme mechanical disadvantage when the stick is almost horizontal. The effective load on the motor will change a lot.

Important details have been left out like how much mass or inertia is being moved and at what frequency.

If the stick must swing 170 degrees it would be best to have a motor at the base of the shaft and gear it down till there is enough torque. The sinusoidal motion can be done with a motion controller. The torque required would be highest when the stick is almost horizontal but a good motion controller will know the angle and apply a torque offset to compensate for the torque required to overcome gravity.

My company as a lot of experience at this raising pipe from horizontal to vertical in applications such as oil fields only we, our customers, use hydraulics. I doubt your system is that big but the problems that must be overcome are still the same.





Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[tbuelna]

I would agree with MikeHalloran, using the kinematic arrangement shown it would be difficult to produce a +/- 170 swing of the output lever without using a very long connecting link. And the motion at the end of the output arm would not be sinusoidal.

There are probably much better mechanisms for producing the motion/force your application requires. Maybe you could provide more details.

 

[jlnsol]

You can use the well known linkage as between the bucket and the hydraulic actuator in an excavator. Such mechanism will amplify the input angle of the oscillating stick i.e. from 85 degr tot 170 degr easily.