Calculating mechanical advantage for mechanism

[tazengr]

Hello,
I have an application where I use a motor to turn a shaft and this shaft is linked to a coupler that moves a piston back and forth.
Basically a four bar linkage that produces linear motion from rotary motion of a motor. I am using the linear motion to puncture holes into PTFE.

Question:
How do I calculate the mechanical advantage gained?
ie
If I know what the torque produced by the motor at a given speed, what is the force that the piston has at the other end?

 

[handleman]

Assuming the impact speed is relatively low, you would use statics.

 

[GBor]

Sounds more like a slider-crank than a 4-bar.

 

[GregLocock]

Alternatively you can use an energy method. Force*velocity at the piston = torque *angular velocity at the motor.

The advantage of this is thatyou don't need to draw a free body diagram.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[desertfox]

Hi tazengr

To find the force at the piston for a given position during the crank rotation you need to produce a velocity and subsquently an acceleration diagram for your mechanism.
You will also need to know the inertia's of the links and piston.
A book like "Mechanics of Machines" by J.Hannah & R.C.Stephens should help with this problem.

Regards

desertfox

 

[zekeman]

Yes, if you want to convert the torque delivered at the input shaft, at the point(s) of impact, you must determine the incremental output motion,dx normal to the plate you are perforating (either graphically or analytically) for an arbitrary small input angle of the shaft,d@
Then you can write
T*d@=F*dx
and
T=F*dx/d@
where
d@ incemental small angle in radians
dx incremental distance
T input torque
But since the energy is absorbed over a small time, one usually adds a flywheel so that the motor which can put energy into the flywheel over 360 degrees of motion while energy is absorbed over a tiny fraction of the angular motion, thus cosiderably reducing the power demands on the motor.