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Hello,

at the moment i am trying to design a crank slider mechanism. This mechanism should be driven with the help of an e-motor. Through this rotation a slider should move.

Unfortunately i have some problems, because i can not find any good books about the calculation of these mechanisms.

For example i do not understand how can i calculate the system to avoid it to come to a stand at the dead points?

 

Can someone help?

 

Thank you in advance!

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You can find a lot about this and many other mechanisms in the newly released textbook Mechanics of Machines, 2nd ed. Please download a copy and encourage your friends to do the same; it is offered without charge to everyone.

DrD

MoM2DraftMar2019r1-compressed.pdf

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Thank you for your fast answer and the book!

I know this kinematik equations. I do not understand how to calculate if there are problems in dead points, for example the system will stop and not move in these points?

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Hello,

the dead points of a crank slider mechanism are reached if all joints are in line. Acc. to my opinion, dead points will be critical only,  if the mechanism is driven by the slider (piston). If it rests in a dead point, it will need some lateral force at the connecting rod to "come out", which an ideal slider can not deliver. In your case it seems, that the mechanism is driven through the crank by an electric motor, which is able to create some lateral force at the dead points. The lateral direction is identical to the tangential direction, in which the joint of the crank shaft is moving at the dead points.

Regards

Alban

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That is also my opinion. The problem is, that the rotation speed of the motor will be very low. Thatswhy i need to calculate it, to know which moment i need to move the system.

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Hello,

I have derived some relation between driving torque and slider force "just for fun" (see attachement). Therefore it should be checked. Any inertial foces are neglected (you wrote, it is a slow motion)

I have used a simplified transfer function from old lecture notes. More accuracy can be achieved deriving this function by means provided by DrD.

At the dead points the required torque becomes zero. This can be explained as follows: At the instant, when the crank joint passes through dead point, it moves laterally with respect to the connecting rod, whereas the slider does not move. The rod just rotates about the joint of the slider. Thus no counter force exists for the crank at this instant.

Regards

Alban

CrankSliderMechanismTorque.pdf

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Thank you for the posts!

I am sorry that i didnt wrote earlier, had much to do. But i found in a book some possible calculations for the forces of the system (do not know if they are rigth.

I wrote a little matlab script to calculate the forces at every possible angle of the crank system. The results show me very high forces at pi and 2*pi of the system. (I think thats because of the singularities.

My question is how to solve this problem. The system should not have fast motions, in the contrary it should move slow and transfer forces! Or is it better to avoid this positions and rotate only between 0 and pi? (I would like to use the whole rotation, if it is possible)

 

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Perilun, you evidently do not like to read. Everything you need for this problem, and a whole lot more, is contained in the book that I gave you above. In Ch 2 you will find the kinematics, in Ch 6 you will find the statics, and in Ch 7 you will find the statics of this system.

The PDF from Alban Kronenberger is also fine, although there is no reason to use the approximate kinematics when the exact relations are just as easy.

 

DrD

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Thank you for your help  i could calculate with your help the crank slider system.

 

Unfortunately i have another question. How can be calculated the accuracy or a backlash of a crank slider system?

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Backlash depends upon clearances at the joints. You will have to specify all of these, and then look at the various possible combinations to find the worst case. It is a long, tedious process.

DrD

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