Slider Crank Mechanism

All Steam Locomotives do!

Tsebytsev did all the theory/tabulation 150 years back.

Exact 180 degree rotation, by applying a force in one direction, can be achieved only when connecting rod and the crank lie in a straight line (at top dead center or bottom dead center).

But at these TDC or BDC there is every chance of mechanism getting locked. In continues rotations, the stored momentum in the 'counter-weight' takes care to maneuver these deadlocks.

Still there could be many options. Discussions could be easier with simple sketches. We should use the wonderful facility provided in this forum to include such simple sketches to make better understanding.

Can the link be made like bell crank with a very small 45 degree bend like link at the end of crank to prevent locking of the mechanism.

Locomotives used two actuators in different phases, and the momentum of the rotors prevent locking. However, some models used oscilating actuators. I think this can be used in this case. The actuator hear point of attachment being a pin with rotation, and the actuator not in the exact direction of the 180 deg movement, but perpendicular to it. THe only problem is that you must have a different control circuit to allow one retraction and one extension per travel. Different rotation points for the actuator would give you different speeds, forces, and movement characteristics per travel. The rest I leave to your imagination.

Hi: it is not possible to get 180° with good transmision rate with only a four bar mechanism, a slider crank or a four bar mechanism with revolution joint connecting all bars. You have to considerer a sixbar mechanism, one of them mentioned in some reply. The inconvenient of this kind of solution is the space that this mechanism will take.

Regards

That is right. One such is as shown below. Crank, Connecting rod, Crosshead with Rack and Pinion can achieve any desired angle including 180 degree. Stroke length and the Pinion diameter will decide the angle of rotation…

sir ,what about the locking of the mechanism as we have to start from 0 degree where the crank and con rod are in the same line and difficulty in starting . And after finishing the stroke it has to again move from 180 degree back to 0 degree where at that point of time crank and con rod will be pushing each other and again difficulty in starting.Good sketch thnks, can u provide a solution for this.

Is this operation on one acuator only? If you use 2 acuators and phase them 90 degrees apart you will never have the lock on both units at 0 or 180 degrees as one will always be at the full pressure of the 90 degrees to 0-180 degrees. This is what we did on the power plants for the stern wheelers. Gave good results for us.
Ric

Many options are possible…

Shown is another arrangement. Since the lever is inclined, the crank will not get locked at the extremes ends

.

The suitable arrangement depends upon, 'purpose, the kind of input force both in vector form, the space etc.

sir we want to convert the linear movement of the pneumatic cylinder into rotary movement.

Yes, attach a pneumatic actuator with the lever. Reversing the air flow will reverse direction of rotation.

sorry sir we have space constrain can you suggest us another mechanism with simple links.

I mean something like this. In this drawing, the stops would be located in the 0 and 6 positions, not in the 9 and 3 positions as the previous ones.

Hi: response n° 12 gives a solution, but also n° 6, with rack and pinion, perhaps better regarding size considerations.

There are many possibilities…

This arrangement could be anther option with space constrains in mind.

Many factors decide the more suitable options. The factors like: exact space available, budget and is it for a one-time project or a repetitive requirement for a product etc…

sir, can we use offset slider crank mechanism with the crank bend at 45 degree at the end for some 10mm to prevent locking of the mechanism with the connecting rod.sir, since assuming you are a pneumatic expert from your profile what can be used as pneumatic fail safe circuit in case of a rupture of the pneumatic hose also what can be used to position the pneumatic cylinder in its intermediate position rather than extreme ends.

Can you come up with a simple sketch?

we have to rotate the top platform to 180 degree from the default resting position.In photo its lifted for 30 degrees

What if...

exactly...

same as you explained in reply 13.

elaborating this:

1:

2:

3:

4:

5:

SIR, MY DOUBT IS IN THE THIRD STAGE OF YOUR MECHANISM WONT IT LOCK THE MECHANISM WHILE STARTING .SINCE BOTH THE CRANK AND THE CON ROD SUCH THAT THE PISTON ROD ARE ALIGNED IN THE SAME LINE.GREAT SKETCH THANKS A LOT.

Yes, I get your point.

If a 'counter weight' is attached at a suitable position, it will continue to progress at the same direction of travel surpassing the dead-center…

Or, simply locate your load platform in an upper position related to the ringe. It will cause the center of gravity of the load to be more to the left in respect to the sketch, what will make the system work when the actuator is in the aligned position. No need for counterweight this way.

Still no Guarantee that it will break everything down at Dead Center--Try out

This model should perform...

• Oops! I mean this instead:

"Still no Guarantee that the system will NOT jam and break down at the 0 angle."

The overhung deadweight will cause more harm.

Was the original problem not for getting a continuous translation to rotation?

Other one-stroke deviations entered in between.

Tsebytcheff saw it all coming 150 years back.

THE MECHANISM WAS TO CONVERT TRANSLATIONAL RECIPROCATING MOTION TO ROTATIONAL BUT ITS NOT CONTINUOS IT WILL START FROM O DEGREE AND END AT 180 DEGREE AGAIN IT WILL START FROM 180 DEGREE AND WILL END AT 0 DEGREE.THE PROBLEM IS LOCKING OF THE MECHANISM WHILE AT STARTING CAN YOU PROVIDE ME A SOLUTION.THE MOTION IS CONTINUOS AND ITS INTERMITTANT MOTION.

If Post#29 is your FINAL problem statement ,then the solution is very straightforward:

RACK pushed by Hydraulic Cylinder . Cylinder controlled by solenoid controlled multiport valve.

Full Rack motion= say 250 Degrees,whereas we need rotation 0.>>180 degrees.With equal overtravel space. Means Cylinder will never reach stroke limits.

At 0 deg. and 180 deg.(Rotation) you have 2 limit switches--which operate circuits disabling excess Cylinder/Rack travel either way.

Now you can happily use translation to 0>>180 deg. Rotation. Nothing will Jam.

SIR WE ARE LIFTING SOME 300 KG WITH THAT PLATFORM IN SCISSOR LIFT SO ITS DIFFICULT USE COUNTER WEIGHT .

Please look at the fig1 and fig5 (#20)…

At 0 degree and at 180 degree, the plunger has to push to initiate the rotation of the table. At these positions the cylinder is inclined and not in line with table pivot. So what is the trouble?

When the table is vertical at 90 degree, the plunger is at transition (if out then in). Only in this position the mechanism could lock. So counter weight.

If 300+ kg is table + Job, you don't need to have heavy weight to counter. May be 10/15 kg might work. All you need to disturb and pass that 90 degree.

sir can you tell me on which software you have drawn the sketches .Thanks for your help.

CAN YOU PLEASE EXPLAIN US ON THAT MECHANISM.