How to make a Scotch Yoke Mechanism

Anmol_Sekhon

Hi there! Was wondering how to create a scotch yoke mechanism? I've tried both pin slot mate and tangent mate but can't seem to get it to work. Anyone have any ideas about this sort of mechanism? Not much on the web about this topic.

Here is what I'm working with and I want that grey pin that is in the blue wheel to be in the other grey part and want the motion to be side to side:

matthew_stacy

@Anmol_Sekhon, in lieu of what might be called functional-contact-mates, there is a work-around for this.  In Onshape, one approach is to insert construction geometry (that can be hidden) into an assembly.  Often times sketches or surfaces will suffice for the construction geometry.  Both offer a lighter computational footprint than parts.

For your scotch yoke, Revolute-mate the crankshaft to the global origin.  Refer to this example.  Then Slider-mate the yoke to the global origin.

At this point it is helpful to insert a sketch into the assembly (drawn in the part studio) with a point at the center of the crank pin.  Revolute-mate this sketch to the crank pin.  This sketch will translate with the crank pin, but its orientation is decoupled from crank rotation.

Then Slider-mate a feature on the center-line of the yoke slot with the crank-pin-center sketch. 

When you animate the crankshaft rotation, the yoke is now constrained by two slider mates such that it can only slide laterally and the slot must stay centered on the crank pin.

eric_pesty

In @matthew_stacy's example above the pin-slot mate works fine and there is actually no need for the intermediate sketch.

https://cad.onshape.com/documents/be5945925f9befff0067f727/w/504bd68762f0420ca9c34b2f/e/b16f929d7a01f9e08e04f4bd

The one constraint for this to work is that the "z" axis has to match for the pin slot so once you add "real world" manufacturing clearance it can be a bit trickier, the best way would be to define a pair of explicit mate connectors (assuming the parts are modeled in the same part studio with the correct clearances.
Or if that's not possible you can use the "extra sketch" trick as shown above but change the "crank pin center" mate from a "revolute" to a "cylindrical" mate and then it can handle axial mis-alignment.

dirk_van_der_vaart

Here's an example,
https://cad.onshape.com/documents/de9727ecc28a568a041dcead/w/6b25da7c4b7956ac39324d86/e/a033faff5dfc13b2a9a2e488
Just mates, no extra sketch

eric_pesty

This makes me think it would be nice sometimes to have a pin-slot mate that also allowed free movement long Z. Just like the revolute also has the "cylindrical" equivalent. 

Created the following IR: https://forum.onshape.com/discussion/18747/new-slot-mate-variation/p1?new=1

eric_pesty

Also, if you create the pin separately so it can rotate relative to the crankshaft, you can get something similar to @dirk_van_der_vaart 's example without needing a square pin:

https://cad.onshape.com/documents/be5945925f9befff0067f727/w/504bd68762f0420ca9c34b2f/e/b16f929d7a01f9e08e04f4bd?renderMode=0&uiState=62e054a932824b2a0d05927d

This allows the slider to move along Z. In order to not be stuck in the Z direction, you either need the point sketch intermediate part or allow the pin to be a separate part that rotates relative to the crankshaft...

It looks like @Anmol_Sekhon has the pin as a separate part already so if the pin can be allowed to rotate the planar mate (with realigned mate connectors) in this example would work.