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I need to convert 7.62cm/3in linear motion from an actuator to 150-180° (150 is minimum) rotary motion. I am aware of the existence of gears, but would probe here to see if there is some simpler method. I could use rods mounted in some way. It has to be compact and rugged, however. I'd also be interested in formulas for this type of conversion. Thanks!

Edit: I found this contraption Mystery device for microlinear precision, whilst googling for scotch yoke. Is this a scotch yoke? If not, what is it? I can probably awfulhack one of these with a file, some aluminium rods and strips. But it would be prettier if I bought one.

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  • $\begingroup$ You won't need gears as there are commonly used methods often used in backhoes and cherry picker trucks to move the arms almost 180 degrees with a single hydraulic piston. I have not tried the drawing tools here yet so I will leave it to others to provide more detail. But if you search for photos of backhoes or power line maintenance trucks you will see a type of linkage that does what you need. $\endgroup$
    – Entrepreneur
    Commented Jun 17, 2017 at 16:01
  • $\begingroup$ @Entrepreneur Well, this is a linear actuator. I suppose the same principle applies. Can you provide a link to what you mention? $\endgroup$
    – user2497
    Commented Jun 17, 2017 at 17:29
  • $\begingroup$ Another simple way to convert linear motion into rotary motion is with a scotch yoke, look up the Bourke Engine to see how these operate. $\endgroup$
    – William Hird
    Commented Jun 17, 2017 at 19:38
  • $\begingroup$ it is essentially a rack and pinion with different gear shape than the typcal involute. In this case a cycloid function, which is the second common, but considerablybrarer gear shape. $\endgroup$
    – joojaa
    Commented Jun 18, 2017 at 3:03
  • $\begingroup$ This youtube video is crude but you can definitely see the 3 moving parts and how they interact. The red line is the hydraulic cylinder. The white line is a link that connects the 2 green arms. youtube.com/watch?v=tb4jiscwhvw $\endgroup$
    – Entrepreneur
    Commented Jun 18, 2017 at 3:21

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As suggested a rack and pinion will provide a linear transfer function and avoids complex linkages. A variant on the rack and pinion is to replace the straight rack teeth with a taught cable and the pinion teeth with a pulley. This mechanism is sometimes referred to as a capstan and bowstring.

https://en.wikipedia.org/wiki/Capstan_and_Bowstring

If you need a guarantee against slippage then the cable can be wrapped multiple times and attached to the pulley at the half-way point.

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There are commonly used methods used in backhoes and cherry picker trucks to move the arms about 180 degrees with a single hydraulic piston. If you search for photos of backhoes or power line maintenance trucks you may see a type of linkage.

Here is a photo

enter image description here

This video is crude but you can definitely see the 3 moving parts and how they interact. The red line is the hydraulic cylinder. The white line is a link that connects the 2 green arms.

backhoe linkage

In this second video the link proportions and pivot points are way off. But the essential pieces are there and the concept is the same.

Converting Linear to Rotary motion

The transfer function won't be linear though.

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  • $\begingroup$ What about a small rack and pinion then? $\endgroup$
    – user2497
    Commented Jun 25, 2017 at 10:48

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