Pneumatics Math Help

[GeeTwo]

Not meaning to contradict any of the notes above, but in designing such a joint, I find it convenient to first look at the amount of work to be done. In the case of the 90 degree rotating arm, this would be π/4 (90 degrees in radians) * arm length * [average] force required at end of arm. The result will be in ft-lb or N-m, or other similar units. Then, divide this amount of work by the working pressure on the cylinder (perhaps 50 lb / in²) which is the smallest possible displacement (that is, area times stroke) to do this amount of work. Don't forget to convert feet to inches!

Multiply this minimum displacement by a nice safety factor (1.5 if you don't mind the stroke being rather stately, or perhaps 5 to 10 if you want fast action). This will allow you to come up with a fairly small set of reasonable "stock" cylinders. If you use Bimba cylinders, the "power factor" (first two digits of original line cylinders part number) is the area of the piston in tenths of a square inch, and the remainder is the stroke length, so to calculate the displacement of a Bimba cyldinder, just multiply power factor * stroke / 10. (Example: an SR 1715 cylinder's displacement is 17 * 15 / 10 = 25.5 in³.)

THEN, for each candidate cylinder, figure out the appropriate mount points. Shorter, thicker cylinders will mount closer to the pivot but will require larger forces, possibly meaning thicker plates and bolts and certainly better precision. Longer, thinner cylinders will mount further from the pivot (requiring more maneuvering room), but the forces will be smaller and probably more reliable, especially if your manufacturing is subject to error.

[Ari423]

I love the complicated physics in this problem as much as the next guy, but it might be easier if you can mount something like this rotary actuator next to the pivot point. This will also have the added benefit of providing a constant force throughout the rotation as opposed to a dynamic force with the linear actuator. Most pneumatics suppliers sell some version of this.

Disclaimer: I have not actually used one of these (because I haven't had the need) so I don't have any real world experience to say whether or not it's easier.

[ollien]

Originally Posted by Ari423

I love the complicated physics in this problem as much as the next guy, but it might be easier if you can mount something like this rotary actuator next to the pivot point. This will also have the added benefit of providing a constant force throughout the rotation as opposed to a dynamic force with the linear actuator. Most pneumatics suppliers sell some version of this.

Disclaimer: I have not actually used one of these (because I haven't had the need) so I don't have any real world experience to say whether or not it's easier.

Pretty sweet. A bit pricey, but nothing too crazy. Would love to see a place I could purchase these other than Bimba, because of their crazy turnaround times.

McMaster sells them, but at a uh.. rather hefty price.