• The distance to move the piston rod along the Y-axis, and
  
• The angle to rotate point A around point B so that it connects with axle C
  

(2019-09-26, 3:59)N. W. Perry Wrote: I can't quite figure out how to calculate the angle and vertical distance of a piston rod on a rotated crankshaft, specifically using LDCad's Selection Info tool. Here's a diagram:


So the crankshaft has been rotated -49.5 degrees. The piston rod (A-B) will move up the Y-axis, but I don't know that distance (it's slightly less than the current Y distance between A and C). Also, point B will remain at its current X position but point A will rotate around point B to connect with the axle at point C. The angle of that rotation is the second thing I don't know.

No doubt this has been tackled many times before, but I'd like to know the precise way to figure it out (rather than downloading an already finished model). It's either slightly more difficult, or vastly simpler than I realize.   (And I do know about the piston placement script in LDCad, but AFAIK it doesn't recognize these old-style piston parts or brick-built cylinders.)

Again, the two values I need to find are:

• The distance to move the piston rod along the Y-axis, and
  
• The angle to rotate point A around point B so that it connects with axle C
  

(2019-09-26, 7:34)David Manley Wrote: There is a feature in LDCad which may help you do this automatically. See the following image:

[image snipped]

If you select the three points you are interested in, then right mouse click a part, the "Selection" menu pops up. At the bottom of the menu, there is a menu item "Selection info". Click it and a number of options are shown. For example, to get the angles shown in my diagram, I have "Show Info" set to yes, "Show distance" set to no and "Show angles" set to yes.

Try this and see if you can get the information you are looking for.

(2019-09-26, 13:20)N. W. Perry Wrote: Yep, that's the same selection info tool I mentioned. Trouble is, I can't figure out quite how to use its features to get the precise measurements I need, because I don't yet have all three of the points I'm interested in. Looking at my diagram, I can't just take the angle of ABC, because A and B will have moved up the Y axis to a new position, which I don't know. So the angle I need is A'B'C, and first I need to find A' (and by extension, B').

Alternatively, if I knew the angle, I could get the new Y position perfectly easily by using part snapping.

(2019-09-26, 14:09)Philippe Hurbain Wrote: Yes, you need to do a bit of trigonometry here...

(2019-09-26, 14:09)Philippe Hurbain Wrote: Yes, you need to do a bit of trigonometry here...

We want to turn the piston arm so that its tip is at the distance between red pin and initial axis of arm (vertical here). Since I put the gear on the origin, this distance is directly pin X coordinates (13.856), but you can also derive it in the general case from LDCad measurements as sin(16.10°)*49.96. Note that the blue parts are just helpers parts to get the right measurements points.
Once we get this value, we calculate piston arm rotation angle as arcsin(13.856/60)=13.35°. Rotate the arm by this value and use snapping on red pin to correctly position the pistion arm...

• We need to know the "x" distance from A to C, which forms an opposite triangle leg to angle B.
  
• We also know the hypotenuse of that triangle (BC), and we know angle B, so we can use sine to solve for "x" because sine is the ratio of opposite (x) to hypotenuse (BC); thus:
  
• sin(B) = x / (BC) and sin(B)*(BC) = x
  
• Once the piston rod is rotated, it will become the hypotenuse (B'C) of a new triangle, with a known length of 60.
  
• x is still the opposite leg to a new angle B', which we need to find. Since we want an angle instead of a length, we use the inverse of sine, or arcsin; thus:
  
• arcsin( x / B'C ) = B' or arcsin( x / 60 ) = B'
  
• Rotate the piston rod around point B by angle B', then use part snapping to move the rotated point A onto point C (and consequently, point B onto point B').
  

(2019-09-27, 5:44)N. W. Perry Wrote: (angle B) [sin] * (distance B-C) / 60 = [sin-1] =

(2019-09-26, 19:25)Roland Melkert Wrote: -take the x distance between a and c -> lx
-take the piston length (b-a=60) -> lp
-cos(cc)=lx/lp
-snap the piston using a to c
-rotate the piston over c using angle a 90-cc angle
-snap the piston head to b

(2019-09-27, 6:48)Philippe Hurbain Wrote: Yes, you got the key sequence right

(2019-09-27, 7:13)Philippe Hurbain Wrote: Looks simpler, but ...I can't make to work?