PETRI 7 - GEAR WHEELS

All the gear wheels found in this camera were common spur gears. The spur gears are used to control the rotation of the axis of the film and indicator.

There are three spur gears connected to eachother. The largest spur gear has 36 teeth, the other two are identical and have 24 teeth. So the velocity ratio is 1,5.

The largest spur gear is able to turn a 120°, made possible by an attached part.

So for winding the film, the big aluminum circle on the outside is turned. By turning this circle a mechanism will make sure that GEARa is turned a 120°.

Since GEARa has 36 teeth and has rotated a third of a cirle, only 12 teeth had contact with GEARb and GEARc. 

GEARb and GEARc have each 24 teeth, by rotating 12 teeth each they have both rotated a 180°.

The gear attached to the indicator has 80 teeth minus two because of the small piece left out.

When we carefully remove the indicator wheel, we are able to see the axis of GEARb.

We know this axis of GEARb rotates a 180°. 

When rotating, the axis is actually pressing a mechanism that pushes the indicator wheel forward one tooth at a time. 

And that's how a simple turn below the camera is transformed to one small movement of the indicator.

Calculations:

-> Velocity Ratio

Vr = Na / Nb = Da / Dp

Na = number of teeth of gear a = 36
Nb = number of teeth of gear b (and c) = 24

Da = pitch diameter of gear a = 14 mm
Db = pitch diameter of gear b (and c) = 9 mm

Vr= 1,5


-> Diametral pitch

Pd = Na / Da = 36 / 14 = 2,57
Pd = Nb / Db= 24 / 9 = 2,66

Pd = 2,6


-> Center to center distance

CtoC= (Da + Db) / 2 = (14 + 19) / 2 = 11,5 mm
CtoC = (Na + Nb) / ( 2 x Pd) = (36+24) / (2 x 2,6) = 11,5 mm

CtoC = 11,5 mm