PETRI 7 - ADJUSTMENTS

Because my research is almost coming to an end, I'll discuss some obstacles I encountered and therefore would change if I had the chance.

The main problem is the assembly and disassembly of the camera. Don't get me wrong, I never had to break a part during the disassembly. All the parts are connected with each other on a certain way that allows it to disconnect them again.

It's even so that the disassembly went a lot more smoother than the assembly. And that's my problem.

For example, the assembly of the blades of the shutter or diaphragm iris is so delicate work, even by using a pincet.


But I found a way to make things easier.

All the small metal blades aren't screwed to a solid part. They are connected by placing them on the right place, and than putting an other part over it, hoping everything fits and comes together.

here you see a blade of the shutter with no solid connection

it's the same story for the blades of the diaphragm iris

this is were the blades must be placed on

this black disc must be placed exactly on the blades in order to keep them in place

here you see the blades, there are no holes for a solid connection

If each blade would have at least one hole, then at least one spacing ring and screw could hold the pieces together on a solid part. This would make the assembly much easier.

here I placed the spacing ring above a blade of the shutter

a blade of the shutter with spacing ring and tiny screw

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

PETRI 7 - MATERIALS

Oddly enough, there aren't many different kinds of materials used to build this camera. Most of the parts are made out of steel or aluminum.

It's sometimes hard to tell though, because most of the parts are painted black, to make sure the inside of the camera is as dark as possible. By checking the magnetism of each part, it's easy to distinguish steel from aluminum.

aluminum lens casing

aluminum lens casing

aluminum top -> the inside is partially painted black

aluminum top

aluminum parts

aluminum casing of the body

aluminum casing of the body

steel parts (are attracted by the magnet)

steel backplate of the camera (attracted by the magnet)

steel backplate of the camera (attracted by the magnet)

steel rotating ring

Many parts are made out of aluminum because of the small weight of the material and its easy to machine and process. That's why the body of the camera is made out of this material.

But parts that need to rotate or move and can't afford to break or deform, it's best to use steel. Steel weighs more than aluminum but its toughness makes it more resistent to deformation.


Parts like small gear wheels and screws are made out of brass. Brass is an alloy of copper and zinc. It's also not magnetic and has a typical yellow-brownish color.

Special parts like the lenses, the viewfinder, hand grip, lens cap and so on are made out of certain kinds of plastic.

PETRI 7 - SPRINGS

During the disassembly, I came across a lot of springs. Some of wich are realy tiny, what makes it hard to notice them in the first place.

The first kind is the tensile spring. These are used in a system that needs to be cocked, and eventually be returned to its original setting.

tensile spring in the film winding mechanism

tensile spring in the shutter mechanism

tensile spring in the indicator mechanism

Another kind is the torsion spring. These are normally connected by small screws and are located directly on the axis of the moving parts, or nearby.

I also calculated the spring rate of the two largest tensile springs.

Calculations:

-> Spring 1

k = F / u

u-normal : 10 mm

u-streched : 18 mm

F = 0,380 x 9,81 = 3,73 N

u-difference = 0,008 m

k = F / u = 466 N/m

-> Spring 2

k = F / u

u-normal : 8,5 mm

u-streched : 11 mm

F = 0,012 x 9,81 = 0,118 N

u-difference = 0,0025 m

k = F / u = 47 N/m

PETRI 7 - CONNECTIONS

Now that I've explained the fundamental principles behind the Petri 7 analog point-and-shoot camera, it's time to take a closer look at the mechanical components, how they are connected with eachother, the used materials and so on.

First of all, I like to discuss the used connections because I was suprised how easy the disassembly turned out to be.

Two main techniques were used.

• Overall, all the parts and components are connected by tiny screws.
• In the lens however, all the round large parts are threaded, and can be screwed in one another.
  
  

tiny screws for the gear wheel

tiny screws for the components of the shutter mechanism

threaded part of the lens

threaded lens

threaded lens casing