How the Minox escapement works

Minox escapement front view

While you can still find a little bit about how the shutter blades work on the internet, things look really bad with the escapement mechanism. In addition, the escapement is complicated and hard to see inside the opened housing of the camera. In addition, the movement sequence, the kinematics, can only be guessed at when the mechanism is at a standstill.

I therefore created videos of what happens after the shutter is released with the camera housing open. I show these with the corresponding explanations on this page. To my knowledge, videos like this have not been published on the internet before, so stay tuned.

Why does the Minox need an escapement?

If you simply let go of the blade locking arm preloaded by a spring , it pulls the the locking pins down so quickly that the 1/1000 s occurs on its own. The Minox does not need any escapement for this shutter speed as explained here.

If you want the shutter to stay open longer, you have to control the movement of the blade locking arm differently in terms of speed.

To understand this description, I recommend reading the article about the function of the shutter if you haven’t already.

As we know, both locking pins are attached to the same blade locking arm:

Blade locking pins attached to the blade locking arm. The short pin locks the opening blade, the longer pin locks the closing blade.

In order to keep the shutter open for a specific time, the following process is necessary after it has been released:

  1. The blade locking arm is released by the trigger. A torsion spring at the pivot point of the arm sets it in rotation. The rotation is slowed down by the escapement.
  2. The arm starts moving and releases the opening blade via its short pin. Exposure begins.
  3. From this moment on, the arm continues to run for exactly the desired exposure time.
  4. Exactly after that the escapement disengages and the blade locking arm runs unbraked with maximum rotation to its end stop.
  5. Meanwhile the long pin releases the closing blade. Exposure ends.

The distance that the long pin still has to travel in phase 4 before it releases the closing blade  does not matter because the pin is moved at maximum speed.

Rick Oleson (rickoleson-brightscreen.com) worked intensively on the Minox shutter a long time ago and wrote one of the two standard articles about it. So I asked him to review my description of these 5 steps. Thankfully, he did this in detail and checked and confirmed the sequence also with the help of my slow motion video (see below).

The decisive factor is the speed at which the pins swing downwards in phase 3. This speed is always the same, regardless of the shutter speed selected and is regulated by the escapement. But what is the right speed?

The speed is easy to calculate if you consider that the longest possible shutter speed is 1/2 s. In this case the blade locking arm must be braked from the beginning of it’s movement. Since the two pins differ in length by 1.7 mm, the blade locking arm has to cover this distance in exactly 0.5 s. The required speed v of the blade locking arm during phase 3 can therefore be calculated:
v = 0.0017 m / 0.5 s = 0.034 m/s

The maximum speed can also be easily calculated. At a selected shutter speed of 1/1000 s it is maintained throughout the whole time of phase 3, i.e
v = 0.0017 m / 0.001 s = 1.7 m/s

Strictly speaking, the movement of the pins is not a straight-line (translatory) movement, but a circular one. This is because the blade lock arm is a lever that rotates around its attachment point. For our considerations here, this is negligible.

The task of the escapement

The task of the escapement is to precisely control the angular speed of the blade locking arm. Depending on its starting position, which is specified by the speed dial, the lever, braked by the escapement, runs along the sector gear of the escapement until it releases the lever. This distance is precisely dimensioned so that the release occurs when the exposure time is reached. Then the lever accelerates to maximum speed. This causes pin 2 to immediately release the closing blade and the shutter closes.

The following video shows the release of the shutter and the subsequent movement of the escapement and locking pins with the camera open at a shutter speed of 1/2 s. Note that at this slowest shutter speed, the lever is braked the entire time by the escapement. There is no acceleration to maximum speed here. Watch out for the rotation of the blade locking arm (far right) with the pins (bottom middle):

So the escapement mechanism has to ensure that the speed of the large sector gear arranged at the top is as uniform as possible. The designer chose an anchor escapement mechanism, like the one used in mechanical watches to realise this function.

How does an anchor escapement work?

History of the anchor escapement

Escapement mechanisms have been used for mechanical watches for a very long time. The oldest clock in the Western world with an anchor escapement dates back to the year 1670. Such a clock was also described in China as early as 1100. The Swiss anchor escapement still used today dates back to 1798. This further development made it possible to use it in portable watches and also in the Minox.

The regulation of a clock

The idea of the anchor escapement is to use a spring-mass system for torsional oscillation. The oscillation system provides a highly precise period of oscillation that drives a double lever (anchor) that controls the rotation of the escape wheel. The following video of the well known watch manufacturer Tissot shows such an escapement. Note the balance wheel with the torsion spring. These two parts create the time base, on whose accuracy the entire clock depends:

The Minox escapement

A clockwork places extremely high demands on the accuracy with which the gears and ultimately the clock hand are driven. This is necessary so that the clock displays correctly over a long period of time. But this accuracy is not required with the Minox shutter, as the accuracy only needs to be ensured for 0.5 s. That would be a ridiculously low requirement for watches, which has led to a greatly simplified design for the Minox.

This meant that the balance wheel and the balance spring could be completely dispensed with. Instead, the anchor was used as the oscillation element. There is also no torsion spring on the anchor. Instead, you use the elastic kickback when the anchor comes into the final position. The pallets on the anchor, which minimize friction on the escape wheel in watches, were also omitted. The friction surfaces are even made quite roughly.
Here we see the Minox escapement asssembly removed from the body:

Minox escapement removed from the body
Minox escapement front / side / bottom view. Dimensions  20 x 18 x 12 mm, weight 2.7 g

 

How the escapement works

1 anchor, 2 escape wheel, 3 barrel, 4 sector gear, 5 end stop

The rotation of the blade locking arm (not visible) is controlled by the sector gear (4). The torsion spring (not visible) installed in the barrel (3) drives the sector gear via the pinion attached to the top of the barrel. The escape wheel (2) is in engagement with the gear ring of the barrel via its pinion located below (not visible). The escape wheel is controlled by the anchor (1) and therefore cannot move freely. The anchor is pushed back by the end stop (5) and is thereby kept in oscillation. In this way, the anchor imposes a rotational speed on the entire gearbox that corresponds to the oscillation frequency of the anchor.

The following video shows the whole process in slow motion. Pay attention to the interaction of the anchor and escape wheel. At the bottom right you can also clearly see how the blade locking arm moves.

Minox B escapement in 8x slow motion

The individual parts of the escapement

The escapement essentially consists of 8 parts. There are 4 parts that are movable, namely the 3 gears and the anchor. There is also the gearbox cover, the anchor holder and two fastening screws. All parts are made of metal and are precisely manufactured. As you will see, the quality of the finishes is not comparable to that of watch movements. But this is not necessary here for the reasons already mentioned. Also pay attention to the dimensions and weights provided to get an idea of how tiny these pieces are!






0: anchor
1: gearbox cover
2: escapement wheel
3: sector gear
4: barrel
A: gearbox cover screw
B: sector gear screw

 

 

 

Warning: If you want to continue using your Minox, do not disassemble the escapement! I sacrificed a Minox B for this because I wanted to take a closer look at every part. In my opinion it is not possible to reassemble the escapement mechanism. I tried but failed because of the drive spring in the barrel. There is no chance of winding up the hair-thin spring again or attaching it to the slot of the pin in the barrel mount. So do not loosen screws A or B under any circumstances!

We’ll start with the most crucial and fastest moving part, the anchor:

Minox escapement anchor, 8 x 5 x 1 mm, weight 0.15 g

As you can clearly see in the picture on the right, the anchor has no pallets made of jewels. The surfaces that touch the escape wheel are even rough. Nevertheless, the accuracy of the oscillation for the shutter speeds is very good. So more precision is not required here. The bearing pins, on the other hand, are of excellent precision.

We now look at the escape wheel as an equally important part. It consists of the actual serrated escapement wheel, the attached pinion and the bearing pins. Note the excellently manufactured teeth of the pinion! The wheel shown here comes from a spare parts box and is pretty corroded. Original parts installed are usually flawless.

Minox escapement wheel with pinion. Wheel Ø 6.6 mm, shaft Ø 0.3 mm, pinion Ø 2.2 mm, shaft Ø 0.3 mm, total weight 0.07 g

The escape wheel’s pinion is in engagement with the barrel’s ring gear. This realizes the most important property of the escapement mechanism. It slows down the otherwise unhindered movement of the barrel, which is driven by its internal spring. Let’ have a closer look at this module.

Barrel, lid and spring
Barrel complete, gear Ø 18.6 mm, shaft Ø 0.3 mm, total weight 0.31 g

This assembly consists of a cavity that houses the  drive spring, the spring itself and the cover. The spring is hair-thin (cross-section 0.7 x 0.03 mm) and 220 mm long. One end is attached to the outer edge of the barrel, the other end is connected to the Minox housing. Let’s take a closer look:

Barrel mount

Above we see the bearing location of the barrel in the housing. The bore takes the shaft of the barrel pinion. Note the slot in the pin. The spiral spring is attached to this slot.

Drive spring, 1 graduation line = 0.1 mm. Cross-section 0.7 mm x 0.03 mm, 220 mm long

Above we see the spring itself. It is usually wound in a spiral in the barrel. The pinion can be seen in the next picture, which shows the barrel assembled.

Barrel mounted in housing, escape wheel, sector gear and gearbox top removed. Pinion Ø 2.2 mm, shaft Ø 0.3 mm

Now let’s move on to the sector wheel, which controls the rotation speed of the drive lever and thus the blade locking arm.

Sector gear, total weight 0.14 g

 

How the sector gear controls the drive lever

You can clearly see here how it engages to the pinion of the barrel. In this way, the reference speed generated by the escapement is transferred to the sector gear. This in turn controls the drive lever via its flank, which is designed as a cam:

Cam of sector gear (green) and engaged drive lever (red)

The photo above shows the drive lever (red) moving to the end position at maximum speed after being released by the cam (green) of the sector gear.

Summary

We learned why the mechanical Minox needs an escapement. To do this, we looked at escapement mechanisms in general, as they are used in mechanical watches. Due to the comparatively very short running times of the escapement in the Minox, the design could be greatly simplified. The individual parts were introduced and their purpose was explained. A unique slow motion video shows for the first time how the escapement works when the Minox is open. This makes the interaction of all parts immediately clear.

The question remains open as to how the different shutter speeds are implemented on the Minox, from the shutter speed selection to the escapement, the blade locking arm and the shutter blades. I will explain this in the next article.

Back to Minox repair

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