Ribbon Breakage of the Mechanical Focal-Plane Shutter in Film Cameras and Shutter Tension Adjustment

1. Mechanical clock-level linkage structure of horizontal travel focal plane curtain shutter

Before the popularity of metal longitudinal shutters, most cameras in the last century used silk-woven rubber horizontal shutters:

• Independent spring rollers for front and rear curtains:
• There are two metal winding hollow shafts (Drum Rollers) with precision coiled springs distributed inside the fuselage;
• First Curtain (Front Curtain): pulled by two extremely thin, high-strength special pure silk fabric drawstrings (Shutter Ribbons);
• Second Curtain (Rear Curtain): Both ends of the entire curtain are directly wrapped and fixed on the main shaft roller;
• Mechanical time delay for slit formation (Slit Width):
• When the shutter is pressed to wind up, the two curtains overlap and lock and roll back to the left axis;
• When the shutter is released, the front curtain is first violently pulled to the right by the clockwork spring to open;
• After the set microsecond time delay, the escapement claw is released, the rear curtain spring is started, and the rear curtain quickly follows and closes to the right;
• Two curtains create a uniform flying gap whose width is controlled by time in front of the film exposure window, completing full-frame photosensitization.

2. Physical pathology of shutter cord (Ribbon) breakage and detachment

The shutter cord is the extremely fragile tissue within the entire mechanical camera that bears the highest shear fatigue stress:

• Silk fabric fiber aging and moth-eaten mildew:
• The curtain tape is woven from extremely fine high-density black silk or natural long-staple cotton, with a thickness usually only 0.15mm ~ 0.20mm, and is bonded to the edges of the metal curtain strips with special natural bone glue or shellac;
• After half a century of air deliquescence and tens of thousands of violent ejection pulls, the silk fibers undergo molecular fatigue hardening and brittle fracture;
• Mechanical paralysis at the moment of rupture:
• A soft "click" sound is heard at the moment of winding up, and the reflector can be lifted when the shutter is pressed, but the curtain in front of the exposure window is soft and tilted to one side, and the rear curtain completely loses traction and cannot be reset;
• Extremely difficult to repair: Replacing the cord requires disassembling the entire reflector box or Barnacle movement, using modern synthetic polyester ribbons of precise thickness, repositioning and bonding with natural shellac paint on strictly parallel positioning fixtures.

3. The mechanical tension imbalance of the film’s ‘half overexposed and half black’ (Shutter Capping)

Even if the cord is not broken, the most common problem with old mechanical cameras is the serious uneven exposure of the picture at high speeds (1/500s or 1/1000s):

• Causes of severe tearing due to left and right exposure:
• In the developed film, the left half of the picture is normally exposed or even bright, while the brightness of the right half is sharply attenuated, and the rightmost side even becomes completely black**!
• Physical phenomenon of rear-end collision - Shutter Capping:
• The old grease inside the front curtain spring shaft dries up and produces viscous damping, causing the front curtain's flying speed to slow down;
• The rear curtain spring is still strong and powerful, and even because the tension of the rear curtain spring is adjusted too high, the rear curtain directly catches up with the front curtain in the middle of speeding!
• The width of the slit is compressed to 0 when it passes near the center line of the film (the front and rear curtains are tightly closed);
• Result: The second half of the film did not receive any photons at all, resulting in severe black border truncation on the right side!

4. Specification for precise fine-tuning of ratchet tension on double spring shafts of base plate

There are two micro ratchet speed adjustment screws (marked Front and Rear) reserved at the bottom of professional film movements:

1. Homemade simple laser/photoelectric shutter tester:

• It is strictly prohibited to adjust the spring tension by blind hearing! The human ear simply cannot tell the difference in milliseconds between 1/1000s and 1/500s;
• Use open source Arduino to assemble a dual-point shutter speedometer including a photodiode and a laser head to measure the instantaneous light passage time of the slit flying through the left, middle and right ends of the exposure window.

2. Tension release and fine-tuning incremental rule:

• Locate the gear lock spring pawl (Click Pawl) at the bottom of the front and rear curtains;
• Use a clock screwdriver to gently open the claws and completely reset the spring tension to zero and release it;
• Basic preload setting: Usually use a flat-head screwdriver to rotate the front curtain spring clockwise to preload 34 turns (or count 1520 ratchet teeth), and preload the rear curtain 2.5~3 turns;
• Dynamic Balance Approximation: Test gradually on the shutter tester for 1/1000s. If the rear curtain is intercepted, use a screwdriver to tighten the front curtain gear clockwise by 2 teeth (to increase the running speed of the front curtain), or release the rear curtain gear by 1 tooth in the reverse direction;
• Until the tester shows that the slit light passage time error at the left, middle, and right positions is within the ±10% tolerance, lock the anti-loosening fastening screws, and the shutter is completely restored to clock-level absolute accuracy!

---