Overview
ISO 3820:1978 specifies the dimensions and design requirements for sprockets used with 8 mm Type S motion‑picture raw stock and processed film. The standard defines tooth geometry, spacing, working height, tolerances and recommended film paths so sprockets reliably engage perforations without damaging film or introducing instability during transport.
Key topics and technical requirements
- Scope: Applies to feed, hold‑back and combination sprockets for 8 mm Type S film; tables give root diameters and tooth data for sprockets with 12 to 84 teeth.
- Tooth spacing and indexing: Teeth are equally spaced at an index angle of 360/N degrees (N = number of teeth). Index‑angle tolerances: ±1 minute of arc for 12–24 teeth and ±30 seconds for 25–84 teeth.
- Working height and film slip: Specified tooth working height H = 0.66 mm (0.026 in). Maximum accommodation for pitch difference (film slippage) is 0.046 mm (0.0018 in).
- Film thickness: Typical design assumes film thickness around 0.15 mm (0.006 in); tables and root diameters are provided for this thickness (notes explain recomputation for other thicknesses).
- Minimum film path radii: Minimum convex film path radius R1 = 3.962 mm (0.156 in) (important to avoid flutter and perforation interference).
- Root diameters and tables: Root diameters for drive, combination and hold‑back sprockets (Dd, Dc, Dh) and other geometric data are tabulated (metric and inch values).
- Tooth shape and hobbing: Tooth profiles are generated from basic racks/hobs (Table 3). Two hob ranges are specified (12–24 and 25–84 teeth) with guidance on acceptable manufacturing deviation (≤ 0.00305 mm in many cases).
- Film path and guide interaction: Guidance on wrap length, lateral profile, flange/guides, allowable sprocket eccentricity (≤ 0.025 mm) and lateral weave/wobble limits (≤ 0.025 mm at root circle).
Applications and who uses it
- Film equipment manufacturers designing sprockets, projectors, cameras and printers for 8 mm Type S film.
- Maintenance technicians and service engineers ensuring replacement sprockets meet original geometry and tolerances.
- Film archivists and restoration labs repairing vintage equipment and preserving film without causing perforation damage.
- Parts suppliers and toolmakers producing hobs and jigs for sprocket cutting.
Benefits include consistent film transport, reduced flutter, minimized perforation wear and improved picture steadiness.
Related standards
- ISO 1700 - Cinematography: 8 mm Type S raw stock cutting and perforating dimensions (referenced for perforation pitch and tolerances).
Keywords: ISO 3820:1978, sprocket dimensions, 8 mm Type S, motion‑picture film, sprocket design, perforation pitch, tooth profile, film slippage, hobs, sprocket tolerances.