Overview
ISO 677:1976 - "Straight bevel gears for general engineering and heavy engineering - Basic rack" defines the dimensional characteristics and profile of the basic rack used as the reference tooth form for straight bevel gears with involute teeth. The standard is concerned with a basic rack that has a constant bottom clearance and establishes how rack dimensions scale with module or diametral pitch. It also defines the term reference line and gives the relationship between the basic-rack profile and the cutting tools used to generate bevel teeth.
Key topics and requirements
- Scope: Dimensional characteristics of the basic rack for straight bevel gears used in general and heavy engineering; applies for gear pairs where the tip angle of one gear equals the difference between the designed shaft angle and the root angle of the mating gear.
- Profile form: Standard basic-rack profile corresponds to bevel gears with involute teeth and substantially straight flanks.
- Key geometric parameters:
- Pressure angle: 20° (standard value for the profile).
- Tooth height: specified as 2.20 × module (dimensions in millimetres when module is used).
- Tooth tip position and thickness: tooth tips lie at a distance from the reference line equal to the module; tooth thickness measured on the reference line equals the width of the space.
- Root fillet radius: normally up to 0.30 × module; where meshing permits this may be increased up to 0.35 × module.
- Scaling: Profile dimensions shown for module m = 1 (or diametral pitch P = 1) must be multiplied by the module or by the inverse of the diametral pitch for other sizes.
- Manufacturing relation: The standard explains how the basic-rack profile relates to the cutting tools that generate bevel teeth (the cut tooth is a portion of an octoid), useful for tool design and machining processes.
- Definitions: Clear definitions of basic rack and reference line for consistent application.
Applications and who uses it
ISO 677 is practical for:
- Gear designers and mechanical engineers specifying straight bevel gear geometry.
- Gear manufacturers and toolmakers who need the reference rack profile for cutter design and gear cutting.
- Quality/inspection engineers using the standard profile and dimensional rules for acceptance and interchangeability.
- Industries using heavy-duty bevel gearing (e.g., industrial gearboxes, marine, mining, heavy machinery) where standardized tooth form and reliable meshing are required.
Related standards
- ISO 678 - Straight bevel gears for general engineering and heavy engineering - Modules and diametral pitches (referenced for module / pitch sizing).
- ISO Recommendation R 677-1968 - predecessor document transformed into ISO 677:1976.
Keywords: ISO 677, straight bevel gears, basic rack, bevel gear standards, involute teeth, gear profile, module, diametral pitch.
