Gears

Gear Teeth Types Involute (common) Cycloidal Mainly in watches

Involute Generation http://www.youtube.com/watch?v=lWuKl4rG2HA

Pressure Angle Mating gears must have the same pressure angle 14.5° +Less bearing load -Low bending strength 20° +Less undercut -High bearing load

Nomenclature

Diametral Pitch Ratio of teeth to pitch diameter ‘Teeth per inch’

Spur Gears Teeth are parallel to the axis of the gear Advantages: Cost Ease of manufacture Availability Disadvantages Only works with mating gear Axis of each gear must be parallel

Helical Gears Teeth are at an angle to the gear axis (usually 10° to 45°) Advantages Smooth and quiet due to gradual tooth engagements (up to 5,000 ft/min) More tooth engagement allows for greater power transmission Disadvantage More expensive Resulting axial thrust component

Helical Gear Videos Parallel Orientation http://www.youtube.com/watch?v=Qcgjsor1Q-Y Cross Orientation http://www.youtube.com/watch?v=ZpJuyK842RQ

Worm Gears Gears that are 90° to each other Advantages Disadvantage Quiet/smooth drive Can transmit torque at right angles No back driving Good for positioning systems Disadvantage Most inefficient due to excessive friction (sliding) Needs maintenance Slower speed applications

Worms Multiple starts 1,2, & 4 typical Enveloped style

Double-Enveloped Worm Gears

Bevel Gears Gear axis at 90°, based on rolling cones Advantages Right angle drives Disadvantages Get axial loading which complicates bearings and housings

Spiral Bevel Same advantage over bevel as helical had over spur gears Teeth at helix angle Very strong Used in differentials

Racks Provide rotary to linear motion Various Materials Nylon Acetal Steel Stainless

Anti-Backlash Spur Gears Reduces backlash in gear transmissions Spring styles Helical (shown) Torsion

Shaft Attachment - Clamp Does not mar shaft Set gear at any orientation Disadvantages Limited torque Two components

Shaft Attachment – Fairloc® Hub Rigid coupling Clamps circumferentially One piece Disadvantages Limited torque Expensive

Shaft Attachment – Hub w/ S.S. Use two set screws Align 90° from each other Grind/machine flats on shaft Disadvantage Marring from screw Screws loosen from vibration

Shaft Attachment - Keyed Sacrificial component to limit torque Variety of styles to choose (Feather Key shown) Must design with standard sizes (gear must be broached)

Design Chart Categories of Gears Types of Gears Efficiency (%) Parallel Axes Gears Spur Gear Spur Rack Internal Gear Helical Gear Helical Rack Double Helical Gear 98 to 99.5 Intersecting Axes Gears Straight Bevel Gear Spiral Bevel Gear Zerol Gear 98 to 99 Nonparallel and Nonintersecting axes Gears Worm Gear Screw Gear Hypoid Gear 30 to 90 70 to 95 96 to 98

Quality Class http://www.perrytool.com/technical_agma.shtml

For more information on gearing http://www.sdp-si.com/D805/D805cat.htm#tech