Metric gears are defined by module, which designates the center-to-center distance between successive teeth. This criterion of gear selection is used in countries which have adopted the metric system and is intended to standardize gear selection.

 

How Metric Gears Work

To define a gear's module, the pitch diameter (the teeth bearing circumference) of the gear is divided--in millimeters--by the number of teeth. If the gear is measured using inches, it is specifically referred to as the English module to avoid confusion.

 

Selecting Metric Gears

 

Types of Metric Gears

Any type of gear can be a metric gear, including spur gears, hypoid gears, differential end gears, worm drives and worm wheels, gear stock or pinion wire, racks and pinions, and miter and bevel gear sets.

 

Dimension Specifications of Metric Gears

More than just module should be studied when considering metric gears. Gears mate via teeth with very specific geometry. Pitch is a measure of tooth spacing and is expressed in several ways.

 

Mounting Specifications of Metric Gears

Consider the gear center, bore diameter and shaft diameter. The gear center can be a bored hole or an integral shaft. The bore diameter is the diameter of the center hole. The shaft diameter corresponds to the diameter of the shaft the gear mounts to, or the diameter of an integral shaft, if present. Metric gears can be mounted on a hub or shaft. A hub is a cylindrical projection on one or both sides of a herringbone gear, often for the provision of a screw or other shaft attachment mechanism. Hubless gears are typically attached via press fit, adhesive or internal keyway.

 

Shaft mounting choices include the following:

  • Keyway: One or more cutouts exist in the gear bore for exact mounting on the shaft. 

 Image credit: TT Net

  • Set Screw: The gear is attached to the shaft by screws through the hub.

 

Image credit: Direct Industry

  • Hub Clamping Screws: The gear is attached with a screw that squeezes the inner diameter of the hub to a tight fit around the shaft.

Image credit: How Stuff Works

  • Split: The hub is split into several pieces that are tightened down by a separate clamp to grip the shaft.

Image credit: SDP/SI

  • Simple Bore: A straight bore designed for adhesive attachment.

Image credit: Monarch Bearing

 

Metric Gear Applications

Application requirements should be considered with the workload and environment of the gear set in mind.

  • Power, velocity and torque consistency and output peaks of the gear drive so the gear meets mechanical requirements.
  • Inertia of the gear through acceleration and deceleration. Heavier gears can be harder to stop or reverse.
  • Precision requirement of gear, including gear pitch, shaft diameter, pressure angle and tooth layout. The module is included for metric gears.
  • Handedness (left or right teeth angles) depending the drive angle.
  • Gear lubrication requirements. Some gears require lubrication for smooth, temperate operation.
  • Mounting requirements. Application may limit the gear's shaft positioning.
  • Noise limitation. Commercial applications may value a smooth, quietly meshing gear.
  • Corrosion resistance. Gears exposed to weather or chemicals should be especially hardened or protected.
  • Temperature exposure. Some gears may warp or become brittle in the face of extreme temperatures.
  • Vibration and shock resistance. Heavy machine loads or backlash, the deliberate surplus space in the circular pitch, may jostle gearing.

  • Operation disruption resistance. It may be necessary for some gear sets to function despite missing teeth or misalignment.

Metric Gear Materials

Gear composition is determined by application, including the gear's service, rotation speed, accuracy and more.

  • Cast iron provides durability and ease of manufacture.
  • Alloy steel provides superior durability and corrosion resistance. Minerals may be added to the alloy to further harden the gear.
  • Cast steel provides easier fabrication, strong working loads and vibration resistance.
  • Carbon steels are inexpensive and strong, but are susceptible to corrosion.
  • Aluminum is used when low gear inertia with some resiliency is required.
  • Brass is inexpensive, easy to mold and corrosion resistant.
  • Copper is easily shaped, conductive and corrosion resistant. The gear's strength would increase if bronzed.
  • Plastic is inexpensive, corrosion resistant, quiet operationally and can overcome missing teeth or misalignment. Plastic is less robust than metal and is vulnerable to temperature changes and chemical corrosion. Acetal, delrin, nylon, and polycarbonate plastics are common.

  • Other material types like wood may be suitable for individual applications.

Resources

 

Wikipedia - The Module System

 

SDP/SI - Elements of Gear Technology


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