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A gear is a rotary wheel having teeth which mesh with other thoothed wheels.  As the gear turns it transmits torque to another gear or shaft.  The larger the gear the slower it rotates and depending on the combination of the gears, the different sizes can either increase or decrease the force or speed.



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Gear Index


Gear Shaft Axis Position Design Classification

  • gear intersecting axis 1Intersecting Shaft  -  They are designed to transmit power and motion between 90 degree angular intersecting shafts having beveled gears.
    • Bevel Gear  -  Used to transmit power between shafts that intersect at a 90 degree angle.  They are used in applications where a right angle gear drive is required.
      • Hypoid Bevel Gear  -  Like a spiral bevel gears, but with hyperbolic pitch surfaces instead of conical ones.
        • Face Gear  -  Looks like a bevel gear that is limited to 90° intersecting axes. This is a circular disc with a ring of teeth cut in its side face.
      • Spiral Bevel Gear  -  They have a curved angle of teeth placement.  It is more angled and also provides gradual teeth to teeth contact than that of straight bevel gears.
      • Straight Bevel Gear  -  They have a conical pitch surface, but the teeth are straight, consistently tapering toward the apex of the system.
      • Zero Bevel Gear  -  The combination of both spiral and straight gears having curved teeth that are placed straight on the conical surface.
  • gear parallel axis 1Parallel Shaft  -  They are designed to transmit power and motion between parallel shafts.  The angle between driving and driven shaft is 0 degree. 
    • Helical Gear  -  These are cylindrical gears whose teeth are not parallel to the axis of rotation.  The teeth are angled and appear as a segment of a helix.  Helical gears can transmit power between parallel or right angle axes.
      • Single Helical Gear  -  Single helical gears have a single row of angled teeth cut or embedded around the perimeter of the gear body.
      • Double Helical Gear  -  These gears have a groove in the middle, between the teeth, whereas herringbone gears do not.  This arrangement cancels out the axial forces on each set of teeth, so larger helix angles can be used.
      • Harringbone Gear  -  A type of double helical gear in which the two teeth track touches each other rather than being separated by a channel.  This forms a V shaped structure.
    • Sprocket Gear  -  A sprocket has a shaft as part of the gear that usually interacts directly with some part of the machinery, whereas gears can and often do push against each other first, then use that collective movement to influence some larger mechanical process.
      • Bushed Sprocket
      • Chain Sprocket
      • Double Plus Sprocket
      • Drive Sprocket
      • Drum Sprocket
      • Duplex Sprocket
      • Idler Sprocket
      • Industrial Sprocket
      • Multi-standard Sprocket
      • Quick Disconect Sprocket
      • Shaft Sprocket
      • Simplex Sprocket
      • Steel Split Sprocket
      • Taper-lock Sprocket
      • Triplex Sprocket
    • Spur Gear  -  A cylindrical shaped gear in which the teeth are parallel to the axis.  This causes the gears to produce radial reaction loads on the shaft, but not axial loads.
  • gear non par non int axis 1Non-intersecting and Non-parallel Shaft  -  They are designed to transmit power and motion between 90 degree angle non-intersecting, non-coplanar shafts.
    • Screw Gear  -  A pair of cylindrical gears used to drive shafts where the teeth of one or both members of the pair are of screw form.
    • Worm Gear  -  This gear transmits power through right angles on non-intersecting shafts.  Worm gears produce thrust load and are good for high shock load applications but offer very low efficiency in comparison to the other gears.
      • Non-throated Worm Gear  -  A helical gear with a straight worm.  Tooth contact is a single moving point on the worm drive. both the worm and the driven gear is not throated
      • Straight Throated Worm Gear  -  Only the contour of the gear teeth is modified to get the increased amount of contact surface.
      • Double Throated Worm Gear  -  The teeth profile of the gear as well as the shape of the worm threads are modified to achieve better engagement.  The modification is in line with the circular shape of the gear teeth.  The engagement is achieved throughout the length and width of the worm gear.


Gear Teeth

  • Straight Gear Teeth  -  The teeth axis is parallel to the shaft axis.
  • Inclined Gear Teeth  -  The teeth axis is at some angle.
  • Curve Gear Teeth  -  The teeth are curved on the gear rim surface.


gear type 1Gear Design Classification

  • External Gear  -  Those type of gears which have their cogs formed on the outside surface of a cylinder or cone.
  • Internal Gear  -  A cylindrical shaped gear, but with the teeth are inside the circular ring.  It can mesh with a spur gear.
  • Rack and Pinion Gear  -  A rack is a circular gear, and a pinion is a linear gear.  Together, they turn rotational motion into linear motion
    • Gear Rack  -  A linear shaped gear which can mesh with a spur gear with any number of teeth.
    • Pinion Gear  -  Is cylindrical in shape.


gear velocity 1Gear Velocity

  • Low Gear Velocity  -  Having a velocity less than 3 \(\frac{m}{s}\).
  • Medium Gear Velocity  -  Having a velocity between 3 \(\frac{m}{s}\) and 15 \(\frac{m}{s}\).
  • High Gear Velocity  -  Having a velocity more than 15 \(\frac{m}{s}\).


Gear Glossary


  • gear depth 1Addendum -  The radial distance of a tooth from the pitch circle to the top of the tooth.
  • Addendum Circle  -  Basically an imaginary circle that passes through the addendum of gear teeth.
  • Arc of Approach  -  The curve traced by the pitch point form the beginning to the end of engagement.
  • Arc of Contact  -  The path traced by a point on the pitch circle from the starting of the engagement to the end of the engagement for a given pair of teeth.
  • Arc of Recess  -  The portion of the path of contact form pitch point to the end of the engagement.
  • Axial Pitch  -  The distance parallel to the axis between corresponding sides of adjacent teeth.
  • Axial Thickness  -  The distance parallel to the axis between two pitch line elements of the same tooth.


  • Backlash  -  The difference between the tooth space and the tooth thickness.  It measures along the pitch circle.  As in the manufacturing of the gear it is desirable to keep the backlash as close to zero but in actual practice some amount of backlash is provided to prevent jamming of the teeth because of the thermal expansion or tooth errors.
  • Base Circle  -  A circle from where involute portion of tooth profile will be produced.
  • Base Diameter  -  The diameter of the base circle of a gear.
  • Billet  -  A part machined from a forged piece of metal rather than a casting.
  • Bottom Land  -  The bottom portion of a gear tooth which will be extended below the pitch circle.
  • Brittleness  -  A tendency to fracture without appreciable deformation.


  • Case Hardning  -  Adding carbon to the surface of a mild steel object and heat treating to produce a hard surface.
  • Center Distance  -  The center distance of 2 spur gears is the distance from the center shaft of one spur gear to the center shaft of the other.
  • gear center 1Center-to-center Distance  -  The center distance of two spur gears is the distance from the center shaft of one spur gear to the center shaft of the other.
  • Circular Pitch  -  The measured distance along the circumference of the pitch diameter from the point of one tooth to the corresponding point on an adjacent tooth.
  • Circular Thickness  -  Thickness of a tooth measure along the circumference of the pitch circle.

  • Clearance  -  It the radial distance from the top of the tooth to the bottom of the tooth in a mating gear.  A circle passing through the top of the mating gear is known as clearance circle.
  • Compound Gear Drives  -  A compound gear is made up of two gears solidly connected. Often they are machined from the same stock or keyed to the same shaft.
  • Contact Ratio  -  The ratio of length of arc of contact to the circular pitch.  It is the denotation of number of pairs of teeth engaged in one contact
  • Contact Stress  -  The maximum compressive stress within the contact area between mating gear tooth profiles.
  • Cross Pin Shaft  -  A hardened shaft which installs into the case and keeps the spiders securely in place.  In semi-float applications, it also prevents the axles from sliding inward into the carrier case.
  • Crowned Teeth  -  They have surfaces modified in the lengthwise direction to produce localized contact or to prevent contact at their ends.  Crowning can be applied to all types of teeth.


  • Dedendum  -  The circle drawn through the bottom of the teeth.
  • Dedendum Circle  -  It is the circle drawn through the bottom of the teeth.
  • Diametral Pitch  -  The ratio of the number of teeth to the pitch circle diameter.



  • Face of the Tooth  -  The surface of the gear tooth above the pitch surface.
  • Face Width  -  The width of the gear tooth measured parallel to its axis.
  • Flank of the Tooth  -  The surface of the gear tooth below the pitch surface.
  • Force  -  The push or pull of an object resulting in a change from rest or motion.
  • Friction  -  The mechanical resistance to the relative movement of two surfaces.


  • Gear Backlash  -  The difference between the tooth space and the tooth thickness, as measured along the pitch circle.  The backlash should be zero, but in actual practice, some backlash must be allowed to prevent jamming of the teeth due to tooth errors and thermal expansion.  But too much backlash also should be avoided.  That leads to the gear chatter in the gear boxes.
  • Gear Chatter  -  Noise due to vibratory torque transmission through the gears.
  • Gear Profile  -  The tooth thickness, diametral pitch and pressure angle all go into determining the gear tooth profile.  These factors are determined by the desired contact ratio between mating parts of the gear.
  • Gear Ratio  -  The number of teeth of the larger gear to the smaller gear.
  • Gib Head Key  -  These are keys tapered on the top surface to ensure a tight fit and having a raised head on one side so that its removal is easy.  Gib keys are generally rectangular or square keys.


  • Helix Angle  -  The tooth angle relative to the central axis.  Helix angles typically range from 10° to 45° depending on the design requirements.  Gears with 0° helix angles are called spur gears.  Helical gears (over 0°) increase contact ratio and improve gear noise, but also produce an axial load that has to be accounted for in the bearing design.
  • Helix Hand  -  The direction of the helix angle expressed as either right or left.  Looking down the central axis of the gear, a right hand helix angles from left to right down the tooth face away from you.  The opposite is true for a left hand helix.
  • Hooke's Law  -  The amount of spring force, compressed or stretched, is proportional applied to the spring deformation.


  • Interference  -  The contact between mating teeth at some point other than along the line of action.



  • Key  -  Usually made from steel and is inserted or mounted between the shaft and the hub of the component in an axial direction to prevent relative movement.
  • Keyseat  -  A recess in the shaft.
  • Keyway  -  The recess (slot or groove) in the hub to receive the key and thus securely lock the component.
  • Kinetic Friction  -  The force opposing two objects rubbing together that are moving relative to each other.


  • Longitudinal Stress  -  The stress imposed on the long axis of any shape. It can be either a compressive or tensile stress.


  • Module  -  It is the ratio of pitch circle diameter millimetres to the number of teeth.


  • Normal Tooth Thickness  -  The tooth thickness measured normal or perpendicular to the central axis of the gear, also at the pitch diameter.
  • Normal Plane  -  A plane normal to the tooth surfaces at a point of contact and perpendicular to the pitch plane.


  • Outside Diameter  -  The diameter of the circle that contains the tops of the teeth of external gears.


  • Path of Contact  -  The path travelled by the point of contact of two teeth from the starting of the engagement to the end of the engagement.
  • Pitch  -  The pitch represents the size of each tooth on the gear.
  • Pitch Circle  -  An imaginary circle that divides the gear teeth in two portion, top lands and bottom lands.
  • Pitch Point  -  It is a common point of contact between two pitch circles.
  • Pitch Surface  -  It is the surface of the imaginary rolling cylinder that the toothed gear may be considered to replace.
  • Path of Contact  -  The path traced by the point of contact of two teeth from the beginning to the end of the engagement.
  • Polar Pitch  -  The proportion of the quantity of teeth to the width of contribute circle millimeter.
  • Pressure  -  The force exerted perpendicular to the surface of an object and is expressed as force per unit area.
  • gear pressure angle 1Pressure Angle  -  The pressure angle figures into the geometry or form of the gear tooth.  The angle between the direction of contacting force on the contact point of the tooth and direction of motion of that point on the tooth profile.
  • Profile  -  It is the curvature contained by the face and flank of the gear tooth.



  • Rectangular Key  -  They are wider than their height and are sometimes called flat keys.  The extra key width allows it to transmit greater torque without increasing the depth.  An increase in depth means a weaker shaft due to a reduction in effective shaft cross-sectional area.
  • Root Circle  -  A circle that passes through the root of gear teeth and the diameter of root circle.
  • Round Pitch  -  The separation between a point of a tooth to a similar purpose of the adjoining tooth, estimated along periphery of the pitch circle.



  • Teeth Number  -  The number of teeth of the gear.
  • Tooth Depth  -  It is the radial distance between the addendum and dedendum circles of a gear.
  • Tooth Face  -  The surface between the pitch line element and the tooth tip.
  • Tooth Space  -  The width of space between the two adjacent teeth measured along the pitch circle.
  • Tooth Surface  -  The total tooth area including the flank of the tooth and the tooth face.
  • Tooth Thickness  -  The width of the tooth measured along the pitch circle.
  • Top Land  -  The top portion of a gear tooth which will be extended above the pitch circle.

  • Torque  -  A type of force that is applied to an object that results in the object rotating around an axis.
  • Torque Speed  -  A motor converts electrical energy into mechanical energy by the rotational motion.
  • Total Depth  -  The radial distance between the addendum and the dedendum circles of a gear.  It is equal to the sum of the addendum and dedendum.
  • Transverse Tooth Thickness  -  The tooth thickness measured normal or perpendicular to the helix angle—also at the pitch diameter.



  • Velocity  -  The rotational speed of a gear.  Is expressed as the distance a point along the circumference of the pitch circle will travel over a given unit of time.


  • Whole Depth  -  The distance from the top of the tooth to the bottom of the tooth.

  • Woodruff Key  -  A semi-circular disc and fits into a circular recess in the shaft machined by a woodruff keyway cutter.  These keys are mostly used in machine tools and automobile shafts and cannot carry the same load as long parallel keys.
  • gear pressure angle depth 1Working Depth  -  The depth of engagement, when two mating gear meshes with each other. 





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Display #
Base Circle
Center-to-center Distance
Circular Pitch
Circular Thickness
Diametral Pitch
Gear Ratio
General Information about Gears
Number of Teeth
Outside Diameter
Pitch Diameter
Root Diameter
Whole Depth
Working Depth

Tags: Glossaries Classification