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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 Types

  • Bevel Gear  -  Most commonly 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.
  • Bull Gear
  • Double Helical Gear  -  A variation of helical gears in which two helical faces are placed next to each other with a gap separating them.  Each face has identical, but opposite, helix angles.  Employing a double helical set of gears eliminates thrust loads and offers the possibility of even greater tooth overlap and smoother operation.
  • External Gear  -  Those type of gears which have their cogs formed on the outside surface of a cylinder or cone.
  • 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.
  • Gear Rack  -  A linear shaped gear which can mesh with a spur gear with any number of teeth.
  • Helical Gear  -  Has teeth that are oriented at an angle to the shaft, unlike spur gears which are parallel.  This causes more than one tooth to be in contact during operation and helical gears can carry more load than spur gears. Due to the load sharing between teeth, this arrangement also allows helical gears to operate smoother and quieter than spur gears. Helical gears produce a thrust load during operation which needs to be considered when they are used.
  • Herringbone Gear  -  Very similar to the double helical gear, but they do not have a gap separating the two helical faces.  These gears are typically smaller than the comparable double helical and are ideally suited for high shock and vibration applications.
  • 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  -  They are like the spur gear with an infinite pitch radius with their axis of rotation places at infinity parallel to that of pinion.  It is therefore the pinion that rotates while the rack translates.
  • Screw Gear  -  A pair of cylindrical gears used to drive non-parallel and nonintersecting shafts where the teeth of one or both members of the pair are of screw form.
  • Sprocket Gear  -  A sprocket usually interacts directly with some part of the machinery at issue, whereas gears can and often do push against each other first, then use that collective movement to influence some larger mechanical process. 
  • 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.  Spur gears tend to be noisier than helical gears because they operate with a single line of contact between teeth.  While the teeth are rolling through mesh, they roll off of contact with one tooth and accelerate to contact with the next tooth. 
  • 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.  Due to this low efficiency, they are often used in lower horsepower applications.


Gear Glossary


  • Addendum -  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.


  • 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.
  • Center-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.

  • 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.
  • 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.
  • 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.
  • 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.


  • Hooke's Law  -  The amount of spring force, compressed or stretched, is proportional applied to the spring deformation.




  • 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.




  • Outside Diameter  -  The overall diameter of the gear.


  • 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.
  • Pressure 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.



  • 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.
  • Root Diameter  -  The diameter at the bottom of the tooth.
  • 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 Space  -  The width of space between the two adjacent teeth measured along the pitch circle.
  • 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.



  • 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.
  • Working Depth  -  Basically 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
General Information about Gears
Outside Diameter
Pitch Diameter
Root Diameter
Whole Depth

Tags: Types of Glossaries