See: Add GearPair
Terminology:
Mesh  two external gears can mesh (called External Mesh), or one external gear can mesh with one internal gear (called Internal Mesh). The default is External Mesh.
DrivingGear  the input gear to the GearPair.
DrivenGear  the output gear from the GearPair.
Edit the GearPair:


There are three tabs in the GearPair dialogbox. 
Gear Mesh and # Teeth Mesh  see Terminology above
 or 
Centre Distance  (ReadOnly) see centredistance calculation in Adjustments tab NumberofTeeth, z1 (Minimum =5) The number ofteeth on the inputgear  called the DrivingGear NumberofTeeth, z2 (Minimum = 5) The number ofteeth on the DrivenPart  or the output from the GearPair. 



Gear Segment Parameters Notes: A GearSegment is not a complete gear. Gear 1 or Gear 2 can be a GearSegment Gear # Segmentation
To reduce the numberofteeth of Gear 1 or Gear 2
Each Gear will have an integer numberofteeth. 
Adjustments GearSlip (degrees) These parameters move Gear 1 and Gear 2 around the rotating centre of each Part at the PinJoint. This parameter rotates the gear profile, not the Part. TopTip: To keep the gears in mesh: GearSlip 2 = –Z1 ( Z2 × GearSlip 1) 

Clearance (move gears apart), jr This parameter increases or decreases the centredistance between the two gears. It is intended to add a small amount of Backlash (Play) between the gears  and thus it should normally be a +ve value. Although backlash may be introduced by increasing the CentreDistance, it is more typical to thin the gearteeth. The minimum value should be enough to accommodate a Lubricating Film. CentreDistance Calculation If External Mesh = Clearance + (Module×(NumberofTeeth Gear1+NumberofTeeth Gear2)÷2) If Internal Mesh = Clearance + (Module×(NumberofTeeth Gear1+NumberofTeeth Gear2)÷2) Notes: Recommend Clearance  approximately.
An alternative way to get a small amount of clearance is to reduce the size of the gear teeth and not to change the centredistance. We do not offer this design parameter. 
Gear Tooth Parameters Edit these Gear Parameters to define the gear teeth. 

Module m = P.C.D (in mm) ∕ NumberofTeeth. In words, it the number of teeth for each Ømm. 

PressureAngle α  default = 20º Standard gears are typically 20º. Other standards' are: 14º (weaker, quieter), 17.5º, 22.5º and 25º (stronger, noisier). 

Addendum, ha  default = m. The radial height of the gear tooth from the pitch circle to the tip of the tooth. 

Dedendum, hf  default = m * 1.25. The radial depth of the Gear tooth below the Pitch Circle to the root of the tooth. It is usually larger than the Addendum so there is clearance for the Gear Teeth to mesh without binding. If 0.25<m<1, Dedendum is typically = m *.1.4 If m>1, Dedendum is typically = m *.1.25 

RootRadius, rf  Default is 0.3 * m The small fillet between the Flank and the Root of the Gear Tooth. Note: In reality, if the gear is manufactured from a Hob/Rack Cutter, then the root of the gear is a Trochoid. 

# Points along Profile The number of 'facets' along the gear tooth flank (and around the two Root Radii). To display Gears more accurately, increase the number of points. Otherwise, 4 is adequate, 10 is good. There is a CPU overhead when drawing GearPairs, do not increase the number of facets without a good reason. 

For information: Working Clearance = Dedendum  Addendum Working Depth = Addendum * 2 Tooth Depth = Addendum + Dedendum Default Values Pitch Circle Diameters: d1= z1*m ; d2=z2*m, Addendum = 1.0*m; Dedendum = 1.25*m ; Root Radius =~0.3*m Tip Diameter = d + 2*m Root diameter = d  2.5*m Centre Distance = (d1 + d2) / 2 

Gear Parameters all ReadOnly These parameters are for Gear Analysis only. ContactRatio: Each gear tooth is in contact with another gear tooth as it moves through the meshing position. The ContactRatio gives the numberofteeth that are in contact with another, on average, as they pass through the meshing point. A contact ratio between 1 and 2 means that contact alternates between two and one pairs of teeth in contact at any one time. Gears that have a 'high' contactratio are smoother and quieter. The contactratio of an Internal GearPair is higher than that of a similar External Gearair, even greater than 2. ContactRatio should not be less than 1.1. ContactRatio should be a minimum of 1.2 for working gears. If the ContactRatio is too low, then consider these options:
Working Clearance = Dedendum  Addendum The distance between the top land of a gear tooth and the bottom land of the gear with which is meshed. Gear 1 PCD & Gear 2 PCD PCD = Pitch Circle Diameter. PCD = Module * NumberofTeeth, (m*z) Normal Backlash (mm) This is the gap (backlash) between the gearflanks you can measure with a 'feeler gauge'. Normal Backlash is a function of the Centre Distance Adjustment parameter ONLY. Angular Backlash  Gear 1 (deg) This parameter gives the maximum rotation of Gear 1 if you do not move Gear 2. Angular Backlash is a function of the Centre Distance Adjustment parameter ONLY. Angular Backlash  Gear 2 (deg) This parameter gives the maximum rotation of Gear 2 if you do not move Gear 1. Angular Backlash is a function of the Centre Distance Adjustment parameter ONLY. 
Image Courtesy of the Internet  an image available from so many sources it is difficult to know which is the original source. 


Term 
Definition 

Addendum: 
the height of the gear tooth above the pitch circle diameter 
Backlash: 
the angle the output shaft of the gearbox can move without the input shaft moving 
Base Circle: 
an imaginary circle used in involute gearing to generate the involutes that form the tooth profiles 
Bevel Gears: 
used for rightangle applications. There are two types of bevel gears which are straight and spiral 
Center distance: 
distance between the axes of two meshed gears  Length of the Lineof Centres 
Circular Thickness: 
the thickness of the tooth on the pitch circle. 
Dedendum: 
the depth of the tooth below the diameter of the pitch circle. 
Diametrical Pitch: 
the teeth per inch of the diameter of the pitch circle 
Differential Gear: 
a bevel gear which allows two shafts to rotate at a different speed. 
Gear: 
a wheel with teeth that meshes with another wheel with teeth to translate motion. 
Gear Center: 
the center of the pitch circle. 
Gear Train: 
two or more gears meshed by their teeth. A gear train generates power speed through the meshed gears rotating 
Gear Ratio: 
the ratio between the numbers of teeth of meshing gears. 
Helical Gear: 
gear with the gear teeth cut at angles 
Line of Contact: 
the line or curve along which two tooth surfaces are tangent to each other 
Involute: 
the curve which describes a line which is unwound from the circumference of the gear 
Pinion: 
a small cogwheel which fits into a larger gear or track. 
Pitch Circle: 
the curve of intersection of a pitch surface of revolution and a plane of rotation 
Pitch Diameter: 
the diameter of the pitch circle 
Pitch Radius: 
the radius of the pitch circle 
Planetary Gears: 
a system that consists of three components: the sun gear, ring gear, and two or more planet gears. The sun gear is in the center, the ring gear is the outermost gear, and the planet gears are the gears surrounding the sun gear inside the ring gear. 
Pressure Angle: 
the angle between the lineofaction and the normal (90º, perpendicular) to the surface of the tooth 
Spiral Bevel Gears: 
shafts whose axes are perpendicular (90º) to each other and are used in rightangle applications 
Spur Gear: 
connect parallel shafts which have involute teeth that are parallel to the shaft 
Sun Gear: 
a gearwheel that rotates around its own axis and has other gears (planet gears) that rotate around it 
Torsional Strength: 
the measure of the amount of torque that a radial shaft can sustain during its rotation in a mechanical system 
Working Depth: 
the max depth a tooth of one gear extends into the tooth gear of a mating gear 
Worm Gear: 
a gear with one or more teeth with screwed threads 
Image 
Term 

Crowning: 

EndRelief 

Topping and SemiTopping 

End Radius and Edge Radius/End Relief Top Round/Semitopping


Definition of Circular Pitch, p Circular Pitch = pitch circle circumference(pi.d) / number of teeth(z) ; p = Π.d / z Module = Pitch Diameter(d) / Number of Teeth(z) ; m = d/z Circular Pitch(p) / Module(m) = pi ; p/m = Π Pitch Diameter(d) = module(m)*number of teeth(z) ; d = m.z α 