<< Click to Display Table of Contents >> Navigation: MechDesigner Reference & User Interface > DialogBoxes > Dialog: GearPair 
See: Add GearPair
Gear Mesh: 
Interlocking gearteeth that allow a torque and motion to be transmitted from one shaft to a different shaft. 
External Gear Mesh : 
The gears have teeth that engage and diverge out from their centersofrotation. 
Internal Mesh : 
One of the gears has teeth that converge in to its centerofrotation. 
Simple GearPair : 
Two gears that rotate about fixed centers. 
Epicyclic GearPair : 
One gear orbits around the center of the other gear. 
Edit the GearPair:
The GearPair dialogbox is now open. 
There are three tabs in the GearPair dialogbox. Define tabAdjustments tabParameters tab 
Mesh  see Terminology above oExternal  or  oInternal ReadOnly Center Distance  See also centerdistance 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 output gear  the DrivenGear in the GearPair. 
Notes: A GearSegment is a gear that must oscillate. Gear 1 and/or Gear 2 can be a GearSegment Gear # Segmentation ◉Complete Gear  a normal full gear ◉Gear Segment  a gear that is not complete, it has fewer numberofteeth. To reduce the numberofteeth of Gear 1 or Gear 2:

GearSlip (degrees) These parameters move Gear 1 and Gear 2 around at the PinJoint. This parameter rotates the gear teeth, not the Part. TopTip: To keep the gears in mesh: GearSlip 2 = –Z1 ( Z2 × GearSlip 1) 

Clearance (move gears apart) This parameter changes the length of the LineofCenters that is between the two Gears. Usually, Clearance is a +ve value for External Gears, and a –ve value for Internal Gears. See Notes on recommended backlash. An alternative way to change the clearance is to reduce (by machining) the size of the gearteeth and not to change the centerdistance. We do not include a parameter for this deign option. However, it is the standard method to provide backlash in commercial gearboxes. Talk with the machinist. centerDistance Calculation If External Mesh, then centerDistance = Clearance + (Module×(NumberofTeeth Gear1+NumberofTeeth Gear2)÷2) If Internal Mesh, then centerDistance = Clearance + (Module×(NumberofTeeth Gear1+NumberofTeeth Gear2)÷2) Notes: Recommend Backlash/Clearance. •Minimum: 0.006 × (centerDistance)0.5 •Maximum: 0.024 × (centerDistance)0.5 Also, I have read that: •Minimum normal backlash = 0.03×module + 0.05 mm If the torque reverses each machine cycle, then you should aim for the minimum recommended backlash. 
Use Gear Tooth Parameters to define the size and shape of the gear teeth. 

Module m = P.C.D (in mm) ∕ NumberofTeeth. 

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

Addendum, ha  default = m. The radial height of the gear tooth from the Pitch Circle to the top 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. The Dedendum is usually larger than the Addendum to give clearance for the teeth. If 0.25<m<1, Dedendum is usually = m *.1.4 If m>1, Dedendum is usually = 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 Total Depth = Addendum + Dedendum 
ALL READONLY ContactRatio: ε ContactRatio should not be less than 1.1. ContactRatio should be a minimum of 1.2 for working gears. 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 center Distance Adjustment parameter ONLY. Angular Backlash  Gear 1 (deg) jΘ1 This parameter gives the maximum rotation of Gear 1 if you do not move Gear 2. Angular Backlash is a function of the center 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 center Distance Adjustment parameter ONLY. 

ContactRatio The ContactRatio gives the numberofteeth that are in contact, on average, as they pass through the meshing point. A contact ratio between 1 and 2 means that contact alternates between one and two pairs of teeth 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 GearPair, even greater than 2. If the ContactRatio is too low, then consider these options: •Decrease the pressureangle •Increase the numberofteeth •Increase the workingdepth 
Useful Gearing Calculations and Equations
TO OBTAIN 
FROM KNOWN 
USE THIS FORMULA 

Pitch Diameter 
Module, m, numberofteeth, N 
D = mN 
Circular Pitch 
Module 
pc = m.π = D.π ∕ N 
Module 
Diametrical Pitch, Pd 
m = 25.4 ∕ Pd 
NumberofTeeth 
Module, m, Pitch Diameter, D 
N = D ∕ m 
Addendum 
Module, m 
a = m 
Dedendum 
Module, m 
b = 1.25m 
Outside Diameter 
Module, m, Pitch Diameter, D, or numberofteeth, N 
Do = D + 2m = m (N + 2) 
Root Diameter 
Pitch Diameter, D, Module, m 
DR = D – 2.5m 
Base Circle Diameter 
Pitch Diameter & Pressure Angle 
Db = D cos μ 
Base Pitch 
Module, m, & Pressure Angle, μ 
pb = m π cos μ 
Tooth Thickness at Standard Pitch Diameter 
Module, m 
Tstd = π. m ∕ 2 
center Distance 
Module, m, numberofteeth, N 
C = m . (N1 + N2) ∕ 2 
Contact Ratio for Spur Gears ( 1 < CR < 2 ) 
Outside Radii, Base Circle Radii, center Distance, Pressure Angle 
CR = (√R012 – Rb12 + √R022 – Rb22 – C sin μ) ∕ m π cos μ 
Backlash (linear) 
Change in center Distance, ΔC 
B = 2( ΔC )tan μ 
Backlash (linear) 
Change in Tooth Thickness, ΔT 
B = ΔT 
Backlash (linear) along Lineofaction 
Linear Backlash along Pitch Circle, B 
BLA = B cos μ

Backlash, Angular 
Linear Backlash, D 
Ba = 6880 B ∕ D (arc minutes) 
Min. No. of Teeth for No Undercutting 
Pressure Angle, μ 
Nc = 2 ∕ sin2 μ Nc (20º) = ~17 Teeth 
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 centers 
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
