How to Draw Gear in Autocad 2018
Use the Gear dialog box to enter specific values for a gear for DIN or ANSI.
Module (DIN) - m
Ratio of the pitch diameter to the number of teeth. Ordinarily, module is understood to mean ratio of pitch diameter in millimeters to the number of teeth. The English module is a ratio of the pitch diameter in inches to the number of teeth.
Diametrical Pitch (ANSI) - p
The ratio of the number of teeth to the number of inches of pitch diameter - equals number of gear teeth to each inch of pitch diameter. Normal Diametral Pitch is the diametral pitch as calculated in the normal plane and is equal to the diametral pitch divided by the cosine of the helix angle.
Number of Teeth - z (DIN) / N (ANSI)
Enter a value for the number of teeth.
The condition for no undercutting in a standard spur gear is given by the expression:
z >= 2 * hap° / (sin(alpha)) 2
Pressure Angle - alpha (DIN / Phi (ANSI)
The angle between a tooth profile and a radial line at its pitch point. Standard Pressure Angles are established in connection with standard gear-tooth proportions. Most spur gears are cut to operate at pressure angles of 20 or 25°.
Helix Angle - beta (DIN) / Psi (ANSI)
The angle that a helical gear tooth makes with the gear axis at the pitch circle unless otherwise specified.
Profile Shift or Addendum Modification - x
The Profile Shift will be taken to avoid undercuts by small number of teeth, to get a given center distance and to raise the load capacity. If the factor is positive the Addendum will be bigger and if the factor is negative the Addendum will be smaller, also the diameters will change.
Standard value: -0.7 < x < +0.7
Addendum Coefficient - hap*
Distance between basic rack reference line and basic rack root line.
(Standard value hap*=1)
hap = ha/mn
Dedendum Coefficient - hfp*
Distance between basic rack reference line and basic rack head line. (Standard value hfp*=1.25).
Top Clearance Coefficient - ca*
Coefficient of distance between outside diameter of first gear and root diameter of second gear in pair. (Standard value ca*="0".25)
Fillet Radius Coefficient - roa*
coefficient of fillet radius at rack cutter. (Standard value ca="0".25)
hfp* - hap* <="0".295 ->roa* = (hfp* - hap*) / (1-sin(alpha))
hfp* - hap* 
0.295 -> roa* = ((1+sin (alpha_n) / cos (alpha_n)) * ((PI / 4) - (hfp* - hap*)* tan (alpha_t))
alpha_n - Pressure angle in plane perpendicular at teeth direction
alpha_t - Pressure angle in radial plane
Length
Determines the length of the gear.
da (DIN)/ do (ANSI)
Outside (tip) diameter
da = dt + 2 * x * mn + 2 * hap° * mn
da = - dt + 2 * x * mn + 2 * hap° * mn
dt (DIN)/ D (ANSI)
The diameter of the pitch circle. In parallel shaft gears the pitch diameters can be determined directly from the center distance and the numbers of teeth by proportionality. The pitch circle is the radius which is equal to the distance from the gear axis to the pitch point.
dt = z * mn / cos beta
df (DIN)/ DR (ANSI)
Root diameter: Diameter of the root circle.
df = dt + 2 * x * m - 2 * hfp° * mn
df = - dt + 2 * x * mn - 2 * hfp * mn
Root Circle: A circle coinciding with or tangent to the bottoms of the tooth spaces.
hfp
Root Diameter Coefficient
hfp* = (ha+ca)mn
hap
Source: https://knowledge.autodesk.com/support/autocad-mechanical/learn-explore/caas/CloudHelp/cloudhelp/2018/ENU/AutoCAD-Mechanical/files/GUID-26A705C9-3056-481F-B847-2CB08E50D406-htm.html
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