diff --git a/freecad/gears/features.py b/freecad/gears/features.py
index 5ca5988..aa1bca1 100644
--- a/freecad/gears/features.py
+++ b/freecad/gears/features.py
@@ -194,7 +194,8 @@ class InvoluteGear(BaseGear):
                         "computed", "transverse_pitch", 1)
 
     def add_tolerance_properties(self, obj):
-        obj.addProperty("App::PropertyLength", "backlash", "tolerance", "backlash")
+        obj.addProperty("App::PropertyLength", "backlash", "tolerance",
+            "The arc length on the pitch circle by which the tooth thicknes is reduced.")
         obj.addProperty("App::PropertyBool", "reversed_backlash", "tolerance", "backlash direction")
         obj.addProperty(
             "App::PropertyFloat", "clearance", "tolerance", "clearance")
@@ -358,7 +359,8 @@ class InternalInvoluteGear(BaseGear):
         obj.addProperty("App::PropertyFloat", "root_fillet", "fillets", "a fillet for the tooth-root, radius = root_fillet x module")
 
     def add_tolerance_properties(self, obj):
-        obj.addProperty("App::PropertyLength", "backlash", "tolerance", "backlash")
+        obj.addProperty("App::PropertyLength", "backlash", "tolerance",
+            "The arc length on the pitch circle by which the tooth thicknes is reduced.")
         obj.addProperty("App::PropertyBool", "reversed_backlash", "tolerance", "backlash direction")
         obj.addProperty("App::PropertyFloat", "head", "tolerance", "head_value * modul_value = additional length of head")
         obj.addProperty("App::PropertyFloat", "clearance", "tolerance", "clearance")
@@ -887,7 +889,8 @@ class CycloidGear(BaseGear):
 
     def add_tolerance_properties(self, obj):
         obj.addProperty("App::PropertyFloat", "clearance", "tolerance", "clearance")
-        obj.addProperty("App::PropertyLength", "backlash", "tolerance", "backlash in mm")
+        obj.addProperty("App::PropertyLength", "backlash", "tolerance",
+            "The arc length on the pitch circle by which the tooth thicknes is reduced.")
         obj.addProperty("App::PropertyFloat", "head", "tolerance", "head_value * modul_value = additional length of head")
 
     def add_cycloid_properties(self, obj):
@@ -969,7 +972,8 @@ class BevelGear(BaseGear):
         obj.addProperty("App::PropertyFloat", "clearance", "tolerance", "clearance")
         obj.addProperty("App::PropertyInteger", "numpoints", "precision", "number of points for spline")
         obj.addProperty("App::PropertyBool", "reset_origin", "base", "if value is true the gears outer face will match the z=0 plane")
-        obj.addProperty("App::PropertyLength", "backlash", "tolerance", "backlash in mm")
+        obj.addProperty("App::PropertyLength", "backlash", "tolerance",
+            "The arc length on the pitch circle by which the tooth thicknes is reduced.")
         obj.addProperty("App::PropertyPythonObject", "gear", "base", "test")
         obj.addProperty("App::PropertyAngle", "beta","helical", "angle used for spiral bevel-gears")
         obj.gear = self.bevel_tooth
diff --git a/pygears/bevel_tooth.py b/pygears/bevel_tooth.py
index a425252..1d4246e 100644
--- a/pygears/bevel_tooth.py
+++ b/pygears/bevel_tooth.py
@@ -30,6 +30,7 @@ class BevelTooth(object):
         self.z = z
         self.clearance = clearance
         self.backlash = backlash
+        self.angular_backlash = backlash / (z * module / 2)
         self.module = module
 
         self.involute_end = arccos(
@@ -108,7 +109,7 @@ class BevelTooth(object):
         intersection_point = intersection_line_circle(xy[i], point, r_cut)
         xy = array([intersection_point] + list(xy[i+1:]))
         xyz = [[p[0], p[1], 1] for p in xy]
-        backlash_rot = rotation3D(self.backlash / 4)
+        backlash_rot = rotation3D(self.angular_backlash / 2)
         xyz = backlash_rot(xyz)
         return(xyz)
 
diff --git a/pygears/cycloid_tooth.py b/pygears/cycloid_tooth.py
index 866a8e7..342a30d 100644
--- a/pygears/cycloid_tooth.py
+++ b/pygears/cycloid_tooth.py
@@ -40,6 +40,7 @@ class CycloidTooth():
         self.da = self.d + 2 * (1 + self.head) * self.m
         self.di = self.d - 2 * (1 + self.clearance) * self.m
         self.phipart = 2 * pi / self.z
+        self.angular_backlash = self.backlash / (self.d / 2)
 
     def epicycloid_x(self):
         def func(t):
@@ -92,7 +93,7 @@ class CycloidTooth():
         pts_outer = transpose([pts_outer_x, pts_outer_y])
         pts_inner = transpose([pts_inner_x, pts_inner_y])
         pts1 = vstack([pts_inner[:-2], pts_outer])
-        rot = rotation(self.phipart / 4 - self.backlash)
+        rot = rotation(self.phipart / 4 - self.angular_backlash / 2)
         pts1 = rot(pts1)
         ref = reflection(0.)
         pts2 = ref(pts1)[::-1]
diff --git a/pygears/involute_tooth.py b/pygears/involute_tooth.py
index 767f231..a350116 100644
--- a/pygears/involute_tooth.py
+++ b/pygears/involute_tooth.py
@@ -69,6 +69,7 @@ class InvoluteTooth():
         self.involute_rot2 = 1 / self.z * \
             (pi / 2 + 2 * self.shift * tan(self.pressure_angle_t))
         self.involute_rot = self.involute_rot1 + self.involute_rot2
+        self.angular_backlash = self.backlash / (self.d / 2)
         self.involute_start = 0.
         if self.dg <= self.df:
             self.involute_start = sqrt(self.df ** 2 - self.dg ** 2) / self.dg
@@ -81,7 +82,7 @@ class InvoluteTooth():
         y = array(list(map(fy, pts)))
         xy = transpose([x, y])
         rotate = rotation(
-            self.undercut_rot + self.phipart / 2 - self.backlash / 4)
+            self.undercut_rot + self.phipart / 2 - self.angular_backlash / 2)
         xy = rotate(xy)
         return(array(xy))
 
@@ -91,7 +92,7 @@ class InvoluteTooth():
         x = array(list(map(fx, pts)))
         fy = self.involute_function_y()
         y = array(list(map(fy, pts)))
-        rot = rotation(self.involute_rot - self.backlash / 4)
+        rot = rotation(self.involute_rot - self.angular_backlash / 2)
         xy = rot(transpose(array([x, y])))
         return(xy)