Gear data:
Pressure angle, 20°. Module, 12 mm: Number of teeth, 25.
Gear calculations:
Pitch-circle diameter = module x no. of teeth
= 12 x 25 = 300 mm Addendum = module = 12 mm Clearance = 0.25 x module
= 0.25 x 12 = 3 mm Dedendum = addendum + clearance = 12 + 3 = 15 mm Circular pitch = n x module = n x 12 = 37.68 mm Tooth thickness = 1 x circular pitch = 18.84 mm
(a) Draw the pitch circle and the common tangent.
(b) Mark out the pressure angle and the normal to the line of action.
(c) Draw the base circle. Note that the length of the normal is the base-circle radius.
(a) Draw the addendum and dedendum circles. Both addendum and dedendum are measured radially from the pitch circle.
(b) Mark out point A on the addendum circle and point B on the dedendum circles. Divide AB into three parts so that CB = 2AC.
(c) Draw the tangent CD to the base circle. D is the point of tangency. Divide CD into four parts so that CE = 3DE.
(d) Draw a circle with centre O and radius OE. Use
Fig. 24.17 Unwins construction-stage 1
Fig. 24.17 Unwins construction-stage 1
Addendum Dedendum
Addendum Dedendum
this circle for centres of arcs of radius EC for the flanks of the teeth after marking out the tooth widths and spaces around the pitch-circle circumference.
Note that it may be more convenient to establish the length of the radius CE by drawing this part of the construction further round the pitch circle, in a vacant space, if the flank of one tooth, i.e. the pitch point, is required to lie on the line AO.
The construction is repeated in Fig. 24.19 to illustrate an application with a rack and pinion. The pitch line of the rack touches the pitch circle of the gear, and the values of the addendum and dedendum for the rack are the same as those for the meshing gear.
If it is required to use the involute profile instead of the approximate construction, then the involute must be constructed from the base circle as shown in Fig. 24.14. Complete stage 1 and stage 2(a) as already described, and mark off the tooth widths around the pitch circle, commencing at the pitch point. Take a tracing of the involute in soft pencil on transparent tracing paper, together with part of the base circle in order to get the profile correctly oriented on the required drawing. Using a French curve, mark the profile in pencil on either side of the tracing paper, so that, whichever side is used, a pencil impression can be obtained. With care, the profile can now be traced onto the required layout, lining up the base circle and ensuring that the profile of the tooth flank passes through the tooth widths previously marked out on the the pitch circle. The flanks of each tooth will be traced from either side of the drawing paper. Finish off each tooth by adding the root radius.
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