2.3.3.Testbendingfatiguestrengthcalculation
To
avoid fatigue rapture of teeth it’s required to assign target local
stress value 
on risky bending point of fillet surface to permissible stresses
:
.
The above statement is calculated separately for pinion and wheel. Design-basis local bending fatigue strength is to be defined per the following formula:
for wheel
,for pinion
,
where
–bending
load ratio;
–tooth
shape and stress concentration ratio;
–equivalent
number of teeth;
–tooth
gradient ratio; 
.
Designs of cylindrical gearings
The following guideline for the design of gearing should be assigned: pinions and wheels are to be made of thermo strengthened steel.
Figure 3. Design of reduction unit parts:
а) – Shaft-pinion with open tooth rim; b) – estimate design of reduction unit output shaft with plain section to be under the wheel and with further mounting of the latter to the shaft having large radial tension c), d) and e) – wheel design; f), g) – wheel design of small sizes up to 250 mm, with low step (в) and shallow pits (г); д) – single-disc wheel
Smaller
toothed wheel of the gear, pinion, can be made integral with the
shaft (shaft-pinion) or separately from the latter. The shaft-pinion
design is used for small (
mm)
and medium (
mm)
range reduction units as well as for mass production applications as
being more reliable and cost effective. The forgings are used as part
blanks. Figure 3 displays the example of shaft-to-pinion design.
The wheel comprises nave, rim and disc. Nave serves for wheel to shaft coupling and for transmitting the torque. Rim takes those forces which are applied on wheel teeth. The discconnectsrimwithnave.
The wheel configuration depends upon part blank fabrication method. It’s assumed that the blanks are produced by open die forging technique in the form of solid discs. In order to save labor consumption the wheels are made in the shape of massive discs having small pits alongside the flat end (Fig.3g) by reducing the volume of mechanical treatment, or with low step on the side of basic flat end (Fig.3 c). In particular, this is reasonable as far as small size wheels (up to 250mm) are concerned. Larger wheel sizes generally are made as of disc configuration (Fig.3e)
2.3.4. Reduction unit wheels layout method
The main purpose to initiate layout on is to define overall dimensions of the part, allowing to assess possibilities to allocate reduction unit within given mounting space. Overall dimensions are measured as length L, width B and height H (Figure 4) in approximation.
The layout is performed in the following sequence:
1. Axle
to axle space 
is measured and axles of both input and output shafts are drawn. For
this reason it’s 1 mm paper should be used. Scaleofthedrawingtobe1:1.
2
. Then the profiles of the wheels are drawn. Dash-and-dot lines
are made corresponding to pitch surface of pinion diameter 
and wheel diameter 
.
Figure 4. Single step reduction unit Layout beginning.
Overall
dimensions of toothed gears are drawn: the flat end surfaces of the
wheels are displayed, the distances between them assumed equal to 
,
then flat end surfaces of pinion are shown with spacing between them
as equal to 
;
then the surfaces of the wheel tips are drawn, which diameter to be
equal to 
for the wheel and 
for the pinion. In coupling one of tip
surface lines for instance of the pinion is drawn in dash-and-dot
lines; teeth pit surfaces are drawn, ensuring spacing between teeth
surfaces equal to 
mm
in coupling.
3. Upon
tooth rim width ratio being as 
and 
the length of the wheel nave can be taken as equal to wheel rim width
.
4. The gaps between running wheels and inner surfaces of housing walls are set as being as follows for example:
the gap between the surface of the wheel tooth tips and inner housing wall of the reduction unit
;
the gap between flat end of the wheel and inner housing wall of the reduction unit
;
thegapbetweeninputshaftaxleandsidesurfaceofinnerhousingwallofthereductionunitcanbesetpreliminaryequalto
,
upon further development of the design the position of side walls can be defined more precisely.
5. The overall dimensions L,B* and H are then defined.