I have had literally hundreds of requests for an explanation of this term, its seem to confuse and scare many people, so here goes.
Why the
confusion? - due in part, to the many names used to describe this
process, addendum modification, correction, addendum modification coefficient,
profile shift, long addendum, short addendum. These terms are often used
in the wrong context and most are a definite term in its own right.
Why modify? - addendum modification is applied
in order to reinforce the gear root especially to avoid undercut in pinions
with small numbers of teeth and also to decrease the relative sliding
speed and Hertzian stress of the mating flanks. Addendum modification
is therefore a simple means for improving the load capacity of gear pairs.
It is achieved by removing the active portion of the involute away from the base circle and so obtain better contact conditions due to increased radius of curvature and reduction in sliding contact.
During manufacture, the depth of insertion of the cutter determines the tooth thickness. In a pair of mating gears, if the cutter is inserted so that the gear reference diameter just touches the cutter reference line (where the cutter tooth thickness is equal to the cutter gap width), then 'standard proportion' gears are generated. This would have zero backlash for a given centre distance, however, it is usual to insert the cutter a little deeper to provide backlash.
This feature can also be used by the designer to provide correction, addendum modification or profile shift to increase the modification beyond the minimum necessary to avoid undercutting and also to apply it to pinions having more than the number of teeth at which undercutting commences.
The reason is to remove the active portion of the involute away from the base circle and so obtain better contact conditions due to increased radius of curvature and reduction in sliding contact.
Addendum modification is positive
if it increases the tooth thickness and negative if it decreases the tooth
thickness.
This modification or radial displacement of the cutting rack can
now be defined as a factor: Addendum Modification Coefficient
(x) = The amount by which the reference
line is offset from the reference diameter when divided by the module
mn.
i.e., x
= addendum modification/mn
Long addendum
- a gear with a positive addendum modification coefficient (x)
is said to have a 'long addendum'. Its value is defined as (1+x)mn
Short addendum
- a gear with a negative addendum modification coefficient
(x) is said to have a 'short addendum'.
Its value is defined as (1-x)mn
For a far more In-Depth view, see Profile and Lead Correction
