Page 16 - Gear Technology Solutions

1 Dimension Sheet Explanation Update

1.1 What Items have been Added?

Several output items have been added to the Special Methods Dimension-
Sheet in GEMS. Some of those items are known from the traditional CAGE
Dimension-Sheet [1], others have been added in order to account for straight
bevel and face gears as well as for super reduction hypoid gears (SRH). In this
manual the new output items are explained and illustrated.

1.2 Tooth Thickness Convention for Straight Bevel Gears

Of particular interest for manufacturers of straight bevels gears is the tooth
thickness convention and definition. Many straight bevel and spiral bevel gear
manufacturers lean in their strength calculations and documentations towards
ISO, AGMA, DIN and other national standards. All of those standards have in
common to define the tooth thickness at the pitch point in the normal direction.
Presently it is common for bevel and hypoid gears to calculate the tooth
thickness as the relationship between the angular location of the convex and
the concave flanks. This relationship is transferred with the CMM download file
to the metrology device. After the measurement of one or more teeth (or slots)
the average difference between the flank locations according to the tooth
thickness angle is shown as “Tooth Thickness Error” in linear dimension. That
means, the tooth thickness is expressed as an arc around the rotational axis of
the gear and the tooth thickness error is a linear dimension along the same
arc. Although this convention works extremely well for tooth thickness
corrections, it does not reveal the normal chordal tooth thickness and the
normal chordal tooth thickness error. In addition, the tooth thickness
measuring points generally do not match the pitch line and are not exactly
located at the mean face positions as defined by the standards. The reason for
this is that the CMM grid center point is per definition the tooth thickness
reference point. Depending on the applied edge reductions, the grid center
point will not match the flank center and its relation to the pitch point cannot
easily be revealed due to the curved lead of spiral bevel gear teeth. A
universal solution for this problem has been developed in the GEMS flank form
and CMM grid generator in connection with an evaluation applet in the
Dimension-Sheet software.

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