Spiral Bevel Assy

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 Verifying properassembly. Use themeasurementmethod to assemble both gears, making

sure that the matched teeth are properly engaged. Only after positioning both

gears at their mounting distances should

 you measure backlash to verify proper as-sembly. In cases where only the pinion is

marked with a mounting distance, posi-tion the pinion at its mounting distancefirst, then adjust the gear position for therequired backlash.

To obtain further verification, you canperform an optional contact patterncheck. This involves painting the toothsurfaces on one gear with a thin markingcompound and rotating the assembledgears under light load. The c ompoundtransfers from one gear to the other and

shows how the teeth contact. Numerouspublications contain charts with typicalcontact patterns (ANSI/AGMA 2008-B90and ANSI/AGMA 2005-B88). The patternsseldom look exactly like the published ex-amples and frequently require some in-terpretation.

Technicians familiar with contact pat-tern checking may believe that this tech-nique is the preferred method of assem-bly. However, the manufacturer hasalready performed this check as part of 

the testing (to determine mounting di-mensions) and interpreted the contactpatterns so you don’t have to.s

Figure 4 — Measurementsrequired for mounting bothoverhung and straddle-mountedbevel gears in a typicalgearbox.

POWER TRANSMISSION DESIGN s MARCH 1994 53

Backlashmeasurement

Normalbacklash

Top view of teeth

Tooth outer end

Pitch circle

Figure 3 — Normalbacklash in a set of

spiral bevel gears ismeasured perpen-dicular to the toothsurface at the outer

end (heel) of thetooth, using a

dial gage.

Straddle mountedgear subassembly

Assembled gearbox

Overhung mountedpinion subassembly

Shim thickness= MDV – VMD

Shim thickness= MDH – HMD

Housing

HMD

VMD

MDV

MAP

WP

MAG

MDG

WG

MDP 

MDH

MDH =

MDV =MD

G– W

G+ MA

G

MDP – WP + MAP