6.4 Tooth bearings
Regardless of how accurate the gear itself may be, poor tooth bearing not only causes oscillation and noise but also
have bad effect on gear’s life span.
Refer to Fig. 2. Extracted Tooth bearing on gear from JIS B 1741-1977 (old)
JIS B1741 (old)「Tooth bearing on Gear」stipulates percentage of tooth bearing mark as follows.
As for Tooth trace direction, it is percentage (%) of mean value bc of Length of tooth bearing for Effective lengthof trace - b’. As for Tooth depth direction, it is percentage (%) of mean value lc of tooth bearing width for Working
depth- h’.
Note∗ For edge of gear tooth with chamfering, Effective length of trace is after deducting chamfered area. For different Effective lengths of Tooth trace between Pinion and Gear, take the shorter side.
Tooth bearing percentage of Tooth trace direction =bc
× 100(%)
b'(a) Cylindrical gear
Spur gear

Tooth bearing percentage for Tooth depth =lc
× 100(%)
h'(b) Straight bevel gear
Spiral bevel gearb'bcb'bclc

lc

h'h'
Helical gearb'(c) Cylindrical worm gear pairWorm wheelbcb'lch'bc

lch'Fig. 2 Tooth bearing
Refer to Fig. 3 for Bevel gear with Crowning and empty load. It is desireable that centre of tooth bearing inTooth trace direction is about 60% of Length of tooth
trace from heel.

10060HeelToeFig. 3 Tooth bearing for Bevel gear with Crowning.

96Percentage of tooth bearing for Worm gear pair is for
Worm wheel engaged with Worm gear.
In general, Tooth bearing to inflow side of flank ofWorm wheel is not desirable. It is desirable for Tooth
bearing centre in Tooth trace direction to be biased
towards outflow side to make clearance at inflow
side. (Refer to Fig. 4)

Fig. 7 Engagement for Bevel gear with Crowning{Gleason Company, INSTALLATION OF BEVEL GEARS(1965)}
Coniflex® Bevel Gear
(Straight bevel gear with Crowning)Ⓡ mark is Gleason Works trademark

Fig. 4 Tooth bearing for Worm wheel
Revolving direction
for Worm gear(a)

Inﬂow side

Spiral bevel gear(b)

Outﬂow side

Fig. 5 Inflow clearance for Worm gear pair
｛A few problems of lubricating oil for Worm gear pairand research work for machine. Volume 8, No. 4 (1956)
written by Dr. Waguri and Dr. Ueno from Yokendo Co.
Ltd.｝

(a)
(b)
Fig. (a) shows proper assembly method, (b) is assembled off centre from location of Top cone. Pleaseobserve the difference in position for Tooth bearing.
Fig.8 Ideal tooth bearing for Bevel gear

Inflow
clearance

Upper side

Worm gear

Lower side

Fig. 6 Line of contact for Worm gear pair (2 numberof threads) and Tooth bearing for standard Wormgear. Quoted literature is the same as Fig. 5.

Upper side

Lower side

Spiral bevel gear
(Pinion: Shape of teeth is left hand)

Redir voecing
lv ion 270°
t

360°
270°
90°

Start

Upper side

180°

0°

End

180°

360°

ofcontact

90°

Lower side
Upper side

Lower side97

Table 4. P ercentage of tooth bearing for Cylindricalgear (Spur and Helical gears)
Class

Percentage of tooth bearing
Tooth trace direction

Tooth depth direction

ClassPercentage of tooth bearing
Tooth trace direction

Tooth depth direction

A

Above 70% of Effective length Above 40% of Effective lengthof Tooth traceof Tooth profileA

Above 50% of Effective length Above 40% of Effective lengthof Tooth traceof Tooth profile

B

Above 50% of Effective length Above 30% of Effective lengthof Tooth traceof Tooth profile

BAbove 35% of Effective length Above 30% of Effective lengthof Tooth traceof Tooth profile

CAbove 35% of Effective length Above 20% of Effective lengthof Tooth traceof Tooth profile

C

Above 20% of Effective length Above 20% of Effective lengthof Tooth traceof Tooth profile

Table 6. Percentage of tooth bearing for Bevel gear
ClassABC98

Table 5. P ercentage of tooth bearing for Worm gear
pair (Worm wheel)

Percentage of tooth bearing
Tooth trace direction

Tooth depth direction

Above 50% of effective length Above 40% of Effective lengthof Tooth traceof Tooth profile
Above 35% of Effective length Above 30% of Effective lengthof Tooth traceof Tooth profile
Above 25% of Effective length Above 20% of Effective lengthof Tooth traceof Tooth profile

Table 7. T able for Tooth bearing classification andSystem of accuracy
ClassSystem of accuracy for
Cylindrical gear

System of accuracy class for
Bevel gear

JIS B 1702-1960 (old)

JIS B 1704-1973

A1, 21, 2B

3, 43, 4C5, 65, 6