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Technology of watch production - part 41

In the case of pinion teeth the main reason for the replacem ent of the

epicycloid is the need to reduce the angle of approach, which in turn makes

it possible to inc reas e the efficiency of the gear transmiss ion.

FIGURE' 3. Replacement o f the epicycloidal arc by a
circular arc

FIGURE 4. Profiles of pinion teeth tips

The line of action of a m eshed pair is the locus of the points of contact

of the two mes hing contours on a given plane .

In the case of involute teeth the line of action is the straight line pass ing

through the pitch point

and tangent to the two base circles (Figure 5, a ) .

The s egment

is the active portion of the line o f action .

In the case of cycloidal tooth contours the line of action cons ists of two

arcs of the generating circles

A P

, meeting at the pitch point

(Fig

ure 5, b ) .

The follower face and the driver flank a r e i n action till the line of centers

01 02

is reached, while the driver face and the follower flank are in action

beyond the line of centers .

The augles through which the driver and follower teeth move from the

beginning of their contact to the line of centers are called t h e a n g l e s o f a p

p r o a c h , while the angles f rom the line of centers to the end of contact are t h e

a n g l e s o f r e c e s s i o n. In the case of cycloidal gearing the angles of

approach are

cx1

and

ctz

and the angles of recession are

�1

and

�z

(Figure 5 , b ) .

1 6 1

The sum of the angles of approach and recession for each wheel must be

equal to or larger than its angular pitch:

IX + p

where

is the

number of teeth on the wheel .

The values of the angles of approach and recession fo r the basic gear

pairs of watch mechanis ms are given in Table 1 . If thes e gear pairs had
teeth with involute contours, the angles of approach would have been be

tween 2 0 to 2 5 ° . The advantage of the watch gearing over involute tooth
form gearing is clear considering the fact that the friction losses decrease

with decreas ed angle of approach.

FIGURE 5.

Line of action :

a-involute profiles; b-cycloidal profiles.

TABLE

Angles of approach and recession in watch gearing (shortened)

Number ofteeth :

Angular pitch

Angle of

Pinion tooth form

lvheel -

zw:

of pinion

approach

recession

(ace. to Fig.4)

pinion- zp

Zw = 60

Zp = 6

60°

17°44'13"

42" 1 5'47"

Zw = 70

5 1 °25'43"

1 1°30'28"

39°55' 1 5"

Zp = 7

Zw = 60

Zp = 8

45°

7°23'40"

37°36'20"

Z w=

45°

7°17'30"

37°42'30"

Zp = 8

Zw = 80

45°

6°59'5"

38°0'55"

Zp = 8

Zw= 75

360

1 °20'7"

340S9'53"

Z p = 1 0

Zw= 80

36°

1°14' 1 2"

34°45°48"

Z p = 10

1 6 2

In watch gearing the line of action will deviate from the theoretical

curve

( Figure 5, b ) at those s ections where the circular arcs do not

coincide with the epicycloids . This deviation disturbs the trans mission

ratio of the meshing pairs (within the limits of one pitch ), and in consequence

the moment transmitted is altered and the accuracy is disturbed. This is
the main s hortcoming of watch gearing.

In order to reduce this deviation to a minimum , the active portion of the

wheel tooth p rofile

(see Figure 3) and of the pinion tooth p rofile (Fig

ure 4 , shape

C),

are made to approximate the epicycloid as clos ely as pos

sibl e .

The basic paramete rs of watch gearing a r e given i n A ppendix 2 (Standards

of the Glavchasprom (Watch Industry C ente r ) ) .

The tolerance o n the center distance must b e narrower f o r cycloidal and

watch gears than for involute gears .

Deviations in the c enter dis tance will produce , in the case of cycloidal or

watch gearing, a contact either between two hypocycloids or two epicycloids

and this obviously will disturb the trans mis sion ratio . In involute gears

the transmission ratio is not affected by s uch deviations .

The influence of radial wheel run- out o r an increas e or decreas e in its

diameter is s imilar to that of a deviation in the center distance . A ccordingly,

narrow tole rances are fixed for thes e param eters .

Jewel bearings are used in watches in order to reduce the friction los ses

in the b earings and to increas e the efficiency of the gear transmis s ion.

Photographs of a wheel, pinions , and a claw clutch are given in Figure 6 .

FIGURE 6. Toothed watch parts

a-chronograph wheel; b-third-wheel pinion; c-cannon pinion; d-claw clutch.

Wheels and pinions have spur teeth. The claw clutch and the winding

pinion have face teeth - formed and angular .

I n order t o obtain high performance from the gear transmission i t is

neces sary that the wheels and pinions have accurate dimensions and that
the wheels have a class 8 or

s urface-finish quality, and the pinions a

1 6 3

class 1 1 to 1 3 finish. In order to ensure accurate center distances in

plates and bridges , the holes are sized (see Chapter VI ).

TOLERANCES ON GEARING ELEMENTS

Accuracy classes and tolerances for 2 0° pressure angle involute gearing

elem ents of module 1 mm and above have been established by GOST 1 6 4 3 - 4 6 .

Only departm ental standards exist for involute gearing of module 0 . 1 5 - 1 m m .

Tolerances us ed i n watch industry for watch gearing correspond to the

departmental specifications NPM5 - 24 6 - 5 3 (see Table 2 ).

The departmental specifications establish tolerances for six gearing

elements . The table has been prepared so that the numerical values of the

tolerances for

/lD

!lA

and

are taken with a geometrical -progression

factor of 1 .4 1 , and for

!ld , /ls

and

with a factor of 1 . 2 6 .

Drawing

TABLE 2

Tolerances for basic watch-gearing elements, in microns

Tolerances

Maximum run-out,

outside diameter

Tolerance, external

diameter

1 6 4

Symbol Accuracy

class

lld

llD

Module, mm

a ;;!

..... �

..-<

0 �

E o

..... �

1 5

0
C'>

' o

o .,

E o

� 8

1 9

2 1

Content .. 39 40 41 42 ..