Technology of watch production - part 104

Variation in bearing friction.

The largest variations in the amplitude

occur in the following two cas e s : 1 ) when the watch position is changed
from horizontal to vertical; 2 ) at maximum and m inimum spring tension.

The decrease in amplitude accompanying a change from horizontal to

vertical  pos ition  of the watch  is  due  to  the  increase  in the friction moment
in  the  bearings .  In  the  horizontal  position  (dial  up  or  down )  the  balance
staff  rests  with its  sphe rical  heel  on  the  endstone  face  and the  friction  m o
ment i s   very  small  be caus e  of the  very  small  friction  radius .  In  vertical

pos ition of the watch the balance rests with its two staff j ournals on the

cylindrical surface of the jewel bearings . The friction m om ent is much

larger than in the first case because of the much larger friction radius ,

equal to the j ournal radius .

In order to reduce the difference between the friction mom ents at hori

zontal and vertical pos itions of the watch, the j ewel hole is rounded, using

a radius equal to two hole diam eters , and a s m all land is formed on the
balanc e - s taff heel . The rounding of the j ewel hole al so reduces the influ

ence of the oil on the watch m ovem ent.

Variation in the moment transmitted.

It was m e ntioned earlier that

change  in the  mom ent  trans m itted  from  the  mainspring causes  a  sharp  var
iation  in amplitude .   The  m oment  of the  m ainspring in its  m iddle  working
range  is  roughly proportional  to  the  deflection  angl e .   The  value  of the  am

plitude for a completely wound spring (4 to 5 turns ) and good as s e mbly is

2 8 0 - 3 2 0 ° . I t drops t o 1 4 0 - 1 7 0° when the sp ring i s wound 1 t o 1 . 5 turns .

Watches adjusted at 2 to 2 . 5 turns will the refore advance at the beginning,
and lag at the end (after 24 hours ) . Interference with is ochronism is most

FIGURE 56. Balance with dis-
placed center of gravity

clearly apparent in this last cas e . In o rder to

reduce the difference between the two winding

extre m e s , the maximum and minimum m om ents
of the spring are m ade as near to one another as

pos sible: the maximum difference between them

must not exceed 2 5 "/o .

Pocket-and wris twatch springs are calculated

for a minimum of 45 hours watch ope ration per

winding.

Unbalance of the balance-hairspring system,

or displacem ent of the center of gravity relative

to the axis of revolution, is one of the main factors
p roducing the s o - called pos ition e rror in watches .

This displacement occurs in spite of the high ac

curacy of manufacture of the balance and hairspring

(geom etric dimension s ) , and in spite of the s tatic

p01smg. The displaced c enter of gravity

(Figure 5 6 ) causes an addi

tional  m om ent  during balance  rotation  which  influences  the  watch  in a  man
ner which  depends  on its pos ition.  A  rule  exists  for the  determination  of
the  pos ition of the  center  of  gravity  of the  balance:

1 . If, when the balance passes through its equilibrium position, its

center of gravity is situated below the axis of revolution, the watch will
advance at an amplitude of 1 4 0 to 1 7 0° , and lag at an amplitude of 2 8 0 to

3 2 0° .

2 . If, when the balance passes thrr ugh its e quilibrium pos ition, its cen

ter of gravity is s ituated above the axis of revolution, the watch will advance
at an amplitude of 2 8 0 to 3 2 0°, and lag at an amplitude of 1 4 0 to 1 7 0° .

4 1 3

If the center of gravity is situated on the ho rizontal diam eter, the watch

will not have a pos ition error, as can be seen from the formula

(J) = -

43 200 ·

· COS

�

(If>),

where

(J) =

daily (24 -hour ) watch e rror;

balance weight;

( 7 )

distance between the c ente r o f revolution o f the balance and its
cente r of gravity

moment of hairspring when it is wound one radian;

�

angle betwe en a vertical line passing through the balance c enter
and the ray passing through the balance c ente r and its center of

gravity;

(<1>)

an alternating slowly convergent series arranged according to
increasing values of the amplitude .

The function

(<1>)

is shown in Figure 5 7 . The daily e rror will b e zero

in the following three cas e s :

1 . for

�

90°;

COS

�

= 0;

(J)

= 0;

2 .

for l = 0

w = O;

3 . for

<1> = 220°; S {<t>)

= 0; (1) = 0.

The daily e rro r will be smaller, the s maller the balance weight and the

higher the hairspring moment . It is s e en from Figure 5 7 that small devia
tions of the amplitude value from the value at point

<1>

2 2 0° cause large

changes in the function

(<1>)

and the refo re in the daily erro r . It i s the re

fore more exp e dient to ope rate in the amplitude range from 3 0 5 to 3 2 0° ,

since a variation i n the amplitude over this range has alm ost no influence
on the value of

S (<1>)

, and therefore on the daily erro r .

li-0 80

120

1oo 200 2w 280 JZO Joo·

----

FIGURE 57. The function

(<%>).

The unbalance of the balance wheel is inspected in the following watch

positions : c rown up , crown right, c rown down, crown left. The influence
of the escapement unbalance and clearance fluctuations on the watch e rror

4 1 4

is  s imultaneously inspected in  each  of  these positions .  The  adjustm ent
technique  in the  pos itions  indicated  is  as  follows :  the balance  screws  are
arbitrarily  numbered  as  shown in  Figure  5 9 ,   the  numbering beginning from

the s c rew oppos ite the hairspring stud in the equilib rium pos ition of the

balanc e .   The  spring is  wound  1  to  1 . 5  turns,  and
an  amplitude  of  1 4 0  to  1 7 0°  is  s e t .   Time  indica
tions  from  the  P- 1 2   instrument in  each  of  the  pos i
tions  are  recorded and  the  position  of the  c ente r

of gravity, or the heavy balance sector, is found

from the reco rding. The balance wheel is pois e d

b y reducing the weight o f the screw in the heavy

zone or increas ing the weight of the opposite s c rew.
This is done by placing, or removing, adjusting

washers located under the s c rew heads . Dim en-

sions for the wa shers us ed in "Zvezda " and "Pobeda"

FIGURE 58. Balance-adjusting

balances are given in Figure 5 8 . Washers of var -

washers

ious thickness are available . Two was hers 0 . 0 1 mm

thick will make the watch advance (or lag) 2 0 sec

in 2 4 hours; 0 . 02 m m and 0 . 0 3 m m thick washers will p roduce 40 and

60 sec effects respectively, etc .

Example of the adjustment of a " Pobeda" watch.

The following daily errors were recorded in tests on a P-12 instrument :

At small amplitudes ( 140 - 1 70.) :

Dial up -20 sec, advance ;
Dial down -26 sec, advance ;

Crmvn up - 60 sec, advance ;

Crown right - 80 sec, advance;
Crown down + 40 sec, lag;

Crown left + 60 sec, lag.

2. At large amplitudes (280- 320°) :

Dial up -35 sec, advance ;
Crown up + 30 sec, lag .

T h e difference between the errors i n the two horizontal positions (small amplitudes) is :

- 26 - (

- 20)

6 sec ( advance).

The average error in the two horizontal positions is :

- 20 + ( -26)

23 sec (advance).

The average error in the four vertical positions (small amplitudes) is:

( -60)

(

= -

1 0 sec ( advance).

Using a special table we find that the weight of screw No. 4 should be reduced.

Since the average error is negative in the two positions, the watch will advance. This should be taken

into  account  when re-poising the balance.  If we  reduce the  weight  of the screw  in the heavy sector,  we  also
reduce  the  moment of inertia of the  balance,  and the re-poised watch will advance  even  more.  In  such
cases the inverse  procedure  is used,  of adding to the weight of the opposite  screw ;  this will both poise the

system and improve its oscillation rate. As a rule, the displaced center of gravity does not coincide with the

position of any of the screws and it is usually necessary to reduce or increase the weight of t\vo adjacent screws.

The c enter of gravity of the balance is most conveniently dete rmined by

means  of  a  differential  instrum ent .  A  nomo graph  cons isting of  coordinate
axe s ,   a  number of concentric  circles ,  and a  diagram  of the  balance  and
s c rews  of  "Pobeda"  watches ,   is  trac ed on the  instrument  scale  (Figure  5 9 ) .
The  origin of the  coordinates  is  located at  the  balance  cente r,  and  the  axes
are  directed  along the  lines  connecting  sc rews  1  and  9,  and  5  and  1 3 .   This
selection  of the  axes  corresponds  to  an  equilibrium  position  of the  balance
with the  c rown  up .  Points  of  inte rsection  of  the  axes with the  conc entric

4 1 5

circles  are  marked  in  seconds  and  in  mm  (adjusting-washer  thickne s s ) .
Two  "mobile "  coordinate  axes  (m etal  wire s )  are  m ounted i n  front o f  the
scal e .

The instrument panel ha s four knobs , located i n two pairs and provided

with scales with

24 divisions each, numbered 0 - 2 0 - 4 0 - 6 0 - 8 0 - 1 00- 1 2 0 sec

to the left and to the right. A diagram of the watch in the appropriate test

position in the

P- 1 2 instrument appears below each scal e .

Balance, view from the cock side

FIGURE

59.

Instrument for the determination of the magnitude and direction of displacement of the center

of gravity of the balance-hairspring system

The right- hand knobs are kinematically connected with the horizontal

m obile  coordinate  axis  and  the  left- hand  knobs  are  connected with  the  ver
tical  axi s .   Rotation  of  a  knob  b y  a  definite  number  of  s e c onds  o n  its  s cale
produce s  a  displacement  of  the  co rresponding mobile  axis  on the  nomograph
by  the  same  number  of  seconds .   Each  knob  displaces  the  axis  it  is  asso

ciated with independently of the other. Daily errors of up to

2 min can be

marked in each of the four pos itions on the nomo graph.

The cente r of gravity of the balance is determined as follows :

1 . The differenc e between the daily errors in two opposite directions

(for instanc e , c rown up and crown down ) is determined in the

P- 1 2 instru

m ent at an amplitude of

1 4 0 to 1 7 0° .

4 1 6

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