design_factors

oving  systems  of  indicating  instruments
must  be  supported  to  allow  the  desired
degree  of  rotation  with  a  limited  amount  of

friction  and  eccentricity  between  fixed  and
moving  parts.  The  most  widely  used  means  for
supporting any type of instrument moving system,
be it  a moving coil, iron vane, or moving magnet,
is  that  of  a  jewel  and  conical-pivot  or  cylindrical-
shaft  construction.  The  shaft  or  conical  pivots  are
usually  rigidly  attached  to  the  moving  system  and
rotate between two jewel bearings. Such con-struc-
tion is also being increasingly applied in miniatur-
ized  devices  of  many  sorts.  This  article  will
describe differences of application of the two most
common types of instrument jewel bearing systems
and will discuss the design considerations of each.

Types Of Bearing Systems:

The two types of

bearings to which this discussion will be limited are
the vee-jewel and the ringstone,

Fig. 1

Vee-jewels are used extensively in electric in-

dicating  instruments  in  which  bearing  friction
torque  must  be  kept  to  an  absolute  minimum  in
order  that  instrument  accuracy  will  always  be
within  specified  limits.  For  true  indication  of  the
quantity  that  is  being  measured,  the  restoring
torque  on  the  moving  system  must  either  equal
the  electrical  torque  or  differ  from  it  by  a  con-
stant  amount.  Since  friction  is  not  a  constant
quantity,  the  bearing  friction  must  be  held  to  a

relatively low value for satisfactory instrument cal-
ibration. Commercial instruments utilizing the vee-
jewel  are  small  panel,  switchboard,  hook-on,
portable  and  laboratory  types,  and  exposure  me-
ters,  some  of  which  are  shown  in

Fig.

Fig

. 3

shows two typical moving systems supported by
vee-jewels.

Vee-Jewel Design:

If the weight of a moving

system  operating  with  the  shaft  vertical  were
supported  by  a  theoretically  sharp  point,  the  con-
tact  area  between  pivot  and  jewel  would  be  zero
and  therefore  friction  would  be  zero.  An  instru-
ment such as this would be very short-lived, how-
ever, because shocks and vibration which it would
experience in normal operation would soon flatten
or  distort  the  points.  This  would  produce  friction
much  greater  than  would  have  occurred  had  the
pivot  points  been  slightly  rounded  originally.  For
optimum  design  of  the  jewel  and  pivot  contour,
then,  the  two  major  requirements  of  minimum
friction  and  maximum  resistance  to  deformation
must be balanced against each other.

In many applications of electric indicating in-

struments  using  the  vee-jewel  system,  the  instru-
ment  can  be  operated  in  any  position—with  the
shaft  vertical,  horizontal,  or  in  any  intermediate
position.  It  can  be  shown  analytically  that  the
maximum  friction  error  exists  when  the  shaft  is
horizontal.  For  this  reason  the  remainder  of  the

DESIGN
DATA

Design Factors for Jewel Bearing Systems

By A. C. Lawson

Meter and Instrument Dept., General Electric Co., West Lynn, Mass.

Reprint from Machine Design

discussion of vee-jewels will deal with horizontal
friction only. In this position, as the moving system
is actuated, friction causes the pivots to roll up the
jewel wall until the wall becomes too steep to sup-
port rolling, and slipping occurs. This action is
illustrated in Fig. 4a. The maximum friction torque
resulting at this point is

T = µ W R

cos . . . . . . . . . . . . . . . . . . . . . . . .(1)

where

= Pivot radius, millimeters

= Maximum horizontal friction torque, gram-

millimeters

= Weight of moving system

= Coefficient of static friction between jewel

and pivot

Angle of pivot roll from neutral position to
point on jewel at which slipping occurs,
degrees

A more accurate expression for friction torque in

a horizontal axis system has been derived by
Nylander.

It involves another important design

consideration—end-play of the moving system. The
equation is

T = µW R

cos tan

-1

(2)

where

= End play with the pivot contacting the

spherical surface of the jewel, millimeters

= Jewel radius, millimeters.

Equation 2 is shown graphically in

Fig. 5

which,

for particular values of R21, W, and R1, shows
how end-play affects friction torque. Two interest-
ing observations can be made from this graph:

1. At end-plays of 0.001-inch and greater and a

coefficient of friction of 0.10 or less, the friction
torque is practically independent of jewel radius.

2. With the end-play reduced below 0.001-inch,

an appreciable decrease in friction torque can
be attained even with a relatively large
coefficient of friction.

However, Figs. 4b, 4c, and 6 illustrate that the

normal force at the contact area between jewel and
pivot  increases  rapidly  as  end~play  is  decreased
below  0.001-inch.  It  is  seen,  then,  that  minimum
end-play  cannot  be  the  only  criterion  in  reducing
friction  because  high  wedging  forces  accompany
small  end-play.  If  this  wedging  force  is  not  kept
within certain limits, serious pivot deformation can
occur  resulting  in  increased  friction.  A series  of
experiments conducted by Stott2 showed that for an
included  pivot  cone  angle  above  30  degrees  the
maximum  allowable  stress  increases  with  cone
angle.  In  addition,  the  rate  of  increase  of  bearing
pressure with decrease in pivot radius depends upon
the weight of moving system,

Fig. 7

cos sin

-1

( 1

)

Equations 1 and 2 indicate that for a given

bearing  system  the  friction  torque  varies  linearly
with  moving  system  weight,  provided,  of  course,
the  bearing  surfaces  are  clean  and  polished  so
that    does  not  vary  with  load.  In  most  cases,  the
rolling  angle

is small and Equation 1 reduces to

T = µ W R

p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(3)

In the design of an instrument using a vee-jewel

and pivot system, the coefficient of friction

and

moving system weight

known, the maximum

allowable  friction  torque  is  usually  known,  (it  de-
pends  primarily  on  the  instrument  sensitivity)  and
the pivot radius can be calculated. The jewel radius
is  made  a  maximum  consistent  with  holding  the
eccentricity of the moving system within allowable
limits and the contact stress within the elastic lim-
its of the materials used.

Shock-Resistant Jewel Mountings:

In certain

instrument  applications  shock  forces  sometimes
produce  stresses  in  the  jewel  and  pivot  which
would  exceed  the  maximum  allowable  stresses  if
the jewel were mounted rigidly. The most common
method  for  protecting  the  bearing  system  against
such forces is that of mounting the jewel on a heli-
cal spring. Th the majority of applications, this con-
struction is perfectly satisfactory. For conditions of
extreme  shock  and  vibration,  however,  a  recent
development using sponge rubber has been proved
superior because of its damping characteristics. As
an example, one small panel instrument design, to
meet an extremely stringent military specification,
must  withstand  severe  amplitudes  and  frequencies
of  vibration,  tumbling  in  a  steel  barrel,  and  nine
shock blows ranging from 1200 to 2000 ft-lb. The
acceleration present when such shocks are applied
is in the order of 1000

The basic design approach was essentially that of

protecting  the  most  vulnerable  part  of  the  instru-
ment,  the  moving  system.  This  was  accomplished
by  cushioning  each  jewel  with  a  small  piece  of
sponge rubber.

The internal portion of the jewel screw is so

designed  that  shock  forces  applied  in  any  direc-
tion  will  move  the  jewel  away  from  the  pivot,
Fig.  8.  The  jewel  moves  due  to  its  own  inertia
rather  than  from  armature  acceleration  force

directed through the pivot. When the shock force is
along  the  axis  of  the  pivot,  the  jewel  moves  as  a
result of the blow. The pivot is restrained to a trav-
el of 0.010-inch by a shoulder on the pivot support.
The action of the soft sponge rubber is to damp the
jewel  motion  severely,  thereby  preventing  it  from
rebounding  onto  the  pivot.  Shock  forces  applied
perpendicular  to  the  axis  tend  to  tip  the  jewels,
allowing the pivot to strike the jewel on the periph-
ery of its cone rather than the tip. The armature is
restrained to a travel of 0.010-inch in this direction
also.

Ringstone and Endstone Bearing Systems:

moving system using ringstone jewel bearings has
higher friction torque than a comparable vee-jewel
system  because  of  the  larger  contact  surface  be-
tween  pivot  and  jewel  Some  types  of  electric  in-
struments can tolerate higher friction torque, how-
ever.  Ringstones  can  therefore  be  used  to  advan-
tage  in  producing  much  more  sturdy  bearing  sys-
tems.  For  the  same  weight  of  moving  system,  the
one supported by ringstones has a much lower bear-
ing pressure than the vee-jewel.

Ringstones are used in preference to vee-jewels

under the following conditions:

1. When bearing friction is negligible compared

with  the  active  electrical  torque  or  with
other  friction  or  error-producing  torques.
An  example  is  a  recording  instrument  in
which  the  moving  system  drives  a  pen  across
a  chart.  Here,  the  friction  of  pen  on  paper
far  exceeds  the  friction  present  in  the  bear-

ings.

2. When the instrument in normal operation is

subjected  to  continuous  vibration,  thereby
reducing  bearing  friction  torque.  Aircraft
instruments  mounted  on  an  airplane  panel  are
an example of this application.

3. When the mechanical construction requires that

the shaft extend through the bearing.

4. When side-play of the moving system must be

extremely small.

There are various types and combinations of ring-

stone systems, as illustrated in

Fig. 9

. In instru-

ments such as recorders where the shaft is always
vertical,

Fig

. 9

, a ringstone-endstone combination

is used for the bottom bearing, while the top bear-
ing can be either a ringstone or vee-jewel.

Fig

. 9

shows  horizontal  shaft  ringstone  bearing  systems.
Certain applications require lower friction than can
be  obtained  by  the  straight-hole  ring-jewel.  The
olive-hole  stone  and  the  "bombe"  type  are  refine-
ments  which  give  lower  friction  by  virtue  of  the
smaller pivot to jewel contact area. The bombe type
is used in very special applications where minimum
thrust as well as journal-bearing characteristics are
required.

It is possible to hold the side-play of a moving

system employing ringstones to a lower value than
can be obtained with vee-jewels.

Fig

. 10 shows

dimensions of a typical ring jewel. Note that the
tolerances are given in ten-thousandths of an inch.
The shaft can, of course, be made to the same

Jewel Materials:

Jewels of various materials, such

as glass, sapphire, synthetic spinel, titanium oxide,
beryllium  copper,  brass,  etc.,  have  been  tried  and
tested. Depending upon the moving system weight,
it  has  been  determined  that  borosilicate  glass  and
sapphire  afford  the  optimum  in  initial  friction,
resistance to impact, and long life.3

In recent years, glass has become a substitute for

sapphire  in  some  vee-jewel  applications,  partic-
ularly  small  panel  instruments.  As  a  result  of  an
extensive  development  program  during  the  war
years,  when  the  accessibility  of  sapphire  jewels
from Switzerland was uncertain, glass was sub-sti-
tuted  for  sapphire  in  vee-jewel  applications  where
the  total  moving  system  weight  is  less  than  0.75-
gram.

The 0.75-gram value for glass vee-jewels has

been empirically derived for rigidly mounted jewels
and  results  from  an  exhaustive  series  of  compar-
ative tests on impact and shock resistance of glass
versus  sapphire.  Of  course,  this  moving  system
weight  limitation  can  be  exceeded  if  the  jewel  is
shock-mounted.

Pivot Materials:

The most common pivot mate-

rial for use with vee-jewels is high carbon tool steel
in the form of ground and polished drill rod. After
hardening to a minimum Knoop hardness of 690,
the pivot tip radius is generally put on by tumbling.
Since in ordinary usage the pivot may be subjected
to very high tensile, compressive, and impact
forces, the microstructure of the hardened pivot is

closely controlled.

Some vee-jewel applications require nonmagnet-

ic pivots to reduce the errors produced by magnetic
hysteresis. In such cases monel metal, stainless
steel, or tantalum shanks with osmium tips are
used, the osmium giving a hardness comparable to
the high carbon steel. Pivots of 92 1/2 percent tan-

talum, 71/2 per cent tungsten are also used for non-
magnetic qualities.

Shafts for use in conjunction with ring jewels are

cobalt-tungsten, monel metal, stainless steel, beryl-
lium copper, tantalum, or high carbon steel.

For all pivot applications the material must be

highly polished, and free of surface scratches or
flaws for minimum friction and longest life.

Summary:

Most electric indicating instruments

in use today have a jewel-pivot bearing system. In
applications where minimum friction consistent
with safe loading is the prime factor, vee-jewels and
conical pivots are used. When extremely low mov-
ing system friction torque can be sacrificed in favor
of higher strength or mechanical construction
advantage, ringstones are used.

REFERENCES

1. A. L. Nylander— “Instrument Bearing Friction”,

General Electric

Review

, July, 1946.

2. V. Stott— “Pivots and Jewels in Instruments and Meters,”

Col-

lected Researches of National Physical Laboratories,

Vol. 24,

Paper No. 1, 1931.

3. P. K. McCune and J. H. Goss—“A New Jewel for Indicating

Instrument,”

AIEE Trans

., Vol. 61. 1942.