Page 142 - The Tribology Handbook

Instrument iewels A24

DESIGN

There are two important quantities which must be considered in designing a jewel/pivot system and in assessing its
performance. 'These are the maximum pressure exerted between the surfaces of the jewel and pivot, and the friction torque
between them. These depend on the dimensions and the elastic constants of the two components and can be determined
by the use of the nomogram. 'This is of the set-square index type, one index line passes through the values of the pivot
radius and the: ratio jewel radiuslpivot radius.
The second index line, at right angles to the first, passes through the value of the load on the jewel, and will then also
pass through the values of maximum pressure and friction torque. The example shown is where the pivot radius is 5 thou.
of an inch (0.127 mm), the ratio jewel radius/pivot radius is 3, and the load on the pivot is 27 grams. The resulting pressure
is 282 tons per square inch (4.32 GN/m2), and the friction torque 0.77 dyne-cm (77 nNm).

Loading Remarks

Stack It is generally considered that the crushing strength of steel is about 500 tons per square inch, and experiment has
shown that the sapphire surface cannot sustain pressures much above this without damage. If a safety factor of 2
is introduced then the maximum pressure should not exceed 250 tons per square inch, Unfortunately, an alteration
in jewel and pivot design aimed at reducing the pressure, results in an increase in friction torque and vice versa,
so that a compromise is usually necessary
-.
Impact All calculations have been based on static load on the jewel. SPRING
Impact due to setting an instrument down on the bench,
transport etc., can increase the pressure between jewel and
pivot very considerably, and in many cases the jewel is
mounted with a spring loading, so as to reduce the maximum
force exerted on it. In general, the force required to move the pL EWEL
jewel against the spring should not be more than twice the
static load of the moving system. This spring force must then
be taken as the load on the pivot
EWEL SCREW

A24.3