Page 140 - The Tribology Handbook

Instrument jewels A24

MATERIALS
~
Uswl combinations S’cial pre-cautions
The most usual combination is that of a steel The sapphire crystal has natural cleavage planes,
pivot and a synthetic sapphire jewel. The steel and the optic axis, i.e. the line along which a LOAD
must be of high quality, hardened and tem- ray of light can pass without diffraction, is at LINE
pered, with the tip highly polished. The jewel right angles to these planes. The angle between
also must be highly polished. Diamond jewels this optic axis and the line ofapplication of the
are sometimes used for very heavy moving load is called the optic axial angle a, and
systems. A. slight trace of a good quality lubri- experiment has conclusively demonstrated
cating oil such as clock oil or one ofthe special that, for the best results as regards friction,
oils made for this pu’pose, improves the per- wear etc., this angle should be 90 degrees, and
formance considerably any departure from this produces a deteriora-
tion in performance

0 PERATI N G CON DlTlONS

Arrangmmt Remarks
Vertical shaft The pivot is cylindrical with a spherical end. The
jewel is a spherical cup- This is used, for example, in
compasses and electrical integrating metres. The
AXIS optical axial angle can be controlled in this case

[a)

The pivot is cylindrical ending in a cone with a hemi-
spherical tip. The jewel recess is also conical with
a hemispherical cup at the bottom of the recess.
This is used in many forms of indicating instrument
and again the optic axial angle can be controlled

Horizontal shaft The pivot and jewel are the same as for the vertical
shaft. In this case the optic axial angle cannot be
controlled since the jewel is usually rotated for
adjustment, so that the load on the jewels must be
reduced in this case

(C)

A24.1