Page 300 - Modular design for machine tools

Engineering Design Fundamentals and Single Flat Joint Characteristics 259

40 Tellus 400
(Redwood 320 s at 22°C)
Saxonal grease
36 Vitrea oil 360
(Redwood 4750 s at 22°C)
32 Dry 320
Joint stiffness 10 6 lbf/in 24 240
280
28
200
20
160
16
12
80
8 120
4 Shaped 40 Scraped
temp. 25°C temp. 27.5°C
0 0
75 125 188 250 312 375 75 125 188 250 312 375
Joint preload, lbf/in 2 Joint preload, lbf/in 2
2
Note: Change in magnitude of joint stiffness axis. Mild steel, frequency 90 Hz, area 36 in .
Figure 6-31 Effects of lubricants on joint stiffness (courtesy of Thornley).

3. Surface topography. The dry rough joint has, in general, lower in-
phase and higher quadrature components compared with those of the
smooth joint. As a result, the smooth joint is stiffer than the rough
joint, provided that the joint surfaces have no flatness deviation.
4. Apparent area of contact. Apart from some special cases, i.e., the joint
under higher preload, the apparent area of contact has no effect on
the dynamic joint stiffness.
5. Effects of planform shape. When the apparent joint area is kept con-
stant and the mild steel specimen with shaped dry surface is used,
the planform shape has very little influence on the stiffness, whereas
the surface topography has considerable influence.

Remembering that the magnitude of the interface pressure in these
earlier research activities ranges to various extents, the dynamic behav-
ior of the single flat joint can be summarized as follows.

1. When the static preload increases, the dynamic joint stiffness
increases whereas damping decreases.
2. The interface layers, such as oil and grease, increase both the dynamic
stiffness and the damping capacity. The oil viscosity shows extreme
effects on the increase of stiffness: the more viscous the oil, the
greater the increase of stiffness.