Page 284 - Modular design for machine tools

Engineering Design Fundamentals and Single Flat Joint Characteristics 243

• Realization of the line action of each force on the body. The lid and cup
configuration can facilitate preferable loading passing through the
center of mass, i.e., center of gravity of the body.
• Applying normal preload with flexible cable and dead weight.

These remedies are favorable to ensure the accuracy of the experi-
ment, although they give rise to some difficulties in machining the joint
surface of the lid.

6.3.3 Peculiar behavior of microslip
In relation to the flat joint under preload and tangential loading, at fur-
ther issue is the deflection- (displacement-) dependent characteristic of
the microslip. In accordance with the general sense, the macroscopic-slip
(gross slip) occurs when the following condition is fulfilled, i.e., the rule
of Coulomb friction.
P > Q (6-4)
h
where P tangential load
h
Q normal preload
macroscopic coefficient of friction
This famous principle can also be accepted at the machine tool joint; how-
ever, we must be aware that the microslip is allowed even when the exter-
nal applied load is less than the friction force, i.e., P < Q. Actually, a
h
microslip occurring prior to the start of the macroscopic slip, which obeys
Eq. (6-4), is one of the most characteristic features of the machine tool
joint, and determines definitely the damping capacity of the joint. To dis-
tinguish this microslip, obviously it is better to use the term tangential
force ratio instead of the coefficient of friction under P < Q. 11,12
h
T
In short, the tangential force ratio is equivalent to the coefficient of fric-
tion in the condition of the microslip, and its utmost characteristic fea-
ture is of displacement dependence, as reported first by Courtney-Pratt
and Eisner [29]. Figure 6-20 shows a relationship between the microslip

11
Kirsanova [19] reported that the tangential force ratio is, in general, around one-half
of the static coefficient of friction. For example, in the joint finished by very fine scrap-
ing, the tangential force ratio and static coefficient of friction are 0.14 and 0.28 in dry con-
dition, and furthermore 0.12 and 0.24 in lubricated condition, respectively.
12
Although we don’t have the relevant definition of the microslip, we have the term slip
damping, which can be observed even in the press fit portion of the turbine blade. A
marked suggestion in it is the existence of the optimum pressure, at which the damping
capacity is maximum. In fact, damping of the two-layered beam has been investigated (see
Chap. 7).
Goodman, L. E., and J. H. Klumpp, “Analysis of Slip Damping with Reference to Turbine-
Blade Vibration,” J. Appl. Mech. ASME, Sept. 1956, pp. 421–429.