Dynamic stiffness analysis in tribocontact

This paper presents the methodology and results of analysis for pin-on-disc tribosystems for the “dynamic stiffness” between specimen and counterface materials. Dynamic stiffness is defined in terms of the tendency of the contact between pin and disc to be maintained in the event that separation of the triboelements occurs. Thus both the dynamic stiffness and the more straightforwardly obtained static stiffness are important in the overall behavior of tribotesters since system response is increasingly recognized as an important factor in friction and wear. In the present work, two designs for application of normal load are investigated. One of these is the “dead-weight” technique; the other is essentially the “governor mechanism”. With the dead-weight design the normal load is applied to a rotating disc through a pivoted lever. With the governor, instead of controlling speed by means of displacement, the normal force applied to the rotating disc is established by controlling the speed of an independently driven motor. With both systems the rotating disc contacts a stationary pin. In order to analyze the dynamic interaction between pin and disc it is necessary to model the pin and its supports, as well as the disc and the mechanisms which are used to provide the normal load. These models are represented in terms of lumped parameters. For purposes of analysis and of comparison between the two alternative designs, the initial conditions consisted of a nominal separation between pin and disc, and a zero approach velocity. In tribotesting, such an event might occur with the removal of a relatively large piece of wear debris. While unlikely as such, this initial condition allows comparison between the two configurations. This comparison then determines the dynamic stiffness for the two designs. Interestingly, the governor mechanism provides excellent dynamic stiffness as well as tunable static stiffness. The latter may be useful in a general sense in that friction and/or wear test simulations may be made to resemble closely tribocontact stiffness conditions which occur in practice.