Contact times mechanical definition

Fig. 17 Dwell time and contact time—mechanical definitions.

The book contains two parts each part comprises six chapters. Part I deals with basic relationships and phenomena of gas-solid flows while Part II is concerned with the characteristics of selected gas-solid flow systems. Specifically, the geometric features (size and size distributions) and material properties of particles are presented in Chapter 1. Basic particle sizing techniques associated with various definitions of equivalent diameters of particles are also included in the chapter. In Chapter 2, the collisional mechanics of solids, based primarily on elastic deformation theories, is introduced. The contact time, area, and... 

Functionally, the effective dwell time (EDT) at 90% level can be defined as the time it taken by the forcetime curve to traverse the 90% of the peak height. Likewise, the effective contact time (ECT) is the time between points at 10% of the peak height. The shape of the force curve depends, as we know, on the deformation mechanisms of the powder and therefore, all other variables being equal, EDT and ECT will be different for brittle and plastic materials. Although somewhat useful for material characterization, such variables should not be confused with the universally accepted definitions of contact time (time of contact) and dwell time (time of immobility). 

Mechanical Definitions of dwell and contact times disregard material properties and concentrate on press and punch geometry (Fig. 17). Contact time can be defined as the time the punch is in contact with the compression wheel. Dwell time is defined as the time the flat portion of punch head is in contact with the compression wheel (time at maximum punch displacement, or time when the punch does not move in vertical direction). In dwell time calculations, the length of the punch head flat and horizontal component of punch speed (as determined by RPM and pitch circle diameter) are used. In case of a round head tooling, the dwell time, as defined here, is zero. But it should be kept in mind that mechanical definition is given here as a convention, a yardstick, or a common measure, to compare press speeds for different presses, and its absolute value is meaningless. A proposed convention to quantify linear speed of a press is to use an... 

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