General Contact Conditions

Is the Part Lubricated If it is, or could be, then wear, even without surface engineering, might be reduced by a factor of 1000 compared to running unlubricated. In lubricated systems under high load and at high speed (e.g., cams and tappets, piston bores and rings) there is still the possibility of scuffing. 

Is the Part Unlubricated yet There is a Need to Reduce Friction  

For dry sliding it is important to specify the exact requirements. A low friction coefficient (see Chapter 3) can be defined as 0.1 or less and is generally achieved with polymers such as PTFE, but these have high wear rates. If, without surface engineering, the friction would be unacceptably high, for example, galling between two soft steel parts, then most surface treatments will reduce friction as well as reduce the wear. 

Is the Specific Loading High Lx ads above 100 MPa (14.5 ksi) are considered high, in which case the hardness and thickness, or case depth, of the surface treatment is the critical factor. Both the substrate and the coating must be able to withstand that load. It is important to remember that rolling parts are often under high specific loading. 

What are the Requirements for Reducing Wear In general, the higher the hardness of the surface layer, the lower will be the wear. It is vital to understand the consequences of the wear for example, it may be that an increase in clearance between two parts must be avoided in service. If the wear is concentrated in a small area, then even a low wear rate will lead to a rapid increase in the clearance, and a high surface hardness is needed in that area. If the wear is spread out over a wider area the corresponding increase in clearance will be smaller, and a simpler, less expensive solution may be adopted. 

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Thus, in view of the dominant contribution of contact conditions between the components of the macroscopic properties of an medium, the general problem was again reduced to a problem of bonds. The main ensemble of bonds 0 was derived by an iteration process in which the initial step (k = 0) involved... 

More generally, the condition of a plug depends both on how long (in terms of contact time, s) it has been in the reactor and at what time (in terms of clock time, t) into the run it entered the reactor. Thus we have the following rate and temperature equations ... 

The adsorption of solvents on activated carbon " is controlled by the properties of both the carbon and the solvent and flie contacting conditions. Generally, the following factors, which characterize the solvent-containing waste air stream, are to be considered when selecting the most well suited activated carbon quality for waste air cleaning ... 

The purpose of these examples is to illustrate the modelling principles presented above. For each example, the free and constrained vector spaces are defined, and the known and unknown components of the end effectw acceleration and general contact force vectors are specified. From this information, the type or class of the contact may be identified. We begin our discussion with a simple contact condition, a rigid connection. 

Ihe following discussion shows how to find solutions for more general boundary conditions (1) fixed contact angle at the arbitrary value and (2) fixed contact line at the arbitrary position. Since the current scope is limited to the small interface slope, the Yoimg-Laplace equation can be linearized (Eqn. (3.4)). For the inclined interface, one can seek a superposed solution ... 

We have also allowed the boundary regions to depend on the component being specified. This is required to cover examples such as frictionless contact. These are not the most general boundary conditions that can be conceived. For example the case of frictional contact is not even covered by this scheme. However, we shall see in Chap. 3 that, at least in the plane case, such problems can be handled by methods similar to those used in the frictionless case. The fact is that the methods outlined in later chapters for attacking viscoelastic boundary value problems are all indirect, in the sense that they focus on the boundary quantities, with the aim of determining these quantities everywhere on B. Once this is done, the task of determining any quantity in the interior of the medium is in principle easy. Indeed, it is either a first or second boundary value problem as defined above. 

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We have to stress that the analysed problems prove to be free boundary problems. Mathematically, the existence of free boundaries for the models concerned, as a rule, is due to the available inequality restrictions imposed on a solution. As to all contact problems, this is a nonpenetration condition of two bodies. The given condition is of a geometric nature and should be met for any constitutive law. The second class of restrictions is defined by the constitutive law and has a physical nature. Such restrictions are typical for elastoplastic models. Some problems of the elasticity theory discussed in the book have generally allowable variational formulation... 

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