Rubbing speed

Turboexpanders eurrently in operation range in size from about 1 hp to above 10,000 hp. In the small sizes, the problems are miniaturization, Reynolds Number effeets, heat transfer, seal, and meehanieal problems, and often inelude bearing and eritieal speed eoneerns. In intermediate sizes, these problems beeome less signifieant, but bearing rubbing speeds and vibration beeome inereasingly important. 

Several steps can be taken to maximize the run time for the reciprocating compressor. Since wear is a function of rubbing speed, the piston speed can be kept to a minimum. Chapter 3 made recommendations for piston speed. Reliability problems due to valves are reputed to account tor 40% of the maintenance cost of the compressor. Valves are the single largest cause for unplanned shutdowns. Basically, valve life can he increased by keeping the speed of the compressor as low as practical. At 360 rpm, the valves are operated six times a second. At 1,200 rpm, ihc valves operate 20 times a second or 1,728,000 times in a day. It is not difficult to understand why the valves are considered critical. To keep the reliability in mind, valve type, material selection and application considerations such as volume ratio, gas corrosiveness, and gas cleanliness need attention by the experts. One final note is that while lubrication is an asset to the rubbing parts, it is not necessarily good for valve reliability. 

Worm gears of either type operating at speeds above 2400 rev/min or 610 m/min rubbing speed may require force-feed lubrication. In general, a lubricant of lower viscosity than recommended in the above the table may be used with a force-feed system. 

In general, the higher the pressure or rubbing speed, the more incendive will be the sparks. However, quantitative predictions can be done only within narrowly defined experiments. 

Figure 8-16. Effect of rubbing speed on friction of cast iron lubricated by fatty acids. Data by A. Dorinson, ASLE Trans., 13 (1970) 215-224.

Since m and fe are constant for fixed rubbing conditions (fixed pairs, fixed contact pressure, fixed rubbing speed), we can write 1. 

The interplay of those influences which inhibit adhesion and those which promote it must be recognized in interpreting two commonly encountered aspects of rubbing contact smooth, regular, controlled wear on the one hand, and destructive, self-accelerating catastrophe on the other. Figure 12-25 shows the appearance of two locations on a disk of hardened steel used in a pin-and-disk wear experiment at a contact pressure of 1069 MPa (10,900 kg/cm ) and a rubbing speed of 0.508 m/s with... 

Figure 13-3. Influence of contact pressure on wear. Slider 120° cone of SAE 1095 steel. Rubbing speed 20 cm/s. Lubricant purified...

Figure 13-4. Types of wear behavior for pins against Stellite rings rubbing speed 68.5 cm/s lubricant hexadecane. (a) Aluminum pin, load 9.6 N. (b) 60/40 Brass pin, load 7.35 N. (c) Bronze pin, load 12.25 N. Data by Hirst and Lancaster [6].

The effect of such operating parameters as load and rubbing speed on the quantitative relation of dry and lubricated wear is neither simple nor obvious. Table 13-4 shows two direct comparisons one of data from the work of Kerridge and Lancaster [15] and the other of unpublished data... 

Abrupt transitions of the wear of brass riders against hard steel from a domain of relatively low rate to one of substantially higher rate were observed by Archard and Hirst [7] in response to load, by Hirst and Lancaster [30] in response to rubbing speed and by Lancaster [31] in response to ambient temperature. The regime of low wear rate was associated with evidence for the participation of surface films of oxide in the wear process, whereas the debris produced at high wear rates was predominantly metallic. The relation between these two kinds of wear behavior in terms of variables such as load, speed and temperature is quite complex [32]. 

Although it has not been established by systematic study, the operating parameter that determines whether the wear process is adhesive transfer and oxidation or oxidation and denudation is most likely rubbing speed, which in the ultimate analysis means interfacial temperature. If the temperature is high enough, both the rider and the track will acquire a coherent film of oxide which will effectively block adhesive transfer of metal from the rider to the track. Below some critical temperature only the more activated sites will be oxidized, which affords an opportunity for transfer of metal from unoxidized sites on the rider to the track oxidation of the transferred metal on the track is probably a consequence of its activated condition there. There is no clear-cut behavioristic demarcation between metallic transfer and the oxidation/ denudation process in the loss of material from the rider. Observers have frequently reported that wear experiments whose steady state proceeds by oxidation/denudation at a moderate rate may have as the initial stage severe wear with metallic debris (e.g. [39, 41]). 

Figure 13-18. Influence of contact pressure on wear rate under constant load. Hardened steel lubricated by white oil at 23.2 N load, 50.8 cm/s rubbing speed, (a) Course of wear. (b) Wear rate as a function of contact pressure. Data by Dorinson and Broman [4].

The effect of reiterative feedback, therefore, is a major consideration in the construction of wear models that realistically take into account the influence of changing physical parameters on the course of wear. In some cases these changes are the result of the course of wear itself, such as the decrease of contact pressure as the conjunction area enlarges under constant load. In other cases, the externally imposed magnitude of a parameter such as load or rubbing speed will determine the influence that reiteration of contact will have on the course of wear. 

The problem of wear when the fluid film lubricant is no longer intact is associated with the asperity contact of structured surfaces. The contact behavior of such surfaces was discussed in Chapter 12 wear models governed by asperity contact were described in Chapter 13. Theoretically the laws controlling fluid film thickness can be coupled with asperity contact models to yield quantitative descriptions of the course of wear. In this section we shall deal with those cases in which the function of the lubricant is only to provide a fluid film separating the two rubbing bodies, and the events at the contact, once it is established, are determined by the interaction of mechanical parameters such as load and rubbing speed with the properties of the contacting interface. 

The kind of wear behavior characteristic of mixohydrodynamic lubrication is shown in Fig. 14-2. The left-hand diagram shows the distance-dependent depth-rate of wear as a function of the contact pressure for a series of four rubbing speeds. The right-hand diagram shows... 

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