Fretting prevention

Most of the cases of fretting met with in practice appear to fall into two distinct classes according to whether or not the surfaces involved in the component are intended to undergo some relative motion. If the surfaces are not intended to move, then the first objective should be to prevent slip, either by eliminating the source of vibration, or by increasing the friction between the surfaces. It is believed that the success of certain soft metal electrodeposits in reducing fretting may be due to the improved fit, and hence possibly increased friction, which is obtained from their use. If the displacements cannot be controlled in this way, it may be possible to interpose a thin sheet of an elastic material which can accept the relative movement without slip. 

Mechanical effects Corrosion can often be initiated or intensified by the conjoint action of mechanical factors. Typical examples include the presence of inherent or applied stresses, fatigue, fretting or cavitation effects. Inhibitors that are effective in the absence of some or all of these phenomena may not be so in their presence. In fact it may not always be possible to use inhibitors successfully in these situations and other methods of corrosion prevention will be required. 

Herbeck and Strohecker used machines designed particularly for comparing the merits of lubricants in preventing fretting corrosion of antifriction bearings. One provided for both oscillating conditions and combination radial and thrust loads to simulate service. Another was concerned primarily with thrust bearings and correlated satisfactorily with the radial load tester. 

The fluorescence intensity of fluorescent proteins is pH dependent and most fluorescent proteins are less fluorescent at lower pH mainly because of a reduction in absorbance. Since the absorbance of the acceptor determines the FRET efficiency, changes in the acceptor absorbance spectrum due to pH variations can be wrongly interpreted as changes in FRET efficiency. Thus, a pKa well below physiological pH is recommended to prevent artifacts due to pH changes inside cells. This is especially challenging if the fluorescent proteins are to be targeted to acid cellular compartments, for example, endosomes, lysosomes, or plant vacuoles. 

Thrust collars shall be positively locked to the shaft to prevent fretting. 

A complete FRET (Forster resonance energy transfer) system based on chlorophyll in the pores of FSM materials was accomplished by Kuroda s group.149,150 They first functionalized the FSM samples with 3-aminopropyl groups to guarantee an ideal position of the macroscopic chlorin units (in the pore center) and prevent their denaturation. Then they ligated chlorophyll derivatives that possess 3-(triethoxy-silyl)-A-methylpropan-l -amine groups to the pore walls. Zinc (for the FRET donor) and copper (for the FRET acceptor) were chosen as the central ions of the chlorins, which made it possible to initiate and record an efficient FRET process (Fig. 3.14). 

Prevention. Various design changes can minimize fretting wear. The machinery should be designed to reduce oscillatory movement, reduce stresses or eliminate two-piece design altogether.75 Some examples of possible approaches to consider are ... 

K Muller studied the effectiveness of various lubricants, including free molybdenum disulphide powder, in preventing fretting. His assessment of effectiveness was based on the friction energy per cycle, which is not in fact a satisfactory criterion for prevention of fretting. He found that initially molybdenum disulphide powder was very effective in reducing friction, but subsequently there was a steady deterioration until the performance was almost identical with that in unlubricated tests. 

Figure 37-24 Common probes and dyes for real-time PCR. f/J Double-stranded DNA dyes show a significant increase in fluorescence when bound to DNA (hv = excitation light). (2) Adjacent hybridization probes. Fluorescence resonance energy transfer (FRET) is illustrated between a donor and acceptor fluorophore.The x indicates phosphorylation of the 3 terminus of the probe to prevent polymerase extension. (3) FRET between a labeled primer and a single hybridization probe. (4) Hydrolysis probes are cleaved between the reporter and quencher, resulting in increased fluorescence. (5) Hairpin probes are quenched in the native conformation, but increase in fluorescence when hybridized. (6) Hairpin primers retain their native, quenched conformation until they are incorporated into a double-stranded product (Modified with permission of the publisher from Pritham GH, Wittwer CT Continuous f/uorescent monitoring of PCR.J Clin Ug Assay 1998, 21 404-412. 1998 Clinical Ligand Assay Society, Inc.)...

Figure 2. Schematic depiction of fluorescent resonance energy transfer (FRET) between the subunits of cAMP-dependent protein kinase (55) used for detecting cAMP. In the absence of cAMP, most of the kinase is in the holoenzyme state, which facilitates FRET between the fluorescein-labelled catalytic (C) subunits and the ihodamine-labelled regulatory (R) subunits. Excitation of fluorescein gives substantial re-emission from rhodamine at the expense of fluorescein. In the presence of high concentrations of cAMP, the R subunits undergo conformational change that dissociates them from the C Subunits and prevents FRET. Therefore, excitation of fluorescein results in fluorescein emission rather than rhodamine reemission (Reproduced with permission from ref. 55. Copyright 1991 Macmillan Maga2dnes).

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