Pre-loading

Figure 4.41 shows the Stress-Strength Interference (SSI) diagrams for the two assembly operation failure modes. The instantaneous stress on the relief section on first assembly is composed of two parts first the applied tensile stress,. v, due to the pre-load, F, and secondly, the torsional stress, t, due to the torque on assembly, M, and this is shown in Figure 4.41(a) (Edwards and McKee, 1991). This stress is at a maximum during the assembly operation. If the component survives this stress, it will not fail by stress rupture later in life. 

An approximate relationship is eommonly used to determine the torque for assembly, M, for a given pre-load, FfShigley and Misehke, 1996). It is a standard formulation for bolts and fasteners determined from experiment and is related to the frietion found in the eontaeting surfaees of the parts on assembly. 

The shear stress, t, due to the assembly torque diminishes to zero with time, the preload, F, remaining constant, and so the stress on the solenoid section is only the direct stress,. v, as given in equation 4.75 (see Figure 4.41(b)) (Edwards and McKee, 1991). A second reliability can then be determined by considering the requirement that the pre-load stress remains above a minimum level to avoid loosening in service (0.5 S/)min from experiment) (Marbacher, 1999). The reliability, R, can then be determined from the probabilistic requirement, P, to avoid loosening ... 

Before a probabilistic model can be developed, the variables involved must be determined. It is assumed that the variables all follow the Normal distribution and that they are statistically independent, i.e. not correlated in anyway. The scatter of the pre-load, F, using an air tool with a clutch is approximately 30% of the mean, which gives the coefficient of variation, = 0.1, assuming 3cr covers this range, therefore ... 

Consider the point xj. to be the mean value of a variable and x - i and xj. i the extremes of the variable. The extremes can be determined for each variable by assuming they exist 4cr away from the mean, which covers approximately 99.99% of situations. For example, for the pre-load force, F, the extremes become (for fip = 10 000 N) ... 

The von Mises stress, L, is then determined for various values of pre-load, E, using the above method. Equally, we eould have used Monte Carlo Simulation to determine an answer for the stress standard deviation. The answer using this approaeh is in faet o- 36 MPa over a number of trial runs. 

The maximum pre-load, ean be taken as a proportion of the proof load, Fp, typieally 0.9 for permanent assemblies (Shigley, 1986) ... 

Therefore, the eontribution of the pre-load, F, to the overall stress is given by ... 

The same proeess ean be performed with the other variables to provide Figure 4.45. It shows that the largest varianee eontribution is provided by the torque on assembly whieh affeets the pre-load, F. 

A parasitie pre-load ean be designed into the bearing by varying the eireular eurvature of the pad with respeet to the eurvature of the shaft. 

Bearing pre-load is another important design eriterion for tilting-pad bearings. Bearing pre-load is bearing assembly elearanee divided by maehined elearanee... 

A pre-load of 0.5-1.0 provides for stable operation beeause a eonverging wedge is produeed between the bearing journal and the bearing pads. 

The variable C is an installed elearanee and is dependent upon the radial pivot position. The variable C is the maehine elearanee and is fixed for a given bearing. Figure 13-7 shows two pads of a five-pad tilting-pad bearing where the pads have been installed sueh that the pre-load ratio is less than one, and Pad 2 has a pre-load ratio of 1.0. The solid line in Figure 13-7 represents the position of the journal in the eoneentrie position. The dashed line represents the journal in a position with a load applied to the bottom pads. 

Fig. 57 Schematic representation of the relationships between development, chamber saturation and pre-loading with solvent vapors (ace. [8]).

Figure 40.20 shows a typical relief valve. System pressure simply acts under the valve disk at the inlet of the valve. When the system pressure exceeds the pre-load force exerted by the valve spring, the valve disk will lift off of its seat. This will allow some of the system fluid to escape through the valve outlet. Flow will continue until the system pressure is reduced to a level below the spring force. 

The lubrication of leadscrews is similar in essence to that of slideways, but in some instances may be more critical. This is especially so when pre-load is applied to eliminate play and improve machining accuracy, since it tends to squeeze out the lubricant. Leadscrews and slideways often utilize the same lubricants. If the screw is to operate under high unit stresses - due to pre-load or actual working loads - extreme-pressure oil should be used. 

Vasodilators are a group of dtugs, which relax the smooth muscle cells of the blood vessels and lead to an increased local tissue blood flow, a reduced arterial pressure and a reduced central venous pressure. Vasodilators reduce the cardiac pre-load as well as after-load and thereby reduce cardiac work. They are used in a variety of conditions including hypertension, cardiac failure and treatment/prevention of angina pectoris. Major groups are Ca2+-channel blockers (e.g. dihydropyridines), NO-donators (e.g. organic nitrates), K+-channel openers (minoxidil), phosphodiesterase inhibitors (e.g. sildenafil), Rho-kinase inhibitors (e.g. Y27632) or substances with unknown mechanism of action (e.g. hydralazine). Inhibitors of the... 

It should be stressed that only those surfaces that actually come in contact with the sample need to be bio-compatible and the major parts of the valve can still be manufactured from stainless steel. The actual structure of the valve varies a little from one manufacturer to another but all are modifications of the basic sample valve shown in figure 13. The valve usually consists of five parts. Firstly there is the control knob or handle that allows the valve selector to be rotated and thus determines the load and sample positions. Secondly, a connecting device that communicates the rotary movement to the rotor. Thirdly the valve body that contains the different ports necessary to provide connections to the mobile phase supply, the column, the sample loop if one is available, the sample injection port and finally a port to waste. Then there is the rotor that actually selects the mode of operation of the valve and contains slots that can connect the alternate ports in the valve body to provide loading and sampling functions. Finally there is a pre-load assembly that furnishes an adequate pressure between the faces of the rotor and the valve body to ensure a leak tight seal. 

Finally we mention in this section the non-catalytic selective bromination of aniline by the application of a zeolite pre-loaded with Bt2 as a slow release reagent (ref. 27). Aniline, dissolved in CCI4 was treated with Br2 adsorbed onto various zeolites and zeolite CaA was found to be most selective for monosubstitution (92%). The addition of organic bases improved the performance, probably due to scavenging of HBr. Also the toluidines could be monobrominated with this system with >95% selectivity. 

Capsules with high enzyme loading and activity prepared by templating BMS spheres can also be used as biomimetic reactors [89]. For example, PGA/ PLL capsules with pre-loaded urease (prepared via a BMS sacrificial template) are capable of catalyzing the hydrolysis of urea and have been shown to induce the exclusive formation of CaC03 particles inside the capsules [89]. 

Recently, a general method to prepare protein-based particles with tailored properties through MS sphere templating was reported [98]. In this approach, polyelectrolytes are infiltrated into the mesopores of MS spheres pre-loaded with... 

COOH groups of the PE (PAA or PGA) and the -NH2 moieties of the pre-loaded protein, using l-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) as a catalyst [99]. Negligible enzyme desorption (<0.1 %) is observed from cross-linked lysozyme-loaded MS spheres after exposing the samples to an aqueous solution for 48 h, while about 25 % of the immobilized lysozyme is desorbed under the same conditions when the lysozyme is not cross-linked. 

To add the efTects of dead load to the SDOF calculation, the resistance will be adjusted by the magnitude of the dead load (refer to the pre-load discussion in Section 7.2.5). For an alternate means of handling dead load, refer to chapter 12. 

Counterflow Experiments Cells are once again pre-loaded with a high concentration of unlabeled substrate. The cells are then placed in a medium containing a low concentration of labeled substrate or a low concentration of an alternative substrate which utilizes the same transport system. The investigator then measures the rate of entry of the labeled substrate or of the alternative substrate. 

---