Hinges

Since many different plastics are flexible (Chapter 1) they are used to manufacture hinges. They can operate in different environments. Based on the plastic used they can meet a variety of load performance requirements. Land length to thickness ratio is usually at least 3 to 1. 

Otherwise problems in service immediately or shortly after initial use delamination occurs. Immediate post-mold flexing while it is still hot is usually required to ensure its proper operation. 

Hinges depend not only on processing technique but using the proper dimensions based on the type plastic used. Dimensions can differ if the hinge is to move 45° to 180°. If the web land length is too short for the 180° it will self-destruct due to excessive loads on the plastic s land. 

A press, a homemade toggle job, or a hot-stamping machine can be used to perform the cold working operation. When heat is required (usual requirement) the male forming die should be about 132 to 138°C (270 to 290°F). Pressure should be maintained for about 10 seconds. This time can be reduced if the product still retains residual molding heat or is preheated. The recommended preheating temperature is from 80 to 110 C (175 to 230°F). 

Forming the hinge cross-section by using an extruder die results in a hinge with poor flex life. Because hinges are formed in the direction of the polymer flow, they cannot be sufficiently oriented when flexed. However, if an extruded hinge is formed by the take-off mechanism while the polypropylene 

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Interestingly, there are many proteins with two domains that show a very clear hinge-bending motion with an obvious functional significance. Such domains have often been reported in the literature, but were never detected on an automated basis. 

Fig. 5. Rigid-body analysis of citrate synthase, using two X-ray structures (after Hayward and Berendsen, Proteins 30 (1998) 144). The decomposition of the protein into two domains (dark gray and white) and two interconnecting regions (light gray) is shown, together with the hinge axis for the closing/opening motion between them.

G., Berendsen, H.J.C. The essential dynamics of thermolysin Confirmation of the hinge-bending motion and comparison of simulations in vacuum and water. Proteins 22 (1995) 45-54. 

The first technique is very intuitive. Out of the few proteins that could be crystallized in a number of different conformations, adenylate kinase is probably the best-studied example. By combining nine observed crystal structures and interpolating between them, a movie was constructed that visualized a hypothetical path of its hinge-bending transition (jVonrhein et al. 1995]). 

In an early study of lysozyme ([McCammon et al. 1976]), the two domains of this protein were assumed to be rigid, and the hinge-bending motion in the presence of solvent was described by the Langevin equation for a damped harmonic oscillator. The angular displacement 0 from the equilibrium position is thus governed by... 

The effective moment of inertia / and the friction coefficient / could easily be estimated. The force constant k associated with the relative motion of the lobes was determined from an empirical energy function. To do so, the molecule was opened in a step-wise fashion by manipulating the hinge region and each resulting structure was energy minimized. Then, the interaction energy between the two domains was measured, and plotted versus 0. 

B. R. Brooks and M. Karplus. Normal modes for specific motions of macromolecules Application to the hinge-bending mode of lysozyme. Proc. Natl. Acad. Sci. USA, 82 4995-4999, 1985. 

Many elements of ehemists pietures of moleeular stmeture hinge on the point of view that separates the eleetronie motions from the vibrational/rotational motions and treats eouplings between these (approximately) separated motions as perturbations. It is essential to understand the origins and limitations of this separated-motions pieture. 

Example The distance between two ends of a large, flexible molecule can provide information about its structural properties or its interaction with solvent. Analysis of an angle can reveal a hinged motion in a macromolecule. 

Fig. 13. Semirigid piping systems (a) two-hinge arrangement (b) universal-type arrangement.

Dual-beUows assembhes, ie, universal-type expansion joints, are particularly vulnerable to squirm, and can experience elastic squirm at one-fourth the pressure of an individual bellows. When large amounts of offset are encountered, as is often the design basis, a pinwheel effect occurs because of unbalanced pressure forces. This effect tends to rotate the center-spool pipe which may lead to bellows mpture. Eor this reason the center spool should always be stabilized by hinges or tie-rod lugs to prevent such rotation. 

Fig. 5. Vacuum caUbrator for pipe and tubing extmsion A, molten tube from die B, tank C, hinged cover with gasket D, siting rings E, circulated and temperature controlled water E, water level G, vacuum and H, inside of pipe open to atmospheric pressure (15).

Although the concept of polymer blends is sometimes a route for a voiding the development of new polymers, it often has been an integral part of the utiliza tion of new polymer chemistry, eg, the commercial success of PPO hinged on the advantages of its blends with PS. 

A number of alternative pricing systems have been proposed that hinge on turbine efficiency and the relative pricing of fuel and electricity. AH... 

Heat Pumps. A heat pump is a refrigeration system that raises heat to a useful level. The most common appHcation is the vapor recompression system for evaporation (qv) (Fig. 14). Its appHcation hinges primarily on low cost power relative to the alternative heating media. If electricity price per unit energy is less than 1.5 times the cost of the heating medium, it merits a close look. This tends to occur when electricity is generated from a cheaper fuel (coal) or when hydroelectric power is available. 

The success of a reverse osmosis process hinges direcdy on the pretreatment of the feed stream. If typical process streams, without pretreatment to remove partially some of the constituents Hsted, were contacted with membranes, membrane life and performance would be unacceptable. There is no single pretreatment for all types of foulants. Pretreatment methods range from pH control, adsorption (qv), to filtration (qv), depending on the chemistry of the particular foulant. Some of the pretreatment methods for each type of foulant are as foUow (43—45) ... 

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