Chain drives, described

There are many different silent chain designs and configurations. There are high-performance silent chains that handle large amoimts of power at very high speeds. This chapter covers selecting silent chain drives that use chains and sprockets that conform to the ASME B29.2 standard. Consult the manufacturer for help with selecting chains not described by the standard or drives with more than two sprockets. 

Many roller and silent chains can be easily placed on the sprockets of a drive by hand. Pull the ends of the chain together around one sprocket with the rollers seated in the sprocket teeth. Hold the ends in place on the sprocket and connect the chain endless as described earlier. Figure 14-25 shows the final connection on a roller chain drive. 

Many chain and sprocket problems are described or shown in Table 15-1. Maintenance personnel should have a copy of Table 15-1 on hand whrai inspecting a chain drive or conveyor. The actions that may be required to correct these problems are briefly described in Table 15-1 and are discussed later in this chapter. 

In high-speed rollo- chain drives, wear elongation may be Umited to 1% or less. This is due to the pin links and roller links wearing at different rates, as described in chapter 3. 

The weight of material in the buckets on the loaded side of an elevator chain causes the elevator to momentarily mn backwards if, during operation, the power is intermpted or there is a failure in the driving system. Because this could be a ha2ard to operating personnel, as well as damage to the elevator, a backstop, similar to that described for a belt conveyor, should be used. 

Pitch diameter (of either the drive sprocket or driven sprocket) is generally on a theoretical circle described by the centerline of the chain as it passes over the sprocket. 

Exchange of unimers between two different types of block copolymer micelles has often been referred to as hybridization. This situation is more complex than for the case described above because thermodynamic parameters now come into play in addition to the kinetic ones. A typical example of such hybridization is related to the mixing of micelles formed by two different copolymers of the same chemical nature but with different composition and/or length for the constituent blocks. Tuzar et al. [41] studied the mixing of PS-PMAA micelles with different sizes in water-dioxane mixtures by sedimentation velocity measurements. These authors concluded that the different chains were mixing with time, the driving force being to reach the maximum entropy. 

Polycarboxylic acid synthases. Several enzymes, including citrate synthase, the key enzyme which catalyzes the first step of the citric acid cycle, promote condensations of acetyl-CoA with ketones (Eq. 13-38). An a-oxo acid is most often the second substrate, and a thioester intermediate (Eq. 13-38) undergoes hydrolysis to release coenzyme A.199 Because the substrate acetyl-CoA is a thioester, the reaction is often described as a Claisen condensation. The same enzyme that catalyzes the condensation of acetyl-CoA with a ketone also catalyzes the second step, the hydrolysis of the CoA thioester. These polycarboxylic acid synthases are important in biosynthesis. They carry out the initial steps in a general chain elongation process (Fig. 17-18). While one function of the thioester group in acetyl-CoA is to activate the methyl hydrogens toward the aldol condensation, the subsequent hydrolysis of the thioester linkage provides for overall irreversibility and "drives" the synthetic reaction. 

From the previous Section it is expected that the application of the flow field will primarily affect the last two order parameters, i.e. the orientation and conformation of the chain. In the discussion of the nucleation dynamics, it is helpful to separate the contributions from the kinetic and thermodynamic processes. The first represents the fundamental timescale to form a nucleus, the prefactor, and the second describes the driving force for the phase transition based on the position of the system in the phase diagram. 

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