This chapter provides guidelines and procedures for selecting silent chain and sprockets for drives.The use of silent chain for conveying is not discussed here; such information can be obtained fromthe chain manufacturer.In general, silent chain transmits power efficiently, smoothly, and quietly at speeds and loadsthat exceed the capability of most other chains and belts. Figure 7-1 through Figure 7-3 showtypical silent chain drives. Power capacity ranges from a fraction of a horsepower to more than2000 hp and speeds can exceed 7000 ft/min. Many silent chains employ two pin joints that wearvery uniformly as they operate. This means that each pitch of these chains elongates consistently,so they deliver smooth, low-vibration, performance throughout their life. Figure 7-4 shows thisuniform wear. The initial cost for silent chain and sprockets is often higher than for other types ofchain, but long life and reduced life cycle costs often justify the added expense.There are many different silent chain designs and con figurations. There are high-performancesilent chains that handle large amounts of power at very high speeds. This chapter covers selectingsilent chain drives that use chains and sprockets that conform to the ASME B29.2 standard. Consultthe manufacturer for help with selecting chains not described by the standard or drives with morethan two sprockets.GENERAL GUIDELINES FOR SILENT CHAIN DRIVE SELECTIONOPTIONS ENCOUNTERED IN DRIVE SELECTIONSelecting a silent chain drive consists of choosing a chain and sprockets that best satisfy the speci fiedrequirements. In most cases, this is a multistep process, the first step of which is obtaining a chainand sprockets that fit in the available space and that will work at the required loads and speeds.Often, more than one combination of chain and sprocket sizes will work in a given situation, so itis good practice to consider two or three alternatives. A final selection is made from the alternativesby considering other important factors such as cost, stock availability, and desired life.Most chain manufacturers offer drive selection manuals and technical assistance to aid in theselection of their products. Many also offer computer programs to simplify the selection process.One should consult the chain manufacturer for help when selecting drives that have large capitalcosts or operate in unusual and extreme conditions.CHAIN PITCHGenerally, small pitches operate smoother at high speeds. Larger pitches offer higher torquecapacity, but at lower speeds.CHAIN WIDTHSilent chain power capacity is rated in units of power per unit of chain width. For any given pitch,increased chain width results in greater chain strength, increased power capacity, and increasedcost. When several chain widths satisfy the speci fied requirements, narrower chain widths aregenerally more readily obtained and less costly. Wider chains will provide a more robust, shockresistant drive.DK4023_C007.fm Page 201 Wednesday, August 17, 2005 9:08 AM 2006 by American Chain Association202 Standard Handbook of ChainsSERVICE FACTORSTable 7-1 lists service factors. Service factors are used to account for the load characteristics ofthe power sources and the driven machinery. The proper service factors must be used when selectinga silent chain drive to obtain satisfactory operation and long drive life. Failure to use correct servicefactors can result in premature drive failures.NUMBER OF SPROCKET TEETHSprockets should have at least 21 teeth for long drive life and smooth operation. Drives usingsprockets with fewer teeth are likely to have increased vibration and noise because of chordal action(see Figure 5-2 and Figure 5-3, and the explanation used for roller chain). Each sprocket must belarge enough to accommodate the bore and keyway speci fied for the shaft on which it