Silent Chain Drives

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 from the chain manufacturer. 

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. 

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For silent chain drives, pinions with 21 teeth or more are recommended for general applications and at least 25 teeth are recommended for high-speed applications. The recommended minimum is 17 teeth. 

Properly engineered silent chain drives having ratios as great as 12 1 will perform satisfactorily. However, it might be more economical to consider a compound drive where the ratio is 8 1 or larger. 

Silent Chain. Table 3-21 gives the typical service factors for silent chain drives [5]. 

Tables 3-22 and 3-23 give the horsepower ratings per inch width for silent chain drives [5,15].

What are the two most important factors in the life of a silent chain drive ... 

FIGURE 1-23 The SS Great Britain of 1845. It used the hrst silent chain drive of importance. 

FIGURE 1-24 The silent chain drive of the SS Great Britain. 

Silent chain has all of the advantages of roller chain listed above, except for one. Silent chain drives require nearly as precise alignment as gear drives to obtain peak performance and service life. In addition,... 

Silent chain drives are quieto and smooths than roller chain drives, and in some cases arc quieter than gear drives. 

Silent chain drives are more compact than gear drives when the shafts must turn in the same direction. 

A few silent chains are used in conveyors, but most silent ehains are used in high-speed drives. Shaft speeds of silent chain drives generally range from about 100 rpm to more than 10,000 rpm. The amount of power transmitted may be as litde as a fraetion of 1 hp or it may be more than 2,000 hp. 

Lubrication and environmental considerations are not discussed here. Silent chain drives generally run at higher speeds than other types of chain drives. Oil bath or oil stream lubrication is usually specified in the power ratings. These types of lubrication require a casing, which shields the drive from most contaminants. 

The shaft sizes for larger sprockets are usually selected before selecting the proper hub size. This is so whether it is for a drive or a conveyor. The data and examples given in this section are based on sprockets for engineering steel chain, but they apply equally well to shaft selection for roller chain or silent chain drives. 

GENERAL GUIDELINES FOR SILENT CHAIN DRIVE SELECTION... 

Selecting a silent chain drive consists of choosing a chain and sprockets that best satisfy the specified requirements. In most cases, this is a multistep process, the first step of which is obtaining a chain and 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 it is good practice to consider two or three alternatives. A final selection is made from the alternatives by considering other important factors such as cost, stock availability, and desired life. 

Table 7-1 lists service factors. Service factors are used to account for the load characteristics of the power sources and the driven machinery. The propa- service factors must be used when selecting a silent chain drive to obtain satisfactory operation and long drive life. Failure to use correct service factors can result in premature drive failures. 

Drive ratios of up to 12 1 are possible with silent chain drives. However, it is usually more economical and practical to use ratios of 8 1 or less. Higher ratios are often better achieved using a two-stage drive. 

Selection of a complete silent chain drive begins with the gathering of as much of the following... 

Shafts should be aligned to within 0.050 in./ft, or 0.25 degrees for most single-strand roller chain drives and conveyors. That same limit applies to most silent chain drives up to 1-in. wide. A somewhat larger limit may be used for engineering steel drives. 

This credo, as promnlgated by the then Association of Roller and Silent Chain Manufactnrers (ARSCM) soon led to a series of projects to build and operate chain test equipment in the laboratory. From those projects were developed the horsepower curves covering all basic sizes of standard roller drive chains. These were inclnded in a hardbound chain design manual, published by ARSCM in 1955, and entitled Design Manual, Roller and Silent Chain Drives. That edition and its softbound snccessors, pnblished in 1968 and 1975, have seen nearly 30,000 copies distributed by the association. 

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