Sprocket Tooth Forms

The major function of a sprocket is to transmit torque. So, the main considerations in designing a sprocket tooth form are as follows. 

There are many similarities between the teeth of sprockets for roller chain, silent chain, the various engineering steel chains, and flat-top chains, and yet there are important design and conceptual differences. There are also basic differences in the terminology of design factors and even in the way the teeth are expected to interface with their chains. 

The major differences probably arise from the intended function of each type of sprocket tooth. For example, the sprockets for roller chain absorb pitch elongation from wear in one way, but the sprockets for engineering steel conveyor chain absorb that same pitch elongation in a very different way. 

On roller chain sprockets, elongation is absorbed by the rollers riding farther out on the working face of the sprocket tooth as the links pass around the sprockets. A trace of the path would be vaguely parabolic. The link would move in, seat briefly on the sprocket bottom diameter, and then move out again. 

Chain-sprocket interaction is very complex in theory. It can be even more so when the effects of wear and elastic deformation of the chain and sprocket under load are considered. A discussion of these effects is beyond the scope of this book. This book must limit the scope to the tooth forms and dimensions as specified by ASME B29 standards and accepted practice. The following sections discuss basic tooth designs for roller chain, silent chain, engineering steel chain, and flat-top chain. 

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Standard silent chains use a standard sprocket tooth form described by ASME B29.2. There also are nonstandard silent chains, such as high-performance, duplex, and reversible silent chains. Those nonstandard chains often require special sprockets that may be unique to a specific manufacturer. It is critical that chains and sprockets are fully compatible, so it is good practice to consult a chain s manufacturer for detailed sprocket specifications. A typical sprocket for duplex silent... 

FIGURE 4-34 Layout of standard sprocket tooth form for silent ehain. 

Engineering Steel Drive Chain Sprocket Tooth Form General... 

The sprocket tooth form for engineering steel drive chains is specified in the ASME B29.10 standard. A drawing of the tooth form and the equations for computing the main dimensions are shown in Figure 4-36. It differs from the tooth form for roller chain in that the pitch line clearance and bottom diameter are slightly smaller than the theoretical root diameter. These differences permit the use of a less precisely made tooth form for engineering steel drive chains than the machine-cut tooth form for roller chains. Engineering steel chain drives are often operated in locations where mud, dirt, ore, rock dust, etc. get into the chain. These drives are often exposed to the weather. Pitch line clearance and the undercut bottom diameter both help provide proper chain-sprocket action under such adverse conditions. 

Flat-Top Chain Sprocket Tooth Forms Type A Tooth Form... 

Cast The teeth on cast iron sprockets are formed in a special pattern and chilled to create a hardened wearing surface at the tooth area. 

Sprockets are made in various ways and from many materials. Manufacturing methods and materials are chosen to produce a tooth form with the precision and surface finish that is needed. Added processing, such as heat treatment, is done to give the load-carrying capacity, rotating speed capability, and service life that the sprockets must have. 

The ASME B29.1 standard defines the dimensions, tolerances, and tooth form for standard roller chain sprockets. These will be discussed in the design section later in this chapter. 

There are two forms of sprockets for double-pitch roller chain. One is designated as single-cut (Figure 4-10) and the other as double-cut (Figure 4-11). Note the difference in tooth form. The term effective teeth indicates the number of teeth that engage with the chain during one revolution of the sprocket. 

Sprockets for engineering steel offset sidebar drive chain are usually made of cast steel or are flame cut from steel plate. The hubs and other parts are usually fabricated. The standard tooth form is defined in ASME B29.10. Figure 4-22 shows the sprockets for a double-strand special version of a 6-in.-pitch drive chain that conforms to the ASME B29.10 standard. These sprockets use the American National Standards (ANS) tooth form and were machine cut to ensure aUgnment of the double-strand chain under heavy loads. These sprockets were made from alloy steel and were heat treated. 

From the discussions here, one can see that a wide range of sprocket styles are available for most types of engineering steel chain. The tooth form and dimensions must properly match the chain type and dimensions. Other than that, it might scan that the chains of this class can be operated on any type of wheel, but that is not the case. Certain wheel styles will not work with certain chains in some applications. For example, a roUer chain will not work on a traction wheel. Normally a certain style of sprocket is almost always used for a given type of chain. All other sprocket styles are used only in special cases. 

The tooth form for flat-top chain sprockets is covered by ASME B29.17 and will be discussed later in the design section of this chapter. This section considers sprockets only for standard flat-top chains described in this book. The only flat-top chains covered in this book are unit link chains with IVi-in. pitch. Thus the number of styles of sprockets discussed here is very limited. 

Figure 4-26 shows the standard tooth form for roller chain sprockets. The figure also shows the equations needed to compute all of the dimensions needed to draw and make the tooth form. Just as for sprocket diameters, the first and third terms, P and are standard dimensions and can be obtained from Table 2-1. The second term, N, must be supplied by the sprocket designer. Tolerances on critical limiting dimensions are given in ASME B29.1 or may be obtained from the sprocket manufacturer. 

The standard tooth form is generated from the path of the chain roller as it moves from the chain pitch line of the tight strand into the pitch circle of the sprocket. One can lay out the tooth form by following the terms and equations in Figure 4-26. Note that radius R of the seating curve and radius F of the topping curve include the necessary clearance allowances between the sprocket and the engaging rollers. 

The tooth form shown in Figure 4-26 is a theoretical tooth form for the specific number of teeth. It is designed so that the chain rollers ride out toward the tips of the sprocket teeth as the chain wears and elongates. Figure 4-27a shows a new roller chain on a new sprocket. Figure 4-27b shows a worn roller chain on a new (or unworn) sprocket. These drawings illustrate how the chain rollers ride further out on the sprocket teeth as the chain elongates from wear. 

There are many ways to produce sprocket teeth, and the actual tooth form may not exactly match the theoretical tooth form because of that. When standard space cutters or milling cutters are used, the actual tooth form matches the theoretical tooth form only at 56 teeth. When hobs or shapers are used, the actual tooth comes very close to the theoretical tooth form for all numbers of teeth, but the actual tooth form matches the theoretical only when the cutting tool is based on a specific number of whole teeth. Cast, powder metal, or molded plastic teeth may vary from the theoretical tooth form in various ways depending on how the pattern, die, or mold is made. 

Figure 4-34 shows the layout of a standard tooth form, expressed in terms of the pitch and number of teeth. It can be seen from the figure that the angle between the left and right face of a given tooth (60°7207AO decreases as the number of teeth is reduced. For a 12-tooth sprocket, the angle becomes zero and the tooth faces are parallel. Thus 12 is the minimum number of teeth that can be produced in a standard sprocket. In practice, sprockets this small are seldom used better results are obtained with 17 or more teeth. 

Note that no equation is given to compute the outside diameter. The outside diameter of an engineering steel sprocket is determined by the factors that are used in the layout of the tooth form. 

Typical Tooth Forms for Engineering Steee Chain Sprockets... 

Typical tooth forms for different types of engineering steel chains are shown in Figure 4-41. One can see notable differences between sprockets made for drive chains, conveyor chains, and bar-link chains. 

FIGURE 4-41 Typical tooth forms for engineering steel chain sprockets. 

Replace sprockets before the teeth are noticeably hook shaped. A worn, hook-shaped tooth form is much noisier than a new tooth form. 

The shape (see Figure 3-31) of the standard form of tooth used on the roller-chain sprockets permits the rollers to ride farther out on the teeth as the chain is stretched... 

This chapter is intended to give the reader an overview of the various standardized types of chain and their uses. Chains may be classified in many different ways. From a theoretical viewpoint, ehain is a eontinuous flexible rack engaging the teeth on a pair of gears. Certainly, a sprocket, being a toothed wheel whose teeth are shaped to mesh with a chain, is a form of gear. From a viewpoint based on its history and development, chain is a mechanical belt running over sprockets that ean be used to transmit power or convey materials. Neither of these aspects relate to the praetieal standpoints of either the producer or the user of the product. 

The sprocket must accommodate a reasonable amount of wear, either in the form of pitch elongation of the chain or tooth and bottom (or root) diameter wear of the sprocket. 

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