Roller Chain Selection

When designing chain drives, it is important to adhere to certain layout guidelines to assure optimum drive efficiency and life.

The following basic conditions should be met when selecting roller chain.

1. Drives should operate under ordinary conditions. Ambient temperature should range between 15ºF and 140ºF. Atmosphere should be free of abrasive dust, corrosive gasses and high humidity.  
2. Sprockets should be aligned and mounted on horizontally parallel shafts.
3. The suggested lubrication system and lubricant should be used.
4. Drive should sustain an even load with little variation. Changing load conditions will affect the life of the chain.

Chain selection and chain life will be adversely affected if these conditions cannot be met.

Procedure for selecting roller chain.

1. Required information when selecting a roller chain.
2. Power source.
3. Driven machine.
4. Horsepower being transmitted.
5. RPM and diameter of both the drive and driven sprockets.
6. Center distances of the shafts.
7. Obtain the “Service Factor” from Table I.
8. Multiply the horsepower value by the service factor to obtain the design horsepower value.
9. Refer to the horsepower ratings tables first on page 8 and then pages 9 to 11 to obtain the appropriate chain number and the number of teeth for the small sprocket. Refer to the number of revolutions of the high-speed shaft (the driving shaft when reducing speed, the driven shaft when speed is being increased) and the design horsepower value. Selecting a single-pitch, single strand chain will be the most economical. If single-pitch chain does not satisfy the transmission requirements, multiple strand chain can be used. Smaller pitch chains run smoother than large pitch chains.

Determine the number of teeth of the small sprocket. The number of teeth for the large sprocket is determined by multiplying the number of teeth for the small sprocket by the speed ratio. More than 15 teeth on the small sprocket is recommended. The number of teeth on the large sprocket should be less than 120. Reducing the number of teeth on the small sprocket will reduce the number of teeth on the large sprocket.

Basic Formula for Selecting Drive Chain

1. Chain Speed: S
   

2. Chain Tension: T
   

3. Number of chain pitches: L
   
   

4. Center distance in pitches: C
   
   
   

Chain Wear Factors
Two important factors, resistance to wear and resistance to fatigue, should be considered when determining chain size and quality. Generally, these two factors dictate roller chain life and failure.

WEAR
Under normal circumstances, if a drive has been properly selected, the limiting factor for chain life is wear in the live-bearing area between the pin and bushing. Chain must be replaced when elongation, due to pin and/or bushing wear, prevents proper chain and sprocket contact. Appropriate attention to installation, lubrication and maintenance must be taken to assure maximum chain life.

FATIGUE
The stress variation caused by the maximum tension and slack on the chain during a complete drive cycle is fatigue. When chains are operated beyond their rate of capacity, fatigue becomes a dynamic part of chain failure. Chains will eventually fail from fatigue if subjected to high enough loads. The frequency and magnitude of the overload determine the amount of time the drive can operate before fatigue failure occurs. Satisfactory results in terms of hours or years of service can be obtained with proper roller chain and sprocket selection.