Foctor Affecting Roller Chain Selection

For any application it is necessary to initially establish certain essential information. This typically includes the power to be transmitted, the drive shaft and driven shaft speeds (and hence the reduction ratio), the duty (specified in terms of power source, the severity of load and the design life), the shaft centre distance, the operating environment (i.e. dampness, temperature, etc.) and the likely level of maintenance (thus indicating lubrication types that may be permissible).
Drive selection is at two level, at the transmission type level (i.e. deciding whether to use chain drives, gears or pulleys) and at the component level (i.e. choosing the chain pitch size, number of strands etc.).

Transmission Type Selection
Many factors might influence why a chain drive will be selected for power transmission in preference to gear or a belt drive. Typically, a chain drive will be favoured when:
1. the centre distance between shafts is relatively large (making gears impractical);
2. a synchronous drive is necessary (making wedge pulley belts unsuitable);
3. a high operating efficiency and long life is required;
4. a high torque (or power) at slow speed is needed;
5. operating conditions are severe (pulleys are unsuitable in a wet, hot or abrasive environment);
6. cost have to be minimal;
7. a number of shafts need to be driven in synchronism;
8. shaft alignment and centre distance may be of low precision.

Component Selection Level
The sequence taken in identifying suitable chain drives and then choosing a preferred alternative is described in the flow diagram. A summary of a worked example which follows the procedure taken in this figure is given on a later page of this chapter. The following comments will help to clarify details of the flow diagram and the example.
1. When establishing requirements, essential information will include the drive and driven shaft speed, the power to be transmitted, the shaft centre distance, the operating duty and permissible lubrication types.
2. To determine the number of teeth required for the drive and driven sprockets it is first necessary to obtain the reduction ratio and this is found from knowledge of the shaft speeds. Frequently, though, it is not possible to achieve exactly the reduction required (since sprockets are available only in a limited number of teeth sizes) and so some tolerance on the driven shaft speed may have to be accepted. Also, the greater the number of teeth in the pinion, the quieter and smoother the transmission, and if low noise and very long life are desirable, a large and thus more expensive chain assembly will be necessary.
3. Operating with a severe drive source and/or a driven load will require a more substantial size of chain – and possibly large sprockets also. To provide a reserve of capacity to compensate for more demanding operational conditions, an appropriate Service Factor is used. This is obtained from a table and applied to the actual power to be transmitted thus producing an imaginary power which is then used for determining suitable chain pitch sizes from a Power Rating Chart. To avoid the necessity for having unique charts for each driving sprocket size and multi-strand alternative, a term called the Selection Power is defined. This quantity is calculated by simply applying factor to the actual power to be transmitted so that a single Rating Chart may then be used.
4. Suitable chain pitch sizes, based on power considerations, will be found from referring to the appropriate chain manufacturer’s ‘Transmission Rating Chart’ (found in a catalogue) and using the calculated Selection Power and the drive shaft speed. Generally, the large the chain pitch and the greater the number of strands, the greater the power that can be transmitted.
5. The chain pitch sizes must be checked for practical suitability, sprocket diameter and minimum centre distances must be established so as to ensure that alternatives are possible (see Figure 1).
6. Frequently the designer is faced with a number of viable alternatives comprising different pitch sizes and multi-strand options and each will have inherent advantages and disadvantages. For example, increasing the pitch size results in larger chain links and sprocket diameters but with consequent increased cost and reduced chain tension and speed capability. In contrast, using multi strand chain results in an increase of face width rather than diameter but it too is more expensive because it necessitates having tandem chain and sprocket sets.
7. Full catalogue details of the chosen chain drive should be summarized, together with connecting link type and tensioner (if required) so that mating items can be designer. The required length may be calculated by applying the appropriate equation. Usually it is more practical to know the length in term of the number of link – rounding up to the next integer number if part of a link is calculated, and using a chain tensioner to take up any slack. The chain tension is important when determining shaft forces and hence establishing shaft and bearing suitability.

Figure 1

Chain Power Rating
Many factors influence the amount of power which may safely be transmitted by a chain assembly. Generally the larger the pitch size and the larger the sprocket diameters the greater the power that can be transmitted but the slower the speed that the chain can operate at.
This is because the larger the pin and bush sizes. The link is thus stronger and the bushes have greater resistance to wear for a given power and speed (hence torque and toot load). Conversely, for a given reduction ratio, the larger the chain pitch the larger the sprockets and hence peripheral speed. For a limiting operational chain velocity, smaller pitch chain will operate at higher rotational speeds. In addition to chain size, the recommended maximum operating power and speed will depend upon factor such as the number of teeth in the sprocket, the severity of use, the lubrication system and the number of strands used.

Service Factor
The application suitability of a chain drive depends upon the type of drive and also the class of load. Typically each is categorized into three levels as follow:
Types of Drive
• Steady (e.g. ‘soft start’ electric motors)
• Medium Impulsive (e.g. single-cylinder i.c. engines)
• Highly impulsive (e.g. single-cylinder i.c. engines)
Types of load
• Class 1 for Smooth loads
• Class 2 for Moderate Shock loads
• Class 3 for Heavy Shock loads

Service Factor for the three classes of load severity are displayed below based on a Driving Sprocket with 15 teeth.

Multi-Strand Factors
Using a multi-strand chain increases the power that can be transmitted for a given pitch size but not fully in proportion with the number of strands used. Typically, the following power rating factors are widely used.

Selection (Design) Power
To minimize the number of charts needed in a catalogue it is necessary to calculate the Selection Power ( Or Design Power as it is sometimes referred). This obtained from :

Life Expectancy of Chains
A Chain’s life expectancy can be expressed as maximum percent of elongation. When using up to 67-tooth sprockets, a normal life expectancy is approximately 3% elongation. Thus, a chain should be replaced when its length increases 0,36” per foot on the average, to avoid sudden tension failure. When using sprockets with over 67 teeth, life expectancy is reduced in relationship to foolowing formula:

Permissible chain elongation = 200 Where N is the number of teeth in the larger sprocket.
N

Importance Lubrication
One of the most important, but often overlooked, factors affecting chain life is proper lubrication. Besides minimizing metal-to-metal contact, lubrication provides cooling and impact damping at high speeds. It also reduces corrosion and carries away foreign matters, which is vital in abrasive environmeters.
Lubricating method:

Normal Wear
Wear normally takes place in the pin bushing load-bearing areas. As they wear, the chain gradually elongates. The rate of chain wear is greatly affected by lubrication. When properly lubricated, load-bearing surfaces of the pin and bushing will look shiny and smooth

Bushing and Pin Wear Surfaces

Excessive Wear
If the load-bearing surface show discoloration (brown-red oxide), lubrication is insufficient. Fretting corrosion has set in, and the abrasive oxide produced will greatly increase the wear rate. Among other causes of excessive wear are:
• Tight Chain, insufficient sag in the slack strand. Lessen idler tension or distance between sprockets until slack is 2 to 3 % of the sprocket center-to-center distance.
• Excessive Slack, chain whips and creates noise. Adjust idlers or sprocket distance for proper slack.
• Worn or Misaligned Sprockets, can cause chain overloads and accelerate the wear rate. Replace sprockets when teeth show excessive wear or are hook-shaped.
Proper sprocket size is also important to minimize the wear rate. Use sprockets with a minimum of fifteen teeth for smoothest operation and loogest life. The fewer teeth there are in a sprocket, the greater the wear rate because of high angle of articulation.

Changes in Tooth Engagement Caused by Chain Wear

Galling (Abnormal Wear)
Galling, or the tearing away of metal particles from the load-bearing surface, occurs as a result of inadequate lubrication or excessive operating speed. The mating surface of the pins and bushings actually weld together, then break away as the joints flex over the sprockets. Once started, galling accelerates rapidly and is highly destructive.

Pin Galling

Galling can occur at high speed if lubrication is inadequate or misdirected. Check lubrication system to be sure that:
a. Proper type of lubricant is being used.
b. Lubricant flow is not obstructed.
c. Lubricant is penetrating chain joints

Tension Failure
This type of failure occurs when the ultimate tensile strength of a chain is exceeded (when the chain is subjected to a one-time load greater than it can withstand). Normally, tension failure can be identified by fracture side plates showing a definite yield (a taffy-pull type stretch) in the metal itself.

Side Plate Fracture Due to Tension Failure

Pin fracture, either near the center of the pin or a pin shear failure between the side plates, can also be a result of tension failure. When a chain breaks because of shocks or overloads, all of its components are affected, even though the unbroken parts may appear sound. To avoid repetitive failure, the entire chain should be replaced.

Tension failure can result from any condition which creates improper engagement between link and sprockets. Characterized by the chain riding up on the sprocket teeth. Such conditions include several already mentioned: excessive chain elongation, excessive slack, sprocket misalignment and worn sprockets.

In addition, dirt and foreign matter buildup in the sprocket tooth pockets will prevent proper seating of the chain, creating an overload condition between link and tooth. Sprocket should be checked periodically, and any foreign material accumulation removed.

Another variation of tension failure is cracked bushings. In applications contaminated by dirt or grit, abrasive material may penetrate the links. When it reaches the inside and outside bushing surfaces, this material literally grinds into the bushings during articulation, reducing their wall thickness and lowering chain tensile strength. Eventually the bushings crack under load.

Fatigue Failure
Fatigue failure are a result of repeated cyclic loading beyond the chain’s endurance limit, or rated capacity. Extend of the overload and frequency of its occurrence are factors which determine when fatigue will occur. The overloading can be continuous or intermittent (impulse load).

Continuous overloading may be causes by worn teeth or pocked buildup, imposing overloads with each cycle. Impulse overloads can be from motor overload torque, dynamic overloading due to sudden stops, or impact loading on equipment.
Generally, a fatigue crack stars at the point of highest stress, which is the aperture of the pin or bushing plate. Repeated cyclic stresses cause the crack to extend approximately perpendicular to the pitch line of the chain until plate breaks. Unlike a pure tension failure, there is no noticeable yielding (stretch) of material.

Fatigue Failure

When fatigue failure occurs, the application should be examined for continuous or impulse overloading conditions. Determine the cause of the overload and eliminate it if possible. (Be sure to check sprockets for worn teeth or pocket buildup). If the cause cannot be eliminated, determine the extent of the overload and increase chain size (capacity) to accommodate the operating conditions.


Bushing fatigue is another type of fatigue failure. Such fatigue manifests it self as circumferential crack near the bushing link plate or longitudinally along the length of the bushing. Both types of cracks may also appear in the same bushing. If bushing cracks are evident, do not try to repair the chain. Determine and correct the cause of the failure, then replace the entire chain.

Bushing Fatigue

Stress Corrosion and Hydrogen Embrittlement
These closely related failures are similar in appearance and nature. They appear as crack which initiate at the point of highest stress and tend to extend in an arc-like path parallel
To the rolling grain of the material. Often, more than one crack will appear on a side plate.

This type of failure can be caused by operating in an acidic or caustic medium or atmosphere. Carbon steel and certain grades of stainless steel are subject to stress corrosion cracking when exposed to a corrosive environment. Also, exposure of carbon steel chain to moisture can lead to rusting and stress corrosion cracking.

Stress Corrosion

The reactions of many chemical agents with metals liberate hydrogen, which attacks and weakens the metal grain structure.

If stress corrosion failure occurs, check the installation to see if the chain is exposed to chemicals, gases, moisture or other possible causes. If the chain has been cleaned with a detergent solution, the detergent could be at fault. For cleaning purposes, use only detergent-free fluids. Never use acids, such as in acid bath degreasing. If the detrimental conditions cannot be eliminated, consult REXTON for alternative solutions.