An Introduction to Types of Chains And selection

An Introduction to Types of Chains And selection

Introduction

Chain drives are an important and widely used type of mechanical element. They have two major uses, that is, for power transmissions and for materials handling purposes on conveyors. In addition, specific types of chain are used for supporting loads and for lifting purposes. As a transmission, chain drives can operate efficiently at high loads and they may be used where precise speed ratios are required.

                                               
Chain drives are a very widely used transmission system and can be found extensively in all mechanical industries and even on some civil engineering projects. Applications range from machine tool, marine and aerospace drives, motor cycle and bicycle transmissions. They are also widely used in the nuclear, mining and food industries and on conveying and packaging machinery. For example, in many motor vehicle and shipsı engines, the cam shaft is driven by a chain. Chain drives are used on civil aircraft such as Concorde and military aircraft like the Tornado and even on the Harrier for rotating the jet nozzle mechanism. In addition, railways use chain drives on track laying vehicles and in civil engineering the raising and lowering mechanism for the drop gates used on the Thames Flood Barrier use chains.

Selecting the best type of transmission drive will depend on the specific requirements of the application. Certainly, chain drives are widely used throughout mechanical engineering and, therefore, deserve to be very seriously considered at the preliminary stages of design.

Selection

When choosing an appropriate transmission system, chain drives should be considered for suitability along with gears and pulley systems. Despite having features in common with other types of transmission, chain drives have significant differences also. A comparison of some of the salient characteristics of the various transmission systems is helpful, at the preliminary design selection stage, when considering whether a chain drive is the most suitable for an application.

Chain drives may be likened to pulley drives since both forms of transmission system link driving and driven shafts together causing rotation in the same direction. In the case of timing belts the resemblance is even closer since they, like chain systems, operate synchronously. Chains, though, are inherently stronger than pulley belts because they are normally made from steel rather than a rubber / nylon composite and so can support larger tensions and thus transmit greater power. Also, because links may be added or removed, it means that the need for standard chain length sizes is avoided - as is the case with pulleys. In addition, since chains do not require tension on the slack side it considerably reduces loads on bearings.

Chain drives do not enable quite as compact an arrangement to be achieved as gears, since sprockets do not engage directly together. However, chains are more flexible because they can operate with a large range of centre distances between drive and driven shafts and they more efficiently support the turning force by distributing it onto more teeth than gears. Chain drives are also more tolerant to shaft misalignment and centre location variations and are generally less expensive than gear drives.


Types of Chain

By far the most common type is the roller chain and this is widely used for both power transmission and conveyor applications. A less frequently used type of transmission chain is the inverted tooth or silent chain as it is called. Another type of chain is the Galle chain which comprises just side plates and pins. This chain is now little used in transmission applications and is largely limited to load carrying uses. Finally, lifting chain is specifically designed for the purpose of supporting load and simply comprises a series of welded links and is used in a range of sling and hoist lifting arrangements. The selection of lifting chain and hoist arrangements, though, is outside the scope of this Procedural Guide.

1- Roller Chains

A length of roller chain is made up from a number of links and each chain link comprise side plates, pins, bushes and rollers (see Figure 1). The ends of a chain are joined together when in mesh with the drive and driven sprocket. As the sprocket teeth engage with the rollers, rolling rather than sliding contact occurs - thus making this a very efficient form of transmission element.

FIG 1

Roller chains are available in various standard forms, including stainless steel, plastic (for their self lubricating qualities) and various bespoke forms (especially for use on agricultural and conveying applications). Because of the predominant use of roller chain this type is dealt with in detail later.

Simplex chain

European Standard

Chains manufactured to the above standards are covered by ISO 606 and DIN 8187. These standards cover three versions: see figure 2

● Simplex

● Duplex

● Triplex

The range of pitch sizes can vary between 4 mm, (0.158 in.) to 114.3 mm, (4.500 in.).

They are characterised by a large pin diameter, especially for the larger pitch sizes. This

results in better wear resistance due to the greater bearing area.

The ISO Standard has a simple form of part numbering, for example 1/2 in. pitch duplex

chain would be 08B-2.

● The first two digits are the pitch size in 1/16’s of an inch, therefore 08 = 8/16 or 1/2 in.

● The letter ‘B’ indicates European Standard.

● The suffix two indicates the number of strands in the chain, in this case a duplex chain.

FIG 2

2- Conveyor Chain

Roller chains are specifically manufactured for use on material handling and conveyor equipment. Such chains are widely used in the textile, food processing, mining and agricultural industries, for escalators, elevators and also on modern machine tools.
Special chain is necessary because the requirement and working environment is very different from that of transmission chains. Instead of primarily transferring power between shafts, conveyor chain is required to pull one or a number of loads - usually along a straight line at relatively slow speed. Consequently, conveyor chain is rated in terms of Breaking Load rather than power and speed.
For material handling and packaging equipment in the food processing industries, hygiene is of paramount importance, consequently, stainless steel is widely used and any lubrication has to be carefully contained so as not to contaminate foodstuffs. As long spans of chain are frequently necessary, and fully enclosing the chain is impractical, the ingress of foreign matter can present serious problems.
Although, the roller chain used for conveyor applications is basically similar to that used for transmission chain, generally larger pitch sizes are used and sprocket wheels tend to have much broader teeth as illustrated in Figure 3a. Also, one or both of the outer side plates is often designed to accommodate a special attachment or incorporate an integral fixture - possibly to aid synchronised movement in some mechanical handling mechanism (see Figure 32b).

FIG 3a

FIG 3 b

3- Inverted Tooth (or Silent) Chain

This type of chain has teeth formed in the links and these engage with the teeth in the sprockets. Inverted tooth chain sprockets can be likened to gears but instead of meshing with each other they connect across a greater centre distance by means of a chain (see Figure 4). This type of chain consists of alternately mounted toothed links which are designed such that the link joints articulate during engagement with the sprocket teeth. Due to the compliance of individual links, there is less impact force when a link first engages with sprocket teeth and consequently an inverted tooth chain drive can operate more quietly and at higher speed than roller chain of comparable size.

In addition, because of the articulating, rocker joint feature it means that all sprocket teeth in the arc of contact (or wrap angle as it is sometimes called) fully engage with the mating chain teeth and consequently the load is very efficiently distributed. Some inverted tooth chains are produced with an involute gear profile, this enables a higher power transmitting capability to be achieved compared to roller chains but at extra cost. With good lubrication, inverted tooth chain will typically operate with an efficiency of 95 % but may even be as high as 99%.

FIG 4

Standards reference guide

Lubrication of chains.
The performance of a chain assembly is much improved by lubrication. It reduces friction and can act as a coolant and so will help to prolong life. There are four basic methods which are employed, namely manual, drip feed, oil bath and pressure spray lubrication and each differ in their effectiveness and in installation and maintenance costs.

Manual operation - This method does not need any special implementation requirements and the oil is periodically applied by brush, aerosol or oil can. The frequency of lubrication should be such that the chain is kept wet thus enabling clean lubricant to penetrate the chain joints. The use of too thin an oil viscosity can lead to lubricant 'flinging' off as the chain rotates.

Drip Lubrication - This requires an oil reservoir and pipe feed system to enable a regular, and preferably a controlled quantity of oil lubricant to drip onto the chain.

Oil Bath - The chain assembly is usually contained in a sealed housing in which the lower portion contains a sump of oil lubricant that just covers the chain at its lowest position. The chain passes through the oil bath each revolution which also has a beneficial cooling effect

Pressure Spray - A continuous supply of oil is directed onto the chain from a pumped supply system. Ideally the stream of oil should be supplied to the inner part of the chain just prior to engagement thereby helping to cushion the impact between rollers and teeth. The centrifuging effect on the oil as it is forced around the chain sprocket aids penetration through the chain members and also improves the rate of cooling.

Because the different lubrication methods vary in effectiveness it affects the operating performance in terms of the power and speed at which a chain drive can operate at. The limiting operating boundaries are illustrated on the Chain Drive Rating Chart shown later.

Selection method

Introduction

Chain selected using this method will have a minimum life expectancy with proper installation

and lubrication of 15 000 h.

Symbols, terms and units

Z1 =number of teeth on drive sprocket.

Z2 = number of teeth on driven sprocket.

C =centre distance (mm).

P =chain pitch (mm).

i = drive ratio.

L = chain length (pitches).

In order to select a chain drive the following essential information must be known:

● The power in kilowatts to be transmitted.

● The speed of the driving and driven shafts.

● The characteristics of the drive.

● Centre distance.

Selection summary

1 Select drive ratio and sprockets Z1 =19 teeth minimum

2 Establish selection application factors f1 takes account of dynamic loads Tooth factor f2 (19/Z1) 

3 Calculate selection power = power × f1 × f2 (kW) 

4 Select chain drive.

5 Calculate chain length using formulae 

6 Calculate exact centre distance 

Finally, choose lubrication method.

Application factor f1

Tooth factor f2

Calculate the selection power

Multiply the power to be transmitted by the factors obtained from step two.

Selection power = Power to be transmitted × f1× f 2 (kW).

Select chain drive

From the rating chart, select the smallest pitch of simplex chain to transmit the selection power at the speed of the driving sprocket Z1.

European chain rating chart

European Standard chain drives

Rating chart using 19T driver sprocket

Calculate chain length

To find the chain length in pitches (L) for any contemplated centre distance of a two point drive, use the formula below:

Calculate exact centre distance

The actual centre distance for the chain length (L) calculated by the method above, will in general be greater than that originally contemplated. The revised centre distance can be calculated from the formula below.

where:

P =chain pitch (mm)

L =chain length (pitches)

Z1 = number of teeth in driver sprocket

Z2 = number of teeth in driven sprocket

Roller Chain Terminology

A roller chain consists of a number of inner link assemblies in which adjacent pairs are connected by outer links. An inner link assembly is made up of a pair of inner plates connected by two riveted bushes spaced a set distance apart called the pitch. A roller is free to rotate on either bush. The outer links comprise a pair of outer plateswhich are attached via a pair of riveted bearing pins which are spaced a pitch distance apart. These pins are free to fit inside the inner link bushes and connect inner links as shown in Figure 5.

FIG 5