Chains and their Types
Chains and their Types
A chain is a series of connected links which are typically made of metal. A chain may consist of two or more links. Chains can be classified in many different ways. From a theoretical viewpoint, a chain is a continuous flexible rack engaging the teeth on a pair of gears. 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 can be used to transmit power or convey materials. Chains have the following four basic functions.
- Transmit power.
- Convey objects or materials.
- Convert rotary motion to linear motion, or linear motion to rotary motion.
- Synchronize or to time motion
Chains have the following general advantages over other equipment intended to do the same functions.
- Have controlled flexibility in only one plane.
- Have a positive action over sprockets, no slippage takes place.
- Carry very heavy loads with little stretch.
- Efficiency of a chain joint passing around a sprocket approaches 100 % because of the large internal mechanical advantages of links in flexure.
- Provide extended wear life because flexure takes place between bearing surfaces with high hardness designed specifically to resist wear.
- Can be operated satisfactorily in adverse environments, such as under high temperatures or where they are subject to moisture or foreign materials.
- Can be manufactured from special steels to resist specific environments.
- Have an unlimited shelf life. They do not deteriorate with age or with sun, oil, or grease.
- Straight link chains, which have alternate ‘outside’ and ‘inside’ links. These include chains with rollers and chains that are similar to chains with rollers, but are roller-less.
- Offset link chains, which have all links alike. These include integral link chains, such as bar-link, flat-top, and welded steel chains, where internal rollers cannot be installed.
- To engage the sprocket teeth and thus transfer any sliding action to the internal members of the chain, which are designed for that purpose.
- To serve as a guide or to support a chain and material carried on it on tracks or ways, as is characteristic of conveyors and some bucket elevators.
Types of chains
From industry stand point, the major types of chains are (i) roller chains, (ii) leaf chains, (iii) silent chains, (iv) engineering steel chains, and (v) flat-top chains. (Fig 1)
Fig 1 Types of chains
Regardless of the types of chains, their styles are classified as follows.
Between the straight and offset link chains, the configuration of the tension members of the chains (in these cases, sidebars, as these are engineering steel drive chains) may be the only difference in the construction of the two chains. If pitch and joint details are identical and the sidebar cross-sections are identical, so that the only difference is straight or offset link construction, the chains can be operated over the same sprockets and for the same purpose. But they are not to be ‘mixed’ by inter-coupling links of one into the other.
The differences in use for a given application stem from the fact that straight link chains consist of outside and inside links and that offset links are all alike. For this reason, strands of straight link chains must be used with an even number of links unless one special offset link is used. On the other hand, offset chain strands can be used with either an odd or even number of links. Straight link chains operate equally well in either direction of travel, but offset chains are to operate in a specific direction (referring to closed or open end forward) to obtain the best service.
Each style has advantages. Straight link chain is easier to manufacture and may give a cost advantage. Attachments are more easily provided in straight link designs, and may cause fewer problems in use. For a given strength, a slightly shorter pitch chain can be provided in straight link chain, since space for an offset need not be provided. Although sidebar thickness and cross section are similar, the pitch of the straight link drive chain can be considerably less. In drives, short pitch is reflected in terms of smaller sprockets and quieter operation.
An advantage of offset chain is that a worn drive strand can be easily shortened by removal of a single link. To shorten a straight link roller chain containing a connecting link, a pair of links (one inside and one outside) is removed and replaced with a single offset link. If the straight link chain does not have a connecting link, a section consisting of five links, two inside and three outside links, are to be removed. The central three links are replaced with a two-pitch offset section, and two of the outside links are replaced with two connecting links. It is necessary to take care in such cases that the replacement offset link has offsets that will clear the ends of the adjacent straight links when the chain flexes. There are some applications, mostly with roller-less chains on conveyors or bucket elevators, where offset chains give excellent wear life when operated open end forward.
The major purpose of rollers is to reduce friction, but the rollers in chains have two separate functions, usually being provided by the same roller. These functions are given below.
Rollers in drive chains are generally smaller in diameter than the height of the link plates of the chain. Thus, the link plates serve as guides when the chain engages the sprockets, and may also do so when the chain is riding on guides, as in a bucket elevator.
Rollers on conveyor chains normally have diameters considerably larger than the widths of their adjacent sidebars. This is done for two reasons namely (i) the large rollers, called carrier rollers, carry the sidebars well above the conveyor tracks and thus prevent friction, and (ii) larger rollers have a definite mechanical advantage over smaller rollers relative to rotational friction, and thus help reduce chain pull.
The carrier rollers in most chains are also used to engage the sprockets. However, carrier rollers, equipped with antifriction bearings, are sometimes used as outboard rollers on roller-less chains. Roller-less chain is similar in appearance to chain with rollers, and is used for applications where rollers are not required.
Roller chains are manufactured in several types, each designed for a particular use. All roller chains are constructed so that the rollers are evenly spaced throughout the chain. A major advantage of roller chain is that the rollers rotate when contacting the teeth of the sprocket.
Two types of roller chains are in common use namely (i) single strand, and (ii) multiple strands. In multiple strands, two or more chains are assembled side by side on common pins that maintain the alignment of the rollers in the several strands.
Standard roller chains are defined as pitch proportional, which makes them different from other types of chains with rollers. Nominal dimensions for these chains are approximately proportional to the chain pitch which is the distance between the centres of adjacent joint members. The three most important roller chain dimensions are pitch, roller diameter, and inside chain width. These dimensions determine the fit between the chain and the sprockets.
Roller chains are used for both drive and conveyor applications. There is a separate series of chains for each application area. There are three types of standard power transmission roller chain namely (i) single strand, (ii) multiple-strand, and (iii) double-pitch power transmission roller chain.
The most commonly used chain for drives is the single-strand standard series roller chain. The power rating capacities of these chains cover a wide range of drive load requirements. Multiple-strand roller chains are used to provide increased power capacity without the need for increasing the chain pitch or its linear speed. For a given power load, a multiple-strand chain with smaller pitch can be run at a higher speed than single-strand roller chain of larger pitch. Double-pitch power transmission roller chains are particularly applicable to power drives where speeds are slow, loads are moderate, or centre distances are long. For such applications, the longer pitch results in a lighter and less expensive chain.
Leaf chains are used almost exclusively for lifting and counterbalancing. Tensions are very high, but speeds are slow. Normally the chains work intermittently. The main considerations in the design of the leaf chains are tensile loads, joint wear, and link plate and sheave wear.
The major dimensions of standard leaf chains are proportional to the chain pitch. These proportions are derived from engineering studies and experience. The standard proportions give a good balance of properties needed for a leaf chain to lift large loads while giving acceptable wear life.
The most common use for leaf chain is probably on lift trucks. A leaf chain used on a lift truck is normally under a substantial static load from the trucks lifting carriage. In addition, the chain withstands the nominal working load from carrying the material. It also absorbs shock loads from moving material over uneven surfaces, and withstands inertia loads from picking up the material to be moved.
Just as with roller chains, leaf chains are to have a certain minimum ultimate tensile strength (UTS). That is because yield strength (YS) and fatigue strength (FS) are generally related to tensile strength. However, UTS is not to be used for selecting leaf chains since it can mislead into overloading of the chain.
Yield strength is the major consideration while designing leaf chains. Leaf chains must often lift very large loads, and hence they need high YS and do not get permanently stretched when they lift large loads.
FS is also a major consideration in designing leaf chains. Leaf chains move at low speeds and accrue load cycles very slowly. Since many leaf chains work in the finite-life range (between 10,000 and 1,000,000 cycles), hence FS in this range is very important.
The fatigue limit (FL) is moderately important in the design of leaf chains, mainly because FS in the finite-life range is related to the FL. It is also because leaf chains accrue load cycles very slowly.
Wear is a very important consideration while designing leaf chains. Leaf chains are mostly subjected to joint wear and link plate/sheave wear. Both of these wears reduce the strength of leaf chains.
Joint wear is important for designing leaf chain. When a leaf chain runs over sheaves, the joints articulate. Material is worn off the outside diameter of the pins and the inside diameter of the holes in the articulating link plates. As the material is worn away, the chain not only gets longer, but the load-carrying sections of the pins and articulating link plates get smaller.
Link plate wear, from running over sheaves, is also important consideration for the design of a leaf chain while the sheave wear is normally not a major consideration.
The lubrication is a serious concern in the design of leaf chains. Some leaf chains receive little or no lubrication in service, and the designer is to consider this.
The environment in which a leaf chain works is a serious concern in the design of leaf chains. Most leaf chains are not protected in any way from the surrounding environment. Many leaf chains work outdoors in all kinds of weather.
Leaf chain is an assembly of alternating sets of pin links and articulating links on pins that are free to articulate in the holes of the articulating links. The pin link plates normally are press fitted onto the ends of the pins in the chain. The centre link plates are usually slip fitted on the pins. Leaf chains are intended to run over sheaves, so there is no provision for them to engage a sprocket.
A clevis pin is used to connect the end of a leaf chain to an outside clevis. The outside clevis gets its name because the outer tangs of the clevis fit outside of the articulating link plates of the chain
A connecting link is used to connect the end of a leaf chain to an inside clevis. The inside clevis gets its name because the outer tangs of the clevis fit inside of the chain. The connecting link consists of two connecting pins press fitted into one pin link plate, the required number of centre link plates, and a cover plate. Either cotters or a spring clip is used to hold the cover plate in place. The cover plate is to be an interference fit on the pins to give the connecting link about the same strength as the pin links in the chain.
Leaf chain is a rather simple lacing of pins and link plates. Operating forces are transmitted from the clevis pin or connecting link to the articulating link plates. The articulating link plates then transfer these forces to the next pin, which transfers the forces to the pin link plates and centre plates. And then the sequence repeats.
Some parts must perform two or more functions at the same time. For example, the articulating link plates need high hardness to resist wear, and also need good ductility to withstand large shock loads. The designer is to decide how to make use of these conflicting requirements.
The tension forces on a leaf chain subject the pins to mostly shear with some bending, and they do so while the pins turn in the articulating link plates. The pins act as both beams and bearings in a leaf chain. The pins need enough strength and ductility to transfer the load from the articulating link plates to the pin link plates and centre plates without deforming or breaking. The pins also need high enough surface hardness to resist wear when the joint articulates under heavy loads.
The pin link plates in a leaf chain are mostly tension members, but they are also subjected to some bending. The holes in the pin link plates are significant stress risers that produce high stress concentrations around the holes. The pin link plates must be strong enough to withstand the tensile forces without deforming or breaking, and they must have enough ductility to withstand some bending and resist fatigue. The holes must be made with some special processing to resist fatigue.
The centre link plates in a leaf chain are almost totally tension members. They are subjected to very little bending. The holes in the centre link plates are significant stress risers that produce high stress concentrations around the holes. The centre link plates are to be strong enough to withstand the tensile forces without deforming or breaking, and they must have enough ductility to resist fatigue. The holes must be made with some special processing to resist fatigue.
The articulating link plates in a leaf chain are almost totally tension members. They are subjected to very little bending. The holes in the articulating link plates are significant stress risers that produce high stress concentrations around the holes. In addition, the articulating link plates must transmit very high tensile forces while a pin turns in the holes of the link plate. The articulating link plates may be the most critical parts in a leaf chain. They are to be strong enough to withstand the tensile forces without deforming or breaking, and they must have enough ductility to resist fatigue. The holes must be made with much special processing to resist fatigue. Finally, the articulating link plates must be hard enough to resist wear when the pin turns in the holes under high loads.
Silent chain, also called ‘inverted tooth’ chain, consists of a series of toothed link plates assembled on joint components in a way that allows free flexing between each pitch. The great majority of silent chain is used in drives. Silent chains are made up of stacked rows of load carrying link plates. Increasing the number of rows of links increases the chain width, tensile strength, and load carrying capacity.
Silent chains are made up of stacked rows of flat link plates with gear-type contours designed to engage sprocket teeth in a manner similar to the way a rack engages a gear. The links are held together at each chain joint by one or more pins, which also allow the chain to flex. The design of both the link contour and the chain joint directly influences a chain’s useful load carrying capacity, its rate of wear and service life, and its quietness of operation.
Silent chains from different manufacturers usually cannot be connected together. Standard silent chains are used in a wide variety of industrial drives where a compact, high-speed, smooth, low-noise drive is required.
There are several classes of silent chains, which are not produced by all the manufacturers and are not covered by any standard. Nevertheless, these special silent chains meet important needs. The most common nonstandard silent chains are probably the high-performance drive chains. High-performance silent chains are specially designed to carry greater loads and run at higher speeds than standard silent chains. They generally have unique joint designs with rocker-type pins that virtually eliminate chordal action. They usually require specially designed sprockets. High performance silent chains are available in a wide range of sizes with pitches and in widths and are used on very-high-speed drives where exceptional smoothness and quietness are required. These chains are commonly used in industrial equipment where ultimate smoothness is required.
Other nonstandard silent chains are duplex, conveyor, and specialty chains. Duplex silent chains have teeth extending on both sides of the pitch line to permit the chain to run on serpentine drives where sprockets engage both sides of the chain.
Conveyor silent chains often use flat-back link plates that provide a smooth conveying surface and may use joint designs that resist fouling. Conveyor silent chain is frequently used where exceptionally smooth transport is required. Specialty silent chains are made for specific applications where attachments or unusual configurations are needed.
Engineering steel chains
Engineering steel chains were first developed in the 1880s. They were developed for greater strength, speed, and shock resistance, and for better dimensional control. Early engineering steel chains were designed for difficult conveying applications. Just as with roller chains, engineering steel chains were developed as all-steel products fabricated from rolled shapes. One exception was that rollers, particularly flanged rollers, were made of cast iron, and this exception has continued to the present.
Larger sizes of engineering steel chains were soon developed. Pitch, strength, wear life, and carrying capacity were increased to meet the heavy-duty needs of industry. Engineering steel chains were developed to operate dependably in the most demanding conditions.
Many different types of engineering steel chains are used in a wide variety of applications. Most engineering steel chains are used in conveyors, bucket elevators, and tension linkages. Only a few are used in drives. The main design considerations for these chains are tensile loads, several types of wear, lubrication, and environment. The main design considerations for an engineering steel chain to be used on a drive include the various tensile loads, certain types of wear, roller and bushing impact, and galling.
The dimensions of engineering steel conveyor chains are not proportional to the pitch. Engineering steel chains generally have much larger clearances between moving parts than roller chains of the same size. The clearances between the pins and bushings, the bushings and rollers, and the inner and outer links are proportionally much larger. The larger clearances are provided so that dirt and debris can pass freely out of the bearing areas. The debris is, thus, not as likely to clog the joints of the chain, causing them to bind or seize.
An engineering steel chain in a conveyor or drive may be subjected to all of the tensile loads. However, the tensile loads from centrifugal force, chordal action, and vibration are not very likely to be a major factor. Thus, engineering steel chain must have certain tensile strength properties to withstand the wide range of tensile loads that may be imposed on it.
Ultimate tensile strength is not a major consideration in designing engineering steel chains. That is because YS and FS are only generally related to UTS. Yield strength is an important consideration in designing engineering steel chains. FS in the finite-life range is a very important while designing engineering steel chains. Loads sometimes exceed the FL in some heavily loaded conveyors and drives.
The FL usually is not critical for the design of engineering steel chains. This is because most of the engineering steel chains accrue cycles very slowly and these chains are expected to wear out before fatigue can cause the chains to fail.
Wear is probably the most important parameter while designing an engineering steel chain. Joint wear, roller and bushing wear, and sidebar and track wear all are of great concern for conveyor chains. Joint wear and roller and sprocket wear are the major concerns for drive chains. As the chain runs over the sprockets, the joints articulate and material is worn off the outside diameter of the pins and the inside diameter of the bushings, and as this material is worn away, the chain gets longer
Sprockets for engineering steel chain are designed to accept chain elongation from wear of 3 % to 6 %. When the chain elongates beyond this point, it no longer fits the sprockets and the system does not operate properly.
Roller wear in drive chains usually is not a major concern, but roller wear in conveyor chains may be a serious concern. Wear of the teeth on a small sprocket can impose large shock loads on the chain. Roller and bushing wears, and sidebar and track wears are very important considerations in the design of engineering steel roller conveyor chains.
Lubrication is a major concern in designing engineering steel conveyor chains. Many engineering steel conveyor chains must work with little or no lubrication, and thus material selection is very important.
Environment is a major concern in designing engineering steel conveyor chains. Standard conveyor chains are to work in mildly corrosive conditions. Some conveyor chains are also to work in highly abrasive conditions. Highly abrasive conditions are typically found in mining and material handling. Extreme temperatures are usually not a major concern in designing standard engineering steel chains.
There are many general types of engineering steel chains. Those with steel rollers are perhaps the most widely used on both drives and conveyors. The bushed, roller-less style meets the needs of many conveyor and bucket elevator applications. Welded steel versions of the basic cast chains are now quite popular, and a simple bar-link type is used for slow-moving conveyors and tension linkages.
Tension linkage chains are a series of chain products that are both catalog standard and manufactured for special purposes. The main use of a tension linkage chain is to move a load slowly or intermittently over a given distance. They also are used to reliably hold a load in position when it is not moving. Tension linkage chains generally move back and forth rather than through a complete revolution.
Tension linkage chains are used in a variety of ways. They may be used for hoisting, supporting counterweights, or pulling objects through forming operations. The loads in these applications can range from a few grams to several tons. The wide range of loads requires many different sizes and types of products to meet the different requirements.
Flat-top chains are used almost exclusively on conveyors. In practice, the flat-top chains are basically special types of slat conveyors.
UTS, FS, and FL are not major concerns in the design of flat-top chains but the yield strength is an important consideration while designing flat-top chains.
Wear is the most important parameter in the design of the flat-top chains. Joint wear and top plate and track wears are of the greatest concerns. Top plate and sprocket wears are also of some concern.
Joint wear is a very important consideration in designing flat-top chains. As the chain runs over sprockets, the joints articulate and material is worn off the outside diameter of the pins and the inside diameter of the top plates, and as this material is worn away the chain gets longer. The sprockets for flat-top chains are not designed to accept much wear elongation. Thus, when the chain elongates even a moderate amount, it no longer fits the sprocket and the conveyor does not function properly.
Wear between the top plates of the chain and the track, or wear strips, that they ride on is a major concern in designing flat-top chains. As straight-running chains operate, material is worn off the top plates and wear strips, and the top plates get thinner and weaker. As this type of wear progresses, the chain may start to malfunction or it may break.
As side-flexing chains operate, material is worn off the bevels or tabs of the top plates and the curved sections of track. As this type of wear progresses, the bevels or tabs may break off, allowing the chain to jump out of the track.
Top plate and sprocket wear is not very important in designing flat-top chains because the chain spends only a small part of the cycle articulating on the sprocket teeth.
Lubrication is a major concern in designing flat-top conveyor chains. Many flat-top chains are to work with little or no lubrication. When the chain operates with no lubrication, selecting materials for the top plates and tracks is extremely important.
Environment is an important consideration in the design of flat-top chains. Many flat-top chains operate in very abrasive or corrosive conditions and some flat-top chains operate in very low or very high temperatures. Material selection is critical in designing a chain that work well in these conditions.
Straight-running steel flat-top chain consists of a series of steel top plates with hinge-like barrels curled on each side. Pins are inserted through the barrels to make a joint. Pins are retained by press fits or heading in the barrels of one top plate and are free to articulate in the barrels of the next link. Thus a continuous length of flat-top chain is formed. The joints in straight-running chain permit flexing in only one plane. The barrels mesh with the teeth of a sprocket to drive the conveyor.
Side-flexing steel flat-top chain is similar to the straight-running type with one major difference. The barrels in which the pins are free to turn are specially formed to permit the joint to flex sideways. Thus, the chain can flex in two planes. The amount of side flexing is limited so that the chain retains enough strength and bearing area to work well as a conveyor. As a side-flexing chain is pulled around a curve, it is often pulled up and out of the track. Thus, side-flexing flat-top chains have bevels or tabs to hold them down in the tracks as they round a curve.
In most cases, the connector for flat-top chains is just a connecting pin. The connecting pin is usually either knurled or enlarged on one end to retain the pin in one barrel of the top plate. Sometimes the pin is just a straight pin and relies on a press fit in one end barrel to retain it.
The top plates have three functions in flat-top chain. The barrels on each side of the top plate mesh with the sprocket teeth and drive the conveyor. The top plates are the primary tension members in the chain and they must transfer all tensile loads from one link to the next. The top plates also serve as slats, as in a slat conveyor, and carry the conveyed material.
Pins transfer the tensile loads from the barrels of one top plate to the barrels of the next top plate, and they must do so while the pins are turning in the barrels. The pins in flat-top chains act as both beams and bearings. Pins need high surface hardness to resist wear when the joints flex and they need high strength to carry the conveyed loads.
Uses of chains
The major uses of the different types of chains are in drives (power transmission), conveyors, bucket elevators, and tension linkages. Some standard chains are designed for use in only one of these applications. However, some chains are designed so that they can be adapted to more than one use.
Roller chains and engineering steel chains are used in all types of applications. Leaf chains are used almost exclusively for lifting and counterbalancing. Silent chain is essentially for drive applications, although a few conveying applications exist. Flat-top chain is intended only for conveying.