CHAIN DRIVES By S Vijayarangan Dept of Mechanical
CHAIN DRIVES By S. Vijayarangan Dept. of Mechanical Engineering, Dr. Mahalingam College of Engg. and Tech. , Pollachi 1
General Features Chain drives consist of an endless chain wrapped around two sprockets as shown in the figure below. A Chain Sprocket A Chain Drive Schematic arrangement A Chain Sample The chain consists of number of links connected by pin joints, while the sprockets are toothed wheels with a special profile for teeth. 2
The chain drive is intermediate between belt and gear drives. Chain drives are popular in the transportation industry, such as bicycle, motor cycles and automobile vehicles. They are also used in textile, building construction and material handling machinery. Metal and wood working machines also use chains for transmission of power. Chain drives are used for speed ratios less 10: 1 and with chain (pitch line) velocities less than 15 m/s. In general, they are recommended to transmit power up to 100 k. W. They are available in single or multiple rows as simplex, duplex, triplex chain etc. 3
Types of Roller Chains Single Row Roller Chain Multiple Row Roller Chain 4
Triplex Power Transmission Chain 5
POLYGON EFFECT: 8 The driving sprocket runs at constant (prime mover) speed; but driven sprocket will be running with fluctuating speed because of chordal (polygonal) action. 8 The chain passes around the sprocket as a series of (straight) chordal links. 8 This action is similar to that of a non-slipping belt wrapped around a rotating polygon. 8 The chordal action is illustrated in the figure below, where sprocket has only four teeth. It is assumed that the sprocket is rotating at a constant speed of ‘n’ r. p. m. 6
Fig-(a) Fig-(b) A Sprocket 8 In the Fig-(a), the chain link AB is at a distance of D/2 from the centre of the sprocket wheel and its linear velocity is given by 8 As the sprocket rotates through an angle ( /2), the position of the chain link AB is as shown in Fig-(b). 7
8 In this case, the link is at a distance of (D cos( /2) )/2 from the centre of the sprocket whose linear velocity will be 8 It is evident that the linear speed of the chain is not uniform but varies from Vmax to Vmin during every (rotation) cycle of tooth engagement. 8 This results in a fluctuating and jerky motion. The variation in velocity is given by ( Vmax – Vmin ) [ 1 – Cos ( /2) ] or ( Vmax – Vmin ) [ 1 – cos (180/z) ] 8
Zmin : Minimum Number Of Teeth On A Chain Sprocket: 8 (Vmax–Vmin) l {1–cos(180/z)} shows that the speed fluctuation will get reduced, if the number of teeth ‘z’ is increased. 8 It is observed that when z is 11 teeth, the speed variation is 4%; when z is 17 teeth the variation is 1. 6% and for z = 24 teeth, the variation is less than 1%. 8 Thus, the minimum number of teeth on a driving sprocket is arrived as 17; in practice, it is 19 or 21. 8 Usually, odd number of teeth on the sprocket, recommended in combination with even number chain links, to facilitate more uniform wear of the drive. is 8 In doing so, a particular tooth of the sprocket wheel will not come in contact with a particular link of the chain for every cycle of rotation; this leads to distribution of wear uniformly on all teeth and links. 9
Zmax : Maximum Number Of Teeth On A Chain Sprocket: 8 The chain gets elongated due to wear in the joints. 8 Such worn out chain has a tendency to shift outward upon the teeth of the sprocket because of the centrifugal force. 8 When the sprocket has a large number of teeth, the pitch will be small; i. e. the cavity of teeth will be small; under this situation, even a small elongation of the chain will result in a larger shift to the chain along tooth profiles. 8 Thus the outward shift of the chain limits the maximum number of teeth on the sprocket. 8 For roller chains, the recommended maximum number of teeth for the sprocket wheel is 100 to 120. 10
Classification Of Chains: The chains are classified, on the basis of their use, into the following three groups: a) Holding (leaf) chain b) Hoisting and hauling (or crane) chains. c) Conveyor (or tractive) chains. d) Power transmitting (or driving) chains. a) Leaf Chain 1. They are used to hold a dead weight in machinery, which may require to have some movement. 2. They are not power transmission elements 11
b) Hoisting And Hauling (Or Crane) Chains: These are chains are used for hoisting and hauling purposes in materials handling equipments and cranes These chains are of the following two types. 1. Chain With Oval Links 2. Chain With Square Links 1. Chain With Oval Links The links of this type of chain are of oval shape as shown in the figure below; each link of the joint is welded. The sprockets which are used for this type of chain have smooth and circular arc cavities to receive the links. Such type of chains could be used only at low speeds such as in hoists and anchors for marine works. 12
2. Chain With Square Links: The links of this type of chain are square shape as shown in the figure. Such types of chains are used in hoists, cranes, dredges. Cost of this type of chain is less than that of the chain with oval links, but the kinking occurs easily on overloading. c) Conveyor (Or Tractive) Chains: These chains are used for elevating and conveying the materials continuously. The conveyor chains are of the following two types. 1. Detachable or hook joint type chain 2. Closed joint type chain 13
The conveyor chains are usually made of malleable cast iron. These chains do not have smooth running qualities. The conveyor chains runs at slow speeds of about 3 to 12 kmph. Detachable Conveyor Chain Links Heavy Duty Conveyor Chain Links 14
d) Power Transmitting Chains: 8 These chains are used for transmission of power, when the distance between the centers of the shafts is limited to less than 1 m (rarely up to 2 m). 8 These chains need to have some arrangement or provision for efficient lubrication (either intermittently or continuously). 8 The power transmitting chains are of following three types. 1. Bush (or Block) Chain. 2. Roller Chain 3. Silent (or Inverted Toothed) Chain 15
Bush (or Block) Chain: 8 This type of chain was used in the early stages of development of power transmission. 8 It produces noise when approaching or leaving the teeth of the sprocket because of rubbing between the teeth and the links. 8 Such types of chains are used to some extent as conveyor chain at small speeds. 16
Roller Chain: The construction of a roller chain is shown in fig. It consists of alternate links made of inner and outer link- plates. There are five parts of roller chain namely, pin, bushing, roller, and inner and outer link-plates. The pin is press fitted to two outer link-plates, while the bush is press fitted to inner link-plates. The bush and the pin from a swivel joint and the outer link are free to swivel with respect to the inner link. The rollers are freely fitted on bushes and during engagement turn with teeth of the sprocket wheels. This results in rolling friction instead of sliding friction between the roller and the sprocket teeth and reduces 17 wear.
Construction of a Roller Chain Link Single Row Roller Chain Single link of a roller Chain Outer link of a roller Chain Elements of Roller Chain 18
Elements of a Roller Chain Assembly 19
The pins, bushes and rollers are made of alloy steels. The pitch (p) of the chain is the linear distance between the axes of the adjacent rollers. Roller chains are standardized and manufactures on the basis of the pitch. These chains are available in single-row or multi-row constructions such as simple, duplex or triplex strands. A roller chain is extremely strong and simple construction. It gives good service under severe conditions as well. in There is little noise with this chain which is due to impact of rollers on the sprocket of the wheel teeth. This chain may be used where there is little lubrication. 20
Types of Roller Chains Single Row Roller Chain Multiple Row Roller Chain 21
It is designed to eliminate the evil effects caused by stretching and to produce noiseless running. When the chain stretches and pitch of the chain increases, the links ride on the teeth of the sprocket wheel at a slightly increased radius. This automatically corrects the small change in the pitch. There is no relative sliding between the teeth of the inverted tooth and the sprocket wheel teeth. When properly lubricated, this chain gives durable service and runs very smoothly and quietly. 22
Inverted Tooth Chain Or Silent Chain: ] An inverted tooth or silent chain is shown in figure below. ]Compared with roller chains, the silent chains can be used for higher speed applications. ]They are, however more heavier, more difficult to manufacture and more expensive than roller chains. ]Their applications are limited due to these reasons. 23
Sprocket Wheels: 8 There are different constructions for sprocket wheels as shown in fig. Small sprockets up to 100 mm in diameter are usually made of a disk or a solid disk with a hub on one side (fig a & b). 8 They are machined from low carbon steel bars. Large sprockets with more than 100 mm diameter are either welded to steel hubs or bolted to cast iron hubs (fig c & d). 24
8 In general, sprockets are made of low carbon or medium carbon steels and on rare occasions, stainless steel is also used for sprockets. 8 When the chain velocity is less than 180 m/min, the teeth of the sprocket wheel are heat treated to obtain a hardness of 180 B. H. N. 8 For high speed applications, the recommended hardness is 300 to 500 B. H. N. 8 The teeth are hardened either by carburizing in case of low carbon steel or by quenching and tempering in case of high carbon steel. 25
Chain Cases And Lubrication: ] Chain drives should be enclosed to provide proper chain lubrication, to keep away dirt and grit from the chain and the sprocket and to ensure the safety of the operator. ] The casing should be easy to install and to open for inspection and lubrication. ] Except at very low speeds the chain should not run through the oil. ] A high speed chain running through oil causes an excessive amount of churning which raises the temperature of the oil and which may create high pressure in the case so that leakage may occur. ] Oil disks or rings may be used to distribute the lubricant. 26
Some simple methods of lubricating a chain (Courtesy – Mechanical Design by Peter Childs) 27
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Advantages Of Chain Drives: Chain drives can be used for medium centre distances, where gear drives will require additional idler gears. A number shafts can be driven in the same or opposite direction by means of chain from a single driving sprocket. The efficiency of chain drives is higher than belt drives, at times as high as 98%. A chain drive does not slip and to that extent, it is a positive drive compared to belt drives. The chain drives gives less load on the shafts bearing. Since chains are made of metals, they can carry more load in the given space. 29
Limitations Of Chain Drives: 8 Compared with belt drives, chain drives require proper maintenance, particularly lubrication and slack adjustment. 8 They produce noise during operation. 8 Chain drives can be used only for parallel shafts. 8 The production of chains is relatively high when compared with belt or rope drives. 8 However, it is unsuitable where precision motion is required due to polygon effect and wear in the joints. They also require adjustment for slack, such as tensioning device. 30
- Slides: 30