The main advantages to the use of conveyor chain is its ability to transmit power directly from a driven shaft and to carry loads via its integrated rollers. As conveyor chains can be designed to handle different loads and powers, it has developed over the past 100 years in to a cost effective and reliable method to convey products and materials. Numerous standards have been developed in order to ensure that chains from different manufacturers are compatible, although bespoke precision conveyor chains have been developed for some of the most demanding applications. Modern conveyor chains use precision machining, high quality steel and specialist heat treatments in order to provide reliable, low friction, high strength chains.
Types and Style
There are many types and styles of chains available on the market today. However, there are only a few variations when discussing conveyor chains. The first item that varies is the link plates, which can be either straight or offset (cranked). The second variation is with regard to roller options. Conveyor chains are supplied with a flanged roller, standard plain roller, small or shell roller and bush only (no roller). Material selection is generally based upon application requirements, as are applied heat treatments and coatings.
Special chains have been developed for use in scraper conveyors such as;
Block and bar chains – Inner links are a solid block, joined to the next solid inner block, using parallel steel plates.
Forged link chain – All links in the chain are identical and fabricated from high strength forged materials.
Basic Chain Arrangements
For the most basic conveyors there are three conveyor chain arrangements;
Chain and material sliding
Chain rolling and material sliding
Chain rolling and material carried
The three arrangements above can be used either on horizontal or inclined conveyors.
Conveyor chains can also be used on elevators where the chain and material move vertically, with the material carried by attachments such as buckets.
On very rare occasions the chain will slide and the material will be carried on the conveyor chain. However this type of arrangement is not generally recommended due to the potential wear on both the conveyor structure and chain link plates.
Basic Conveyor Considerations
The general layout of a conveyor should be considered before chain selection. The recommended arrangement is for the loaded strand/s of conveyor chain to be rolling and fully supported on both the working run and the return run, with an allowance made for small amounts of catenary sag. Other arrangements are available for the return strand such as;
Unsupported return strand – The weight of the return strand must be considered.
Return strand supported by rollers – Catenary sag between rollers must be considered
Return strand sliding (i.e. support strips under slats) – Increased frictional loads
It is also possible for the conveyor chain to be sliding on the working run, although increased frictional forces must be considered when making a chain selection.
Safety and Overload Protection
For all arrangements of conveyor, care should be taken to restrict access to moving parts, including the entire run of the chains. Access panels should be incorporated for inspection, installation and repair
If the conveyor is to be started while fully loaded, care should be taken to avoid shock loading of the conveyor chain. A fluid coupling or centrifugal clutch can be employed to gradually increase the torque, facilitating a smooth acceleration.
Damage to the chains may occur if the conveyor jams or becomes overloaded. Torque limiters, fluid couplings or shear pins can be used to prevent overloading of the conveyor chains.
For inclined conveyors or elevators, run back protection should be considered. Due to increased efficiency it is no longer acceptable to rely on the gear box to prevent chain runback. Ratchets, electromagnetic brakes or automatic brakes can be used, although a small amount of run back can occur with the possibly of shock loading the conveyor chain. The best solution is to use a sprag clutch, which prevents all run back and potential shock loading.
In the event of conveyor chain failure, hinged pawls can be employed to lock up against the chain rollers, preventing the chain before the failed link running back and incurring further damage.
Chain pitch is the distance between neighbouring bearing pins and can be metric or imperial, depending upon the series of chain. The recommended chain pitch for a given application usually depends upon bearing loads, sprocket diameter and chain speed, required attachment spacing and cost.
Increasing the pitch of the chain increases the bearing loads and sprocket diameters, but reduces the cost, as less round parts are required for a given length.
Chain Loads and Friction
There are several factors to consider when calculating the loading of the chains.
The first consideration is the total weight of the chain itself, plus any attachments or fixings added to the plain conveyor chain.
The next consideration is resistance to motion, such as friction, inclines and product head loads.
The final consideration is the total weight of product expected to be on the conveyor at any given time.
Chain or conveyor speed is a critical factor when designing a conveyor system. Although often governed by the required through put, low or excessive speed can have a detrimental effect on chain life and conveyor performance.
For example, if the speed is too high or too low, the rollers can cease their rotation, causing flat spots on the rollers and dynamically altering frictional forces over the entire system, resulting in ‘surging’.
‘Surging’ also occurs when the chain makes contact with the sprocket, due to the polygon effect. For example a conveyor chain operating around an 8 tooth sprocket with a nominal speed of 100 ft/min, would actually see a theoretical speed variation between 92.4 ft/min and 100 ft/min. The detrimental effect of sprocket surging is increased by higher speeds or loads, or lower numbers of teeth on the sprocket.
As a general rule, the greater the number of sprocket teeth and the smaller the pitch of the chain, the greater the permissible speed of the conveyor.
Consideration must be given to the operating environment when selecting a conveyor chain for a given application.
For abrasive hostile environments, bush chains are often utilised to remove wear that would occur between a roller and bush. However, removal of the roller would increase the frictional forces, requiring an increase in chain breaking load and power required at head shaft.
Lubrication must be considered. Poorly lubricated chains have a higher resistance to motion, increasing the frictional forces.
Corrosion can cause premature wear or failure of conveyor chains, especially when exacerbated by a ‘surging’ conveyor. Corrosion can be caused by many factors, including condensation during shutdown, chain contaminants, high or low PH level materials, entering and leaving tanks, total or partial immersion and condition of conveyed material (i.e. damp, dry, wet). The chance of 'stress corrosion cracking' can be reduced by increasing the breaking load of the chain or by applying protective coatings.
Non ambient running temperatures can have a dramatic effect on the permissible working load of a chain. At low temperatures, steels encounter a ductile to brittle transition which decreases the breaking load rating of the chain. For low allow steels, -30°C can reduce the permissible working load by up to 75%.
At elevated temperatures above around 200°C, low alloy chains can have their permissible working load reduced by up to 50%. Above 300°C, special materials must be used in the construction of conveyor chains.
Bearing pressure is the pressure exerted between the pin/bush and bush/roller.
For bush/roller bearing pressure the normal maximum, for a conveyor moving at 0.5m/s, is 1.8N/mm2, assuming a good degree of cleanliness and lubrication. If the bearing pressure exceeds this value, but the chain is well lubricated and in a clean environment, then a higher bearing pressure may be acceptable. However, if the conveyor is moving faster than 0.5m/s, then one must examine the rubbing speed, with a maximum value for average conditions set at 0.25.
For very low speeds it is advisable to use bearings within the rollers, to decrease rolling friction, or design a roller with an o/d to bore ratio greater than 3.
If the calculated bearing pressure and rubbing speed both exceed the suggested maximum values, then it is suggested to user a larger chain size with larger rollers, use outboard rollers or use ball bearing rollers.
Chain Pull Calculations
For new applications or modifications to an existing system, where there is uncertainty about the loadings on the chains, chain pull calculations can be performed for various arrangements of chain conveyors and elevators.
The pull calculations take into account the conveyor speed, angle of any inclines, resistances due to friction and bends, product weight, attachment weight, chain weight and running environment.
It is strongly recommended that the conveyor details are supplied to the chain manufacturer rather than attempting the calculations without assistance.
If chain pull calculations are performed for a particular conveyor arrangement, it is possible to determine the approximate amount of power required at the head shaft. Once the power required has been established, it is possible to make an informed decision regarding the drive arrangement, including motor specification.
Effective lubrication of the chain bearing surfaces is essential to obtain optimum performance in addition to minimising power absorption, rate of wear, liability of corrosion and noise.
The lubricant used for chains must be of a grade capable of reaching the bearing surfaces between the bearing pin and bush, and between the bush and roller, and with adequate body to maintain an oil film over the whole of these surfaces. It must also maintain its lubricating properties under operating conditions and be free from corrosive elements.
In all cases the lubricant should be applied immediately after the chain leaves the driving wheel and with the chain running. This is the point of least tension and the most likely position where the lubricant will reach the rubbing surfaces. Chains can be lubricated automatically with drip feed or oil mist spray lubricators or manually with a brush and lubricant. For normal conditions a good quality mineral based lubricant with a medium viscosity is recommended.
Mineral based lubricants carbonise at about 140ºC, thus causing a build up of carbon between pin/bush and bush/roller. For temperatures up to 300ºC a colloidal graphite lubricant suspended in a volatile carrier should be used. Evaporation of the carrier (usually white spirit) leaves a film of graphite on the wearing surfaces, but this will not be retained for a long period and must be re-applied at regular intervals.
Chains operating in abrasive conditions can also be lubricated with a dry lubricant but for extremely abrasive applications grease gun lubricated chain be used:-
Prevention of contamination of lubricant is usually considered in the context of handling food for human consumption. On such applications, vegetable oils or medicinal paraffin may be acceptable, and will provide satisfactory lubrication when applied by the normal methods. If non lubricated chains are essential, a reduction in chain life must be accepted, and taken into account when selecting the chain. For dairies or similar industries the possibility of bottles being marked by a lubricant is not acceptable, and soluble oil or liquid soap and water are widely used. When designing conveyor systems on which product contamination will occur, every effort should be made to avoid proximity of the chain to the product, as this will obviously assist the provision of satisfactory lubrication without running into contamination difficulties.
Chains operating in dirty water environments, such as a sewage treatment works, are often completely immersed, making it impossible to lubricate regularly. Under these conditions, chains are either designed to be replaced frequently, or special materials are used in their construction, to allow continuous operation in wet conditions. Where chains are accessible, grease gun lubrication should be used with water repellent grease, to periodically flush out old grease and contaminants.
Determining Conveyor Chain Range
Initial conveyor chain selection is based upon the required breaking load of the chain. A safety factor must then be applied to ensure that the conveyor chain link plates do not exceed their elastic limit during operation (as this would damage the link plates and extend the pitch). The most common range of safety factor is between 8 and 12 but is dependent upon the application and running environment.
Once the breaking load has been established using the relevant safety factor, a decision must be made between the use of a narrow British standard chain or wide metric chain. This is generally determined based upon the application, with the wider metric series chains offering a wider roller and therefore a larger rolling area, smaller bearing pressures and reduced stresses upon the sprocket teeth.
The design and number of teeth on a sprocket is an important consideration when designing a conveyor. Larger sprockets have a larger diameter, which means the difference in distance between the send and return strands is increased. This in turn increases the depth of the conveyor, or requires a guide to bring the return strand closer to the send strand. Larger sprockets have a greater number of teeth, with a greater number of rollers engaged at any one time. This decreases the loads on each individual tooth, but limits the permissible chain extension allowed before the conveyor chain will not correctly gear with the sprocket.
Larger sprockets also decrease the variation in chain speed when the chain engages/disengages from the sprocket. Chain wheels are in effect a polygon, which causes the chain to rise and fall relative to the sprockets axis. 8 teeth is generally considered the smallest recommended sprocket for a chain conveyor, with a speed variation from a nominal speed of 100ft/min being 92.4 ft/min to 100 ft/min due to the polygon effect.
Sprockets can be manufactured in various arrangements. The most common is solid sprocket consisting of a flat plate and welded boss. If shaft removal is difficult, then split sprockets can be manufactured with flush fitting machined faces, with the two halves of the sprocket being assembled around the shaft. For applications where extreme tooth wear is experienced, then a removable tooth ring can be bolted to a central boss which remains on the shaft. If split in to 3 or 4 segments, then each segment can be replaced without having to un mount the chain from the sprocket.
Fabrication can be implemented in standard mild steel or harder wearing high carbon steel. For maximum sprocket life, the teeth can be flame or induction hardened, although the harder tooth face will inflict wear upon the chain rollers. Sprockets can also be fabricated from stainless steel for high temperature, corrosive or food environments, although plastic sprockets are a cheaper alternative for lighter duty applications in the food industry.