Belt Conveyor Idlers
Belt conveyor idlers are the rollers which are used at certain spacing for supporting the active as well as return side of the conveyor belt. Accurately made, rigidly installed and well maintained idlers are very important for smooth and efficient running of a belt conveyor. Important requirements for idlers are proper support and protection for the belt and proper support for the load being conveyed.
Belt conveyor idlers for bulk materials are designed to incorporate rolls with various diameters. The rolls are fitted with antifriction bearings and seals, and are mounted on shafts. Frictional resistance of the idler roll influences belt tension and, hence, the power requirement. Roll diameter, bearing design, and seal requirements constitute the major components affecting frictional resistance.
Selection of the proper roll diameter and size of bearing and shaft is based on the type of service, load carried, belt speed and operating condition. Fig 1 shows different designs of roller mountings on antifriction bearings.
Fig 1 Different designs of roller mountings on antifriction bearings
Idlers are normally made from steel tubes, uniformly machined all over at the outer diameter and at the two ends of the inner diameter. The tubes are mounted on antifriction bearings over a fixed steel spindle. The ends of the spindles are flat machined to standard dimensions for quick fixing in slots of idler structure. The idlers can also be made of heavy steel tubes for severe service condition (like in material loading section) or cast iron in corrosive application (handling coke etc.).
There are the relative merits of the various antifriction bearings used, and the merits of the seals to protect these bearings from dirt and moisture and to retain the lubricant. Each belt conveyor idler manufacturer chooses a particular bearing and seal arrangement. Much ingenuity has been exercised by these idler manufacturers to provide dependable idlers.
Idler classifications
There are two types of idlers used in belt conveyors namely (i) straight carrying and return idlers, which are used for supporting active side of the belt for a flat belt conveyor and also for supporting the return belt in flat orientation in both flat or troughed belt conveyor and (ii) troughing idler set consisting of 2, 3, or 5 rollers arranged in the form of trough to support the belt in a troughed belt conveyor. The carrying idlers support the loaded run of the conveyor belt, and the return idlers support the empty return run of the conveyor belt.
The length of the straight or troughed idler set is based on the selected width of belt, and desirable edge clearance between belt and roller edges. Fig 2 shows some types of conveyor belt idlers.
Fig 2 Some types of conveyor belt idlers
Carrying idlers are of two general configurations. One is used for the troughed belts and normally consists of three rolls. The two outer rolls are inclined upward while the centre roll is horizontal. There is another configuration which is used for supporting flat belts. This idler normally consists of a single horizontal roll positioned between brackets which attach directly to the conveyor frame.
Return idlers normally are horizontal rolls, positioned between brackets which are normally attached to the underside of the support structure on which the carrying idlers are mounted. Two-roll ‘Vee’ return-idlers are also used for better training and higher load ratings.
Troughing carrying idlers – Due to the increased cross sectional fill depth, troughed belts can carry far greater tonnages than flat belts of the same width and speed. Troughing carrying idlers are sometimes referred to as troughers or carriers, and are the most common type of belt conveyor idler used. Rolls are normally fabricated from steel tube with end disc (bearing housings) welded to the tube ends. Rolls made from high molecular weight polyethylene are used where abrasion, material build-up, or corrosion create short shell life with steel roll idlers.
Troughing carrying idler sets are made with troughing angle (the angle made by the inclined roller with horizontal) of 15-degree, 20-degree, 25-degree, 30-degree, 35-degree, 40-degree, 45-degree, and 50-degree. Troughing angle of 15-degree is applicable only to two roll troughing idlers.
Historically, 20-degree troughing idlers have longer application histories than either 35-degree or 45-degree troughing idlers. As conveyor belt design technology has advanced, allowing greater transverse flexibility, 35-degree troughing idlers have become the most widely used type of troughing idler.
Troughing idlers are made as either in-line or off-set centre roll design. Three roll, in-line, equal length roll troughing idlers are most commonly used and offer the best all-around shape to carry a maximum load cross section. Fig 2 shows a 20-degree in-line troughing idler and an off-set centre roll (normally known as a grain idler) idler. The offs-set centre roll idler, utilizing the wing or side rolls located in an adjacent parallel plane to the centre roll, is used in either the grain industry where thin belts are used or underground mining where height clearance is minimal. Fig 2 also shows a picking and feeder (or picker) idler. This design uses a long (extended) centre roll and short side rolls inclined at 20-degrees to allow maximum product dispersal for inspection or sortation. Unequal length roll troughing idlers are also available where the side rolls are inclined at 35-degrees or 45-degrees.
Impact troughing idlers, sometimes called as ‘cushion idlers’, are used at loading points where impact resulting from lump size, material density, and height of material free fall can seriously damage the belt, if the belt is supported rigidly. Several types of impact troughing idler are available using pneumatic tires, semi-pneumatic tires, heavy rubber covers vulcanized to steel rolls, and individual narrow discs pressed onto a steel tube. This latter type, as shown in Fig 3, is the most common type of construction. Each disc is made of a resilient material such as a soft (40 to 50 durometer) natural rubber, grooved and relieved to allow the rubber to move under impact. The continuous (massed) row of disc gives better support to the belt than most pneumatic or semi-pneumatic types. The resilient discs help absorb energy from impact loads, which can save the belt from impact damage. The discs are sacrificed in favour of reducing the risk of belt damage. Impact troughing idlers with three equal length rolls have the same load rating as standard troughing idlers for a given CEMA (Conveyor Equipment Manufacturer’s Association) class (series).
Flat belt impact rolls as shown in Fig 3 have the same load rating as a single roll return for a given CEMA class. Flat belt type impact rolls are also available in a ‘live-shaft’ design supported by pillow block style bearings. These rolls are frequently used on heavy duty belt feeders and have a much higher load rating. These rolls are not covered by a CEMA standard, but are available from CEMA members on an application-by-application basis.
Fig 3 Different types of conveyor belt idlers
Even though an impact idler provides for some cushioning under the belt to help soften the force and reduce the possibility of damage, the impacting force has to be dissipated. The magnitude of these impact loads and their dissipation is not covered in CEMA load ratings.
Use of an impact style troughing idler as a transition idler is not recommended. Although not classified as idlers, there are a number of designs and configurations of fixed impact bars, impact saddles, impact cradles, and impact / slider beds available. These can solve some specific impact or sealing system application problems, but are not a ‘cure all’ solutions. These are not covered by CEMA standards, but are available from CEMA members on an application-by-application basis.
Belt training idlers, carrying – The normal carrying idlers are the primary devices which control the belt alignment. No self-alignment idlers are needed under well designed, precisely assembled, and maintained belt conveyors. There are transient conditions, however, which can cause conveyor belts to become misaligned despite all efforts to assure proper installation and maintenance. For this reason, conveyor manufacturers also furnish belt training idlers to help control belt alignment in difficult situations. Fig 3 shows a 35-degree troughed training idler.
The training idlers pivot about an axis vertically perpendicular to the centre line of the belt, and when the belt becomes off-centre, the idlers swing about so that the axes of the rolls themselves become canted in a corrective direction. This swinging action about the centre pivot is accomplished in various ways normally associated with the pressure of the off-centre belt against a fixed arm attached to the idler frame.
If the belt is to be reversed, the self-aligning idlers are to be of a type which swings about their pivot in a corrective direction regardless of the belt direction. Those types which depend on friction of the off-centre belt to shift the idler work in both directions of belt movement. Even with properly designed self-aligning idlers, the training of a reversing belt needs very careful alignment of all idlers and pulleys as well as levelling and alignment of the conveyor structure itself.
If belt training idlers are needed, they are to be spaced from 30 metres to 45 metres apart, and at least one training idler is to be used on conveyors less than 30 metres long. Belt training idlers are not to be used in areas of belt transitions.
Fixed guide rolls placed perpendicular to the edge of the conveyor belt are not normally recommended, since continuous contact with the conveyor belt edge accelerates belt edge wear, appreciably reducing belt life. In general, the greater is the belt tensions, the less effective is the training idlers.
Return idlers – Return idlers are used to support the return run of the belt. These idlers normally are suspended below the lower flanges of the stringers which support the carrying idlers. It is preferable that return idlers be mounted so that the return run of the belt is visible below the conveyor frame. Fig 3 shows a typical return belt idler. The value of troughing angle of troughed return idlers are selected from 0-degree, (i.e. straight idler), 10-degree and 15-degree for all widths of belt.
Flat return idlers – The flat return idler consists of a long single roll, fitted at each end with a mounting bracket. Idler roll length, bracket design, and mounting-hole spacing allow for adequate transverse belt movement without permitting the belt edges to contact any stationary part of the conveyor or its frame.
Self-cleaning return idlers – An important consideration with return idler applications is the adherence of materials to the carrying surface of the belt. Such material can be abrasive and wear the shell of the return idler rolls. Or, this build-up can be sticky and adhere to the return idler rolls. A large build-up can cause misalignment of the return run of the belt.
Several types of return idler rolls are available to overcome these difficulties. When sticky materials are a problem, rubber or urethane disc, or rubber coated helically shaped, self-cleaning return idlers can be used. Disc and helical rolls present very narrow surfaces for adhesion and thus reduce the tendency for material build up. This type of return idler sometimes is erroneously called a ‘belt cleaning idler’. Even though such idlers do ‘track-off’ material adhering to the belt surface on the return run, they do not constitute belt cleaning devices. Fig 3 shows self cleaning idlers.
If additional abrasion resistance is needed and sticky material is not the major concern, steel roll idlers with rubber polyurethane or polyethylene covering extend the wear life of the roll. The preferred disc type return idler has massed disc at each end of the roll to provide better belt support, if the belt is running off centre of structure.
Garland or suspended idlers – Garland idlers are available as two-roll, three-roll, or five-roll units. These idlers are also sometimes referred to as catenary idlers. Normally, two-roll units are utilized as return idlers and serve to aid in belt training due to the trough which is formed. Three-roll and five-roll units are used as carrying or impact units. Rubber discs can be utilized on three-roll units to provide additional cushioning. Five-roll units do not use rubber discs. All garland idlers are suspended from the conveyor framework by means of various devices such as hooks or chains.
The suspended design aids in belt alignment and handles large, irregular lumps because of the flexible connections and available vertical move. Five-roll designs offer a deep trough configuration and greater load capacities than conventional three-roll units.
Garland idlers can be furnished with quick release suspensions which allow the unit to be lowered away from belt contact in case of roll failure. Garland design idlers can be utilized on rigid frame or wire rope supported conveyor systems. Fig 4 shows the available types and their configuration under off-centre loads.
Fig 4 Various types of conveyor belt idlers
Return belt training idlers – Return belt idlers can be pivotally mounted to train or align the return belt in a manner similar to the training idlers previously described for the carrying run of the belt. Fig 3 shows return belt training idler. Training idlers for both one-way and reversible belts are available.
In general, a training idler designed for use on a one-way belt travel does not work on a reversible belt. Return belt training idlers are normally more effective than trough training idlers, due to lower belt tension.
Two-roll ‘Vee’ return idlers – On short conveyors, it can be necessary to equip the complete return run with the self-cleaning idlers. On long return belt runs, it is necessary to use these idlers only to the point where the material on the belt surface no longer adheres to and builds up on normal return idler rolls. Beyond this point, standard return idlers can be used.
With the increased use of heavy, high-tension fabric and steel cable belts, the need for better support and belt training has resulted in the development of ‘Vee’ return idlers. A basic ‘Vee’ return idler consists of two rolls, each tilted at a 5-degree, 10-degree, or 15-degree angle. These rolls are either of the garland (suspended) or rigid design. Fig 4 shows two roll ‘Vee’ return idlers.
The ‘Vee’ return idler has some training effect on the belt, while allowing greater idler spacing because of its increased load rating. The trough shape of the belt also tends to reduce or eliminate vibration along the conveyor. ‘Vee’ return-idlers can be supplied with steel rolls, rolls coated with some type of polymer, or spaced disc of rubber, urethane, or other material.
A decrease in wear life of the roll shell or spaced disc can occur with ‘Vee’ return idlers. This is caused by most of the belt weight contacting the roll around one-fourth of the roll length from the centre line of the idler.
Idler spacing
Factors to be considered when selecting idler spacing are belt weight, material weight, idler load rating, belt sag, idler life, belt rating, belt tension, and radius in vertical curves. More complex issues (such as belt flap or vibration stability in wind, and power usage from belt indentation, material tramping, and rolling resistance) are affected less by idler spacing.
For general conveyor design and selection, the belt sag is limited to 2 % of idler spacing at minimum tension conditions. Sag limits during conveyor starting and stopping is also to be considered in overall selection. If too much sag of a loaded troughed belt is permitted between the troughing idlers, the material can spill over the edges of the belt. For the best design, especially on long-centre troughed belt conveyors, the sag between idlers are to be limited.
Tab 1 lists the suggested normal troughing idler spacing for use in general engineering practice, when the amount of belt sag is not specifically limited. The figures given in the table on spacing are to be used in conjunction with the information on sag selection. Spacing is normally varied in 150 millimetres (mm) increments.
Tab 1 Suggested normal spacing of the belt idlersBelt width in mmTroughing idler spacing in mmReturn idler spacing in mmWeight of materials handled in kg/cum1.834.569124501,6501,5001,5001,5001,3501,3503,0006001,5001,3501,3501,2001,2001,2003,0007501,5001,3501,3501,2001,2001,2003,0009001,5001,3501,2001,2001,0501,0503,0001,0501,3501,3501,2001,0509009003,0001,2001,3501,2001,2001,0509009003,0001,3501,3501,2001,0501,0509009003,0001,5001,2001,2001,0509009009003,0001,8001,2001,0501,0509007507502,4002,1001,0501,0509007507506002,4002,4001,0501,0509007506006002,400Some conveyor systems have been designed successfully utilizing extended idler spacing and / or graduated idler spacing. Extended idler spacing is simply greater than normal spacing. This is sometimes applied where belt tension, sag, belting strength, and idler rating permit. Advantages can be lower idler cost (fewer number used) and better belt training.
Graduated idler spacing is greater than normal spacing at high tension portions of the belt. As the tension along the belt increases, the idler spacing is increased. Normally this type of spacing occurs toward and near the discharge end. Extended and graduated spacing are not normally used but if either is employed, care is to be taken not to exceed idler load rating and sag limits during starting and stopping.
Return idler spacing – The suggested normal spacing of return idlers for general belt conveyor work is also given in Tab 1. For conveyor belts with heavy carcasses, and with a width of 1,200 mm or more, it is recommended that the return idler spacing be determined by the use of the idler load ratings and belt sag considerations.
Carrying idler spacing at loading points – At loading points, the carrying idlers are to be spaced to keep the belt steady and to hold the belt in contact with the rubber edging of the loading skirts along its entire length. Careful attention to the spacing of the carrying idlers at the loading points minimizes material leakage under the skirt boards and, at the same time, also minimizes wear on the belt cover. Normally, carrying idlers in the loading zone are spaced at half (or less) the normal spacing suggested in Tab 1. The caution is that if impact idlers are used at loading zones then the impact idler ratings are to be no higher than standard idler ratings.
Good practice dictates that the spacing of idler rolls under the loading area is to be such that the major portion of the load engages the belt between idlers. Rubber disc idlers are normally used before, between, and at the end of the impact bars. Though this spacing is directed by bar length, idler rating selection and elevation are to be considered in the designs.
Troughing idler spacing adjacent to terminal pulleys – In passing from the last troughing idler to the terminal pulley, the belt edges are stretched and tension is increased at the outer edges. If the belt edge stress exceeds the elastic limit of the carcass, then the belt edge is stretched permanently and causes belt training difficulties. On the other hand, if the troughing idlers are placed too far from the terminal pulleys, spillage of the load is likely to take place. Distance is important in the change (transition) from troughed to flat form. This is especially significant when deeply troughed idlers are used.
Depending on the transition distance, one, two, or more transition type troughing idlers can be used to support the belt between the last standard troughing idler and the terminal pulley. These idlers can be positioned either at a fixed angle or at an adjustable concentrating angle.
The selection of Idlers
Earlier, CEMA ratings were based on 90,000 hours Bu (useful bearing life) at 500 rpm. Bu values were around 3 times L10 value. L10 life is considered as a guide for establishing idler ratings. The definition of L10 for the belt conveyor idlers is ‘the basic rated life (number of operating hours at 500 rpm) based on a 90 % statistical model which is expressed as the total number of revolutions 90 % of the bearings in an apparently identical group of bearings subjected to identical operating conditions will attain or exceed before a defined area of material fatigue (flaking, spelling) occurs on one of its rings or rolling elements’. The L10 life is also associated with 90 % reliability for a single bearing under a certain load.
The Bu (useful bearing life) theory was technically correct. However, L10 bearing life is more commonly used and accepted for bearing life calculations and rating. Previous to CEMA publication 502-1996, the CEMA idler selection procedure used idler life (K) factors to calculate an adjusted idler load. Some of these (K) factors were entirely independent of idler load and bearing L10 life. This procedure provided a conservative selection based on load but did not necessarily provide clear data relative to expected idler life.
Rating and idler life
Idler life is determined by a combination of several factors, such as seals, bearings, shell thickness, belt speed, lump size / material density, maintenance, environment, temperature, and the proper CEMA series of idler to handle the maximum calculated idler load. While bearing life is frequently used as an indicator of idler life, it is to be recognized that the effect of other variables (e.g., seal effectiveness) can be more important in determining idler life than the bearings. However, since bearing rating is the only variable for which laboratory tests have provided standard values, CEMA uses bearing for the life of the idler.
Load ratings for B, C, D, and E idlers as per CEMA are based on (i) CEMA B load rating based on minimum L10 of 30,000 hours at 500 rpm, (ii) CEMA C load rating based on minimum L10 of 30,000 hours at 500 rpm, (iii) CEMA D load rating based on minimum L10 of 60,000 hours at 500 rpm, and (iv) CEMA E load rating based on minimum L10 of 60,000 hours at 500 rpm. These loads and L10 life ratings are minimum ratings for CEMA rated idlers. Actual values for load ratings and L10 life for specific series and belt sizes supplied by CEMA manufacturers can be higher. In some cases the idler frame design can be the limiting factor for load with L10 life being a higher value.
Idler selection procedure – There are several conditions which affect idler life. Those considered in the selection procedure are (i) type of material handled, (ii) idler load, (iii) effect of load on predicted bearing L10 life, (iv) belt speed, (v) roll diameter, and (v) environmental, maintenance, and other special conditions. In addition to information provided in the idler selection procedure, the above items are summarized below.
Type of material handled – The characteristics of the material handled have a direct bearing on the idler selection. The weight of the material governs the idler load and spacing, and lump size modifies the effect of weight by introducing an impact factor.
Lump size considerations – The lump size influences the belt specifications and the choice of carrying idlers. There is also an empirical relationship between lump size and belt width. The recommended maximum lump size for various belt widths is described below.
For a 20 degree surcharge, with 10 % lumps and 90 % fines, the recommended maximum lump is one third of the belt width (bw/3). With all lumps, the recommended maximum lump is one fifth of the belt width (bw/5).
For a 30 degree surcharge, with 10 % lumps and 90 % fines, the recommended maximum lump is one sixth of the belt width (bw/6). With all lumps, maximum lump is one tenth of the belt width (bw/10).
Idler load – For the selection of the proper CEMA class (series) of idler, it is necessary to calculate the idler load. The idler load is to be calculated for peak or maximum conditions. The belt conveyor designer is required to thoroughly investigate all conditions relative to calculating idler misalignment load (IML), in addition to structure misalignment. The idler height deviation between standard fixed idlers and training idlers (or other special types of idlers) is to be accounted for either by idler series selection or by conveyor design and installation control.
Although it is recommended that calculated idler load (CIL) be equal to or less than CEMA idler load rating, there is a certain amount of judgment involved in final selection. For example, an experienced belt designer knows that the maximum IML based on belt tension occurs at the head or discharge for a level or incline conveyor. Since belt tension is decreasing from this point towards the tail or loading end, the number of idlers which slightly exceed the CEMA idler load rating can be determined.
Effect of load on predicted bearing L10 life – When calculated idler load (CIL) is less than CEMA load rating of series idler selected then the bearing L10 life increases. There is a relationship for either a tapered roller bearing or a ball bearing idler design. This relationship can be used in conjunction with the type of service or life expectancy of the conveyor system. If the specified design life of the conveyor system exceeds the CEMA L10 life rating at rated load, it can still meet specification based on percent of rated idler load versus calculated idler load (CIL).
Belt speed – The belt speed affects the predicted bearing L10 life. However, suitable belt conveyor speeds also depend upon the characteristics of the material to be conveyed, the capacity desired, and the belt tensions employed. Bearing life (L10) is based on the number of revolutions of the bearing race. The faster is the belt speed, the more is the revolutions per minute and hence, a shorter life for a given number of revolutions. All CEMA L10 life ratings are based on 500 rpm.
Roll diameter – For a given belt speed, use of larger diameter rolls increases the idler bearing L10. In addition, since larger diameter rolls is contacting the belt less because of lesser rpm, the wear life of the shell increases.
Environmental, maintenance and other special conditions – Hostile environmental conditions and the level of commitment to the belt conveyor installation and maintenance affect the idler life. With the above assumed conditions it is apparent that potential idler life is less than the predicted bearing L10 life. Expected or potential idler life can also be limited by shell wear. The wear of shell can vary considerably with each installation. In addition to conveyed material characteristics, environmental, and maintenance factors, idler alignment and belt cleaning can have a considerable effect on shell wear and idler life.
There are certain conditions which affect potential idler life. All of these conditions do not have an exact mathematical basis and hence can be very subjective. The most important phase in the selection of the idle is the identifying of the idler life condition for the application and then arriving at solutions to obtain maximum idler life for that application. Since idler roll configuration, type of bearing, and seal design can vary with each idler manufacturer, it is logical to state that idler life can also vary for a given environmental and maintenance condition. The environmental, maintenance and other special conditions are independent of idler load but can cause idler failure before obtaining predicted L10 life rating.
Special conditions – Idler roll shell material normally used throughout the industry is electric resistance welded steel mechanical tubing. For most belt conveyor applications, this material provides sufficient idler life most economically. For severe abrasive or corrosive conditions, covered idler rolls are available in a variety of materials. CEMA has not compiled a relative wear index or corrosion compatibility index for these various materials. The economic issue versus increased life is to be investigated thoroughly. Some of the generically available materials are (i) steel sleeves, (ii) rubber lagging, (iii) neoprene lagging, (iv) polyethylene sleeves / rolls, (v) carboxylated nitrile, (vi) urethane, and (vii) ceramic. There are numerous grades available in each of these materials which affect the performance.
Another consideration for increasing shell wear life is to use thicker metal shells. Some idler manufacturers customarily supply larger diameter rolls with thicker metal shells and normally offer optional shell thickness for all roll diameters. Idler shell wear life is more of a factor for the return idlers since it normally contacts the ‘dirty’ side of the belt resulting in abrasive wear of the shell. The exception to this is a conveyor system with a belt turnover system. With normal conveyor systems, materials build up on the roll and increase its effective diameter. Since the build-up is never uniform and normally is less at the belt edges, the clean sections of the return roll travel at a slower surface speed than that of the belt. This results in relative slippage, thereby accelerating wear of both the belt cover and the surface of the roll. Hence the life of the roll shell is normally shorter on return belt idlers than on carrying idlers. The material build-up can also aggravate belt training.
Impact forces at conveyor loading points also affect impact idler life. There are several variables available and different designs are used for the impact idlers at the loading points.
Quite often it is desirable to have the return idler spacing at a multiple of the troughing idler spacing to simplify stringer or truss design. However, this is not to be the control for selection.
If the conveyor has long centres, consideration is to be given to using two roll ‘Vee’ returns and increasing spacing. With this choice it is not necessary to use training idlers.
Calculated idler loads are to be repeated for training idlers (if used). Height deviation of training idlers are to be included for IML calculation or controlled by shimming and maintaining closer installation tolerances at these areas of conveyor.
Belt alignment
A belt conveyor is required to be designed, constructed, and maintained so that the belt consistently runs centrally on its mechanical system of idlers and pulleys. To accomplish this, the following conditions are to prevail.
There can be situations when the above procedure is not sufficient and the belt persistently runs to one side. The following corrective measures are to be initiated to prevent side creep.
Summarizing, there are a number of options available to the belt conveyor designer in regard to idler selection. The selection process of the idlers involves exploring of these options, resulting in a reliable, cost effective idler selection.
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