Debswana Best Practice Guideline Mechanical: Belt Alignment Using Self Aligning Idlers ISSUE 0 21 February 2008 Page 1 of 7
CONTENTS PAGE 1 INTRODUCTION 3 2 SCOPE 3 3 REQUIREMENTS 3 APPENDIX A : RECORD OF AMENDMENTS 7 ISSUE 0 21 February 2008 Page 2 of 7
1 INTRODUCTION A belt conveyor must be designed, constructed and maintained so that the belt consistently runs on its mechanical system of idlers and pulleys. This guideline provides some information to assist in achieving this. 2 SCOPE This guideline applies to all those involved in the maintenance and tracking of conveyor belts. 3 REQUIREMENTS 3.1 BELT ALIGNMENT 3.1.1 The tail and head pulleys must be square with the conveyor frame. 3.1.2 All carrying, impact and return idlers must be square with the conveyor frame. Idlers must be in line, and lie in the same horizontal plane. The attachment bolts must be properly tightened. 3.1.3 All frames must be cross-levelled to ensure a cross-section parallel to the ground plane. If one side of the conveyor frame is lower than the other, gravity will force the belt off centre. The requirements of Debswana specification DS 373005 apply. 3.1.4 The belt must be straight and the belt splice square. If side creep occurs only in the vicinity of the belt splice, the splice may not be square with the belt. In general, if the creep follows the belt, there is a problem with the belt. If it remains in one general vicinity, there is a problem with the system. 3.2 BELT WANDER OR CREEP There may be times when the above procedure is not sufficient and the belt persistently runs to one side. The following corrective measures may be initiated to prevent side creep. 3.2.1 While running the belt at the lowest speed possible, find the point of maximum side creep. The idler preceding this point along the direction of belt travel can be adjusted to minimise side creep. Facing the conveyor from the tail end, the idler must be pivoted clockwise to correct side creep on the left and counter-clockwise to correct side creep on the right. Once the belt is centred, change to a higher speed (if possible) and load the belt with material. Continue adjusting until normal operating conditions do not cause the belt to misalign. 3.2.2 If creep persists, ensure that the head and tail pulleys are perfectly aligned. Steer the belt with the carrying or return idlers. Here it must be noted that the steering action cannot be satisfactorily achieved with only one idler set. Normally, for belt to creep consistently in a specific area indicates some structural damage in that area. If the structure cannot be straightened, then the belt must be steered. The location and exact nature of the misalignment of the conveyor is achieved by observation of the running belt over several cycles, both in the loaded and unloaded conditions. Once the steering area is identified, the belt may be corrected by adjustment of several idler sets before and after the location of the peak misalignment. ISSUE 0 21 February 2008 Page 3 of 7
3.3 SELF-ALIGNING TRAINING IDLER Self aligning training idlers can be installed to replace troughing or return idlers. These should only be used in problem systems and should ideally be up to about 15m from any terminal or bend pulley to achieve maximum benefit. 3.3.1 Do not use a training idler in vertical curves, either concave or convex. 3.3.2 Reversible belt training idlers are available for reversible belt conveyors. However, the use of specially designed swivelling idler sets for reversible conveyors is preferred. 3.3.3 Free rotation of the training idlers swivel mechanism is essential for satisfactory tracking results. 3.3.4 At least one training idler may be installed on every conveyor under 10m in length. It must be noted that the training effect of idlers is reduced with increasing tension. The training idlers should be installed at the penultimate carrying idler before the transition at the head end of the conveyor or at a tripper. On the return strand, the training idler may be located at the penultimate return idler before the tail pulley or return bend pulley. 3.4 PERSISTENT CREEP If creep still persists, some or all of the troughing idlers may be tilted not more than 2 degrees from the veryical, in the direction of the belt travel. This can be accomplished by using a steel flat washer between the conveyor frame and the troughing idler footplate. Caution: Troughing idlers which have this tilt built in should not be additionally tilted. Reversible belts should not use tilted idlers, as the misalignment of the belt would be accentuated when it runs in the reverse direction. The effect on belt tension and idler roll shell wear should be considered when tilting the idlers 3.5 LASER ALIGNMENT If none of the above steps solve a belt misalignment condition, the conveyor should be laser aligned and corrective action taken based upon the survey data. NOTE: Maximum tracking effect occurs when the distance between the corrective component and the following component is maximised. 3.6 BELT TRAINING IDLERS 3.6.1 Carrying Strand 3.6.1.1 The normal carrying idlers are the primary devices which control the belt alignment. No self-aligning idlers are needed under well-designed, properly assembled and maintained belt conveyors. There are transient conditions. however, that may 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. ISSUE 0 21 February 2008 Page 4 of 7
3.6.1.2 The training idlers pivot about an axis vertically off-set to the centre line of the belt and when the belt becomes off-centre, they swing about so that the axes of the rolls themselves become canted in a corrective direction. This swinging about the centre pivot is accomplished in various ways usually associated with the pressure of the off-center belt against a fixed arm attached to the idler frame. 3.6.1.3 If the belt is to be reversed, the self aligning idlers must be of a type which will swing about their pivot in a corrective direction regardless of belt direction. Those types which depend on friction of the off-centre belt to shift the idler will work in both directions of belt movement. Even properly designed self-aligning idlers, the training of a reversing belt requires very careful alignment of all idlers and pulleys as well as levelling and alignment of the conveyor structure itself. 3.6.1.4 If belt training idlers are required on the carrying strand of a longer conveyor, they should be spaced no closer than 50m to 75m apart. However, at least one training idler should be used on conveyors less than 30m long. Belt training idlers should never be installed in areas of belt transitions. 3.6.1.5 Fixed guide rolls placed perpendicular to the edge of the conveyor belt are not generally recommended, because continuous contact with the conveyor belt edge accelerates belt edge wear, appreciably reducing belt life. In addition, the pressing action of the belt against the perpendicular roller will very rapidly wear the roller face, which will further increase the danger of belt damage. In general, the greater the belt tensions, the less effective the training idlers. 3.6.1.6 Where belt creep occurs at locations along the length of longer conveyors, it is useful to arrange three self-aligning idlers in consecutive oitches at the start of the misalignment. This will relive the loading on a single self aligning idler and will ease the corrective action. In all other cases, as ingle self aligning idler should be sufficient. 3.6.2 Return Belt 3.6.2.1 Return belt idlers can be pivotally mounted to train or align the return belt in a manner similar to the training idlers described for the carrying run of the belt. These self aligning idlers may be in either single-roll or multiple roll. Flat form or vee form, as required. 3.6.2.2 Training idlers for both one-way or reverse belts are available. In general, a training idler designed for use on a one-way belt travel will not work on a reversible belt. 3.6.2.3 Return belt training idlers are often more effective than trough training idlers due to lower belt tension and the greater span between consecutive idler sets. 3.6.2.4 The location of the return trainer is very important. The return trainer should be located at the penultimate return idler pitch before a terminal or bend pulley. 3.6.2.5 For locations along the length of longer conveyors, it is useful to arrange three self aligning idlers in consecutive pitches, at the start of the misalignment. This will relieve the loading on a single idler and will ease the corrective action. See also 3.6.1.6 above. 3.6.2.6 If belt training idlers are required on the return strand of a longer conveyor, they should be spaced no closer than 100m to 150m apart. ISSUE 0 21 February 2008 Page 5 of 7
3.7 SYSTEM MAINTENANCE It must be stressed that self aligning training idlers can and should never be seen as a substitute for adequate and continuous belt maintenance. A properly implemented belt maintenance program will enable the operations personnel to timeously identify and correct any potential for misalignment in the system. Self-aligning training idlers can never be used as a corrective measure for belts that misalign as a result of material build up on the idlers, or as a result of structural damage to the conveyor supports or spillage. Self-aligning training idlers must be seen as an aid to good maintenance practices and an insurance against spurious overloads or mishaps. 3.8 COMMERCIALLY AVAILABLE UNITS A number of commercially available units are available such as Tru Trac, Hosch Tracker, Sureguide, Melco Tamer, Osborne, Continental Meco etc. It must be noted that the list is by no means exhaustive. The list is also by no means prescriptive and does not exclude other makers from providing self aligning idlers. The units are generally available for both carry and return strands. ISSUE 0 21 February 2008 Page 6 of 7
APPENDIX A : RECORD OF AMENDMENTS Issue 0 : Converted from De Beers Code of Practice COP 002 Issue 1 dated 28 June 2001 ISSUE 0 21 February 2008 Page 7 of 7