PULLEYS
A deeper understanding of conveyor pulley friction Despite significant research, there is still much more to learn. Experts from the University of Newcastle and Elastotec investigate the emerging issues around pulley friction, the complexity of the issue, and the failures that may result. BELT CONVEYORS FORM A CRITICAL part of the materials handling process and have established themselves as the chosen technology for transporting bulk materials at high tonnages over long distances. As the scale of new mining operations continues to increase, this has resulted in an unprecedented demand on these systems to convey material further, faster, and up greater elevation. Significant improvements in the tensile strength of belts and the refinement of dynamic models to assess the transient tensions under starting and stopping conditions have allowed for reduced safety factors, greater transport distance and lift, as well as faster belt speeds. Despite the considerable amount of research that made these achievements possible, much room for improvement remains in understanding the effectiveness of drive systems to transmit the large amounts of power now required by these systems. The drive system forms a critical component of any belt conveyor, tasked with transmitting the force required to start and stop the conveyor, as well as maintain a constant operating
Figure 1: Euler drive friction model.
velocity. This force, known as the effective belt tension, is traditionally transmitted through a drive pulley, to the pulley lagging (if installed), to the rubber bottom cover of the conveyor belt, and eventually to the reinforcing carcass (steel cord or fabric) within the conveyor belt. The effectiveness of this transmission is ultimately defined within a frictional contact between the conveyor belt and the drive pulley surface.
Review of current design methods Drive system design relies on the respective belt tension either side of the drive pulley, denoting T1 as the tight side tension, and T2 as the slack side tension. The difference between these two represents the effective tension (Te) outlined above, and the force required to be transmitted through the drive system to allow the belt to operate. The design method is well understood, based on Euler’s classic ‘rope friction’ model defined below. T1 =eμθ T2 Where, µ = the coefficient of friction between the belt and the pulley surface, and
Physical damage and delamination on a high-tension bend pulley began to show after three months.
θ = the angle of wrap of the belt around the pulley The simplicity of this model naturally has corresponding limitations, the majority of which are also understood within industry. The assumption that the friction is fully developed around the arc of contact relates only to rigid body contact and means that each point of contact around the pulley surface exhibits an equal coefficient of friction. This is not possible for a viscoelastic drive mechanism in shear. The gradual increase in belt tension around the drive pulley, coupled with the varying normal force (see Figure 1) results in a varying degree of viscoelastic slip within the contact, and therefore a varying coefficient of friction. For viscoelastic surfaces, friction may only truly reach is maximum kinetic value when the belt is slipping, or on the verge of slipping. Design standards such as ISO5048 and DIN22101 compensate for this by utilising
Australian Bulk Handling Review: September/October 2021 І 59
