Note that the relative motion around the pulley is constantly increasing as the belt increases its speed more and more in accordance with the increasing elongation (condition of continuity!), but the pulley has a constant circumferential speed. This means that the belt speed and the peripheral speed of the pulley are only equal when the belt runs onto the driven pulley, otherwise the belt speed will be higher or the pulley speed lower. The relative motion on the pulleys, which is always present due to the elasticity of the belt, is called elastic slip (partial relative motion between belt and pulley)! i sprayed some wd40 on the drive and air con belt, i think it helped a bit, but the noise is still there. i know that there is a special spray for squeeky drive belts, do these work well? Its advanced silicone synthetic formula is safe for use on metal, rubber, wood, and vinyl and also protects electrical parts and is perfect for wet environments and marine use. It's also NSF H2 registered making it safe for use in food-related environments. The reduction in peripheral speed between the driving pulley and the driven pulley is directly relatet to a loss in power, because a decrease of the circumferential speed v at a transmitting circumferential force F c means a direct decrease in power according to the P=F c⋅v.

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The figure below shows schematically the distribution of the speed along the belt according to the animation above. Figure: Speed distribution along the belt Thus, the elastic slip S can also be determined by the power loss ΔP with respect to the power P i at the input pulley: Delta P = P_i – P_o = F_c \cdot v_i –F_c \cdot v_o = F_c \cdot \underbrace{(v_i-v_o)}_{=v_i \cdot S} = F_c \cdot v_i \cdot S = P_i \cdot S \\[5px]

Why do belts slip?

The sliding angle φ’ relevant for power transmission can also be expressed by the circumferential force F c to be transmitted. Thus follows with F c=F t-F s or F t=F c+F s: The belt adapts to the different speeds by elastic slip on the pulleys! Circumferential speed of the pulleys As already explained in the section Belt speeds, the belt strains ε and the belt speeds v are directly interrelated. Mathematically this can be expressed as follows: So if the driving pulley generally moves faster than the belt and the driven pulley is slower, then the circumferential speeds of the pulleys are obviously no longer identical (this would only be the case with a complete inelastic belt).This ultimately results in a loss of circumferential speed between the drive pulley (rotating faster than the belt) and the output pulley (rotating slower than the belt).

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The more the belt stretches, i.e. the greater the elastic slip, the greater the difference in belt speeds and thus also in the circumferential speeds of the pulleys. Therefore, the elastic slip S can be defined by the relative loss of speed at the circumference of the input pulley (v i) and the output pulley (v o):The entire wrap area φ can therefore be divided into two zones. In the co-called sliding zoneφ’ a relative motion takes place between belt and pulley. With the acting sliding friction, this zone ensures the transmission of the circumferential force. In the remaining adhesion zone, the belt adheres to the pulley without a relative motion and without force transmission.

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The elastic slip influences the belt speed and thus the power but not the circumferential force or the torque! Such (elastic) stretching or shrinking processes of the belt on the pulleys, which inevitably lead to relative motion, are also called elastic slip. From the belt’s point of view, elastic slip should always be kept as low as possible, otherwise enormous belt wear will occur due to the strong relative motion. The surfaces of pulleys must therefore not be too rough, as one might misleadingly assume due to the increased static friction on rough surfaces!

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As can be seen from the animation below, the belt is running more and more ahead due to the increasing stretching on the driven pulley. This means that the speed of an imaginary point on the belt is always slightly higher than the peripheral speed of the pulley. This corresponds to the relative motion between belt and pulley described above. Sliding slip is the complete sliding of the belt over the entire pulley in the event of overload (complete relative motion between belt and pulley)! Belt speeds Since the circumferential speeds can also be expressed by the rotational speeds n and pulley diameters d (v=π⋅d⋅n), the elastic slip can also be determined as follows: Conversely, the belt section coming from the tight side (and thus maximally stretched up) contracts during rotation around the driving pulley due to the decreasing belt force. The belt shrinks on the driving pulley, so to speak, and thus also results in relative motion and thus in sliding.