VS - Electrical Systems From Stop-Start to Hybridization

t Sliding area Most important parameters to transmit higher torques: ●● increase the belt wrap angle on the pulley ●● increase the belt preload to have higher contact pressure area to reduce belt slippage ●● increase the number of ribs on the belt to get a higher friction coefficient The torque transmission generates belt tension fluctuations (slack span & tight span) which can be critical if the slack span belt tension goes too low, automatic tensioners are required to compensate this. When the torque demand gets too high and generates the complete slip on the belt, the “functional slip” becomes “complete slip” which is critical as it leads to a total loss of torque transmission. Automatic tensioners Automatic tensioners maintain a constant belt tension in the slack span over the engine operating range, they are designed to: ●● To assure a minimal belt preload in any boundary conditions - engine geometrical tolerances - ambient temperature (cold or hot) - belt conditions (new belt, aged belt) ●● To optimize the system lifetime (lower belt preload than static tensioner) ●● To damp some belt vibrations / engine irregularities Tensioners are made of a stiffness element to apply force on the belt in any conditions and a damping element to get a smooth dynamic system behavior, they can be designed with a symmetric or an asymmetric damping behavior, with a mechanical damping function or an hydraulic damping function. High torque transmission H T C In dynamic conditions the warp angle is the seat of some phenomenon at the surface of the pulley, where the belt partially sticks and slips. As the belt is a reversible elastic body, the length of the belt is getting longer when the internal force is changing from the slack span to the tight one. It generates micro-movements of the belt section which is running on the pulley ; this phenomena is called : “functional slip”. As the torque demand increases, the portion of the slip section to the wrap angle increases, leaving less space for the belt to stick and convey torque to the pulley.

H

Wrap angle

H = hubload

T

T

0

0

T 0 = static belt tension

The wrap angle: contact zone between the belt and the pulley

H

Sticking area

Sticking area

Sliding area

t

T

C

Low torque transmission

A naturel sliding is inevitable but it must be limited

H

Sticking area

Sliding area

ng area

t

T

C

High torque transmission

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