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A sprag (clamping element) is a component with increasing height, which stands constantly in friction contact with the inner ring and outer ring.
The sprag design shows at the contact surfaces a sophisticated geometrical shape, the engagement curve, which results in a certain clamping angle (αa, αi) when the sprags get in contact with the round mating parts. This part of the engagement curve is called operating zone.
The engagement curve must guarantee all over the operating zone the basic function: (µ = friction value).
| µ > tan α |
When torque is applied to the clutch the sprags roll in along the engagement curve until a balance of tension in the sprags and mating parts is achieved.
From this moment torque can be transmitted from the inner ring to the outer ring (or the other way round). The size of the twisting angle (movement between inner and outer ring until torque can be transmitted) depends on the applied torque and on the stiffness of the mating parts - this twisting angle should not be mistaken for any slippage.
Equal force acting on all sprags is required from the instant of engagement to full torque transfer. To achieve these factors it is important that all sprags engage simultaneously and with the same angle for torque pickup.
The special geometric shape of the logarithmic spiral along the engagement curve guarantees equal clamping angles between the sprags and round mating parts in each position of the engagement curve. This angle remains equal even with different position angles of the sprags. The logarithmic spiralled engagement curve is prerequisite for precise indexing - even at high indexing frequencies - long operating life and high transmittable torque.
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