The invention relates to a mechanism for the automatic spooling of safety belts, including a belt spindle rotatable in a housing, upon which the belt is affixed and which is engaged by a tensionable retraction spring during the extension of the belt.
Automatic safety belts, such as utilized in particular in automobiles as well as in other types of airborne, surface and waterborne vehicles, should, among others, exhibit two characteristics.
The released belt should be respooled in a reliable manner, which implies a high belt retraction force for the rewind spring. The belt, as worn, should, on the other hand, impede the user as little as possible, which, in turn, implies the lowest possible belt retraction force for the rewind spring.
Several approaches are known through which these, mutually contradictory, demands can be reconciled with one another. For example, a rewind spring with negative spring characteristics may be used. With such a spring a substantially uniform torque is exerted on the belt spindle, independent of the length of extension of the belt. This results, in the very least, that the tension force exerted on the user of the applied belt is not greater than the rewind force at the end of the respooling process. However, the minimal rewind force required for an absolutely reliable re-spooling still acts on the user with the belt applied.
A substantial improvement is attained when an S-shaped spiral spring, wound onto two spindles, with a negative spring curve is used, as known from the prior art. With this known mechanism a substantial reduction in the rewind force acting on the applied belt is achievable, over that exerted at the end of the re-spooling process. However, the winding of the return spring on two spindles requires, necessarily, a detrimental enlargement of the dimensions of the respooling mechanism.
Further, mechanisms are known wherein, additionally to the rewind spring and opposing it, a weaker drive spring engages the belt spindle.
The expense, implied by the additional drive spring is disadvantageous; its dimensions define the retraction length over which the compensation of the rewind spring is effective.