Endless belt power transmission systems or components – Control for variable input to output speed-ratio
Patent
1993-03-04
1994-01-18
Bui, Thuy M.
Endless belt power transmission systems or components
Control for variable input to output speed-ratio
F16H 6300
Patent
active
052795241
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a pedal crank drive for a bicycle, comprising two pedal cranks, which are non-rotatably interconnected by a pedal crank shaft, a chain sprocket, which has at least one gear ring and is mounted to be rotatable relative to the pedal crank shaft, and at least one force-transmitting spring, which is disposed between the chain sprocket and that pedal crank which is adjacent to the chain sprocket, and said spring is adapted to be biased by a forward rotation of the pedal cranks.
In known pedal crank drives of that kind (Swiss Patent Specification 539,538) the force-transmitting spring which is provided between the hub of the chain sprocket and the gear ring, which is rotatably mounted on the hub, consists of a tension spring and serves to overcome the top and bottom dead centers of the pedal crank drive because the force-transmitting spring is tensioned as the hub of the chain sprocket leads the gear as the pedal cranks are tread down and said spring can relax and transmit a corresponding torque to the gear ring at the dead center. Because considerable driving forces must be expected as the pedal crank drive is tread down, the desired effect cannot be ensured unless the force-transmitting spring is so dimensioned that it can store a sufficient part of the force within the limited spring excursion which is available. But it has been found that the desired effect will not reliably be achieved with increasingly harder force-transmitting springs because as the spring force increases the cyclist will be increasingly unable to exert at the dead center of the pedal crank drive a supporting torque which is sufficient to prevent a reverse rotation of the pedal crank so that the relaxation of the force-transmitting spring may result in a distinct deceleration and subsequently in a reverse rotation of the pedal cranks and this is regarded as most disturbing and will oppose a uniform rotation of the pedal crank drive.
For this reason it is an object of the invention so to improve by simple structural means a pedal crank drive of the kind described first hereinbefore that a desirable distribution of the torque through a revolution of the chain sprocket can be achieved whereas an effective assistance of the overcoming of the dead center by a force-transmitting spring disposed between the crank drive and the chain sprocket is still provided.
The object set forth is accomplished by the invention in that a damping spring, which is prestressed in a sense to stress the force-transmitting spring, is provided between the chain sprocket and the adjacent pedal crank.
Because the damping spring is prestressed in a sense to stress the force-transmitting spring, the force-transmitting spring can relax only against the force of the damping spring and the damping force of the damping spring increases as the force-transmitting spring is progressively relaxed. As a result, the load conditions of the pedal crank drive at the dead center are so changed that the torque which is exerted by the cyclist even at the dead center will certainly be sufficient to prevent an appreciable deceleration of the rotation of the pedal crank. As a result, the torque is more uniformly distributed over a revolution of the chain sprocket and this fact will produce desirable results, particularly when relatively heavy loads are exerted on the pedal cranks, as is usually the case during uphill runs.
For its damping function the damping spring must be comparatively soft compared to the force-transmitting spring. For this reason the force of the damping spring should not exceed one-half of the force of the force-transmitting spring. The adaptation of the instantaneous damping force to the load on the force-transmitting spring may be effected by an adjustment of the initial stress of the damping spring, although a change of the initial stress of the damping spring will obviously change also the initial stress of the force-transmitting spring. Because the damping force desirable increases with an increase of the load on the force-transmitting s
REFERENCES:
patent: 3828621 (1974-08-01), Uchino
patent: 5035678 (1991-07-01), Hageman
patent: 5067370 (1991-11-01), Lemmens
Bui Thuy M.
Kelman Kurt
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