Compliant clutch

Details Compliant clutch Compliant clutch

2002-08-09

2004-05-18

Rodriquez, Saul (Department: 3681)

192 clutches and power-stop control

Clutches

Operators

C192S076000, C192S10400R, C192S1050BA

Reexamination Certificate

active

06736249

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to systems and methods for selectively transmitting torque between rotatable members. More particularly, the present invention relates to a compliant clutch having a plurality of engagement members connected to a hub by flexible segments that permit extension of the engagement members to contact a rotatable receiving member, such as a drum.
TECHNICAL BACKGROUND
In many mechanical devices, there exists a need to transmit torque in a variable fashion between two rotating members. Under certain circumstances, such as when a motor is idling or starting to operate, it may be desirable to have the motor disconnected from any load. However, the motor should be connected to the load during normal operation. For example, the engine of a car may be disconnected from the remainder of the vehicle's drive train while idling at a stoplight, and may be reconnected to induce motion of the vehicle.
Several mechanisms exist for disconnecting and reconnecting a rotational load. For example, geared transmissions may disconnect a driving gear from a driven gear, thereby disconnecting a load from a motor. However, geared transmissions are somewhat complex, and typically require that the driving and driven gears be rotated at about the same rate of rotation before they can be reconnected. Additionally, a geared transmission is either fully connected or fully unconnected; there is no in-between state in which torque is transmitted, but relative slippage of the rotational members is still permitted.
Clutches have been developed to provide a more continuous torque transfer. Clutches utilize friction to gradually couple rotational members; since the coupling is not sudden, the rotational members need not be rotating at the same speed, or even in the same direction, for coupling to take place. The friction may operate to ultimately bring the rotational members to the same rotational speed, depending on how the clutch is designed.
Many different types of clutches exist. Some examples are rim types with internal or external expanding shoes, band types, disk or axial types, and cone types. Clutches may be engaged or disengaged manually by a user; for example, a manual transmission in a vehicle uses a clutch that can be selectively disengaged, typically by pressing a pedal. In the alternative, clutches may be engaged or disengaged automatically by some operating characteristic of the machine in which they are used. Centrifugal clutches, for example, may engage or disengage when a threshold rate of rotation of the clutch is achieved. Often, centrifugal clutches take the form of rim type clutches with internal expanding shoes that are spring loaded, so that the shoes contact the rim to transfer torque only when the centrifugal force is large enough to overcome the spring force on the shoes.
Known centrifugal clutches have a number of inherent disadvantages. First, they typically have many parts that must be separately produced and assembled. Each shoe must typically have at least one spring/pin joint combination, and several shoes will often be used; the total number of mechanical parts involved in the production of the centrifugal clutch can easily exceed twenty. Furthermore, known clutches are often quite thick; each pin joint must have a certain minimum length in order to operate. The thickness of the clutch, in combination with the multiplicity of parts required, makes the clutch somewhat heavy. The weight of the clutch contributes significantly to the mass moment of inertia of the entire rotational system, thereby decreasing the efficiency of the machine.
Furthermore, the torque capacity of the clutch depends on a number of factors, including the surface roughness values of the clutch and rim and the outward force with which the clutch presses against the rim. Although the shoes are typically contoured to match the rim, only parts of the shoe will contact the rim until the shoe wears somewhat; the shoes are worn to a smoother finish during use. Thus, the torque capacity will typically change somewhat during use and wear of the clutch. Even when a shoe is fully worn in, the pressure on the shoe is often concentrated at a comparatively small portion of the surface of the shoe. Increasing the number of shoes increases the number of parts, and is therefore a less desirable option for most mechanical clutches.
The manner in which the shoe moves to contact the rim also affects the torque capacity of the clutch. If a shoe moves such that the frictional force of the rim against the shoe tends to increase the pressure of the shoe against the rim, the shoe is termed an “aggressive” shoe. Conversely, if the frictional force tends to actuate the shoe away from the rim, the shoe is a “non-aggressive” shoe. Torque tends to tighten the engagement of aggressive shoes, thereby enhancing their torque capacity. However, aggressive shoes will generally have a far more sudden engagement than nonaggressive shoes; as a result, the aggressive centrifugal clutch does not gradually transfer torque to the load, but rather engages somewhat abruptly. Such abrupt engagement produces higher stresses and may damage mechanical components.
Furthermore, aggressive clutches that are designed or used improperly may induce a condition called “self-locking.” Self-locking occurs when the frictional force is sufficient, alone, to overcome the spring force and hold the aggressive shoes against the rim. When self-locking has occurred in a centrifugal clutch, the clutch may remain engaged, even when the clutch has slowed its rotation below the threshold rate of rotation. As long as the torque transferred by the clutch remains high enough to support self-locking, rotation of the clutch is no longer required for engagement.
A motor coupled to the clutch may thus be fully-loaded at a speed far lower than the minimum load-bearing speed of the motor. As a result, the motor or other mechanical components may suffer damage. Thus, the aggressiveness, and hence the torque capacity, of known clutches has also been limited by the need to design the clutch such that self-locking does not occur.
Although known compliant clutches have provided some improvements over known mechanical clutches. Compliant clutches utilize bending material in place of pin joints and springs to provide motion and restorative force. Thus, the part count, production expense, and weight can be decreased somewhat. Such compliant clutches have found application in the lawn and garden industry, in which many smaller machines such as string trimmers, hedge trimmers, edgers, and the like have a need for variable torque transmittal.
However, known compliant clutches typically have a low torque capacity, partly owing to the fact that the compliant clutches utilize a comparatively simple S-configuration with only two members that can bend outward to contact the rim. Thus, the degree of friction that can be generated by known compliant clutches is quite limited. Additionally, torque is often transmitted through the thinner, compliant members of the clutch, so that the strength of the material used to form the compliant members limits the torque the clutch can effectively handle. In many applications, multiple compliant clutches must be used to generate the necessary torque.
Consequently, there is a need in the art for a clutch that is easily manufactured from a small number of parts, and with a minimum of assembly. Furthermore, there is a need in the art for a clutch that can fit within a compact space, and yet provide a high torque capacity. Such a clutch should preferably provide a torque capacity that is high even before significant wear of the clutch has occurred, and that changes comparatively little when wear occurs. There is a further need for a centrifugal clutch in which torsional stress is not concentrated in thin, compliant members of the clutch. The high torque capacity should preferably be provided while maintaining a comparatively smooth engagement, and avoiding any danger of self-locking.
BRIEF SUMMARY OF THE INVENTION
The present

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