1. Field of the Invention
The present invention concerns a torsional damper comprising at least two coaxial parts mounted to rotate relative to one another within a defined sector of relative angular movement, and circumferentially acting elastic means disposed circumferentially between said parts.
2. Description of the Prior Art
This type of torsional damper is normally incorporated in the design of a friction clutch, particularly for automotive vehicles, in which case one coaxial part carries a friction disk designed to engage rotationally with a first shaft, in practice a driving shaft and the motor output shaft in the case of an automotive vehicle, whereas the other coaxial part is carried on a hub designed to engage rotationally with a second shaft, in practice a driven shaft and the gearbox input shaft in the case of an automotive vehicle.
This type of device is used to permit regulated transmission of rotational torque applied to one of its coaxial parts where the other is itself subject to a rotational torque. It is thereby able to filter vibrations which may arise at any point in the kinematic system in which it is incorporated, extending from the motor to the driven road wheels in the case of an automotive vehicle.
In some applications at least there is a two-fold requirement to be allowed for in the design of torsional dampers. The first requirement is to transmit a moderate torque throughout a greater or lesser degree of relative angular movement. This normally involves the use of elastic means of moderate stiffness. The second requirement is to further provide for the transmission of a high final torque at the final value of such relative angular movement, normally involving the use of elastic means of high stiffness.
The circumferentially acting elastic means normally employed comprise a plurality of elastic members, in practice springs, each of which extends in a substantially tangential direction relative to a circumference of the damper assembly. Each spring is housed in an opening formed in part by windows in a component of one of the coaxial parts of the torsional damper and in part by windows in a component of the other of said coaxial parts.
In the case of a first coaxial part (that carried by the hub, for example), the component in question comprises at least one disk with a transversely disposed coaxial part. In the case of the other coaxial part (that carrying a friction disk, for example), the aforementioned component comprises at least one annular ring with a transversely disposed coaxial part, parallel to the disk of the other coaxial part.
In practice and for equal mechanical loads on the springs, the spring length must be increased to obtain a wider range of relative angular movement for a given value of final torque or a higher value of final torque for a given range of relative angular movement.
The higher the range of relative angular movement and/or the final torque the higher the ratio between the circumferential extent of the windows in the disk and the guide rings necessary to house the springs and the circumferential extent of the material between the aforementioned windows.
The disk and the guide rings are weakened by the incorporation of the respective windows, with the result that there is a mandatory upper limit on the maximum torque which the disk and guide rings can transmit and/or the permitted range of relative angular movement.
Hence the requirement for the transmission of a moderate torque for a greater or lesser degree of relative angular movement and for the transmission of maximum final torque for the final value of relative angular movement is difficult to satisfy.
In practice, and in the case of an automobile, for example, the final value of relative angular movement is relatively high (around 15.degree., for example) and the final torque is relatively low (around 15 mkg, for example), or the final torque is high (around 25 mkg, for example) in which case the final value of relative angular movement is relatively low (around 8.degree., for example). These cases represent the two extremes between which all intermediate solutions are possible.
Various solutions to the problem of reconciling these conflicting requirements have been put forward.
One proposal is to use double torsional dampers, consisting of two torsional dampers in series and disposed axially relative to one another, a first operating at low values of torque and the second operating at higher values of torque, following saturation of the first.
Such double torsional damper devices have the disadvantage that their axial dimension is considerable, unacceptable in at least some applications.
Another proposal is for torsional dampers in which, in order to facilitate the incorporation of circumferentially acting elastic means, such means are fitted at various circumferences (that is, at different radii) in the damper assembly, certain of the elastic members thus being offset radially relative to the others.
However, this radial disposition of the various circumferentially acting elastic members also raises problems, in particular with regard to the axial spacers which normally connect the annular rings which are a feature of the construction of one of the coaxial parts concerned.
The object of the invention is to provide a torsional damper in which it is possible to reconcile the need for transmission of a moderate torque over a greater or lesser degree of relative angular movement and a maximum final torque, even where the springs are disposed on the same circumference of the damper assembly.