1.  Field of the Invention
This invention relates to a flexible coupling for power transmission which is referred to as a low-rigidity coupling. 
2.  Relates Art Statement
In power driving devices like motorcar vehicles, flexible couplings having vibration-absorbing capacity are used, for example, in the in-between linking part between a propeller shaft and a driven device mainly in order that torsional vibrations may not be transmitted to the driven device in the course of power transmission. 
Known flexible couplings of this kind are of a construction wherein drive side connecting elements and driven side connecting elements are arranged alternately in a circular manner and pairs of adjoining connecting elements are linked mutually by means of coupling pieces having elasticity.  More specifically, the drive side connecting element and the driven side connecting element are, as shown in FIG.  6 and FIG.  7, constructed so that an inner sleeve 4' made of metal for receiving therein a bolt for fixing to a drive shaft or a driven shaft is fitted in an outer sleeve 5' made of metal wrapped therearound with a belt strip 7' as a core material for coupling pieces and both inner and outer sleeves 4', 5' are united together by an elastic intermediate member 6' such as rubber interposed between these sleeves 4', 5'. 
One example of such construction is disclosed in Japanese Patent Application Publication (Unexamined) No.  52-140758 (1977). 
Existing flexible couplings are classified into two groups, that is, the elastic intermediate member 6' either fills completely the space between the inner sleeve 4' and the outer sleeve 5' in one group or leaves a void space in the axial direction between the inner and outer sleeves 4', 5' in the other group.  However, what is common to them is that the elastic intermediate member 6' is provided to be located in a position where the coupling is always subjected to compressive deformations in the course of power transmission, namely the position where it is pressed and pinched by the inner sleeve 4' and outer sleeve 5'.  This is contemplated having recourse to the cushioning effect of the elastic intermediate member by avoiding direct contact between the inner and outer sleeves of metal, and there occurs no problem in torque transmission upon low loading and normal loading. 
In torque transmission under high loading, however, the elastic intermediate member 6' interposed between the inner and outer sleeves 4', 5' carries directly the high loads, which elevates the surface pressure.  As a consequence, its elasticity is degraded owing to repeated strong compressions and lifetime is shortened. 
Further problem was that the rotational direction of the flexible coupling as illustrated in FIG.  6 is determined as shown in the arrow line depending upon the arrangement order of the drive side connecting elements and driven side connecting elements, and when this flexible coupling is driven in the reverse rotation in the power transmission system with use of it, spring characteristics in torsional direction are extremely different. 
In the case of the flexible coupling having a partial void space in the axial direction, when the elastic intermediate member 6' is vulcanized and bonded to the inner and outer sleeves 4', 5' it is required to give drawing to the outer sleeve 5' for the purpose of enhancing durability of rubber material.  However, its draw ratio is limited, at most, to the order of 13% owing to the failure limit of metal and consequently, enhancement of durability cannot be expected so much. 
This invention has been accomplished to cope with these prior art problems and to improve them, and has for an object to provide a flexible coupling of a type having a low torsional rigidity and low inclination rigidity.