The purpose of a flexible coupling is to accept misalignment between the shafts of the connected machines. This misalignment may take the form of an axial movement of one or both the machine shafts, an angular misalignment between them or a relative radial (parallel) offset between their axes of rotation - or indeed any combination of these three. All these being accomodated whilst the coupling is rotating at speed and transmitting the machine torque.

Euroflex Transmission Disc Couplings utilise thin laminations or discs pre-assembled to produce a 'disc pack'. The number of discs used is a factor of the required transmission torque and number of anchor points.

TORQUE TRANSMISSION - The Euroflex Disc Coupling is torsionally rigid with zero backlash. Torque is transmitted in tension through the spans of the flexing discs. In the adjacent figure holes B, D & F are assumed connected to the 'driving section' and A, C & E to the 'driven section' of the coupling. As the driving machine rotates the attachment at B will 'pull' the attachment at A round introducing a tensile stress in the 'span' between the two. The same will happen with D & F 'pulling' C & E respectively.

MISALIGNMENT - Misalignment of the attached machines is accommodated by the relative 'out of plane' movement of A, C & E relative to B, D & F. This relative movement introduces bending stresses into the 'spans' of the discs which may be 'steady' in the case of an 'axial' misalignment or fluctuating (fatigue) in the case of 'angular' misalignment. Since the disc span is in tension due to torque transmission, the bending stresses are focused near the anchor points (try pulling a piece of paper and moving you hands transversely!).

SPECIFIC DESIGN PRINCIPLES

1) DISC PROFILE - The Euroflex Coupling uses discs with a polygonal outer profile and a circular central hole. This profile ensures that material is at a maximum in the region of the anchor points where the bending stresses due to misalignment occur and gives rise to the capability to handle high misalignments without limiting the life of the coupling.

2) BOLTING - Euroflex give considerable consideration to the bolting when designing the coupling. The intent of the design is to ensure that under all continuous operating conditions the anchoring of the flexing discs to their respective flanges is sufficiently rigid so as to avoid any slip. Slip during operation can lead to micro-movement of the adjacent components and result in fretting. Correct bolt sizing, material selection and pre-load ensures that no slip can occur and fretting avoided.

DISC STRESSES

• With the coupling rotating at the speed of the driven machine centrifugal stress is introduced into the discs. Under normal fixed speed conditions this is a 'steady' stress.
  
  
• As discussed above, when Torque is transmitted alternate spans in the disc are under conditions of tensile stress. In cases where the connected machinery has no significant torque fluctuations this stress may be considered 'steady'. If, however, either machine (driving or driven) has major fluctuations then the stress will need to be considered as 'alternating' and assessed as a fatigue stress.
  
  
• During installation the coupling will usually be asked to accommodate some level of error in the set distance between the two machinery shafts. In addition, during operation, one of both of the machine shafts may move axially due to thermal (or other) influences. In both cases the coupling discs accommodate the misalignment by bending of the disc spans. In most normal applications this combined axial deflection does not vary rapidly and, as such, can be considered as introducing steady stress.
  
  
• The introduction of angular misalignment again may arise from inevitable limitations in the accuracy of the machinery installation and/or from relative movement of the machine shafts during operation. Such misalignment is again accommodated in the coupling discs by bending of the spans. However, in this instance the bending is fully reversing cyclic, introducing fatigue stresses in the discs.
  
  
• Since all high power metallic flexible couplings can, generally, only accept either an axial of angular deformation at a flexing joint, radial (parallel) misalignment between the machinery shafts is only accommodated by the introduction of two flexing joints - both of which can accept an angular misalignment as outlined above.

A flexible coupling installed in any typical application will have stresses arising from all of the above factors simultaneously.

In the operation of a disc coupling all the induced stresses listed above must be considered in combination and, as such, there is always a 'trade-off' between the permitted limits on any of the factors inducing the stresses. Since Torque & Speed are, generally, defined and fixed this 'trade-off' will be between the three types of misalignment. For example as the required axial deflection increases the allowable angular or radial misalignment must decrease and visa versa.

TORQUE vs MISALIGNMENT