Membrane couplings

A new coupling has been developed that eliminates many of the disadvantages of prior types of designs

Metallic flexible couplings of the flexible disc type have been in use alongside the gear type for many years, due to their absence of lubrication, immunity from the environment, torsional rigidity and some predictable axial flexibility. Original designs had solid flexible elements with a contoured profile which were costly and relatively inflexible. These have evolved into the current designs that use flexible packs comprising identical thin laminations. These are more flexible and cheaper to make.

Early membrane designs consisted of flexible elements in the form of radial spokes. These were eventually replaced by the peripheral link type, due to the lack of sufficient axial flexibility in the radial type.

In the original radial spoke type, all spokes contribute to the transmission of torque. This is distinct from the ring type element that uses peripheral links, of which only half transmit torque. This has the advantage that, in the event of the spokes shearing due to a destructive overload, the transmission of torque disappears and the coupling may be termed as ‘fail safe’.

The disadvantage with this approach is that for a given coupling diameter, the outer and inner rings clamping the membranes at the inner and outer annuli, result in spokes with a free flexing length that cannot provide adequate axial flexibility. As a result, they have been phased out in favour of the ring type element.

In the event of spoke shear from excessive overload, the fractured edges undergo a violent attrition which is unacceptable. In a flammable environment it requires a non sparking material such as Monel which adds to the cost.

In the four- or six- link peripheral link type of coupling, the axial flexibility is considerable. What is more, if the spacer does not have to be removed on site, the hubs fixed to the connected shafts may be reversed and inserted through the inner diameter of the flexible links, extending into the bore of the tubular spacer.

The coupling is not, however, ‘fail safe’. In the event of link fracture, the securing bolt heads normally housed in clearance holes in the adjoining driven flange, will hit the side of the hole and transform the coupling into a crude gear type, immediately putting at risk the bearings of the connected machines.

If the risk of a runaway in a turbine drive is a concern, turbines will be fitted with their own overspeed protection devices. For that reason, they would not need the assistance of a broken down coupling.

In the link type coupling, a torque transmitting link in tension is followed by a ‘dead’ link in compression. The dead links are invariably in a buckled state, especially if the ‘live’ links have suffered a slight slip at the bolt corners, when the buckling becomes severe. At high speeds, the outer dead laminations are subjected to severe twisting by the offset centrifugal force which can be very damaging.

Now, a new coupling developed by Servac International eliminates many of the disadvantages of prior types of couplings. In the design, the flexible element consists of star shaped laminations with radial spokes designed for manufacture by means other than press tools. All bolts directly attaching the flexible elements to the adjoining rigid members are eliminated and replaced by controlled resistance welding. This method provides, for a given coupling diameter, axial flexibilities comparable to a six-bolt peripheral link coupling type.

The attachment of the inner zone of the flexible membrane to the spacer is free of bolts. The outer ends of the spokes are welded to a ring which is bolted to the hub, eliminating the need for critical bolt tightening procedures essential to the peripheral link type couplings.

The ultimate shear strength of the welds is predictable. As a result, the ‘fail safe’ shearing of the radial spokes under gross overload is eliminated and replaced by the shearing of the inner row of welds at a predictable overload torque, which acts as a non-resetable torque overload device. The system thus allows the unbroken flexible membranes to disconnect the drive by freely rotating about the central spacer spigot as well as preventing the spacer from flailing.

The coupling is claimed by its developers to be considerably lighter than the conventional spoke type or the peripheral link type. As a result, balancing is easier and higher rotational speeds are possible.

Figure 1: The star shaped lamination is used to form the two flexible elements of the new coupling. It has 5 radial arms that are free at their outer ends (A) with their inner ends connected to an annular area (B) with a central hole (C). An outer ring (not shown) is attached to the outer ends by resistance welding one or more small welds close to the edge A. The annulus B is similarly welded to the spacer shaft by a cluster of similar welds. The overload torque at which the inner cluster of welds will shear and release the drive is determined by the ultimate shear stress of the weld cluster, and is predictable

Figure 2: The flexible transmission unit S bolted to two hubs. It consists of two flexible membrane packs that are resistance welded to the flanges of the spacer shaft (1) by a cluster of welds (2) that determine the limiting overload by shear. The ring (3) is welded at the outer ends of the spokes ensuring concentricity with the spacer

{{Coupling Size GSX1 gsx2 GSX3 gsx4 GXS5 GSX6 GSX7

Torque Rating Nm 110 230 640 1000 2300 4500 8000

Outside diam A mm 102 120 143 75 207 235 280

DBSE (min) B mm 56 65 70 75 80 95 100Hub Length C mm 40 47 62 90 95 110 120Hub Diam D mm 70 90 105 115 140 160 200Max Bore E mm 40 50 65 75 90 100 140

Max AxialDeflection(Hub to Hub) +/-y mm 2.0 2.0 2.5 3.0 3.0 3.0 4.0Max Axial Force F Kg 17 19 40 52 65 100 180Weight (kg) 3.67 6.05 8.79 17.52 31.98 48.47 73.80}}