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METAL DISC SHAFT COUPLINGS IN DETAIL

CHOOSING TORSIONALLY RIGID FLEXIBLE COUPLINGS


Machinery requirements have created a diverse market for shaft couplings from low cost rubber elements up to massive steel gear couplings. Metal disc couplings have an important place in this market covering most machine requirements with shafts from 4 to 150mm and torques from 1 to 23000Nm or more. These are not simple low cost couplings but should be considered as engineered products with higher prices. In overview, their key features are all metal construction, high speed capacity, backlash free, torsionally stiff and requiring no maintenance. A correctly sized metal disc coupling should last for the lifetime of the machine.

Compared to gear couplings, metal disc designs require no lubrication or maintenance and are backlash free. Grid type couplings are another alternative, although not popular these days – they need maintenance and are torsionally soft. Particularly where long spacer couplings are required, metal disc couplings can be compared with carden shafts (universal joints). The latter suit higher misalignments but do require lubrication and maintenance.

Metal disc shaft couplings transmit torque with misalignments from one shaft to another using disc packs made from laminations of stainless spring steel bushed together at 4 or 6 bolting points. The disc packs are the key to the coupling performance and on larger sizes are optimised by finite element analysis. In operation and under load the laminations flex and separate to a visible extent. Most metal disc couplings consist of two hubs, two disc packs and a central spacer connected by fitted bolts – an arrangement known as double flexing that is capable of angular, radial and axial misalignment. Single flexing designs with one disc pack and no central spacer are capable of angular and axial misalignment but have no radial capacity. Materials used for the hubs and spacer are steel or aluminium alloy.

Correct application of metal disc couplings depends on an understanding of the misalignments present in the machine. Values may be small but are never zero. Angular misalignment is easy to visualise and is often small – for an accurately aligned machine it will be less than 1∘ although some applications require medium values up to 3∘ (rubber or spring couplings) or even higher (universal joints). Radial misalignment is the offset between the two shafts and is also termed parallel misalignment. Values may be significant and depend on the size of the machine but ideally will be less than 1mm. Double flexing couplings, accept radial misalignment by each disc pack applying an equal and opposite flexing. Single flexing couplings cannot do this and if used where radial misalignment is present there will be vibration and possible mechanical damage. Finally there is axial misalignment, also called end float, which is often the result of thermal expansion. Metal disc couplings have a good capacity here, often 2mm or more.

Metal disc couplings start with miniature models and typical is the type 328 from Mönninghoff with alloy hubs for shafts 4 to 24 mm and torques up to 25Nm. Miniature metal disc couplings are used to connect encoders and on small machinery that requires precise positioning. At this end of the market, bellows couplings are an alternative but metal discs are more robust and have a similar value of wind-up at nominal torque around 0.2∘.

As metal disc couplings get bigger, alloy hubs are replaced by steel and a range ofmodular options come in. Typically these relate to the hub-shaft connection and the coupling length. Torques up to 500Nm can be considered as medium range. Torsional stiffness remains high with typically 0.1∘ wind-up at nominal torque. Speed capacity is high which leads to the possibility of transmitting high power, for example a standard coupling can transmit 6000Nm at 4000r/min equivalent to 2.5 MW. In reality industrial machinery rarely requires this sort of power levels, although precision metal disc couplings are available for high specification turbine applications. More generally medium and high torque metal disc couplings are used on line shafts, printing, packaging machinery and pumps particularly in hot environments as ambients up to 150∘ C are permissible.

Modular options allow metal disc couplings to be readily customized to machine requirements. One major consideration is backlash – although the coupling is backlash free, the hub to shaft connection is not when a keyway is used. This can lead to problems if the drive reverses and the solution here is to use clamping hubs, effectively a keyless locking bush that creates a friction connection between shaft and hub. Simple split hubs are economical and easy to use, but transmit lower torques. A proper tapered locking bush should be used for higher torque transmission. Additional benefits of these options are the ability to adjust the radial timing and the fact that no axial hub location is needed.

Another common option is to customise the length of the coupling which is done bychanging the central spacer. Standard and short spacers may be available off theshelf. The longer the spacer, the higher the capacity for radial misalignment. The shorter the spacer, the lower the inertia of the coupling. Large gaps between shaft ends can be bridged with spacer couplings where the central spacer is made to measure, often 0.5 to 2m for machine applications although 6m is possible. The spacer element is welded together and special attention is paid to the squareness of the ends. Balancing may be needed and operation near the critical speed has to be avoided. Ideally the angular misalignment each end should be no more than 0.5∘ to get the most out of the coupling. Higher speeds and even longer lengths are possible by using composite materials for the spacer. Either carbon fibre or glass fibre, which is electrically insulating, can be used. When couplings are mounting vertically theweight should not be supported by the disc packs. A simple and elegant solution in the form of a support button is available.

Coupling torque ratings are set to achieve long life under misalignment, particularly the angular misalignment. In service the leaves of the disc pack flex and ultimately will fatigue and fail. As this is undesirable for machinery applications, ratings are set at a level to match the machine life. The selection process for metal disc couplings should therefore observe the catalogue ratings and three rules are appropriate:

1. All three types of misalignment are important. A good rule is to consider each as a percentage of the maximum and then keep the total within 100%. As an example the following misalignments on a medium sized coupling would be acceptable











2. Reversing drives or loads that introduce reversing torques may require a significant torque derating. Depending on the model and size this can vary between zero and 50%, generally with the higher values on large couplings. The reason is that reversing loads can cause wear on the shanks of the fitted bolts that assemble the disc packs.

3. It is important to apply other service factors appropriate to the application. For most machines with electric motors the operating factor will be between 1.0 and 2.4. Additional factors should be added for starting frequencies above 1 per minute, high starting torques and ambient temperatures outside what is a wide range of -35 to +150∘C.

Metal disc shaft couplings are engineered products that give benefits of high torsional stiffness, zero backlash, capacity for high speed and zero maintenance. They cover a wide performance range and are found in many industries. Examples are the production of car parts, tin cans and cigarettes, test rigs and dynamometers, fans and pumps. Spacer couplings with customized central sections are found in print machinery, ovens and cooling towers.


The full range of Mönninghoff metal disc coupling, are available in the UK through the Techdrives Division of Lenze Ltd, www.techdrives.co.uk

328 Mini Servoflex 1 to 25Nm shafts to 24mm

318 Servoflex  35-500Nm shafts to 55mm

338 Aluminum Servoflex 35-500Nm shafts to 45mm

314 Arcoflex  800-23000Nm shafts to 150mm



Double flexing metal disc coupling with backlash-free shrink disk connection

Miniature metal disc coupling

Metal disc spacer coupling with customised central tube


Actual

Misalignment

Maximum

Permissable

Percentage

Angular

0.7⁰

2⁰

35%

Radiall

0.8mm

2.5mm

32%

Axial

0.5mm

2.6mm

19%



Total

86%