Technical Paper: API-all-steel coupling with optimised performance

Marc Egelwisse

In the Arpex all-steel multi-disk coupling the Flender GmbH, a 100 %-held Siemens subsidiary, is marketing a coupling that has proved itself over many years. Marc Egelwisse, product manager for Arpex couplings, and his team have further developed this series with a focus on three aspects:

  • increase in the specific power,
  • optimisation of the areas of application and
  • reduction of complexity of the type series.

The results show it was worth it:

  • 70 to 100 % higher torque,
  • 40 % greater angular misalignment,
  • 10 % higher maximum speed and
  • 14 instead of the 42 design variants existing hitherto.

“Because of the high torque capacity and the optimised speed and bore capacity of the new N-Arpex we are confident that we are offering what is at present the world’s best-performing coupling”, says Egelwisse.

The torsionally rigid all-steel disk couplings Arpex that have proved themselves over many years enable the power of a motor to be transmitted to, for example, a pump and at the same time shaft misalignments to be compensated in an axial, radial and angular direction. With all-steel disk couplings the reliable transmission of torque between the shafts to be connected is backlash-free, torsionally rigid and flexurally elastic.


The outstanding systemic advantage of these couplings is the wear-resistance and maintenance-freedom afforded by their steel disk packs. Their disks are made from high-strength spring steel. About this Egelwisse says, “Because they are manufactured entirely of steel and contain no wear-prone components of any kind, Arpex couplings are highly stressable. This also enables the very wide range of application temperatures from currently – 40 °C to + 280 °C.“

Until now Flender has offered these couplings in seven different types, each optimised to fit a specific requirement profile. The Arpex ARS standard series, the ARC series for heavy-duty machine applications, and the ARP series designed specifically for pumps have been brought together to form the new, performance-optimised N-Arpex series. This new coupling series with outside diameters of up to 343 mm and a maximum coupling nominal torque of 28,000 Nm was first shown at the Hanover Fair. “The geometrical replaceability of the existing series with the new one is guaranteed in every case,” says Egelwisse.

Best-performing all-steel multi-disk coupling

FEM of the plates

A number of design measures form the basis for the leap in performance now made possible by the N-Arpex. “We have been able to optimise the disk contour and the screw connection so much that both the torque and the displacement capacity have been substantially increased. Our new disk contour gives a more even load distribution, so maximising permissible shaft misalignments and permissible speeds,” says Egelwisse. “Moreover our disk packs now have a new kind of linear bend edge at the screw connection. With this we are raising what has until now been the bending point of the disks under load to a bending line. That is one good reason why the new N-Arpex couplings can support considerably larger shaft misalignments“.

Egelwisse sums up in figures the optimisations so achieved: “We were able to lift the per­missible torques of N-Arpex couplings by 70 to 100 %, depending on coupling size. We succeeded in increasing shaft misalignments – and particularly angular misalignment – from 0.7 to 1°, that is by 40 %. Additionally, bore capacity, the ratio of the outside diameter of the hub core to the coupling bore, was optimised up to a value of 1.3. This means that larger shaft ends can be accommodated without changing the outside diameter. After com­pre­hensive tests we can increase the permissible maximum speed by 10 % to 110 m/sec. This means that we are now offering operators a coupling that thanks to increased power, increased torque and an optimised bore capacity is one size smaller than before. For our customers, besides weight advantages, this naturally also brings advantages for procurement.”

Optimisation of the areas of application

Disk packs now have a new kind of linear bend edge at the screw connection

The previous ARS standard coupling was not designed in accordance with the API-standards Flender changed this by coming out with the N-Arpex series, which also superseded ARS. The N-Arpex is standardly designed to comply with oil and gas standards API610/ISO 13709 and API671/ISO 10441. “Our customers no longer need to decide whether they want to use the coupling for an API or a standard application. Furthermore couplings have been designed for use in potentially explosive environments as defined in Directive 2014/34/EU This means greater safety for operators, because they always have a coupling offering API conformance to fall back on. This also includes the now standard fly-away prevention device for the spacer sleeve.”

Moreover the standard N-Arpex coupling can be applied in an ambient temperature down to – 50 °C. “By optimising by 10 K in the low-temperature range we are meeting the demands of users operating plant systems and pumps in, say, Canada or Russia. They expect and specify a reliable solution for under – 40 °C. Of course we always had a solution to this problem,” says Egelwisse, “but it was made from special materials and so was more expensive. We are now able to meet this low-temperature requirement easily with our standard coupling from the catalogue.”

The existing maximum operating temperature of + 280 °C for the coupling remains unchanged. The N-Arpex is available both as a 3-part standard coupling and as a 5-part, recess-guided version. The spacer sleeve is replaceable for 3- and 5-part versions. The disk pack can be used for all types.

To validate these new Arpex best performances Flender has subjected these couplings to numerous tests on the test floor. The tests include, amongst others, pulsator tests with dynamic torque and rotation tests with combined static or dynamic torque, speed and misalignment. Furthermore low-temperature tests, a high-speed test and fitting/demounting tests were performed.

Individual solutions are available on request: for example, a special DBSE, spacer designed as a torsion shaft, flange variants, with axial-backlash limitation or leakage current insulation, vertical supports, slip hubs, slip flanges or even a non-sparking version.

Naturally, N-Arpex couplings are not only used in pumps, fans and compressors, they are also fitted to generator, turbine, agitator and mixer or water screw drives and similar equipment.

Advantages thanks to reduction of complexity

Bringing together a number of existing series to form N-Arpex reduces complexity – for both manufacturer and OEMs and operators of pump systems. The power range of the existing 42 design variants will in future be covered by 14 variants with the usual shaft end distances for pumps. This will make it easier to choose couplings, stock spare parts and reduce storage costs

Thanks to its high specific power an N-Arpex at least one size smaller than a coupling from the previously existing series can be selected. So power-to-weight ratio, investment costs, operational fitness and cost of procurement also have an impact on the lifecycle costs of a pump system.

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