Extended Controlflex® coupling fixes encoder bearing failures

Repeated encoder bearing failures were causing serious concern at Shotton Paper Company.

Peter Whyatt of Francis & Francis describes how an extended design on a standard Controlflex® encoder coupling eliminated the problem.

A total of 21 motors are involved in the paper-making machinery at Shotton Paper, each of which has been fitted with Swedish-made Leine & Linde Type 5810A encoders driven from the tail end shaft of the nominal 820kW motors via a double bank, metal lamina flexible coupling. However, the frequent fatigue fracturing suffered indicated that the life of the beryllium copper membranes of the coupling was proving to be too limited and unreliable, giving rise to unpredictable failures in the encoder drive.

Such failures resulted in changes to the speed of the rolls, with consequent changes in paper tension causing the paper to tear. Furthermore, serious damage was also inflicted on the encoder bearings, resulting in encoders failing every four to five weeks. This meant that batches of five or six encoders were in continual transit between Shotton Paper and Leine & Linde for repair or total replacement, which was proving to be expensive.

Leine & Linde referred the problem to Francis & Francis and an examination at the site revealed two causes of the problem:

  1. Constant radial vibration by the paper-making machinery which is difficult to eliminate.
  2. No spigot register location on the motor casing for centering the encoder bell housing, thus allowing up to 0.5mm radial misalignment to occur during installation.

As all the motors were installed and in constant use, the site instrumentation engineer was not in a position to consider subsequent machining of a motor casing spigot register, or any subsequent change to the length of the encoder bell housing to accommodate a shorter length of standard coupling.

The distance between shaft ends (DBSE) formed a gap that had to be bridged by the coupling. The duty cycle required was 24 hours a day, 365 days a year, and the life required was two years. Therefore, the coupling duty/design requirements were:-

  • Torque: 0.02Nm
  • Speed: 1500 rpm
  • Radial offset: 0.5 mm
  • DBSE: 42 mm
  • Max. radial shaft load: 20N
  • Max. axial shaft road: 10N

A specially extended design of the standard Schmidt Controlflex® coupling Type CPS-15/1-10K/11K was proposed, comprising two hard anodised extended end discs, each fitted with extended length drive pins and spacer bushes to axially locate the uniquely shaped Dupont Delrin® 107 flexible element.

The whole assembly, complete with standard clamp-lock type end discs was produced to span the same axial length as that required by the failed coupling and to utilise the same radial holes in the bell housing to gain access to the end disc clamp-lock cap head screws.

A carefully monitored test programme was carried out on site over a period of 12 months before the coupling was adopted as the standard for all 21 encoders. The high fatigue life of the Dupont Delrin® 107 acetal resin material, coupled with the uniquely designed shape of the centre element with its long flexible link arms, proved capable of adequately accommodating both the inherent machinery radial vibrations and the bell housing/motor frame misalignment.
This has been done without creating any of the damaging radial reaction forces usually directed back onto the encoder bearings that normally arise from coupling misaligned shafts. Yet it still provides the high torsional stiffness needed to drive the encoder precisely.

The Schmidt Controlflex® coupling consists of two hubs to be attached to the shafts and a centre flex member. This flexible element is affixed to the hubs through shoulder bolts or on pins.

STANDARD Series, Type CPS–.1

IMPULS PLUS Series, Type CPS–.2

The coupling is based on a parallel linkage system. Unlike elastomeric couplings, the Controlflex® disc allows parallel, angular and axial shaft misalignments, while offering higher torque rigidity and maintaining constant transmission of torque and angular velocity.
Because of these features and, since all the forces act in one plane, the reaction forces due to transmission of torque and unavoidable shaft misalignments are considerably smaller when compared with common flexible couplings.

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