Back to the start for coupling design

Edited by the Engineeringtalk editorial team Apr 11, 2006

Engineers go back to the drawing board to produce one piece flexible shaft coupling with high torsional stiffness, low kinematic error, low bearing load and long life.

With the high integrity and more demanding applications that are facing engineers daily, the correct selection of system components is becoming increasingly important.

Shaft couplings are a crucial part of motion control assemblies.

They are designed to provide accurate motion between two shafts where a small amount of misalignment exists.

Failure of these couplings can have a catastrophic effect on the parent machine, in the form of a total loss of drive or position sensing.

There are different designs of flexible shaft coupling.

Very few, however, are suitable for high integrity, high reliability, accurate positioning applications.

Reliance engineers therefore went back to the drawing board to design the Reli-a-Flex coupling.

Using their experience in gear transmission error testing, the engineers modified the in-house gear testing equipment to provide an accurate trace of the kinematic error across various coupling prototypes.

With accuracies of arc seconds and a data input rate of 200kHz, dynamic performance graphs were produced.

A combination of this equipment and finite element analysis enabled the engineers to experiment with different slit configurations to determine the criteria that best suited their requirements.

Their design goals were to produce a one piece flexible shaft coupling with the following performance criteria: high torsional stiffness; low kinematic error; low bearing load; and long life.

High torsional stiffness was achieved by putting the beams radially around the coupling.

In this way, the loads are supported by beams in compression - an advantage over types of coupling where the beams are in spiral pattern and rely on the twisting of the beam to support the drive loads.

In addition, with radial rather than spiral beams, the centre piece is better supported, therefore higher speeds are attainable than with other types of one piece coupling.

Based on the double Hooke's joint principle, using two sets of slits and a solid centre piece, the Reli-a-Flex coupling achieves the lowest theoretical kinematic error.

The second set of slits introduces an equal and opposite angle which allows it to cater for parallel as well as angular offsets.

The double slit pattern also reduces torque fluctuations by smoothing the transition of torque from slit to slit as the coupling rotates introducing negligible axial and radial bearing loads for increased bearing life and accuracy.

Similar looking couplings may have a slit pattern that is a mirror image but in such designs the pivot points do not travel back and forwards with a constant centre piece angle.

They move in and out and the centre piece angle varies causing stress variations and less uniform rotation.

Tested to 50,000,000 fatigue cycles, at its rated torque and 80% maximum offset, the coupling outperforms other one piece couplings and gives predictable performance.

In comparison with a spiral beam coupling, it achieves up to 16 times greater torsional stiffness, 12 times less transmission error and 12 times less bearing load.

This along with reliability makes the coupling suitable for accurate measuring and responsive precision motion control, both for servos and steppers, and allows feedback devices to operate to new and higher standards.

Reliance has continued to develop the Reli-a-Flex coupling standard range which now includes: a minute, yet high performance, 6mm outside diameter micro coupling; a range of insulated couplings designed to isolate delicate measurement equipment from the electrically powered drive with negligible loss of performance; and the optional Reli-a-Grip clamp that allows the Reli-a-Flex flexible shaft couplings to transmit greater torques by achieving a high clamping force without the need to use set screws.

For customers who require something outside the standard range, Reliance has a design, test and rapid manufacturing facility which is autonomous, self-contained and dedicated to providing rapid concept or prototype models.

This manufacturing cell can supply small quantities of custom Reli-a-Flex couplings quickly and with predictable performance without being prohibitively expensive.