Reliance

Engineered to Perform

Reli-a-Flex® Flexible Shaft Couplings

With the high integrity and ever more demanding applications that are daily facing engineers, 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 many different designs of flexible shaft coupling. Very few, however, are suitable for high integrity, high reliability, accurate positioning. Reliance engineers therefore drew on 40 years of experience in the production of precision geared mechanisms and went back to the drawing board to design the Reli-a-Flex® coupling.

Using their experience in gear transmission error testing, Reliance modified their own 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 200 KHz, dynamic performance graphs were easily produced. A combination of this equipment and finite element analysis enabled Reliance engineers to experiment with different slit configurations in order 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
• 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 – a definite advantage over other 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 much 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 Reli-a-Flex coupling's unique patented double slit pattern also minimises torque fluctuations by smoothing the transition of torque from slit to slit as the coupling rotates introducing negligible axial and radial bearing loads - vital for both 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 forward 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 Reli-a-Flex coupling outperforms all other one piece couplings on the market and gives predictable performance.

In comparison with a spiral beam coupling the Reli-a-Flex coupling achieves up to:

• 16 times greater torsional stiffness
• 12 times less transmission error
• 12 times less bearing load

This along with unsurpassed reliability makes the Reli-a-Flex coupling an excellent choice 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.
• The optional Reli-a-Grip clamp 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 unique design, test and rapid manufacturing facility which is autonomous, self-contained and dedicated to providing rapid concept or prototype models. This dedicated manufacturing cell can supply small quantities of custom Reli-a-Flex couplings quickly and with predictable performance without being prohibitively expensive.