Helical beam drive combines three parts in one

Abssac recently enhanced the operation of a medical orthodontic machine by replacing a universal joint, plastic shaft coupling and a piece of shafting with a one-piece helical beam drive shaft unit.

The customer had initially been using a traditional universal joint to connect a hand control, via a piece of shaft and through another plastic flexible shaft coupling, to a small gearbox. Not only was the universal joint expensive, but it was also being utilised at the limit of its angular capability, which was causing cyclic torque variation. A separate piece of bar stock was then used to connect the drive mechanism to the gearbox, which, again, required 3 degrees of shaft misalignment to be accommodated for by a small plastic multi-component shaft coupling. The old design was inefficient, time-consuming to build and had suffered from reliability issues.

Abssac's custom design was machined from a 400mm long piece of aluminium bar stock, 15mm in diameter, to give the required drive shaft length. Then, by adapting the flexure (or beam part of the helical beam shaft coupling), Abssac's design accommodated the two angular misalignments required in the system - but all within a single part.

Utilising a double-start coil flexure at each end to give superior torsional stiffness, the drive shaft could accept angular misalignment of up to 30 degrees at one end, while the other end would accept up to 5 degrees. The final design incorporated a split clamp design to lock the drive shaft into position onto the driven and drive ends.

The new design was then used in four other machines using different lengths as the single part to provide a zero-backlash drive that did not require lubrication or maintenance, and did not suffer from operational issues.

For more information about special flexures, visit www.abssac.co.uk/Products/Specialist-products/101/Specialist_flexures.html.