Flex Tube Could Replace Conventional Driveshaft

A unique segmented and flexible driveshaft designed by an established engineering company in South Korea could replace the double-jointed automotive driveshafts currently used to transmit drive from the transmission to the wheels.

Conventional driveshafts for rear- or all-wheel-drive layouts commonly have universal joints at both ends, each requiring a cruciform member with two forked yokes, not to mention four sets of bearings, seals and snap rings. These can be costly items to manufacture and assemble.

The new Korean-designed flexible prop shaft, on display at a recent Geneva exhibition, is a thick-walled steel pipe, with one end chucked in the spindle head of a water-jet machining center.

During the slow and reversible rotation cycle of the pipe from standstill to 30 rpm for each segment, a high-pressure water jet aimed over the free end makes a rounded zigzag cut around the shaft's circumference.

The cut pattern is comparable to interlocking pieces of a circular jigsaw puzzle. The water-jet nozzle traverses axially along the pipe, and its movement is coordinated with variations in rotational speed during each complete turn of the driveshaft.

These combined movements are digitally programmed through computer-aided design/computer-aided manufacturing processing.

Water jet pressure is said to be about 5,500 psi (400 bar), controlled to a cut width of 0.03-0.04 ins. (0.7-1 mm) by a diamond orifice and focusing nozzle.

After each cut, the detached segments are held steady by a tool to maintain axial alignment with the work in progress. Hardening by heat treatment follows.

Called Flexible Transmission Shaft (FTS), the design is patented by the Monas Pump Co. Ltd. in Kyungki-Do, S. Korea, a company that specializes in eccentric-screw pumps for thick liquids.

The design of such pumps calls for an integral offset drive. That need gave rise to the FTS as a low-cost alternative to the existing universal-jointed shaft for these special-purpose pumps.

The company's industrial use for the FTS design is an ongoing development, and automotive applications are foreseen as vehicle driveshafts and also as drives for power takeoffs and other auxiliaries.

The company says the small amount of backlash inherent in the segmented design might be handled by packing the shaft with heavy grease retained by a plastic sheath or boot, which would cushion the slight lash expected from drive reversals.

Another anti-shock alternative is the high-viscosity silicone fluid used in viscous couplings, which instantly thickens and expands under pressure and also would act as a lubricant.

Torque capacity of the FTS is said to be 40% of the equivalent-diameter solid pipe, with frictional losses matching those of a shaft with conventional CV joints.