William McCombe
Vice President, Engineering
Curtis Universal Joint Co.
Springfield, Mass.

A spring retainer on the pins lets users quickly disassemble individual joints. A splined shaft lets two block-and-pin joints slip axially relative to one another.

A spring retainer on the pins lets users quickly disassemble individual joints. A splined shaft lets two block-and-pin joints slip axially relative to one another.


Perhaps a hundred different types of couplings can join rotating drives when there is just a small misalignment between shafts (<3°). But designs with misalignments approaching 10° or more leave just three choices: Cardan-type universal joints (automotive-style needle bearing and block-and-pin), and flexible shafts.

Automotive-style Cardan joints, also known as cross-andbearing joints, find use in numerous automotive and industrial drives including propeller shafts, axles, steering columns, and transfer tables in metal rolling mills. They work best for high-speed applications when misalignment is less than 15°. Rolling-element bearings in these devices keep friction losses low and efficiencies high. However, a relatively low torque capacity limits their use.

Flexible shafts have almost no limitation on misalignment. Some can work when the direction of motion is skewed 180°. Flexible shafts also do a good job of isolating driven-unit vibration from the drive unit. And they can transmit torque around multiple obstructions in some cases.

The bend radius that a flexible shaft makes, not the absolute angle of misalignment, is the limiting factor. The smaller the radius, the less torque that the devices can transmit. At the mini-mum-bending radius, torque capacity drops to 20% of rated capacity. Also, most flexible shafts are unidirectional, capable of transmitting rated torque in only one direction. When turned in the unwind direction, torque rating falls 30%.

Block-and-pin joints cover probably the widest spectrum of high-misalignment applications. Singly, they work for misalignments to 35°, and as high as 70° when used in pairs. Hardened wear surfaces make them damage tolerant. Block-and-pin joints get the nod for high-torque applications needing minimum maintenance, though they are somewhat speed limited. Another negative: block-and-pin joints are not of a constant-speed design. Output speed sinusoidally rises and falls with each rotation. However, output speed can be made nearly constant by installing two joints in tandem with the pins aligned 90° with respect to each other.

Applications for block-and-pin joints include drives for Gatling guns on the new F-35 Joint Strike Fighter, an articulating mechanism that raises and lowers convertible tops on cars, a drive for in-flight refueling equipment, and drives on commercial washing machines. Many of these applications have a fail-safe spec, which is satisfied by the high torque ratings of these u-joints.