Vice President, Engineering
Curtis Universal Joint Co.
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.
Three types of misalignment
Though shaft misalignment is commonly referred to in terms of degrees, there are actually three types of misalignment: angular, parallel, and shaft displacement.
Angular misalignment is the angle between two nonparallel intersecting shafts, which is by far the most common type. Parallel misalignment refers to shafts that lie in the same direction, but are not coincident. Shaft displacement describes how one component moves relative to the other along a common centerline.
Casing the joint
A mechanism that opens and closes a gate valve for regulating flow of coolant water provides a design example. An 18° offset angle exists between the valve and a dc-gearmotor shaft. Shafts are 0.5-in. diameter and rotating speed is 150 rpm. Marine buildup and scale in the valve may raise startup torque to 200 lb-in.
The combination of high torque, cramped quarters, and a relatively large offset angle make a Cardan joint unsuitable for this application. The cramped geometry also lowers the operating radius of a flexible shaft to about 12 in. A 0.5-in.-diameter flexible shaft handles just 145 lb-in. of torque at that bend radius, and isn't an option.
It turns out that a block-and-pin joint fits in the available space. Calculate dynamic-torque capacity of this joint using the following three values: Speed-angle factor = rpm Misalignment angle = 150 rpm X 18° = 2,700
Select the corresponding operating-use factor:
Static torque rating = Torque Operating use factor = 200 lb-in. 10 = 2,000 lb-in. A 0.5-in.-diameter bore block-and-pin joint easily handles this amount of static torque.
Torque capacity for 0.5-in.-diameter shaft
Maximum angle of misalignment
Up to 20,000 rpm
15° (single joint); 30° (double joint)
Constant velocity; nonconstant velocity;
Limited by bend radius: 75 lb-in.@ 8 in. radius; 117 lb-in. @ 10 in. radius; 145 lb-in. @ 12 in. radius; 172 lb-in. @15 in. radius; 284 lb-in. @ straight
Limited by bend radius. At minimum bend radius, speed must not exceed 80% of shaft rated speed.
Near-constant velocity; highly efficient.
Limited by angle of misalignment: <10°, 3,000-rpm maximum; 10°, 1,500-rpm maximum; 25°, 600-rpm maximum
35° (single joint); 70° (double joint)
Single-joint output speed varies with misalignment angle, from about 3% at 10° to about 13% at 20°. Double joints with pins aligned 90° to each other have nearly constant output speed.
Curtis Universal Joint Co., www.curtisuniversal.com