Two block-and-pin joints transmit motor torque to a feed mechanism in a carpet-stitching machine.

Two block-and-pin joints transmit motor torque to a feed mechanism in a carpet-stitching machine.

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:

Speed-angle factorOperating-use factor
0 to 3,000 10
3,001 to 9,000 20
9,001 to 15,000 40

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.

Coupling type
Torque capacity for 0.5-in.-diameter shaft
Rotational speed
Maximum angle of misalignment
Other considerations
178 lb-in.
Up to 20,000 rpm
15° (single joint); 30° (double joint)
Constant velocity; nonconstant velocity;
Flexible shaft
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.
180° (approximately)
Near-constant velocity; highly efficient.
3,050 lb-in.
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.,