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 factor Operating-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.

 COMPARING JOINTS Coupling type Torque capacity for 0.5-in.-diameter shaft Rotational speed Maximum angle of misalignment Other considerations Cardan 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. Block-and-pin 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.

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