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Try a little math....
The mounting holes on the face plate of the Chiaphua motors are 1.002" from the line of rotation of the rotor, and tapped for 10-32 machine screws.
The minor diameter of a 10-32 screw is 0.1840 inches (ref: Machinery's Handbook 26). This yields a minimum cross section for the screw of 0.0266 square inches. Let's go with the worst case, and assume you are using a low-end SAE Grade 1 screw, made of low carbon steel. Material yield strength is 36,000 psi. Cross section times strength equals a shear strength of 957 pounds. At a one inch standoff distance, the screw is capable of withstanding 79.77 foot-pounds of torque.
Multiply that times two (you are using two screws to mount the motor), and you will see that this simple mounting method can withstand 159.4 foot pounds of torque. And remember, this is worst case, using a low-strength material and calculating yield strength instead of true shear strength. Try a medium or high-quality fastener with a decent yield strength (say ASTM A490, with 130,000 lbs yield), and you will get a much better result.
The Chiaphua motor at full stall exerts 276 oz-in, or 1.4375 ft-lbs, of torque (ref: CCL Industrial Motor Limited spec sheet).
Mounting with two screws would be more than adequate (by a factor of over 100). U-clamps are unnecessary, and just add extra weight to your robot.
-dave
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Y = AX^2 + B.... ehhh, whatever
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