A single universal joint between two shafts will turn at a constant speed. Which statement is correct?

A single universal joint between two shafts will turn at a constant speed. Which statement is correct?

• A. Technician A only
• B. Technician B only
• C. Both A and B
• D. Neither A nor B

Answer: (B)

A single universal joint does not preserve constant speed when the shafts aren’t perfectly aligned. The way the joint transfers rotation—via a cross and two yokes—causes the driven shaft’s speed to speed up and slow down as the input shaft turns. Even if you push the driving shaft at a steady rate, the output shaft will oscillate in speed over each revolution. Only when the shafts are exactly in line (zero angular offset) does the joint transmit motion at the same speed, and that’s the special case where constant velocity is preserved. The math behind it shows this clearly: if the angle between the shafts is φ, the input and output angles satisfy tan(θ2) = cosφ tan(θ1); differentiating reveals the output angular velocity depends on θ1 and varies through the rotation. That’s why the correct statement is that the speed is not constant with a single universal joint. If you ever use two universal joints in a drive with the joints phased correctly, the fluctuations can cancel out, giving a more constant speed, but a single joint by itself cannot.

A single universal joint does not preserve constant speed when the shafts aren’t perfectly aligned. The way the joint transfers rotation—via a cross and two yokes—causes the driven shaft’s speed to speed up and slow down as the input shaft turns. Even if you push the driving shaft at a steady rate, the output shaft will oscillate in speed over each revolution. Only when the shafts are exactly in line (zero angular offset) does the joint transmit motion at the same speed, and that’s the special case where constant velocity is preserved. The math behind it shows this clearly: if the angle between the shafts is φ, the input and output angles satisfy tan(θ2) = cosφ tan(θ1); differentiating reveals the output angular velocity depends on θ1 and varies through the rotation. That’s why the correct statement is that the speed is not constant with a single universal joint. If you ever use two universal joints in a drive with the joints phased correctly, the fluctuations can cancel out, giving a more constant speed, but a single joint by itself cannot.