ASVAB (Armed Services Vocational Aptitude Battery)Mechanical Comprehension (MC)Hard
A technician is using a universal joint to transmit rotational motion between two shafts that are not in a straight line. What is the primary disadvantage of a single universal joint when the angle between the shafts is significant?
- AIt generates excessive heat at high RPMs.
- BIt requires frequent lubrication due to high friction.
- CIt causes non-uniform rotational speed between the input and output shafts.
- DIt cannot transmit torque effectively.
Show answer & explanationAnswer & explanation
Correct answer: C. It causes non-uniform rotational speed between the input and output shafts.
A single universal joint (Cardan joint) does not transmit rotational motion uniformly when the input and output shafts are at an angle. The output shaft will speed up and slow down twice per revolution relative to the input shaft, leading to vibrations and wear, especially at larger angles. This non-uniformity is a key disadvantage.
Why the other options are wrong
- A. While friction leads to some heat, the primary disadvantage at significant angles is not excessive heat, but non-uniformity.
- B. All mechanical joints require lubrication, but this is a general maintenance issue, not the fundamental kinematic disadvantage of a single universal joint at an angle.
- D. It is designed to transmit torque effectively, even at an angle.
Universal Joint (Non-Uniformity)
A mechanical coupling that allows two shafts to rotate at different angles. A single universal joint, when operating at an angle, causes the output shaft to rotate non-uniformly even if the input shaft rotates uniformly.
- Also known as a Cardan joint or Hooke's joint.
- Non-uniformity increases with the angle between shafts.
- Often used in pairs (double Cardan joint) to cancel out non-uniformity.
Memory trick: Angled shafts make the joint 'wobble' the 'ouput speed'.