Question:
A toy is shown on the left. When the wheels of this toy rotate, the fan is also required to spin. If one pair of gears is to be used exclusively, which option will make this possible?
A toy is shown on the left. When the wheels of this toy rotate, the fan is also required to spin. If one pair of gears is to be used exclusively, which option will make this possible?
Show Hint
In toy design questions, look for "crown gears" (wheels with teeth on their flat face) when transmitting motion at 90 degrees. They are the industry standard for low-cost, high-tolerance gearboxes.
Updated On: Jun 25, 2026
- Fig A
- Fig B
- Fig C
- Fig D
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The Correct Option is C
Solution and Explanation
Step 1: Understanding the Question:
This question belongs to the topic of
Machine Design and Kinematic Transmission.
We are designing a mechanism for a toy helicopter where the rotation of the horizontal wheel axle must drive the vertical shaft of the overhead rotor fan. We need to identify which gear pair is capable of transmitting power between these two perpendicular, intersecting shafts.
Step 2: Key Formula or Approach:
We analyze the geometric relationship between the input shaft (horizontal) and the output shaft (vertical):
• The axes of rotation are perpendicular (at 90 degrees) and intersect.
• Power transmission between perpendicular intersecting shafts requires specialized gears such as bevel gears or crown-and-pinion assemblies.
• In plastic toy design, a crown gear (face gear) paired with a spur gear pinion is highly preferred due to low manufacturing costs and high tolerance for axial misalignment.
Step 3: Detailed Explanation:
• Let us analyze each option based on kinematic principles:
-
Option A: Shows a pair of standard bevel gears with straight teeth cut at 45-degree angles. While bevel gears can transmit motion at 90 degrees, they require highly precise alignment and axial thrust support, which is difficult to maintain in a simple plastic toy.
-
Option B: Shows two helical gears that are not oriented correctly to mesh at 90 degrees.
-
Option C: Shows a crown gear (a disc with teeth projecting perpendicular to its face) meshing with a standard spur gear pinion. This is the classic "crown and pinion" drive. It is widely used in toy mechanisms because the horizontal pinion can slide slightly along the axle without losing mesh with the vertical crown gear. This is the most practical and exclusive solution.
-
Option D: Shows a mismatched set of gears that cannot mesh properly.
Step 4: Final Answer:
Option (C) is the most robust and common mechanism used in toys to achieve this motion.
This question belongs to the topic of
Machine Design and Kinematic Transmission.
We are designing a mechanism for a toy helicopter where the rotation of the horizontal wheel axle must drive the vertical shaft of the overhead rotor fan. We need to identify which gear pair is capable of transmitting power between these two perpendicular, intersecting shafts.
Step 2: Key Formula or Approach:
We analyze the geometric relationship between the input shaft (horizontal) and the output shaft (vertical):
• The axes of rotation are perpendicular (at 90 degrees) and intersect.
• Power transmission between perpendicular intersecting shafts requires specialized gears such as bevel gears or crown-and-pinion assemblies.
• In plastic toy design, a crown gear (face gear) paired with a spur gear pinion is highly preferred due to low manufacturing costs and high tolerance for axial misalignment.
Step 3: Detailed Explanation:
• Let us analyze each option based on kinematic principles:
-
Option A: Shows a pair of standard bevel gears with straight teeth cut at 45-degree angles. While bevel gears can transmit motion at 90 degrees, they require highly precise alignment and axial thrust support, which is difficult to maintain in a simple plastic toy.
-
Option B: Shows two helical gears that are not oriented correctly to mesh at 90 degrees.
-
Option C: Shows a crown gear (a disc with teeth projecting perpendicular to its face) meshing with a standard spur gear pinion. This is the classic "crown and pinion" drive. It is widely used in toy mechanisms because the horizontal pinion can slide slightly along the axle without losing mesh with the vertical crown gear. This is the most practical and exclusive solution.
-
Option D: Shows a mismatched set of gears that cannot mesh properly.
Step 4: Final Answer:
Option (C) is the most robust and common mechanism used in toys to achieve this motion.
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