Question

In order to have interference fit, it is essential that the minimum permissible diameter of the shaft should be

• A. Larger than the lower limit of the hole
• B. Smaller than the lower limit of the hole
• C. Smaller than the upper limit of the hole
• D. Larger than the upper limit of the hole

Hint:

Think about the extreme cases within the tolerances. For interference, the smallest shaft must always be bigger than the biggest hole. This ensures a positive interference in all scenarios.

Answer 1

The Correct Option is D

Step 1: Understand the concept of interference fit.
An interference fit is a type of fit between two mating parts (like a shaft and a hole) where the diameter of the shaft is consistently larger than the diameter of the hole. This difference in size causes the parts to be tightly held together by friction and elastic deformation when assembled. To assemble, one part must be forced into or over the other, often requiring force or heating/cooling to create the necessary dimensional difference. Step 2: Define the limits of size for the shaft and the hole.
For both the shaft and the hole, there is a specified basic size, and permissible variations are defined by upper and lower limits.
For the hole:
Upper limit of the hole (\(H_{max}\)) is the maximum allowed size of the hole.
Lower limit of the hole (\(H_{min}\)) is the minimum allowed size of the hole.
For the shaft:
Upper limit of the shaft (\(S_{max}\)) is the maximum allowed size of the shaft.
Lower limit of the shaft (\(S_{min}\)) is the minimum allowed size of the shaft.
Step 3: Determine the condition for interference fit.
For an interference fit to always occur, even under the least favorable conditions within the specified tolerances, the smallest possible shaft must still be larger than the largest possible hole. In other words, the minimum permissible diameter of the shaft must be greater than the maximum permissible diameter of the hole. $$S_{min}>H_{max}$$ Step 4: Relate the minimum permissible diameter of the shaft to the limits of the hole.
The question asks about the minimum permissible diameter of the shaft (\(S_{min}\)). For an interference fit to be guaranteed, this minimum shaft size must be larger than the upper limit of the hole (\(H_{max}\)). If \(S_{min}\) were smaller than or equal to \(H_{max}\), there would be a possibility of clearance or transition fit under certain tolerance conditions. Step 5: Evaluate the given options.
(1) Larger than the lower limit of the hole: This condition does not guarantee interference, as the shaft could still be smaller than the upper limit of the hole, leading to clearance or transition fit.
(2) Smaller than the lower limit of the hole: This would definitely result in a clearance fit, not interference.
(3) Smaller than the upper limit of the hole: This allows for the possibility of clearance or transition fit if the shaft is also larger than the lower limit of the hole but smaller than the upper limit.
(4) Larger than the upper limit of the hole: This ensures that even the smallest allowed shaft is larger than the largest allowed hole, guaranteeing an interference fit.
Therefore, the essential condition for an interference fit is that the minimum permissible diameter of the shaft should be larger than the upper limit of the hole.