NEIEP Mechanics Practice Exam

What type of allowance does a shaft have when a roller bearing needs to be pressed onto it?

1. Positive allowance

2. Zero allowance

3. Negative allowance

4. Variable allowance

The correct answer is: Negative allowance

In the context of fitting a roller bearing onto a shaft, the correct choice involves a negative allowance. When a bearing is pressed onto a shaft, it is typically designed to create a tight fit. The negative allowance indicates that the diameter of the shaft is slightly larger than the inner diameter of the bearing. This ensures that as you press the bearing onto the shaft, it will be held securely in place due to the interference fit. This interference compensates for any potential expansion from heat or wear that may occur during the operation of the machinery, ensuring that the bearing remains securely attached to the shaft without the risk of slipping. The tight fit is crucial for the effective performance and longevity of the bearing and the overall machinery. In contrast, a positive allowance would imply that the shaft is smaller than the bearing hole, leading to a loose fit, which is not desirable for roller bearings that require stability. Zero allowance suggests an exact match in diameter, which might not provide the necessary grip for a secure assembly. Variable allowance refers to a range of fit strategies, but a specific negative allowance is the most appropriate for the situation described.