Fits and Clearances Quiz

Questions and Answers

Why must a hole be drilled smaller if it is to be reamed?

To allow for the metal removed by the reamer.

What determines the classification of fit?

The relationship between the bolt hole and the bolt.

What is a clearance fit?

It is a fit that always results in a clearance when mating parts are assembled.

What is an interference fit?

It is a fit that always results in an interference when mating parts are assembled. Signup and view all the answers

What is a transition fit?

Transition fits are a compromise between clearance and interference fits. They are for when it is desirable for the shaft to be held precisely, yet not so tightly that it cannot be disassembled. Signup and view all the answers

How is the British Standards System (BS) devised?

It consists of 21 types of holes designated by capital letters and 21 types of shafts designated by small letters. From diameters of 0.04" to 19.69". Each hole or shaft is designed with 16 grades of accuracy. Signup and view all the answers

How is the ISO 286 devised?

It is similar to the BS system except it has 28 types of holes and shafts and 20 grades of accuracy. Signup and view all the answers

How is the American National Standards Institute (ANSI) devised?

For metric fits and clearances, it follows the ISO standard. For Imperial-based units, it uses symbols such as RCx for running and sliding fits and FNx for force and shrink fits. Signup and view all the answers

What are the classes of fit in the Newall hole-based system?

Holes are classified as Class A and Class B fits. Class A holes have a closer tolerance than Class B holes. Signup and view all the answers

What is the British Standards BS 4500 system?

It is a system of limits based on standard sized holes and the various fits are obtained by using shafts of varying sizes. Signup and view all the answers

What is one advantage of the BS 4500 system?

It permits the use of standard hole manufacturing tools such as drills and reamers. Signup and view all the answers

When is tolerance called bilateral?

When there is a variation either side of the nominal dimension. Signup and view all the answers

When is tolerance called unilateral?

When one tolerance is zero. Signup and view all the answers

What is a shaft-based system?

A system in which the shaft is constant and the hole varies in size. Signup and view all the answers

What is a hole-based system?

A system in which the hole is constant and the shaft varies in size. Signup and view all the answers

What is allowance?

It is the difference between the nominal dimension and the upper or lower limit. Signup and view all the answers

What is tolerance?

It is the difference between the extreme permissible dimensions. Signup and view all the answers

How is tolerance calculated?

By subtracting the smallest size from the largest size. Signup and view all the answers

What is the difference between baseline and continued/chained tolerances?

Baseline tolerances do not stack up. Continued tolerances stack up and are cumulative. Signup and view all the answers

What is the Schedule of Fits and Clearances?

It is part of the maintenance documentation and contains tables which specify the limits on wear, ovality of a hole or shaft, bow of a shaft and twist of a shaft. Signup and view all the answers

What are the four dimensions covered in wear tables?

<ul> <li>Dimension New: size of part when new.</li> <li>Permissible Worn Dimension: size to which a part may wear before it is rejected as unserviceable.</li> <li>Clearance New: desired clearance.</li> <li>Permissible Worn Clearance: allowable clearance when reassembling the component.</li> </ul> Signup and view all the answers

How are holes tested for ovality?

By using Go/No-Go gauges, internal micrometers or calipers. Signup and view all the answers

How are shafts tested for ovality?

By placing the shaft to be measured on a block and rotating it under a scribing block or dial indicator. Signup and view all the answers

When is a shaft considered to be bowed?

When the centerline of the shaft is not straight. Signup and view all the answers

What are the consequences of a shaft in a bowed state?

Vibration, which can lead to mechanical failures and loosening of fasteners. Signup and view all the answers

How is the amount of bow measured?

By placing the shaft on a surface table and inserting feeler gauges at the point of maximum clearance or by using a dial indicator mounted on a surface gauge. Signup and view all the answers

How is the amount of twist measured?

By placing the shaft on a surface table and using a dial indicator mounted on a surface gauge. Signup and view all the answers

Study Notes

Fits and Clearances Overview

Proper hole dimensions for reaming: A smaller hole is required to account for metal removal by the reamer.
Classification of fit: Determined by the relationship between the bolt hole size and the bolt size.

Types of Fits

Clearance fit: Always results in a space between mating parts upon assembly.
Interference fit: Always results in a tight fit between mating parts upon assembly.
Transition fit: A compromise between clearance and interference fits, allows precise holding while remaining disassemblable.

Standards and Systems

British Standards (BS) System: Comprises 21 types of holes (capital letters) and 21 types of shafts (small letters), covering diameters from 0.04" to 19.69", with 16 accuracy grades.
ISO 286: Similar to the BS system but features 28 types of holes and shafts and offers 20 grades of accuracy.
American National Standards Institute (ANSI): Aligns metric standards with ISO and uses symbols for Imperial-based measurements (e.g., RCx for running fits).

Classes of Fit

Newall hole-based system: Class A and Class B fits; Class A offers closer tolerances than Class B.
BS 4500 system: Standard-sized holes establish limits and various fits are achieved using shafts of fluctuating sizes.
Advantages of BS 4500: Facilitates use of standard hole manufacturing tools like drills and reamers.

Tolerances and Systems

Bilateral tolerance: Variation exists on both sides of the nominal dimension.
Unilateral tolerance: One side of the tolerance is zero, focusing on the other side only.
Shaft-based system: A fixed shaft size with varying hole sizes.
Hole-based system: A stable hole size with varying shaft sizes.

Allowance and Tolerance

Allowance: Difference between nominal dimensions and upper/lower limits.
Tolerance: Range defined by extreme permissible dimensions; calculated by subtracting smallest size from largest.

Tolerance Types

Baseline tolerances: Do not accumulate or stack.
Continued tolerances: Cumulative and stackable.

Schedules and Testing

Schedule of Fits and Clearances: Maintenance documentation includes tables that specify limits for wear, ovality, bow, and twist in holes and shafts.
Wear dimension metrics:
- Dimension New: Size of the part when new.
- Permissible Worn Dimension: Size allowed before rejection.
- Clearance New: Desired clearance.
- Permissible Worn Clearance: Allowable clearance for reassembly.

Testing Methods

Hole ovality testing: Utilizes Go/No-Go gauges, internal micrometers, or calipers.
Shaft ovality measurement: Achieved by placing the shaft on a block and rotating it under a scribing block or dial indicator.

Shaft Conditions

Bowed shaft: Occurs when the centerline is not straight, potentially causing vibration and mechanical failures.
Measurement of bow: Conducted using feeler gauges or dial indicators on a surface table.
Twist measurement: Similar process to bow, identifying twist via a dial indicator on a surface table.

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Description

Test your knowledge on fits and clearances in engineering. This quiz covers key definitions and concepts essential for understanding the relationship between holes, bolts, and fits. Perfect for students in mechanical engineering or manufacturing courses.