16-04-2026 KWA Gr2. HOLE & SHAFT BASIS SYSTEM &TOLERANCE GRADE

What is Tolerance in the limit system?

• The basic size of a component
• The difference between maximum and minimum permissible sizes
• The upper deviation of a hole
• The measured size of a manufactured part

Answer explanation

Tolerance = Maximum Limit − Minimum Limit. It is also equal to Upper Deviation − Lower Deviation. PSC Tip: Questions asking 'difference between limits' or 'maximum permissible variation' always point to Tolerance. Common Mistake: Do not confuse Tolerance with Deviation.

In the Hole Basis System, which dimension is kept constant?

• Shaft size
• Basic size
• Hole size
• Upper deviation

Answer explanation

In the Hole Basis System, the hole size is kept constant and shaft size is varied to obtain different fits. PSC Tip: This is the STANDARD system used in industry. Common Mistake: Students often confuse Hole Basis with Shaft Basis — remember, Hole = House (fixed).

Holes are represented by ______ letters in the BIS limit system.

• Small (lowercase) letters
• Greek letters
• Capital (uppercase) letters
• Roman numerals

Answer explanation

In BIS (Bureau of Indian Standards): Holes → Capital letters (e.g., H, G, F). Shafts → Small letters (e.g., h, g, f). PSC Tip: H = Hole = Capital. h = shaft = small. Common Mistake: Reversing letters for holes and shafts.

Which IT grade represents the MOST accurate tolerance?

• IT16
• IT8
• IT4
• IT01

Answer explanation

IT grades run from IT01 (most accurate) to IT16 (least accurate). Lower IT number = Higher accuracy. IT01 is used for gauges and precision measuring instruments. PSC Tip: IT01 = Best quality. IT16 = Roughest. Common Mistake: Thinking IT16 is more accurate because it is a higher number.

A shaft of 25 mm diameter has limits of 25.02 mm and 24.98 mm. What is the tolerance?

• 0.02 mm
• 0.04 mm
• 25.02 mm
• 0.01 mm

Answer explanation

Tolerance = Max Limit − Min Limit = 25.02 − 24.98 = 0.04 mm. Formula: Tolerance = Max Limit − Min Limit. PSC Tip: In calculation questions, always subtract the smaller limit from the larger. Common Mistake: Using 25.02 − 25 (basic size) instead of the full range.

In which type of fit is the shaft always larger than the hole?

• Clearance Fit
• Transition Fit
• Running Fit
• Interference Fit

Answer explanation

Interference Fit: Shaft size > Hole size. Parts must be forced together (shrinkage fit, heavy drive fit). Used for permanent/rigid joints. PSC Tip: Interference = 'interfering' with each other — they must be forced together. Common Mistake: Confusing Interference with Transition fit.

For which application range are IT grades IT5 to IT7 typically used?

• Casting and sheet metal work
• Gauges and measuring instruments
• Precision engineering and bearings
• General rough machining

Answer explanation

IT5–IT7: Precision engineering, bearings, fits requiring accuracy. IT01–IT4: Gauges. IT8–IT11: General engineering. IT12–IT16: Casting, stamping. PSC Tip: Bearings are precision components — always IT5 to IT7. Common Mistake: Selecting IT8–IT11 for bearings.

What is the lower deviation of a Basic Hole (H) in the BIS system?

• Equal to tolerance
• Positive value
• Zero
• Negative value

Answer explanation

Basic Hole H: Lower deviation = 0 (zero). This means the hole starts exactly at the basic size and tolerance zone goes upward only. PSC Tip: H = Hole Low = Lower deviation is zero. Basic Shaft h: Upper deviation = 0. Common Mistake: Confusing Basic Hole with Basic Shaft deviations.

A transition fit is characterised by which of the following?

• Hole is always larger than shaft with guaranteed clearance
• Shaft is always larger than hole with guaranteed interference
• Tolerance zones of hole and shaft overlap, resulting in either small clearance or small interference
• Shaft and hole have identical sizes with zero deviation

Answer explanation

Transition Fit: Tolerance zones of hole and shaft OVERLAP. The assembled joint may result in a small clearance OR a small interference — it is not certain which. Example: Push fit. PSC Tip: Transition = 'in between' clearance and interference. The key word is 'overlap'. Common Mistake: Thinking transition fit always gives clearance OR always gives interference.

Why is the Hole Basis System preferred over the Shaft Basis System in practice?

• Holes are easier to measure than shafts
• Standard tools like drills and reamers produce standard holes; shafts can be easily turned or ground to any required size
• Shaft Basis System requires more IT grades
• Hole Basis System is only used for clearance fits

Answer explanation

Hole Basis System is preferred because: (1) Standard cutting tools like drills and reamers produce holes of standard sizes. (2) Shafts can be easily turned, ground, or finished to any required size on a lathe. This makes it more economical. PSC Tip: 'House and Furniture' logic — hole = house (fixed), shaft = furniture (adjustable). Common Mistake: Thinking both systems are equally common in practice.