How do you specify limits and fits in a mechanical drawing?

Short Answer:

In a mechanical drawing, limits and fits are specified to control how tightly or loosely two parts, like a shaft and a hole, will join together. Limits are given by showing the maximum and minimum allowed sizes of a part. Fits are described using standard hole and shaft combinations to ensure the required level of clearance or tightness.

To specify limits and fits, engineers use standard notations like H7/g6 or H8/f7, where the letters and numbers define the type of fit and tolerance range based on ISO system of limits and fits. These are mentioned next to the dimensions in the technical drawing.

Detailed Explanation:

Specifying Limits and Fits in a Mechanical Drawing

In mechanical design and manufacturing, it is very important to control how two parts will connect and work together. This is done using limits and fits. These ensure that the assembled parts will function correctly—whether they need to move freely, fit tightly, or stay exactly in place.

Let us understand in simple terms how limits and fits are written and used in mechanical drawings.

What are Limits and Fits

  • Limits define the maximum and minimum acceptable sizes for a dimension.
    For example, if a hole is specified as 50.00 mm with limits of +0.02 and –0.01, the size can be from 49.99 mm to 50.02 mm.
  • Fits describe the relationship between mating parts, like a shaft fitting into a hole. The fit controls whether the parts slide freely, lock tightly, or stay snugly together.

These are based on standard systems like the ISO system of limits and fits, which helps everyone follow the same rules.

How to Specify Limits in Drawings

There are three common ways to show limits in a drawing:

  1. Limit Dimensions
    • You write both upper and lower sizes.
    • Example: 20.01 / 19.98 mm
  2. Plus-Minus Tolerancing
    • You show one basic size with a plus and minus tolerance.
    • Example: 20.00 ± 0.02 mm
  3. Deviation Method
    • You give basic size and separate upper and lower deviations.
    • Example: 20.00 +0.01 / –0.03 mm

These are placed next to the feature (hole, shaft, etc.) in the technical drawing.

How to Specify Fits in Drawings

To define fits, the International Tolerance (IT) system is used. It has two main parts:

  • Letter: Indicates the deviation from the nominal size.
    • Capital letters (like H, G) are used for holes.
    • Small letters (like h, g) are used for shafts.
  • Number: Shows the tolerance grade (how tight or loose the fit is).
    • Lower numbers mean tighter tolerances.
    • Higher numbers allow more variation.

Examples of fits:

  • H7/h6 → Common clearance fit (free movement)
  • H7/g6 → Sliding fit
  • H7/p6 → Interference fit (tight press-fit)

In the drawing, these fits are written next to the size, like:
Ø50 H7/g6

Types of Fits According to Use

  1. Clearance Fit
    • Always a gap between hole and shaft.
    • Used when parts must move freely (e.g., shafts, gears).
  2. Interference Fit
    • Shaft is slightly larger than hole.
    • Tight fit, used when parts must not move (e.g., wheels on axles).
  3. Transition Fit
    • Could be tight or loose depending on actual sizes.
    • Used when parts need precise location with some flexibility.

Importance of Limits and Fits

  • Ensures proper function of machines and assemblies
  • Controls manufacturing accuracy
  • Reduces part rejection
  • Helps in choosing correct tools and machines
  • Supports interchangeability of parts made at different locations

Common Standards Used

  • ISO 286: Standard for limits and fits
  • ANSI B4.1: U.S. standard for tolerance systems
  • Hole Basis System: Most commonly used system where hole size is kept fixed (like H7) and shaft size varies (like g6, f7, etc.)
Conclusion

Specifying limits and fits in a mechanical drawing is an essential part of engineering design. It helps control how parts connect—whether they slide, press, or fit tightly. Limits give the allowed size range, and fits define the type of connection. Using standard notations like H7/g6, engineers can clearly communicate how the parts should be made and assembled. Proper use of limits and fits improves performance, reduces errors, and ensures smooth functioning of mechanical systems.