Short Answer:

Press-fit connections are shown in mechanical drawings using dimensional tolerances, fit notations, and assembly notes. These connections rely on a tight interference between parts, such as a shaft and a hole. To indicate a press-fit, the drawing must clearly show shaft and hole dimensions with appropriate tolerances, like H7/p6 or H6/u5, according to ISO or BIS standards.

Additionally, callouts or notes are added to inform the manufacturer that a press-fit is required. Proper indication helps ensure the parts fit tightly without additional fasteners, and function reliably in the final assembly.

Detailed Explanation:

Indicating press-fit connections in a drawing

In mechanical engineering, press-fit (interference fit) connections are used when one part (like a shaft) is inserted into another (like a hole) with such tight tolerances that they hold together without bolts or welding. This method is widely used in automotive parts, gears, bearings, pulleys, and bushings.

To make sure the fit is correct and tight, it must be properly shown on the engineering drawing. This is done using dimensional tolerances, fit symbols, and clear annotations that guide machinists during manufacturing and assembly.

Ways to indicate press-fit connections

1. Use of limits and fits system

• The most common way to show a press-fit is by using hole-basis or shaft-basis tolerance systems.
• For example, a press-fit might be indicated as H7/p6, where:
  • H7 is the tolerance for the hole
  • p6 is the tolerance for the shaft
• These codes follow the ISO or BIS system of limits and fits.
• The combination gives a negative clearance, meaning the shaft is slightly larger than the hole.

Example:

• Hole Ø30 H7
• Shaft Ø30 p6
  → This will create a tight press-fit between shaft and hole.

2. Tolerance symbols on dimensions

• The drawing will mention exact upper and lower limits for both parts.
• For instance:
  • Shaft Ø20.02 to Ø20.04 mm
  • Hole Ø20.00 to Ø19.98 mm
• This ensures that even at maximum and minimum sizes, the shaft will always be larger and press-fit is achieved.

3. Fit description in drawing notes

• A note can be written on the drawing like:
  • “Press-fit connection between shaft and hub. Use fit H7/p6.”
  • “Ensure interference fit; shrink fitting required during assembly.”
• This alerts the manufacturer or assembler to follow the correct method.

4. Highlighting assembly instructions

• Press-fits often need heat or press machines to assemble.
• The drawing should include any special assembly or installation instructions, such as:
  • “Heat the housing to 100°C before inserting shaft.”
  • “Do not hammer. Use hydraulic press for assembly.”

Why press-fit must be clearly indicated

• Ensures correct dimensioning for both hole and shaft
• Avoids loose or too tight fits that can damage parts
• Helps production teams and quality control understand the expected result
• Reduces errors in assembly or part rejections
• Ensures proper load transfer, vibration control, and reliable operation

Common applications of press-fits

• Fitting gears on shafts
• Mounting bearings into housings
• Pressing pulleys onto spindles
• Fixing rotors in electric motors

These applications require tight control over tolerances, so the drawing must guide the entire manufacturing process.

Conclusion:

To indicate press-fit connections in a drawing, engineers use tolerance symbols like H7/p6, specify upper and lower dimension limits, and write notes that clearly define the interference fit. Properly showing this information ensures that the parts are made and assembled correctly without any gaps or movement. It helps machinists, quality inspectors, and assembly workers produce reliable and well-fitted components essential for mechanical performance.