How to: Inventor’s Design Accelerator: Limits/Fits Mechanical Calculator

Posted February 25, 2013 by Robert Reid:

Designers often calculate and design a shaft-hole fit. This problem influences the properties of the final product and its price of manufacturing. Use the adaptable interface in Design Accelerator to design the ideal fit.

Limits and Fits are used to define tolerances for mating parts. Limits are most commonly used for cylindrical holes and shafts, but they can be used for any parts that fit together regardless of the geometry.

A limit is the upper and lower tolerance for a shaft or hole.

A fit consists of a pair of limits. There are three categories: clearance, transitional, and interference.

Clearance                   

  • The largest shaft is smaller than the smallest hole.
  • Large clearances are for relative motion.
    • A shaft in a bushing
  • Small clearances are for a precise fit that can be assembled without tools.
    • A shaft in a bushing

Transitional

  • Tolerance zones overlap.
    • The largest shaft is larger than the smallest hole (interference).
    • The smallest shaft is smaller than the largest hole (clearance).
  • Precise location that may require tools to assemble.
    • Dowel pins in tooling to locate parts

Interference

  • The smallest shaft is larger than the largest hole.
  • Precise location that requires tools to assemble.
  • Parts do not come apart or move during use.
  • May require heating or cooling to assemble parts.
    • A gear on a shaft

To access the Limits and Fits Mechanical Calculator, In the Assembly, click on the Design Tab:  

Click the black down arrow on the Power Transmission Panel:  

Click the black down arrow Next To Tolerance Calculator, and click on the Limits/Fits Calculator:

This opens the Limits and Fits Mechanical Calculator Dialogue Box:  

Section 1, Conditions:

Determine the fit type for the Hole-basis system or the Shaft-basis system of fits. A similar fit is calculated automatically if the option is checked in the More Options area when the fit calculation system is changed.

Select the type of fits. This option provides an easy way to keep hole or shaft tolerance class on “H” (“h” symbol). You can define any custom fit, if needed. Basic-hole allows the use of standard tooling to bore the hole.

Enter or measure the nominal size of the hole and the shaft.

Set up the nominal diameter as a basic size. For ANSI active profiles, the diameter value is in inches. For ISO active profiles, the diameter value is in millimeters.

Section 2, Tolerances:

Displays the selected tolerance image. The displayed image depends on the specified option in More Options (Tolerance Image pull-down list). The image and dimensions dynamically update to reflect changes made in the Conditions and Tolerance Zones group boxes.

The upper half of the image represents the hole and the lower half of the image represents the shaft. Basic size value is indicated by a bold circle. The tolerance zone is indicated by a yellow, hatched area. The tolerance zone area is defined by two arcs where the arcs represent the upper and lower values of the limit. The arcs are tangent to the basic size circle on the left side.

The dimensions display basic size, maximum clearance or interference, minimum clearance or interference, and distance between midpoints of tolerances.

Section 3,  Tolerance Zones:

Specify the limits and fits. Selected fit is displayed in a graphic form.

Section 4, Results:

To display the Results area on the right side of the Calculation tab, double-click the double line on the right or click the chevron.

The area displays numerical values for the tolerances. Click Calculate to calculate the values.

The following items are displayed in the Results pane:

  • Hole
    • Minimum diameter of the hole (Hole min)
    • Maximum diameter of the hole (Hole max)
    • Upper tolerance of the hole (Hole upper)
    • Lower tolerance of the hole (Hole lower)
  • Shaft
    • Minimum diameter of the shaft (Shaft min)
    • Maximum diameter of the shaft (Shaft max)
    • Upper tolerance of the shaft (Shaft upper)
    • Lower tolerance of the shaft (Shaft lower)
  • For Clearance Fits
    • Minimum clearance diametric value of the fit (Clearance min)
    • Maximum clearance diametric value of the fit (Clearance max)
  • For Transition Fits
    • Maximum interference diametric value of the fit (Interference max)
  • Midpoint
    • Distance between midpoints of tolerance zones (Midpoint)

Section 5, More Tab:

Find   equivalent fit when changing between Hole-basis and Shaft-basis

When you select   a shaft fit symbol In the Tolerance Zones area (fit symbols are in a graphic   form), the Hole-basis fit system and ‘H’ fit symbol is set automatically. The   same behavior occurs for the Shaft-basis fit system.

Change   fit system according to selections in fit chart

A corresponding   fit is calculated automatically, if the box is checked, when the fit   calculation system is changed.

Standard

Use the   pull-down menu to select the standard for the calculation.

Tolerance   zones that are outside the specified range

Change the   color of the tolerance zones.

Tolerance   zones that are within the specified range

Change the   color of the tolerance zones.

Tolerance   Image

Use the   pull-down list to select the type of picture to display in the Tolerances   group box of the Calculation tab.

Example:

  • Shaft-basis system, ISO 30mm Diameter, Max Clearance .08mm,
  • Min. Clearance .03mm. Fit Type is set to Clearance, Preferred Fit/Limits
  • Will be set to a H7/g6 Standard fit, Tolerance Image set to Hole and Shaft:

By clicking on any of the Limits in the Tolerance Zone will show Various Results for that Fit.

Here, the F7/k6 is shown.  Notice the different results and also the Tolerance Picture:

Changing the Tolerance Image to ISO Tolerance Range Image will now show:

The Use of the Limits/Fit Mechanical Calculator can save the end user valuable time by having the correct values calculated automatically according to inputted data! No need to look up the values in Reference Books, thereby possibly choosing the incorrect values!

Contact us if you run into any issues, we can help!

Happy Designing!

-Robert Reid

(*This tip will work for either Autodesk Inventor 2012 or 2013 versions.)

Robert Reid graduated from Middlesex College in New Jersey with a Degree in Mechanical Engineering Technology. With over 25 years of Industrial Machine Design, including Wire Drawing Machinery, Plastic Processing Machinery, & Packaging Machinery, Robert has held various positions within the workforce, including, Manager of Engineering; Sheet Extrusion Dept, Manager of Engineering, Chief Design Engineer. He is currently an Autodesk Inventor Certified Expert.
//

11 comments

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s