Gear and Shaft Fits: A Guide to Selection Based on the Shaft Basis System

The fit between gears and shafts is typically determined using the shaft basis system. In this system, the shaft is the reference part, and it is machined first. The gear bore is then machined to match the shaft. This approach offers several advantages:

High machining accuracy: The shaft's dimensions and geometric precision are easier to maintain.

Enhanced rigidity: The shaft's rigidity improves transmission accuracy.

Convenient assembly and interchangeability: Assembly is straightforward, and the parts are interchangeable.

In certain circumstances, the hole basis system may be employed. For instance, the hole basis system is suitable when the gear material is brittle, the machining precision requirements are stringent, or the shaft's size is substantial and challenging to machine.

The primary fit types for gears and shafts are as follows:

Clearance fit: The gear bore's size is greater than the shaft's size, resulting in a clearance between the two components. Clearance fits are characterized by their loose assembly, but they have a lower torque transmission capacity. They are appropriate for situations involving light loads and minimal impact.

Shrink fit: The gear bore's size is smaller than the shaft's size, resulting in an interference fit between the two components. Shrink fits are characterized by their tight connection and strong torque transmission capacity. However, assembly is more difficult, and the materials and machining precision of both the shaft and gear must be high.

Transition fit: This fit type falls between clearance and shrink fits. Transition fits combine the benefits of both clearance and shrink fits, making them suitable for applications involving moderate loads and impact.

The choice of fit type depends on a thorough evaluation of the following factors in light of the specific application:

Power transmission: The amount of power being transmitted

Load characteristics: The load's smoothness and impact severity

Shaft and gear materials and machining precision: The materials and machining precision of both the shaft and gear

Assembly considerations: The assembly process and ease of assembly

Clearance fits should be prioritized when selecting a fit type. Only when clearance fits cannot satisfy the application's requirements should transition or shrink fits be considered.

Here are some recommendations for selecting fits for gears and shafts:

Clearance fits such as H7/h6 or H7/g6 can be used in situations with low power transmission and smooth loads.

Transition fits like H7/k6 or H7/m6 can be used in applications with moderate power transmission and some impact.

Shrink fits like H7/p6 or H7/s6 can be used in situations with high power transmission and significant impact.

In addition, the fit should be adjusted based on the specific application's requirements. For instance, in situations with high operating temperatures, the interference allowance should be increased; in applications with stringent precision requirements, the clearance or interference allowance should be reduced.