Reference System Tolerance System: How To Choose The Hole Reference And The Axis Reference?

In mechanical design, the reference system (Basis System) is an important principle to determine the coordination relationship of parts, which directly affects the assembly accuracy, processing cost and the convenience of subsequent maintenance. Usually, the design engineer needs to make decisions between the hole datum (Hole Basis System) and the shaft reference (Shaft Basis System) when selecting the tolerance fit. How to choose reasonably according to the application scenarios? This paper will systematically analyze the characteristics, applicable scenarios and influencing factors of the two benchmark systems.

1. The concept of the benchmark system and the tolerance system

The base system tolerance system refers to the determination of a fixed size reference part in the coordination design, and the selection of the appropriate coordination tolerance on its basis.for instance:

The hole reference system (Hole Basis System) adjusts the size and tolerance of the shaft with the size of the hole, so that the fit to achieve the desired effect.

Axis reference (Shaft Basis System) adjusts the size of the hole and the tolerance to meet the design requirements.

In the standard tolerance marking method, the dimensional tolerance of the reference part is usually fixed, such as:

Hole reference system H7 / f7 (hole H7, shaft f7)

Axis reference h7 / F7 (shaft h7, hole F7)

2. The difference between the hole reference system and the shaft reference system

3. How to choose the right base system?

1. Selection of the hole reference system

The hole reference system is more common in mechanical design due of its convenience and versatility, and is applicable for the following scenarios:

✅ Mass production, standardized assembly: such as rolling bearings, standard keyway, pin connection, etc., the use of fixed hole size can reduce the complexity of processing and inventory management.

✅ The processing difficulty of holes is higher: because the processing of holes usually involves drilling, boring, hinge and other processes, it is more difficult to adjust than the shaft processing, so the fixed hole size can improve the production efficiency.

✅ scenarios with high assembly requirements: such as precision bearing seat, to ensure that the bearing can be easily installed without damage, using H7 and other standard tolerance can ensure interchangeability.

2. Choose the axis reference system

Although it is common, the shaft may be more appropriate in the following special cases:

✅ Special working conditions, such as sliding bearing coordination: in some high-precision applications (such as hydraulic piston, special guide rail), the diameter of the shaft must be strictly controlled, and the hole can be adjusted appropriately.

✅ Axis machining accuracy requirements are high: for the parts that need to ensure the consistency of shaft diameter, such as high precision spindle, the use of fixed shaft diameter can improve machining consistency and reduce error.

✅ The force direction is special, and the hole can be adjusted: for example, in some crankshaft applications, the tolerance of the hole can be slightly adjusted to accommodate the deformation or temperature change of the axis.

• 4. Tolerance selection in different application scenarios

• In mechanical design, in addition to the selection of reference system, it also needs to consider the coordination type (gap coordination, transition cooperation, surplus coordination).for instance:

• 1. Rotary shaft and bearing seat (hole reference system, H7 / g6)

• Scene: The motor shaft and the bearing are fit

• Requirements: Ensure that the bearing can be easily loaded into the seat hole and have appropriate gaps to maintain lubrication

• Selection: H7 (hole fixed), g6 (shaft adjustable)

• 2. Precision sliding guide rail (shaft reference system, h7 / F7)

• Scenario: linear guide rail and slide block

• Requirements: Ensure the accuracy of the guide rail, and allow the fine-tuning of the slider hole

• Selection: h7 (shaft fixation), F7 (hole adjustment)

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• 3. Overess mating connector (hole reference, H7 / n6)

• Scene: Flywheel and shaft fit

• Requirements: a certain amount of surplus to ensure firm connection

• Selection: H7 (hole fixation), n6 (shaft adjustment)

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5. Factors influencing the selection of the benchmarks system

In the actual design, the factors influencing the selection of the base system include:

Processing cost: if the machining shaft is more economical than the machining hole, the hole base is preferred; otherwise, the shaft base is selected.

Convenience of assembly: during mass assembly, the hole reference system is easier to exchange parts and improve production efficiency.

Material and heat treatment: In some cases, the material of the shaft may require special treatment (such as carburization, quenching), when the shaft reference system may be more suitable.

Temperature influence: If the operating temperature of the equipment is high, consider the influence of thermal expansion on the size, and choose a more appropriate reference system with the tolerance range.