Quality Gearbox & Gear Reducer factory

The micrometer is a widely used precision measuring tool. Based on its manufacturing accuracy, it can be classified into two grades: Grade 0 and Grade 1, with Grade 0 offering higher precision and Grade 1 being slightly lower. The manufacturing precision of a micrometer is primarily determined by its indicating error and the tolerance of flatness and parallelism between its anvils.  Based on these precision requirements, micrometers are suitable for measuring part dimensions with IT6 to IT10 tolerance grades.

For assembly (fit) dimensions (such as bearing seats or flange end cover spigots), a micrometer must be used for measurement, and the results should be recorded with two decimal places. For other non-fit dimensions (such as external dimensions), calipers or steel rulers may be used for measurement, and the results should be recorded with one decimal place.

1. Keyway Width Sides (Flanks): Ra = 3.2 μm Rationale: This precision grade ensures that the key and the keyway sides can be assembled smoothly and have a sufficiently large actual contact area for transmitting torque. This effectively prevents stress concentration, fretting wear, and the initiation of fatigue cracks. Achieving the Finish: This surface roughness can typically be achieved directly through precision milling, broaching, or shaping without the need for additional finishing processes (such as grinding or honing), making it very cost-effective. Application: This is the standard selection for the vast majority of general-purpose machinery (e.g., gearboxes, transmission shafts, pulleys, couplings) under conventional load and speed conditions. 2. Keyway Bottom Surface: Ra = 6.3 μm Rationale: The bottom surface is a non-working face. Its primary function is to provide radial support and positioning for the key during assembly. An Ra=6.3μm requirement is smooth enough to ensure the key can seat properly at the bottom without interference, yet economical enough as it does not directly contribute to torque transmission. Achieving the Finish: This roughness is easily achieved in a single pass through conventional milling. Critical Point: For the bottom surface, the quality of the root fillet radius is more critical than the roughness. Machining must ensure the fillet is smooth, free of sharp corners, tool marks, or micro-cracks. These defects are primary sources of stress concentration and can easily lead to shaft failure initiating from the end of the keyway under alternating torque.

This standard specifies the technical requirements for the design tolerances of keyway width, symmetry, and the keyway root fillet on shafts and hubs.This standard is applicable to the design of keyways on both shafts and hubs.The technical requirements for the keyway root fillet specified in this standard are critical to the service life of the shaft. Design, machining, and inspection must strictly adhere to this standard. Based on experience, over 50% of shaft failure cases are attributed to inadequate design and machining of the keyway root fillet and its symmetry.

The purpose of preliminary heat treatment is to improve machinability, eliminate internal stresses, and prepare a suitable metallurgical structure for subsequent final heat treatment. Common processes include annealing, normalizing, aging, and quenching & tempering (also known as conditioning).

"A fatigue crack initiation point formed at the root of the keyway. The reason why a small fillet or sharp corner at the keyway root easily becomes a fatigue source can be understood by comparing it to the tear notch on a instant noodle package. Due to our general lack of in-depth research and understanding regarding keyways, it is easy to overlook their importance. This often leads to the keyway root being machined into a sharp corner (with a very small fillet, almost a right angle) unless special attention is paid."

Stress concentration also has many beneficial applications in daily life. A prime example is the tear notch on the packaging of the snacks we eat. These notches are typically serrated or triangular-shaped. This design intentionally creates points of high stress concentration, allowing the package to be torn open with very little force. Conversely, you might have encountered packaging with rounded tear notches; these require more force to open because the rounded shape reduces the stress concentration effect.

When the root fillet radius of a keyway is very small, approaching zero (effectively a sharp corner), it creates a significant stress concentration. The presence of this stress concentration severely reduces the load-bearing capacity of the shaft.