In the field of mechanical engineering, the shaft, as a crucial component for transmitting power and motion, holds immense importance. However, a successful shaft design goes beyond mere conception; it demands an in-depth understanding of various design elements. In industrial production lines, the performance and reliability of shafts directly impact the efficiency and lifespan of mechanical systems. This article will take you on a journey to explore shaft design.
I. Classification of Shafts
Classification by Load Conditions:
Transmission Shafts: Those simultaneously subjected to bending and torsional loads, commonly found in components like gearbox shafts.
Drive Shafts: Those subjected primarily or solely to torsional loads, such as the drive shafts in automobiles.
Stub Shafts: These endure bending loads without torsion. They can be further categorized as rotating stub shafts, like railway vehicle axles, or stationary stub shafts subjected to static loads.
Classification by Shape:
Straight Shafts: These include plain and stepped shafts. Stepped shafts have specific roles, such as facilitating component installation and ensuring uniform strength across shaft segments – a common practice in machinery. Terminology related to stepped shafts includes:
Shaft Neck: The supported portion of the shaft where bearings are installed.
Shaft Head: The section where components like hubs are mounted.
Shaft Body: The portion connecting the shaft neck and shaft head.
Shaft Extension: The protruding end of the shaft.
Crankshafts: Primarily used in reciprocating machinery, especially in specialized applications.
Flexible Wire Shafts: Composed of tightly wound layers of steel wire, these shafts offer flexibility in transmitting torque (torsion and rotation) to various locations.
In most cases, shafts are manufactured as solid components. However, to reduce weight (as seen in large water turbine shafts, aircraft engine shafts, etc.) or fulfill operational requirements (such as passing through other components or accommodating lubrication), hollow shafts are also utilized.
II. Shaft Design Process
Material Selection: Consider the requirements for strength, stiffness, and wear resistance of the shaft. Determine the appropriate heat treatment method for the shaft, taking into account process demands, manufacturing processes, and material availability and pricing. Commonly used materials include 40, 45, 50, and 40Cr through quenching or tempering.
Note: Carbon steel is more cost-effective than alloy steel and is less sensitive to stress concentration. Alloy steel boasts superior mechanical and heat treatment properties but comes at a higher price. It is often used for shafts subjected to heavy loads, limited dimensions or mass, and those requiring high wear resistance and corrosion resistance. Alloy steel is more sensitive to stress concentration. When designing alloy steel shafts, it's crucial to avoid or reduce stress concentration through structural means and minimize surface roughness values.
Initial Calculation: Use an analogy method to estimate the diameter 'd' or preliminarily calculate 'd' based on torque. Pay attention to the influence of keyways on shaft strength. When a single keyway is present, increase the shaft diameter by 3% to 5%; with two keyways, increase it by 7% to 10%.
Structural Design: Shape, dimensions, calculations, and drawings should be developed concurrently.
Precise Calculation: For ordinary shafts, check the strength of 2 to 3 critical cross-sections using the equivalent bending moment method. For vital shafts, further verify strength using the safety factor method. Shafts requiring high stiffness should undergo stiffness calculations, while high-speed shafts require vibration stability calculations.
III. Shaft Structural Design
Shaft failures can occur in the forms of fracture, wear, vibration, and deformation. Shaft design should encompass sufficient strength, stiffness, good vibration stability, and a rational structure. For high-speed shafts, vibration stability is an additional concern.
Principles of Shaft Structural Design:
(1) Rational Force Distribution: Strive for uniform strength while minimizing stress concentration.
(2) Precise Component Positioning: Ensure accurate, simple, reliable, and easily detachable positioning of components on the shaft.
(3) Good Manufacturability: Consider ease of manufacturing.
Content of Structural Design:
Determine the reasonable external shape and overall dimensions of the shaft.
Structural design should be based on known factors:
Schematic layout of shaft assembly (position of components on the shaft).
Main parameters and dimensions of transmission components.
Power and rotational speed transmitted by the shaft.
The typical procedure for shaft design generally involves:
a. Drafting the arrangement and assembly scheme of components on the shaft based on the overall layout of the mechanical transmission system.
b. Material selection for the shaft.
c. Estimating the minimum diameter of the shaft.
d. Conducting structural design of the shaft.
e. Performing load-bearing capacity verification, including strength, stiffness, and vibration stability assessments.
f. Based on the load-bearing capacity verification results, either finalizing the design or refining it.
g. Creating detailed part drawings of the shaft.
Conditions for Shaft Structural Design to Meet:
(1) Precise Component Positioning: Components on the shaft and mounted on the shaft must be in accurate working positions.
(2) Ease of Disassembly and Adjustment: Components on the shaft should be easy to dismantle and adjust.
(3) Good Manufacturability: The shaft should be manufacturable with good process feasibility.
Common Fixation Methods and Considerations
Axial Fixation of Components on the Shaft:
(1) Shaft Shoulder and Collar (Pedestal): Comprised of positioning surfaces and transition radii. The transition radii should be smaller than the chamfers on the components (otherwise, components won't fit snugly). Generally, the single-sided height of the positioning shaft shoulder (h) is taken as h = 0.07~0.1 times the shaft diameter (d), and the collar width (b) should be ≥1.4h.
(2) Coordinating with Rolling Bearings: The height of the shaft shoulder or collar coordinating with rolling bearings should be referenced from manuals. Generally, the height of the shaft shoulder does not exceed 2/3 to 4/5 of the thickness of the inner ring to facilitate bearing disassembly.
(3) Elastic Retaining Rings, Nuts, Bushings, etc.: The inner diameter of the bushing and the shaft are generally a dynamic fit. The structure and dimensions of the bushing can be flexibly designed based on requirements, but the wall thickness of the bushing is usually greater than 3mm (note: not suitable for high-speed shafts).
(4) Shaft End Retaining Rings and Tapered Surfaces.
(5) Fastening Screws and Locking Rings, etc.
(6) Key: Provides both axial and circumferential fixation. Offers good structural feasibility but requires drilling holes in the shaft, potentially weakening its strength and disrupting rotor balance.
(7) Pin: Good structural feasibility, provides both axial and circumferential fixation. Requires drilling holes in the shaft, affecting its strength and rotor balance.
(8) Interference Fit: Simple structure, provides both fixation and circumferential fixation, but with poor positioning accuracy.
(9) Nut: Threads are made on the shaft surface, weakening the shaft's strength but offering strong load-bearing capacity and reliable fixation.
(10) Shaft End Pressure Plate: Threads are drilled in the shaft end without weakening its strength, providing strong load-bearing capacity and reliable fixation. Suitable for the shaft end.
To ensure reliable axial positioning, the length of the shaft segment coordinating with parts like gears and couplings should generally be 2~3mm shorter than the hub width.
Circumferential Fixation of Components on the Shaft:
Circumferential fixation of components on the shaft can use keyways, splines, pins, interference fits, and expansion sleeve connections.
Keyways, splines, interference fits, pins, form-fits, and elastic rings are collectively referred to as shaft-hub connections.
Considerations
(1) Keyway openings on the shaft should be on the same machining line to reduce clamping times. If the diameters of the segments with keyways are not significantly different, use keyways of the same width to reduce tool change frequency.
(2) Half-moon keys should be opened on the same side; otherwise, they will weaken the shaft more severely.
(3) When using feather keys, the surfaces of the keys and the bottom of the hub slot should have a 1:100 slope.
Manufacturing and Installation Requirements:
(1) To facilitate assembly and disassembly, create a stepped shaft, and all components on the shaft should be able to reach the coordinating positions without interference.
(2) To facilitate processing and inspection, the diameter of the shaft coordinating with parts should be a rounded value and selected from the standard diameter series (see GB/T2822-2005); the parts coordinating with rolling bearings should meet the dimensions of the rolling bearing inner hole.
(3) Threaded shaft segments should leave room for tool retraction slots, and the diameter of threaded shaft segments should comply with standard thread diameters.
(4) Ground shaft ends should have grinding wheel traverse slots or edge trimming.
(5) Except for specific structural requirements, for easy part assembly and deburring, the shaft end should have a 45° chamfer.
(6) Parts with interference fits are typically fitted with guiding tapered surfaces (with a slope angle of generally 10°) to facilitate smooth insertion.
(7) The radii on the same shaft should be as consistent as possible, with the same width for tool retraction slots and the same chamfer dimensions.
Reducing Stress Concentration and Improving Loading Conditions:
(1) Avoid abrupt changes in shape to ensure uniformity among stepped shafts at different levels.
(2) Use fillets at section transitions and avoid them being too small (specific dimensions can be found in relevant manuals).
(3) Avoid drilling transverse holes or grooves on the shaft. If necessary, the hole's edge should be rounded (the hole opening should be chamfered), and blind holes should be avoided.
(4) In the structure of important shafts, use unloading slots B, transitional collar rings, or notched fillets to increase the radius of the shaft shoulder's rounded corners and reduce local stress.
(5) Make unloading slots B on the hub (reducing the thickness on both sides of the hub) to also reduce local stress at interference fits.
(6) Cut unloading slots on the hub's end faces at the interference fit locations.
(7) Ensure the diameter at the interference fit location is larger than the adjacent part's diameter.
(8) Provide rounded corners for tool retraction slots.
(9) Increase the radius of the collar ring and shaft shoulder, and if necessary, introduce an intermediate collar.
Improving the Surface Quality and Increasing Fatigue Strength of the Shaft:
(1) The rougher the surface of the shaft, the lower the fatigue strength. Therefore, minimize the surface roughness values on the shaft and at the rounded corners (especially when using materials sensitive to stress concentration).
(2) The distance between the keyway end and the stepped part should not be too small to avoid damaging the transition fillet and reduce the chances of multiple stress concentration sources overlapping.
(3) The smaller the radius at the root of the keyway, the more severe the stress concentration. Therefore, specify its size on the part drawing of important shafts.
(4) Avoid printing or leaving unnecessary marks on the shaft, as they can become sources of initial fatigue cracks.
(5) Common surface strengthening methods: high-frequency quenching and other heat treatments; chemical heat treatments such as carburizing, cyaniding, and nitriding; strengthening treatments like rolling and shot peening. (Rolling and shot peening can induce compressive stress on the shaft's surface, thereby enhancing its fatigue resistance.)
| Surface Quenching Treatment and Hardened Layer for Shafts | |||
| Performance Requirement | Operating Conditions | Hardened Layer Depth/mm | Remarks |
| Wear Resistance | Light Load | 0.5~1.5 | For small and medium-sized shafts, the hardened layer depth can be calculated as 10%~20% of the shaft diameter (up to the upper limit for shafts with a diameter of 40mm or above). |
| Heavy Load or Impact Load | 2.0~6.5 | ||
| Fatigue Resistance | Periodic Bending or Torsion | 3.0~12 | |
| Chemical Heat Treatment Methods for Shafts | ||||||
| Infiltrated Element | Process Method | Common Steel | Surface Layer Structure | Layer Depth/mm | Surface Hardness | Function and Characteristics |
| C | Carburizing | Low carbon steel, low alloy steel | Carbides + Martensite + Residual Austenite after quenching | 03~1.6(usually 0.8~1.2) | 57~63HRC (usually 58~ 62) | Carburizing and quenching can enhance surface hardness, wear resistance, fatigue strength, and ability to bear heavy loads. However, it requires higher treatment temperature and may lead to significant workpiece deformation. |
| N | Nitriding | Al-containing low and medium alloy steel, medium carbon chromium alloy steel, Austenitic stainless steel, etc. | Alloy nitrides + Nitrogen-containing solid solution | 0.1~0.6(usually 0.2~03) | 700~900HV | Nitriding can increase surface hardness, wear resistance, anti-adhesive ability, fatigue strength, corrosion resistance (except stainless steel), and resistance to temper softening. Nitriding offers higher hardness and abrasiveness compared to carburizing but comes with higher cost. Nitriding temperature is lower, leading to less workpiece deformation. However, longer nitriding times can result in increased brittleness of the layer. |
| C,N | Nitrocarburizing | Low and medium carbon steel, low and medium carbon alloy steel | Carbide nitrides + Nitrogen-containing Martensite + Residual Austenite | 0.25~0.6(usually 0.3~0.4) | 58~63HRCI | Nitrocarburizing can enhance surface hardness, wear resistance, and fatigue strength. The process temperature is lower than carburizing, resulting in less workpiece deformation. However, achieving a thick layer can be challenging. |
| Low-temperature Nitrocarburizing (Soft Nitriding) | Carbon steel, alloy steel, cast iron, stainless steel | Carbide nitrides + Nitrogen-containing solid solution | 0.007~<0.02 | 50~68HRC | Low-temperature nitrocarburizing improves surface hardness, wear resistance, and fatigue strength. It is performed at a low temperature, resulting in minimal workpiece deformation, and offers lower hardness compared to general nitriding. | |
Key Considerations in Shaft Structural Design
(1) The length of the shaft shoulder should be slightly shorter than the hub length; otherwise, reliable positioning of the hub (axially) will be compromised.
(2) For the shaft end coordinating with bearings, the shaft shoulder or locating bushing must exceed the height of the bearing inner ring. Otherwise, bearing disassembly will be hindered.
(3) Pay attention to whether the length of the key affects the installation of axially positioned components, and be mindful of the location of the keyway.
In the field of shaft design, a thorough understanding of key elements and practical recommendations is crucial for ensuring success. By considering aspects such as load analysis, material selection, dimension optimization, and connection methods, you can create shafts with outstanding performance, reliability, and stability.
