Understanding Shafts: Types, Materials, and Applications

What is a Shaft?

A simple shaft definition is that they are essential mechanical components whose primary purpose is to transfer power and motion between various elements within a system while maintaining a strong, durable, and safe interface.

In most cases, shafts have a circular or annular cross-section that is extruded up to a certain length. Therefore, their geometric appearance is cylindrical. Typically, there are mounting structures at the ends as well to allow assemblage with other mechanical components via appropriate fasteners.

Shafts, while seemingly basic in terms of design, are quite complex, requiring a strong conceptual understanding of various engineering disciplines like design, heat transfer, stress analysis, and manufacturing.

Types of Shafts

A mechanical shaft, while having the same core purpose, comes in many types. There are numerous types of shafts that differ in terms of design features, load-bearing qualities, and applications.

In the following sections, we will discuss some of the typical shaft types.

Transmission Shaft

Transmission shafts transmit power between components in mechanical systems. These systems include engines, gearboxes, drive assemblies, etc. Typically, a transmission shaft transfers rotary motion and torque from one part of a machine to another.

For example, in automotive gearboxes, multiple transmission shafts transfer the input power from the power source to different devices. By using the correct gear ratios, the input torques and speeds are converted to the desired values.

Machine Shaft

Machine shafts are ubiquitous in mechanical devices. Mainly, machine shaft is a general term for the all-purpose mechanical shaft. They are very similar to transmission shafts but with the additional purpose of being an integral structural and functional component of the machine.

Depending on their application, a machine shaft may have design features like keyways, threads, or splines.

For example, an engine shaft like a crankshaft or camshaft has several features like counterweights, cams, and cooling channels that allow them to not only transmit torque but also perform other important functions in the engine.

Flexible Shaft

Flexible shafts are useful for applications where a rigid shaft is impractical or impossible to use. This can be due to space or motion constraints. Their flexibility comes from their unique construction featuring coiled or braided metal wires, like a rope. Owing to this build, a flexible mechanical shaft can bend and twist without losing the ability to transmit rotational motion.

Axle

Axles are automotive shafts that support and drive critical components wheels. They transmit power from the engine to the wheels while also bearing the vehicle’s weight. Due to this, they are subject to intense loading conditions, which require them to be highly durable and resistant to bending forces.

Propeller Shaft

Propeller shafts are special-purpose transmission shafts with the sole purpose of transmitting rotational power to propellers, primarily in marine applications such as ships.

Similar to an engine shaft, they connect the motor to a propeller, whose blades then generate thrust to push the body through a fluid (typically air or water). Propeller shafts have special requirements like corrosion resistance, high dynamic stability due to propeller vibrations, and high torsional and bending strengths.

Stub Shaft

Stub shafts are short shafts that primarily serve as interfacing devices in mechanical devices rather than load-bearing components. Generally, their use is in gearboxes. A stub shaft usually has just enough material that it extends a bit from a larger shaft or housing. Its free, cantilever end features splines or keyways to attach other coupling components for power transmission.

Common Materials for Manufacturing Shafts

When it comes to the choice of shaft material, there is an even wider range than the types of shafts. The correct choice depends on the design goals, application, and budget. The following discussion highlights some of the common materials for manufacturing shafts.

Types of Stresses in Mechanical Shafts

A mechanical shaft typically undergoes several forms of mechanical loadings during operation. Therefore, shafts endure different types of mechanical stresses. It is important to understand this for a designer as this information is critical in determining the shaft’s dimensions, material, and manufacturing techniques.

Shear Stress

Shear stress develops in the shaft due to torsional loads, which try to slide one part of the cross-section in one direction and the other in the opposite direction, as is seen in the image. Since the main purpose of shafts is to transmit torque, designing them to withstand the consequent shear stress is a priority.

For torsional loading conditions, the shear stress is calculated as:

Where:

• τ = Shear stress (N/m²)
• T = Torque (Nm)
• r = Distance of forcing point from the center (m)
• J = Polar moment of inertia (m⁴)

Bending Stress

Bending stresses are very important in a mechanical shaft as shafts tend to bend under loads and gravity. Typically, as a shaft gets longer, it is more prone to bending and requires greater care to prevent any bending-related damage.

They induce both tensile and compressive stresses in the shaft material, which are cyclical as well if the shaft rotates. Bending stress is the highest at the outermost points of the shaft, getting smaller as it gets closer to the center.

The shaft bending stress formula:

Where:

• σ_bending ​ = Bending stress (N/m²)
• M = Bending moment (N·m)
• y = Distance of the forcing point from the neutral axis (m)
• I = Moment of inertia (m⁴)

Axial Stress

In many cases, shafts also carry axial loads, either in the form of pulling (tensile) or pushing (compressive). The most common example is the spindle shaft of a CNC machining center, where cutting forces act in both radial and axial directions.

Axial stress is generally of lesser concern than radial stresses as shafts are stronger along their axial direction. It is, however, important to consider when axial forces are high or axial elongations matter.

To calculate axial stress, the following formulation is useful:

Where:

• σ_axial  = Axial stress (N/m²)
• F = Axial force (N)
• A = Cross-sectional area of the shaft (m²)

Thermal Stress

Shafts operate in extreme conditions where temperatures can be significantly higher than the ambient conditions. In some cases, shaft temperatures can grow to an extent that causes notable thermal expansion or thermal softening.

A good example of this is in CNC machines. The spindle shaft is very close to heat sources like the bearings, which generate enough heat to cause the shaft to expand by a few microns. In terms of CNC machining, where accuracies are in the order of µm, this is significant.

To compute thermal stress in a mechanical shaft, engineers use the following equation:

• σ_thermal = Thermal stress (N/m²)
• E = Young’s modulus of the material (N/m²)
• α = Coefficient of thermal expansion (1/°C)
• ΔT = Change in temperature (°C)

It is clear that the thermal stress grows linearly with temperature and Young’s modulus, highlighting how important the selection of shaft material is for different applications.

Fatigue Stress

Fatigue stress occurs in a mechanical shaft due to cyclic loading (loading that oscillates with respect to time). The continuous, repetitive loading/unloading cycle makes the material weak over an extended period of time, increasing risks of failures like cracking.

While there is no one-shot formula to quantize fatigue stress in shafts, there are several ways to express it for performance evaluation. Engineers use concepts like stress endurance limits, mean stress in a loading cycle, and number of cycles to failure (S-N curves) to evaluate and compare fatigue resistance.

Vibration

Vibration is an unavoidable occurrence in mechanical devices. In a mechanical shaft, there are several sources of vibration. Vibrations are not desirable as they cause the shaft to deform, and thus, develop stress. For example, in cases of chatter in machining, the shaft sometimes vibrates so much that the resultant stresses permanently damage the spindle shaft.

Some common vibration sources in shafts include:

• Internal: Mass imbalance
• Mounting Devices: Bearings, couplings
• External: Cutting forces in CNC machines
• Involuntary: Road bumps

It is a complex concept, but to put it simply, a mechanical system exhibits different vibrations and behaviors are different frequencies. At some fundamental frequencies, the vibration response of a mechanical shaft may be very high (also known as resonance).

To avoid issues, engineers should design shafts in a way that such fundamental frequencies fall outside the range of operating frequencies. For example, if an engine shaft is supposed to rotate at up to 5000 RPM, its fundamental frequencies should ideally be at higher frequency levels of 6000 RPM and above.

Design and Manufacturing Considerations for Custom Shafts

There are many ways in which a mechanical shaft may not perform as intended. To avoid such occurrences, there are certain good practices that design and manufacturing professionals follow to ensure minimal chances of failure.

Dimensioning

At the basic level, a shaft with incorrect dimensions may not assemble with other components. For instance, an oversized shaft would not fit well with other components like gears or sleeves, potentially damaging them.

The dimensioning is also directly linked to the weight of the shaft. Larger diameters and lengths, and smaller holes, increase a shaft’s mass, and vice versa. One of the reasons to be careful with dimensioning is to reduce mass to avoid unnecessary assembly weight, material costs, and rotary inertia.

At a more technical level, as was discussed previously, a poorly dimensioned may not withstand the loads in its operating environment and fail prematurely. Imagine an engine shaft with a small diameter. Due to its lack of strength, it would bend or fracture under mechanical loads.

Tolerancing

Since all types of shafts are parts of a larger assembly, defining the correct tolerances in the engineering drawing is a critical factor. An effective assembly between shafts and components like hubs, bearings, sleeves, etc. is highly dependent on the tolerances.

In this regard, there are several rules to follow. For instance, the design engineer should be aware of shaft and hole-basis tolerancing, tolerance classes (interference, transition, and clearance), and material dependencies.

Improper tolerancing can have drastic effects such as component failures, low mechanical efficiency, safety issues, and high maintenance costs.

Form

The form of a mechanical shaft refers to qualities like cylindricity, coaxiality, and runout. A shaft must be as close as possible to a perfect cylinder, otherwise, it will have an uneven mass distribution about its axis, which can damage components like bearings and decrease efficiency. Similarly, if a shaft has different features with variable diameters, all of them should be coaxial to avoid similar issues.

Furthermore, poor form tolerances can also hinder assembly procedures. For instance, a shaft with a curved/bent axis may not fit inside a sleeve despite having the correct dimensions.

In most cases, engineers use the universal Geometric Dimensional & Tolerancing (GD&T) system to define the dimensional and form tolerances of shaft designs.

Surface Finish

Surface finish is another consideration for mechanical shaft manufacturing. The most important factor is frictional forces between the shaft and mating components. If the surface is rough, there is high friction at the mating interface, which can cause uneven mounting pressure, damage, and wear.

Therefore, depending on the type of fit and coupling, designers generally define an appropriate surface finish level for manufacturers. Furthermore, having a smooth surface finish also aids with other issues like fatigue resistance, micro-stress concentrations, and thermal conductivity.

Balancing

Shafts transmit rotary power. Since they rotate in the process, mass balancing is a critical issue. If a mechanical shaft has a mass imbalance, it adds extra imbalance loads to load-bearing components like bearings, decreases mechanical efficiency, and increases the chances of failure.

Moreover, the impact of imbalance forces increases with speed, so the issue gets more crucial in high-speed applications like CNC machines. Take an engine shaft for instance; it is finely balanced with counterweights to maximize the performance and stability of automotive engines at speeds in the order of thousands of RPM.

Generally, balancing is the last manufacturing process a shaft goes through before mechanical testing. Manufacturers use techniques like cylindrical grinding and balancing rings to ensure that the mass distribution around the shaft’s axis is even.

Applications of Mechanical Shafts

Mechanical shafts are present in a huge variety of machinery, covering nearly every industry from home appliances to aviation. The following is a list of their most common applications.