The mechanical shaft is the backbone of machine design. These simple but essential rotating components are used to transfer power or motion from one place to another, such as to a different part of a machine or to another machine altogether.
Found in motors, gearboxes, pumps, and many other machines, shafts are designed to withstand strong twisting forces and high bending loads as they rotate. That means proper shaft design and manufacturing are a top priority for engineers in fields like automotive, industrial manufacturing, and power generation.
This article explains the basics of shaft machining, looking at how processes like CNC turning are used to craft robust shafts for all sorts of 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
Shafts play a vital role in transmitting torque and motion in mechanical systems. Depending on their shape, design, load distribution, and application, shafts are classified into different types.
Understanding these types helps in selecting the right shaft for specific machinery and industrial needs. Below are the major types of shafts explained in detail:
1. Transmission Shafts.
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.
Examples of transmission shafts:
- Counter shafts
- Line shafts
- Overhead shafts
- Spindle shafts
They are typically made from mild steel or alloy steel and often use keys and couplings for torque transfer.
2. Machine Shafts.
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.
Examples:
- Crankshaft in an engine
- Tailstock shaft in a lathe machine
- Spindle shafts in drilling and milling machines
Machine shafts are specially designed to suit the unique geometry of each machine, and they often combine complex shapes like crank throws and stepped sections.
3. Line Shafts.
A line shaft is a type of transmission shaft used in factories and workshops to transmit power from a central motor or prime mover to different machines. These shafts were especially common in older manufacturing plants.
- They run along the length of the workshop.
- Pulleys, belts, and gears are attached to distribute power.
- Strong materials like alloy steel are used to handle long-distance torque transfer.
Although modern plants now use individual motors for machines, line shafts still illustrate how power distribution worked in earlier mechanical systems.
4. Spindle Shafts.
A spindle shaft is a short, rotating shaft that supports cutting tools, workpieces, or gears in a machine. Spindles are usually hollow or tapered to hold tools or workpieces securely.
Applications include:
- Lathe machine spindles
- Drill press spindles
- Milling machine spindles
- Automotive wheel spindles
Spindle shafts require high precision and are often made from hardened steel to resist wear and vibration.
5. Crankshafts.
A crankshaft is a specialized type of machine shaft that converts reciprocating motion (up and down movement) into rotary motion. It is commonly used in engines.
- Found in internal combustion engines of cars, trucks, and motorcycles.
- Equipped with crank throws and counterweights for smooth rotation.
- Made from forged steel or cast iron for strength and durability.
Crankshafts are among the most complex shaft designs due to their critical role in engine performance.
6. Flexible Shafts.
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.
Applications:
- Speedometer cables in vehicles
- Flexible drill attachments
- Dental tools
- Remote control devices
They are especially useful where motion needs to be transmitted around corners or in confined spaces.
7. Stepped Shafts.
Stepped shafts have different diameters along their length, forming “steps.” These steps are created to accommodate bearings, gears, pulleys, or couplings.
- Provide better support for machine components.
- Reduce stress concentration at critical points.
- Common in electric motors, pumps, and gearboxes.
Stepped shafts are manufactured using precision machining to maintain concentricity between the steps.
8. Hollow Shafts.
Hollow shafts are designed with a hollow centre instead of being solid. This reduces weight while maintaining high strength.
- Lighter than solid shafts but can transmit similar torque.
- Used in aerospace, marine, and automotive applications.
- Provide space for fluid, cables, or lubrication inside.
Hollow shafts are costlier to produce but highly efficient for high-speed machinery.
9. Splined Shafts.
A splined shaft has ridges or grooves (splines) cut along its length. These splines fit into matching grooves in gears, pulleys, or couplings, allowing torque transfer without slipping.
Applications:
- Gearboxes in automobiles
- Aircraft engines
- Industrial machinery
- Marine propulsion systems
Splined shafts are highly reliable for transmitting power where alignment and accuracy are critical.
10. Propeller Shafts (Drive Shafts).
A propeller shaft, also called a drive shaft, is mainly used in automobiles to transfer power from the gearbox to the differential.
- Common in rear-wheel drive vehicles.
- Must withstand torsional stress and vibration.
- Typically, hollow and balanced to prevent noise.
Propeller shafts are often equipped with universal joints (U-joints) to allow flexibility during motion.
11. Dead Shafts.
A dead shaft is one that does not rotate but serves as a support for rotating components.
Examples:
- Axle shafts in carts or trailers
- Roller shafts in conveyors
Dead shafts are fixed in position and allow mounted elements like wheels or rollers to rotate around them.
12. Counter Shafts.
A counter shaft is a type of transmission shaft used in gearboxes. It lies parallel to the main shaft and carries gears that mesh with the main shaft gears.
- Found in vehicles and industrial machines.
- Helps reduce or increase rotational speed.
- Works in combination with clutches and gear trains.
Counter shafts play a vital role in speed reduction or torque multiplication.
Materials used for shafts
The choice of material is critical for shaft performance and longevity. Most shafts are made from mild steel, but for higher strength requirements, alloy steels such as nickel-chromium steel, chromium-vanadium steel, and nickel steel are preferred.
Standard grades for conventional shafts include 45C8, 40C8, 50C4, and 50C12 carbon steels. These materials are typically hot-rolled, cold-drawn, and ground to improve mechanical properties.
Key Material Properties Required:
- High tensile strength for torque and bending loads
- Good wear resistance to endure friction and stress
- Heat-treatability for enhanced hardness and fatigue strength
- Low notch sensitivity for stress concentration resistance
Standard dimensions of shafts
Mechanical shafts up to 25 mm in 0.5 mm steps.
Standard dimensions for transmission shafts – step length:
- 25 mm to 60 mm – 5 mm step.
- 60 mm to 100 mm – 10 mm steps.
- 110 mm to 140 mm – 15 mm steps.
- 140 mm to 500 mm – 20 mm steps.
For machine shafts, standard sizes are up to 25 mm with 5 mm step lengths. For shafts, standard lengths are 5m, 6m and 7m, but generally 1m to 2m is taken.
Manufacturing of shafts
The shaft is manufactured by hot rolling. The strength of the shafts is higher in cold rolling as compared to hot rolling, but cold rolling leads to high residual stresses which cause deformation of the shafts during machining. The forging process is used to manufacture shafts of larger diameters.
After rolling is completed, the shafts are end-machined, with one end of the shaft mounted on a check and the other end of the shaft supported by a lathe turret. For finishing the shaft, the tool is fixed in the tool holder and when the power is turned on, the chuck starts rotating the shaft.
Micrometre is used to check the concentricity of the shaft before machining and customize CNC turning, facing, grooving, taper turning and many other operations according to the usage.
Applications such as high volume and CNC are best suited for final machining processes. Machining can also be done with CNC double-ended machines where the shaft is clamped between the tool rotation and the fixture.
To achieve concentricity and roundness, the rotating tools should be opposite each other at the centreline. Drive shafts and motors are usually manufactured using this process.
Advantages of Shafts
- They are less likely to jam.
- They need less maintenance in comparison to chain system.
- They have high torsional strength.
- They have high value of polar moment of inertia.
- They are very strong and less likely to be failure.
- Internal shape of a hollow shaft is hollow therefore they require less material.
- Hollow shaft has low weight in comparison with solid shaft for same value of torque transmission.
- They have high radius of gyration.
Disadvantages of Shafts
- Due to loose coupling, they have power loss.
- They vibrated while rotation.
- They generate a constant noise.
- Manufacturing and maintenance costs are high.
- Difficult in manufacturing.
- Changing the speed of shaft is not easy.
- Due to mechanical problems the downtime is longer.
- Oil dripping from overhead shafting.
- Use of flexible couplings like leaf spring coupling can result in loss of velocity between the shafts.
- If failure takes place in shafts, then it takes huge time in repairing.