Knowledge About an Axis: Definitions, Materials, Types, and Shaft Design

What is an axis?

A shaft is basically a rotating part of any machine, having a circular cross-section, used to transmit power from one part to another or from a power generating machine to a power absorbing machine. To transmit power, one end of the shaft is connected to the power source and the other end is connected to the machine. Shafts can be solid or hollow as desired, with hollow shafts helping to reduce weight and provide advantages.

General description of the axis

Shaft is one of the very important elements used in machines. They are used to support rotating parts such as pulleys and gears. They are supported by bearings located in the rigid machine casing. Gears and pulleys located on the shafts help transmit motion.

Many other rotating elements are keyed onto the shaft. The shafts are subjected to bending moments and torsions due to the reaction forces of the members they support and the torque generated by the transmission of power.

The shaft always has a circular cross-section and can be hollow or solid. Shafts can be classified as crankshafts, linear shafts, articulated shafts or flexible shafts, but linear shafts are usually used to transmit power.

Shafts are usually designed as steep cylindrical rods so that they have different diameters throughout their length, although shafts with a constant diameter are easy to produce.

The magnitude of stress in a stepped shaft varies with its length. Shafts with uniform diameters are not suitable for disassembly, assembly, maintenance, and these shafts create complications when fastening the parts mounted on them, especially bearings.

Shaft type

01 drive shaft

These shafts are stepped shafts used to transmit power between one source to another machine that absorbs the power. Mounted on the stepped portion of a shaft gear, hub or pulley for transmitting motion.

02 mechanical axis

These axes are located inside the assembly and are an integral part of the machine.

03 axle shaft

These shafts support rotating elements, such as wheels, which may be mounted in housings with bearings, but the shafts are non-rotating elements. These are mainly used in vehicles.

04 spindle

These are the rotating parts of the machine that house the tools or workspace. They are short shafts used in machines.

Shaft material

Typically mild steel is the material used for shafts. If high strength is required, alloy steels such as nickel-chromium, nickel, and chromium-vanadium steel are used. They are formed by hot rolling and cold drawing and grinding. The materials usually used for conventional shafts are 50C12, 50C4, 45C8, 40C8 grade carbon steel.

The material used for the shaft should have the following properties:

1. The material should have high strength;

2. The material should have high wear resistance;

3. The material should have heat treatment properties;

4. The material should have good mechanical properties;

5. The material must have low notch sensitivity.

Standard size of shaft

Mechanical axis

Up to 25 mm in steps of 0.5 mm.

transmission shaft

Standard dimensions of shafts - step size

25mm to 60mm - 5mm steps

60mm to 100mm - 10mm step

110mm to 140mm - 15mm steps

140mm to 500mm - 20mm steps

Machine axes are available in standard sizes up to 25 mm in steps of 5 mm. For shafts, standard lengths are 5m, 6m and 7m, but generally take 1m to 2m.

stress in shaft

The stress induced in the shaft is:

Shear stress due to torque transfer (torque due to torsional loading);

Bending stresses, which are compressive or tensile in nature, due to forces acting on mechanical elements such as pulleys and gears and the self-weight of the shaft;

Combined stresses caused by bending and torsional loads.

The maximum allowable shear stress of design stress is:

1. The shaft is 56000KN/m2, and there is room for the keyway.

2. The shaft is 42000KN/m2 without keyway allowance.

The maximum allowable bending stress is:

1. The shaft is 112000KN/m2, and there is room for the keyway.

2. The shaft is 84000 KN/m2 without keyway allowance.

Shaft manufacturing

The shaft is manufactured using hot rolling process. The strength of the shaft is higher when cold rolling compared to hot rolling, but cold rolling causes high residual stresses that cause the shaft to deform during processing. The forging process is used to create larger diameter shafts.

After the rolling is completed, the end of the shaft is processed, one end of the shaft is mounted on the inspection machine, and the other end of the shaft is supported by the turret of the lathe. To finish the shaft, the tool is held in place by the tool holder, and when the power is turned on, the chuck starts rotating the shaft.

Dial indicators are used to check the concentricity of the shaft before machining and perform various operations such as turning, facing, grooving, and taper turning depending on the purpose. High-volume, CNC and other applications are best suited for final machining processes. It can also be machined with a CNC double-ended machine with the axis clamped between the tool rotation and the fixture.

To achieve concentricity and roundness, rotating tools should face each other on center lines. Drive shafts and motors are often manufactured using this process.

Shaft drive

We know that shafts are used for power transmission, so the formula used to calculate power transmission is: P = 2πnT/60

Where, P is the transmitted power (W); n is the revolutions per minute (RPM); T is the torque in Nm.

Speed of axes for various applications:

1. Machinery: 100~200;

2. Woodworking machinery: 250~700;

3. Textile industry: 300~800;

4. Light machinery workshop: 150~300;

5. Countershaft: 200~600.

Shaft design

Shafts can be designed through two different processes based on different load considerations:

1. Strength-based shaft design Drive shafts are usually susceptible to bending moments, torques, axial pulls, and combinations thereof. Typically, bearings are loaded with a combination of torsional and bending stresses.

Bearing tensile stress:

Tensile stress=P/A

Among them, A= (π/4) x D2, D is the diameter of the shaft in mm.

Bearing bending moment:

Bending stress = (MbxY)/ I

Among them, Mb=bending moment; Y=D/2, where D is the diameter; I = moment of inertia = (πxD4)/ 64

Bearing torque:

Torsional stress=Mt x R/J

Among them, Mt=twisting moment; R=D/2, where D is the diameter; J=polar moment of inertia = (πxD4)/32

Shaft design based on rigidity

A driveshaft is said to be rigid on the basis of torsional stiffness if the shaft does not twist too much.

{Mt/J} = {(G x թ)/ L}

Among them, Mt=torque in N unit mm; J=polar moment of inertia=(πxD4)/32; D=diameter of shaft (mm); ө=twist angle; G=stiffness modulus N/mm2

Advantages and Disadvantages of Shafts

Advantages of shaft

They are less likely to get stuck;

They require less maintenance than chain systems;

They have high torsional strength;

They have high polar moment of inertia values;

They are very strong and unlikely to fail;

The internal shape of hollow shafts is hollow, so they require less material;

For the same torque transmission value, a hollow shaft is lighter than a solid shaft;

They have a very high radius of gyration.

Shaft Disadvantages

They have power loss due to loose coupling;

They vibrate as they rotate;

They produce a constant noise;

Higher manufacturing and maintenance costs;

Difficult to manufacture;

Changing the speed of an axis is not easy;

Long downtime due to mechanical issues;

Oil droplets on overhead shafts;

The use of elastomeric couplings (such as leaf spring couplings) can result in speed losses between the shafts;

If a shaft fails, it will take a lot of time to repair.