What is shaft?
A shaft is a rotating machined element used to transmit power from the source of the machine to other parts of the machine. The shaft is generally circular in cross-section. It is one of the essential parts of any machine. Because without shafts, the machine will not be going to transmit power.
The pulley & gears are generally placed on the shafts, which will help to transmit motion. Not only pulley and gear but also many other rotating elements can mount on the shaft with the help of a key.
For power transmission, one end of the rotating shaft is connected to the power source & the other to the machine. It may be solid or hollow, depending upon the type of application. The hollow shaft reduces its weight and gives an added advantage.
The geometry of the shaft is different as per the application. Sometimes it is straight & sometimes it is stepped. The straight shafts are supported & guided with bearings to transmit power.
Stepped shafts are for mounting different gears or hubs to transmit power. With every step, its diameter changes with the length.
The bearings at the ends of the shafts for the guiding & supported on the rigid structure to absorb vibration developed. An E-clip or Circlip in the groove of the shaft prevents it from coming out from the bearing.
Depending upon the geometry & application of the shafts, it will be subject to bending & torsional stress.
Types of shafts
Shafts are divided into four types.
- Transmission Shaft
- Machine Shaft
Transmission shafts are the stepped shafts used to transmit power from one source to the other machines. On the stepped portion of the shaft gear, pulley or hub mounts to transfer motion.
Eg. Counter shafts, line shafts, and overhead shafts.
Machine shafts are the inside part of the machine assembly. If we consider the example of the car engine, the crankshaft is considered as a machine shaft.
The axle is the type of shaft that supports the rotating element like a wheel, which fits in housing with bearing. The axle is the non-rotating element.
Eg. Axle in the automobile.
The spindle shafts are a rotating part of the machine. It is for holding workspace or tool. The spindle is the short shaft that is used in all machines.
Eg. Spindle in lathe machine.
The manufacturing process of the shaft.
In general, shafts can manufacture with a hot rolling process. If we compared the strength of the shaft as per the hot rolling & cold rolling process, cold rolling is best. But it develops high residual stresses, which deform the shaft when it is machined. For manufacturing of huge diameter shafts forging processes is used.
After the above processes, it is subjected to the end working process. In this process, one end of the shaft is loaded on the check & the other end is supported with the turret of the lathe machine.
To finish the shaft, the tool holds on the toolpost. Once the power gets ON, the chuck starts rotating the shaft. Dial gauge can be used to check the concentricity of it before machining. Various operations like facing, turning, taper turning, grooving, and others were performed as per the use.
For high-volume applications, CNC is the prime choice for the end working process. Also, it can be performed with CNC double-end machine. In this process, the shaft holds between the fixtures and the tool rotates to do machining. To achieve roundness and concentricity, rotating tools are faced each other in the centreline.
Motor and transmission shafts are generally made with this process.
The materials of the shaft
Generally, the material of the shafts is mild steel & carbon steel with grades 40 C 8, 45 C 8, 50 C 4, and 50 C 12. Nickel, nickel-chromium, or vanadium are the material used for the shaft in high-strength applications.
The material having the following properties.
- High strength ability.
- High machinability.
- Low sensitivity factor.
- Good heat transmission properties.
- High wear-resistant properties.
The standard sizes of the shaft.
Following are some standard sizes and lengths are using in nowadays.
|Standard sizes of the transmission shaft||Step sizes|
|25mm to 60mm||5mm step|
|60mm to 100mm||10mm step|
|110mm to 140mm||15mm step|
|140mm to 500mm||20mm step|
|5, 6 7 meters|
The standard Sizes of the Machine shafts are up to 25mm with a 5mm step.
Speed of shafts for different applications.
Speed of shafting is dependent upon the application in which these shafts are using. Following are some common speeds in different applications.
|Applications||Speed in RPM|
|Light machine shop||150-300|
There are two different processes to design shafts. Each process has its different loading considerations.
Design of Shaft on a Strength basis.
Transmission shafts are generally subjected to the axial tensile force, bending, torsional moment, and their combinations. In most cases, shafts are subjected to combined bending and torsional stresses.
Shaft is subjected to tensile stresses,
Tensile stress= P/A
A= (Pi/4) x D^2
- D=Diameter of shaft in mm
Shaft is subjected to bending moment
Bending stress= (Mb x Y)/I
- Mb= Bending Moment
- Y = D/2, (D= Diameter)
- I = Moment of Inertia (Pi * D^4/64)
Shaft subjected to Torsional moment
Torsional stress = Mt x R/J
- Mt= Torsional Moment
- R = D/2, (D= Diameter)
- J = Polar Moment of Inertia (Pi * D^4/32)
Design of shaft on Rigidity basis.
The transmission shaft is said to be rigid on the basis of torsional rigidity basis if it does not twist too much.
(Mt/J) = (G x θ/ L)
- Mt = Torsional Moment (N-mm)
- J = Polar Moment of Inertia (Pi * D^4/32)
- D=Diameter of shaft in mm
- θ = Angle of twist
- G = Modulus of rigidity (N/mm^2)
Power transmitted by the shaft.
As we know, shafts are used to transmit power. Generally, the below formula is used to transmit the power.
P=2 * Pi * N * T / 60 in watt.
- P = Power Transmitted
- N = Speed in RPM
- T = Torque in in N-m
Advantages of the shafts.
- It having high torsional strength.
- It’s having a high moment of inertia and radius of gyration.
- In a hollow shafts, the material cost are too low.
- Hollow shafts are light in weight, but they are equal in power transmission compare to a solid shafts.
- As shafts are too strong so, there is no chance of failure.
Disadvantages of the shafts.
- The manufacturing process & cost of the shafts are high.
- As shafts are too noisy in operation.
- Any failure of the shafts takes too much time to repair.
- Changing shafts speed is too difficult.
- Power loss due to use of wrong coupling.
- Causes vibration.
A shaft is a rotating machine element used to transmit power from one part of the machine element to another part of the machine or from a power-producing machine to a power absorbing machine. The shaft is generally circular in cross-section.
The shaft is generally classified into four parts.
1. Transmission Shaft– It is for transferring power from the source to the drive unit.
2. Machine Shaft– It is a part of the machine. The shaft of the gearbox is a suitable example.
3. Spindle- It is a rotating part of the machine for clamping workpieces.
4. Axle– It is a part of the vehicle drive system.
The shaft is a rotating circular element that transfers a power generated by the source into a rotating motion to the machine part with gears & bearings. It is part of connecting the power source to the machine element.