Today we share a wave of dry goods about shafts, including the definition, materials, types and designs of shafts.
What is Shaft?
A shaft is basically a rotating part of any machine that has a circular cross-section and is used to transfer power from one part to another or from a power generator to a power absorber. To transmit power, one end of the shaft is connected to the power source and the other end to the machine. The shaft can be solid or hollow as needed, and the hollow shaft helps to reduce weight and provides advantages.
General description of the shaft
The shaft is one of the very important elements used in the machine. They are used to support rotating parts such as pulleys and gears, they are supported by bearings located in a rigid machine housing, gears and pulleys located on shafts help to transmit motion.
Many other rotating elements are mounted on the shaft by means of keys. They are subjected to bending moments and torques due to the torque generated by the reaction forces and power transmission of the members supported by the shaft.
The shaft always has a circular cross-section, which can be hollow or solid. Shafts can be classified as crankshafts, linear shafts, articulated shafts, or flexible shafts, but linear shafts are typically used to transmit power.
Shafts are usually designed as steep cylindrical rods, so they have different diameters over their entire length, although shafts with constant diameters 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 the bearings.
The type of shaft
Propeller shafts
These shafts are stepped shafts that are used to transfer power between one source to another machine that absorbs power. Mounted on the stepped part of the shaft gear, hub or pulley for the transmission of motion.
Examples: Overhead shafts, spools, subshafts, and shafts for all plants.
Mechanical switches
These shafts are located inside the assembly and are an integral part of the machine.
Example: The crankshaft in a car engine is the machine shaft.
Axle axles
These shafts support rotating elements, such as wheels, which can be mounted in housings with bearings, but the shafts are non-rotating elements. These are mainly used in vehicles.
Example: Axles in a car.
Spindle
These are the rotating parts of the machine, which houses tools or workspaces. They are short shafts used in machines.
Example: A spindle in a lathe.
Materials for shafts
Usually mild steel is the material used for shafts. If high strength is required, alloy steels such as nickel-chromium, nickel-nickel, chromium-vanadium steels are used. They are formed by hot rolling and cold drawing and grinding, and the material usually used for conventional shafts is 50cccc8 grade carbon steel.
The material used for the shaft should have the following properties:
The material should be of high strength;
The material should have high abrasion resistance;
The material should have heat-treated properties;
The material should have good mechanical properties;
The material must have a low notch sensitivity factor.
The standard size of the shaft
Mechanical switches
Up to 25 mm in steps of 05 mm.
Propeller shafts
Standard dimensions of the shaft - step size;
25 mm to 60 mm - 5 mm steps;
60 mm to 100 mm - 10 mm step size;
110 mm to 140 mm - 15 mm steps;
140 mm to 500 mm - 20 mm step size.
The standard size of the machine axle is up to 25 mm with a step size of 5 mm. For shafts, standard lengths are 5m, 6m and 7m, but generally take 1m to 2m.
Stresses in the shaft
The stresses caused in the shaft are:
shear stress due to torque transmission (torque due to torsional load);
Bending stresses due to forces acting on mechanical elements such as pulleys and gears and the self-weight of the shaft, of which the nature of them are compression or tension;
Combined stresses caused by bending and torsional loads.
The maximum allowable shear stress of the design stress is:
1.The shaft is 56,000 kN m2 with a keyway margin.
2.The shaft is 42,000 kN m2 without keyway allowance.
The maximum permissible bending stress is:
1.The shaft is 112,000 kN m2 with a keyway with margin.
2.The shaft is 84,000 kN m2 without keyway allowance.
Manufacture of shafts
The shaft is manufactured using a hot rolling process. The strength of the shaft is higher when cold rolled as compared to hot rolling, but cold rolling results in high residual stresses, which causes the shaft to deform during machining. The forging process is used to make shafts with larger diameters.
After the rolling is completed, the shaft is end-machined, one end of the shaft is mounted on the inspection, and the other end of the shaft is supported by a turret of the lathe. To finish the shaft, the tool holds the turret in place, and when the power is turned on, the chuck starts to rotate the shaft.
The dial indicator is used to check the concentricity of the shaft before machining, and to perform a variety of operations such as turning, face cutting, grooving, taper turning, etc., depending on the application. Applications such as high-volume, CNC, etc., are best suited for the final machining process. It can also be machined with a CNC double-end machine tool with a shaft clamped between the tool rotation and the fixture.
To achieve concentricity and roundness, the rotating cutter should be opposite each other at the centerline. Propeller shafts and motors are typically 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=2 nt 60. where p is the power transmitted (w); n is the number of revolutions per minute (rpm); t is the torque in n·m.
Shaft speeds for various applications:
1.Mechanical: 100 200;
2.Woodworking machinery: 250 700;
3.Textiles: 300 800;
4.Light machine shop: 150 300;
5.Countershaft: 200 600.
Shaft design
Shafts can be designed through two different processes depending on the load considerations:
1. Strength-based shaft design transmission shafts are usually susceptible to bending moment, torque, axial tension and their combinations. Typically, bearings are subjected to a combination of torsional and bending stresses.
Bearing tensile stress:
Tensile stress = p a
where a=(4)xd2,d is the diameter of the shaft in mm.
Bending moment of the bearing:
Bending stress = (mbxy) i
Where, mb = bending moment; y=d 2, where d is the diameter; i=moment of inertia = (xd4) 64
Bearing torque:
Torsional stress = mtxr j
Where, MT = torsional moment; r=d 2, where d is the diameter; j = polar moment of inertia = (xd4) 32
2. Shaft design based on rigidity.
If the shaft does not twist too much, the propeller shaft is said to be rigid on the basis of torsional stiffness.
mt/j} =
Wherein, MT = torque mm in N; j = polar moment of inertia = (xd4) 32d = diameter of the shaft (mm); = torsion angle; g = modulus of stiffness n mm2.
Advantages and disadvantages of shafts
Advantages of the shaft
They are less likely to get stuck;
They require less maintenance than chain systems;
They have high torsional strength;
They have a high polar moment of inertia value;
They are very robust and less likely 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 in weight compared to a solid shaft;
They have a high radius of turn.
Disadvantages of the shaft
They have power losses due to loose coupling;
They vibrate as they rotate;
They produce constant noise;
high manufacturing and maintenance costs;
Difficult to manufacture;
Changing the speed of the shaft is not easy;
Longer downtime due to mechanical issues;
oil droplets from elevated shafts;
The use of elastic couplings, such as leaf spring couplings, results in a loss of speed between shafts;
If the shaft fails, it takes a lot of time to repair.