Accurately calculate shaft strength and prevent shaft breakage

In fact, it is becoming more and more common to meet the non-standard installation size of a particular user-specific device. Many technically strong motor supporting households have seized market share. On the one hand, they have tried hard to compress the external dimensions of the equipment, and on the other hand, they have tried to greatly increase the potential of the equipment, which has driven the demand for small-capacity and high-power high-power-density motors. However, the performance of the designed motor achieved this goal perfectly, but the mechanical characteristics of the motor shaft were overestimated and no rigorous strength calculations were performed.

The design of shaft parts is very particular and involves many aspects. First, the selection of materials and the study of selected material properties; second, determine the safety factor; third, the strength calculation. Neglecting any design factor can have fatal consequences.

Material and selected material properties

The motor shaft and shaft extension keys are usually 45 steel, and the allowable stress at normal temperature, static load and general working conditions is 186 MPa.
The mechanical properties during stretching and compression (normal temperature, static load) are specifically: yield limit 353 MPa, strength limit 597 MPa, molding properties: elongation 16%, section shrinkage 40%.

Since the elongation is more than 5%, 45 steel is a plastic material. According to the mass percentage of carbon, carbon content is 0.45% (between 0.25% and 0.60%), which is medium carbon steel; according to the quality of steel (containing sulfur ≤0.040%, phosphorus is ≤0.040%), it is high quality carbon steel. 


Safety factor

The safety factor is related to many factors. Excessive safety factor will waste material; too small safety factor may make the component unsafe. Generally consider the following aspects:
(1) Accuracy of load estimation;
(2) Simplify the accuracy of the process and calculation method;
(3) uniformity of materials;
(4) the importance of construction;
(5) Working conditions of parts, mitigation of self-weight and other factors. The value of the general component is: plastic material is 1.5 to 2.0 according to the yield limit; brittle material is 2.0 to 5.0 according to the strength limit. The above allowable stress is 186 MPa, indicating a safety factor of 353/186 = 1.9.


Motor shaft extension strength check calculation

(1) Allowable stress determination
The shaft and shaft extension keys are 45 steel, and the allowable stress (tensile and compression) [σ] = 186 MPa. Allowable shear stress [τ] = (0.6 - 0.8) × [σ] = 1212 MPa, allowable compressive stress [σjy] = (1.7 - 2.0) × [σ] = 316 MPa.

(2) Strength check calculation steps
1. Motor rated data: power P, speed n, service factor SF.
2. The parameters of shaft extension and shaft extension are: shaft diameter d, key width b, key height h, key length l.
3. axis cross-sectional area F = πd2 / 4.
4. Torque M = P × SF / Ω, where Ω = 2πn / 60.
5. The torsional section coefficient of the 5 axis is W = πd3/16.
6. shaft strength conditions: τ = M / W <= [τ]
7. shaft extension keyway and shaft extension key strength condition: σjy = 2M/d/Fjy<=[σjy], where Fjy = l×h/2.

Reprinted from the network