Machine part principle 2 - 2

- The machine is highly economical, shown in:

+ The effort and cost for design is minimal,

+ Materials for manufacturing machine parts are cheap and easy to supply.

+ Easy to process, manufacturing cost is the least,

+ The price of the machine is the lowest.

1.1.3. Steps to design a machine

Before starting to design a machine, we must master the design task,

need to know the following figures:

- Number of machines to be manufactured. How many to manufacture?

- Machined products. Shape , size, material, accuracy?

- Machine productivity . How many products need to be processed in 1 hour?

- Machine life, or how long does it take to use the machine until it is discarded?

- Requirements for machine size and weight?

- What are the characteristics of the environment in which the machine will work?

- Other requirements?

The design work is carried out in 7 steps:

1. Determine the operating principle and working mode of the machine. It is advisable to refer to existing machines to select the appropriate operating principle. The working mode of the machine and the machine structure are related to the selection of the values ​​of the calculation coefficients during the determination process.

size of machine part

2. Make a general diagram of the whole machine and diagrams of machine parts. The diagram must satisfy the requirements of the design task. It is necessary to make several machine diagram options, then compare and choose the best option.

3. Determine the load acting on the machine, machine parts and each machine detail. This is an important step. If the load is not determined correctly, we will design a machine that is either not durable enough or not economically viable.

4. Calculate and design machine parts. Determine shape, size, and draw.

structure of each machine part

5. Establish technological process for processing each machine part.

6. Establish the process of assembling machine parts and assembling the whole machine.

7. Create design documents for the machine. Create drawings, descriptions, and instruction documents.

use and repair machine.

1.1.4. Steps to design a machine part

To carry out the fourth step in the machine design process, we must calculate the design of each machine part in turn. Before carrying out the design of the machine part, it is necessary to know the data related to the machine part:

- Loads acting on machine parts: intensity, direction, direction, point of application and its characteristics.

- The life of the machine part. Normally the life of the machine part is equal to the age of the machine.

The life of the machine, in some cases only a fraction of the life of the machine.

- Working conditions of machine parts.

- Requirements on materials, weight , size.

- Machining capabilities of the mechanical facility that will manufacture the machine parts.

Designing a machine part usually proceeds through 7 steps:

Figure 1-2: Axis calculation diagram

1. Create detailed machine calculation diagram - diagram the structure of machine parts.

2. Place loads on the detailed machine calculation diagram (Figure 1-2).

3. Select materials for manufacturing machine parts.

4. Calculate the main dimensions of machine parts according to durability conditions or load conditions.

hard case

5. Select other dimensions and draw the structure of the machine part.

6. Test machine parts for durability, hardness, heat resistance, and knife resistance.

If not guaranteed, the size must be increased, if too much, the size of the machine part must be reduced.

7. Make detailed manufacturing drawings of machine parts. They fully show the shape, size, tolerance, surface quality, materials, heat treatment methods, and technical requirements for processing and assembly.

1.2. Overview of requirements for machines and machine parts

When determining the dimensions of machine parts, we need to pay attention to the following points:

- The load acting on machine parts is very complex and difficult to determine accurately, so we only determine the main load components, the secondary components are considered by an adjustment coefficient, called the load coefficient.

- Formulas used in calculating machine part design have 3 types: formulas

exact formula, approximate formula, and empirical formula.

+ Precise formula, built on the basis of Mathematics and Physics theory. Using precise formula, in all cases we always get correct results. In the field of machine part design, this type of formula is very rare.

+ Approximate formulas are built on the basis of having to make assumptions. For example : assuming the material is homogeneous, isotropic , or absolutely rigid. Calculation results, when using approximate formulas, are considered accurate when the conditions of the problem coincide with the assumptions. The further the conditions of the design problem are from the assumptions, the less reliable the calculation results are. In approximate formulas, people put in coefficients to adjust the accuracy of the calculation results, taking into account the deviation between the actual conditions of the problem and the assumed conditions. When designing, we must choose reasonable values ​​for the coefficients. This type of formula is very popular in the field of machine part design.

+ Empirical formulas, or empirical formulas, are built on the basis of statistics of results obtained from experiments, or from experience in using machines. Design calculation results using empirical formulas are only accepted when the conditions of the problem coincide with experimental conditions, or coincide with experience in use. In conditions other than experiments and experience, they cannot be used .

- There are dimensions of machine parts that can be determined accurately through only one calculation. There are also dimensions that must go through two or more calculation steps to get the correct result, because there is not enough data to calculate accurately right away.

- A machine part usually has many dimensions, only the dimensions of the main sections should be calculated (including sections involved in assembly, sections with high stress values, sections that often have damage). The remaining dimensions will be selected during the process of drawing the structure of the machine part. Select according to the assembly conditions with the parts.

other details, according to the rationality, aesthetics of the structure, or according to the experience of the person

design .

- In each step of calculating the machine part design, there may be many options that satisfy the requirements of the problem, we should analyze and choose 2 to 3 most reasonable options to continue calculating. In the final step, we need to compare and choose the best option as the design result.

- Currently, there are many computer programs (application software) used to calculate and automatically draw machine parts, machine components, and even machines. When using them, we need to choose the appropriate software for the design problem, and must have a firm grasp of machine part design knowledge to effectively use the above application software.

1.3. Loads and stresses

1.3.1. Loads acting on machines and machine parts

Loads acting on machines and machine parts include force, moment and pressure. Load is a vector quantity, determined by the parameters: intensity, direction, direction, point of application and characteristics of the load. In which:

- Force , symbolized by the letter F, unit of measurement is N, 1 N = 1 kg.m/s.

- Bending moment, symbol is M, unit of measurement is Nmm.

- Torque, symbol is T, unit of measurement is Nmm.

- Pressure, symbol is p, unit of measurement is MPa, 1 MPa = 1 N/mm 2 .

Classification of loads. Let's get acquainted with some names of loads, and their characteristics:

- Constant load is a load whose direction, direction, and intensity do not change.

over time. The diagram of the constant load is shown above ( Figure 1-3).

- Variable load is a load in which at least one of the three quantities (direction, direction , intensity) changes over time. In actual machine part calculations, it is common to encounter loads with variable intensity; the diagram of the variable load is shown above (Figure 1-4).

- Equivalent load, is the conventional constant load, equivalent to the regime

The variable load acting on the machine part. In other words: when calculating the machine part subjected to variable load, we must use a constant load mode equivalent to the variable load mode in terms of strength and service life of the machine part.

- Fixed load is a load whose set point does not change during the operation of the machine part.

- Moving load, is a load whose application point moves on the machine part, when the machine

work.

- Nominal load is the load acting on the machine part in theory.

- Calculated load . When working , a machine part, or a part of a machine part , must bear a load greater than the nominal load. The additional load can be caused by vibration , or by a load concentrated on a part of the machine part. The machine part must be designed so that the part that bears the large load does not become weak.

So we have to calculate the machine details according to the load greater than the nominal load, this load is called calculated load.

1.3.2. Stress

Stress is the force that appears in the elements of a machine part when the machine part is subjected to a load.

Stress is a vector quantity, it is determined by direction, direction, and intensity.

The unit of measurement of stress is MPa, 1MPa = 1N/mm 2 .

Stress is divided into two groups:

- The symbol for normal stress is  . The direction of normal stress coincides with the direction of the method.

The line of the element is separated from the machine part.

- Shear stress is denoted by . Shear stress has direction parallel to the plane of the segment

elements are separated from machine parts.

Corresponding to the applied loads, stress is divided into the following types:

- Tensile stress, denoted by σ k ,

- Compressive stress, denoted by σ n ,

- Bending stress, denoted by σ u ,

- Contact stress, denoted by σ tx , or σ H ,

- Stamping stress, denoted by σ d ,

- Torsional stress, denoted by τ x ,

- Shear stress, denoted by τ c .

In addition, stress is also divided into constant stress and changing stress :

- Constant stress, also known as static stress, is stress whose direction, magnitude, and intensity do not change over time. The diagram of static stress is shown above (Figure 1-5).

- Changing stress is stress that has at least one quantity (direction, direction, intensity ) changing over time. Stress can change randomly, or change periodically. In calculating machine part design, we often encounter the type of stress that changes periodically, or almost periodically. The diagram of cyclic stress change is shown above (Figure 1-6).

A stress cycle is defined by the parameters:

- Maximum stress σ max ,

A stress cycle is defined by the parameters:

- Maximum stress σ min ,

- Average stress σ m ; σ m = (σ max + σ min ) / 2,

- Stress amplitude σ a ; σ a = (σ max - σ min )/2,

- Stress cycle factor r; r = σ max / σ min , or r = σ min / σ max , when σ min = 0.

Figure 1-5. Static stress diagram Figure 1-6. Changing stress diagram

Based on the value of the stress cycle coefficient r, stress is divided into the following types:

- Stress changes cyclically, when the stress cycle has r ≥ 0.

- Stress changes symmetrically when the stress cycle has r < 0.

- Static stress is a special case of changing stress, with r = 1.

With the same stress value, but different r, the ability of the stress to destroy the material is also different. Machine parts subjected to static stress have a longer life than machine parts subjected to dynamic changing stress, machine parts subjected to symmetrically changing stress have the lowest life.

1.4. Fatigue resistance of machine parts

1.4.1. Fatigue failure

When a machine part is subjected to static stress and is damaged , it is called static stress failure . In other words, the machine part does not have enough static strength. Calculating the machine part to prevent this type of failure is called static strength calculation.

When a machine part is damaged by changing stress, it is called fatigue failure, or the machine part does not have enough fatigue strength. Calculating the machine part to prevent this type of failure is called fatigue calculation.

When the static stress exceeds the limit stress value, the machine part is suddenly damaged.

The fracture is rough and new, and under microscope observation shows leakage of metal grain structure (Figure 1-7).

Figure 1-7: Fracture due to insufficient

static resistance