Related contents of tooth surface strain

1 Definition of tooth surface strain

Strain is a common phenomenon of gear tooth surface defects, which is that before the heat treatment of parts, excessive material is removed from the tooth surface in the last process of tooth surface processing, resulting in slight dent on the tooth surface.

2 Mechanism of tooth surface strain

In the process of machining tooth surface, because of the vibration of the worktable, the tool is not cleaned, the workpiece is stuck with tiny iron chips, or the cooling and lubricating oil are mixed with iron chips, or the tool is seriously worn, which will cause strain on the tooth surface.

3 tooth surface strain

Tooth surface strain can be divided into slight strain and severe strain, as shown in Figure 1 and Figure 2.

4 Detection method of tooth surface strain

Commonly used surface roughness measurement methods include comparison method, touch needle method, light cutting method, interference method and impression method, etc.

comparative law

Comparison method is a detection method which uses roughness template with known height parameter value to compare with the measured surface, and judges the roughness of the measured surface by human senses, and also by means of magnifying glass and microscope.

Touch needle method

Touch needle method, also known as needle tracing method, uses the touch needle to vertically contact and slide slowly along the measured surface. Because the measured surface is rough and uneven, the touch needle is forced to move up and down in the direction perpendicular to the measured surface. The up and down displacement of the touch needle is converted into electrical signals by sensors, which are amplified by electronic devices, phase-sensitive detection and power amplification, and then the automatic recording device is pushed to directly draw the enlarged figure of the measured contour, and the Ra value or Rz value can be obtained by data processing according to this figure.

Light cutting method

When the optical cutting method is implemented, the light passes through the condenser, slit and objective lens in the tube, and immediately becomes a flat ribbon beam, which is projected onto the measured surface at an inclination angle of 45, and then reflected by the measured surface to form a cross-sectional profile figure of the measured surface. Then, the figure is amplified by another objective lens and projected onto the reticle, and the surface roughness is measured by the profile curve formed by its intersection with the measured surface. The surface roughness measuring tool using this method is a light cutting microscope. This method is suitable for measuring surface roughness with Rz and Ry of 0.5 ~ 60 mm, but it requires manual sampling and the measurement efficiency is low.

Microscopic interference method

When the micro-interference method is implemented, the shape error of the measured surface is displayed as interference fringe pattern by using the principle of light wave interference, and the microscopic part of these interference fringes is enlarged by using a microscope with high magnification to measure, so as to obtain the roughness of the measured surface. The surface roughness measuring tool using this method is called interference microscope. This method is suitable for measuring surface roughness with Rz of 0.025～0.8 mm

Impression method

Impression method is a non-contact indirect measurement method. Its principle is to use some plastic materials to make a block impression and stick it on the measured surface of the part, print the surface contour of the part on the impression, and then measure the impression to get the roughness parameter value. This method is suitable for large and heavy parts and surfaces which are difficult to be measured directly by instruments.

5 Harm of tooth surface strain

There are five main hazards of tooth surface strain.

1 Affect the wear resistance of tooth surface: the coarser the tooth surface, the smaller the effective contact area of meshing tooth surface, the greater the pressure, and the faster the wear, as shown in figure 3;

2 Affect the fatigue strength of gear parts: there are large valleys on the surface of rough parts, which are sensitive to stress concentration like sharp corners and cracks, thus affecting the fatigue strength of parts, as shown in Figure 4;

3 Affect the corrosion resistance of the tooth surface: the rough surface easily makes corrosive gas or liquid penetrate into the inner metal layer through the microscopic valleys on the surface, causing corrosion of the tooth surface;

4 Affect the contact stiffness of gear parts: contact stiffness is the ability of the joint surface of parts to resist contact deformation under the action of external force;

5 Affect the measurement accuracy of gears: the surface roughness of the measured surface of parts and the measuring surface of measuring tools will directly affect the measurement accuracy, especially in precise measurement.

Limitations of measurement methods in practical application

The above-mentioned measuring methods have limitations in the practical application of large-scale automobile gear shaving processing:

1 In the industry, there is no standard sample block of surface roughness related to the shaving surface, and there is no description of the judgment of strain, which easily leads to no evidence in actual work. The judgment of strain only depends on the experience of inspection and technicians, and there are many human factors, which can not be objective and fair;

2 The needle touching method is most commonly used, but for the strain located at the edge of the gear, it can not completely cover the strain during the sampling process, and there are still some problems such as limited measurement and excessive data error, and the obtained data is not reliable enough;

3 The instruments used by the light cutting method and the interference method are not equipped by most manufacturers, which can not meet the requirements of issuing test results quickly in the mass production process. In addition, because the impression material can not completely fill the valley bottom of the measured surface with slight unevenness, the measured values need to be corrected;

4 On-site operators and inspectors do not have detection means, and the inspection time is too long, which easily leads to waiting in the processing process and low production efficiency.

Establishment of tooth surface strain standard

In the past, the judgment process of tooth surface strain was as follows: before heat treatment, when the tooth surface strain was found, the tooth surface roughness and experience were measured to evaluate whether it met the design and technological requirements.

This method belongs to back testing. In order to eliminate the tooth surface strain phenomenon in the bud, this paper summarizes the research conclusions of our company's gear design, combines the research results of early wear phenomenon in gear after-sales failure for many years, and establishes the Visual Inspection Standard of Tooth Surface Quality in Gear Shaving Process together with the process research department. It is convenient to adjust the processing parameters in time, eliminate the strain problem in the bud, and judge and deal with the unqualified products by quickly judging whether the surface of the parts is pulled or not in the production site. The Visual Standard for Tooth Surface Quality in Shaving Process is divided into three levels: qualified, concession and scrap, and the scrap level is shown in Figure 5.