Rachaduras na retificação de engrenagens: causas e medidas de prevenção

Cracks when grinding gears: causes and prevention measures

May 22, 2024 Roberto Magalhães

I. Process Requirements for Modern Gear Grinding of Hard Tooth Surfaces

1. Grinding areas in gear grinding – The involute tooth profile section

In modern gear grinding of hard tooth surfaces, the grinding area grinds only that part of the involute tooth profile that is above the starting circle and below the ending circle of the involute.

2. Non-ground zones in gear grinding – Gear root

Modern hardened gear surfaces have the following benefits when the gear root is not ground in the gear grinding process:

(1) Prevents the hardness reduction of the gear root after heat treatment by maintaining a negative stress layer formed on the gear surface and root after carburizing, quenching and shot peening. This significantly improves the gear's resistance to bending fatigue and load capacity.

(2) The narrow bottom of the gear root groove, poor heat dissipation and significant variation in the remaining material in the excessive curve drastically affect the working conditions of the grinding wheel. This can easily cause burns and cracks during gear grinding.

(3) Poor grinding conditions at the bottom of the gear root groove make the grinding grains on the outer circle of the grinding wheel prone to falling and wearing out, thereby affecting the grinding quality of the gear.

(4) In terms of tooth breakage resistance, the gear root must have a certain amount of root cutting. Without a certain amount of root cutting, inevitable protrusions will occur at the gear root during gear grinding. This will lead to a serious concentration of stress, greatly affecting the breaking resistance of the teeth. The occurrence of such protrusions is absolutely unacceptable.

In conclusion, not grinding the gear groove root can improve the bearing capacity of the gear, avoid damage during gear grinding, improve the quality of gear grinding, reduce the load in the grinding process, and increase productivity.

3. Pre-grinding with a milling cutter for preliminary shaping of the teeth

(1) Introduction to pre-grinding plates

Traditional gear hobs are no longer sufficient for the above-mentioned process requirements. Therefore, it becomes crucial to use a pre-grinding cutter equipped with a contact angle during the milling phase. The distinguishing element of a pre-grinding plate, compared to a standard plate, is in the upper part of its cutting teeth, which employ a cutting edge with a contact angle, as shown in the figure below.

Schematic diagram of gear tooth shape before grinding

At the root of the gear tooth, a certain amount of root cutting is carried out. The purpose is to preform the root of the gear being processed and remove most of the excess from the tooth surface, leaving an even margin for precision machining into the thickness of the tooth. After carburizing and tempering, grinding of the tooth root is no longer necessary.

(2) Requirements for gear tooth shape before grinding:

The gear grinding margin must be uniform;

There must be a defined undercut at the gear root before grinding;

The involute curve of the gear after grinding must be long enough.

(3) Improvements in pre-grinding cutters

Initial use of pre-grinding cutters exhibited the following problems:

Insufficient arch envelope formation at the tooth root, below average smoothness, visible tool marks and suboptimal surface roughness.

The problem of protuberances appearing near the initial circle of the involute in the process of grinding the teeth was quite serious.

After long-term targeted research and analysis, we identified the issues as:

Increase in the amount of milling subsidy;

Significant deformation after heat treatment;

Inherent deficiencies in pre-grinding cutters.

Due to the insufficient formation of the envelope line of the original pre-ground cutters, we proposed the idea of redesigning them, approaching the following aspects:

Increase the external diameter of the pre-grinding cutters;

Increase the number of tool lines on the cutters;

Design with variable pressure angle;

Increase the amount of root digging appropriately, ensuring the strength of the gear.

The above requirements were agreed with tool manufacturers with domestic technical capabilities, jointly developing and producing a new type of pre-grinding cutter suitable for heavy gear processing. The new pre-grinding cutter not only completely solved the previous problems, but also resulted in a very smooth root area of the processed gear, producing excellent results.

(4) Cracks and burn marks are not allowed in gear grinding.

The process of gear tooth fracture typically begins with the formation of tiny fatigue cracks, which gradually expand. Therefore, national and international gear standards specify: no cracks or burn marks are allowed on the hardened gear surface after gear grinding.

II. Characteristics and causes of cracks in hard tooth surface gears

1. Characteristics of grinding cracks in hard tooth surface gears

Grinding cracks are the most typical surface cracks. Its vertical depth generally does not exceed 0.5 mm, with shallow ones only reaching 0.010-0.020 mm. Although they can sometimes exceed 1 mm, this is relatively rare.

2. Causes of crack formation in hard tooth surface gears

The consensus both nationally and internationally is that grinding cracks are caused when the tensile stress of grinding exceeds the fracture strength of the material. The factors that directly affect the tensile stress in grinding are:

(1) Heat treatment:

Grinding cracks in gears with a hard tooth surface occur mainly in parts that have been subjected to carburizing, quenching and low-temperature tempering. Therefore, the quality of heat treatment is closely related to crack rectification and is a very important factor.

Excessive residual austenite increases local tensile stress.

Insufficient tempering, too low tempering temperature or insufficient tempering time affect the carbon content in martensite and the welding or reduction in the size of martensite microcracks, thereby affecting the fracture toughness of martensite. The grinding heat generated during grinding produces large thermal and structural stresses, resulting in grinding cracks.

Large deformations during carburizing and quenching heat treatment result in uneven grinding tolerance or increase the grinding tolerance of teeth.

(2) Grinding process:

Since cracks occur during the grinding process, grinding technology is an essential factor that cannot be ignored.

Large grinding tolerances can generate excessive grinding heat, causing thermal and structural stresses. These stresses, combined with the tensile stress in grinding, increase the tendency for cracks to form.

Unreasonable pairing of cutting quantities.

Improper grinding wheel selection.

Cooling oil temperature too high or insufficient oil quantity.

III. Technological measures to prevent cracks in hard tooth surface gears

1. Heat Treatment Measures

Materials that are more sensitive to grinding cracks are more prone to crack formation during grinding. Reducing the material's sensitivity to cracking reduces the likelihood of cracking occurring.

Materials such as 20CrMnTi and 20Cr2Ni4A are more sensitive to grinding cracks, and this sensitivity varies according to different carburizing heat treatment specifications.

Therefore, it can be appropriately regulated and reduced by changing the carburizing, quenching and tempering processes. The following measures are adopted for this reason:

(1) Reduce the quenching temperature of carburized parts: For gears made of 20CrMnTi, carburize at 930°C, quench directly after carburizing, and when the quenching temperature decreases from 860°C to 830°C, serious grinding cracks may be eliminated without changing the grinding conditions.

(2) The surface carbon concentration should be appropriate, controlled within the range of 0.7% to 0.9%. The carbon concentration gradient must be gradual, ensuring good surface resistance and stress distribution.

The carbon content of heavy-duty gears should be controlled at the lower limit, which makes it easier to control the size and shape of carbides. When the carbon content is controlled at the upper limit, it will increase the tendency of residual austenite formation, increase carbide, surface oxidation and the tendency of tooth root strength reduction.

According to relevant data, the United States has controlled the carbon concentration on the surface of heavy-duty gears to about 0.65%.

(3) The less sufficient the tempering, the greater the sensitivity to grinding cracks. Therefore, complete tempering is essential to increase the ductility of the carburized hardened surface, allowing residual stresses to balance or reduce and improve surface stress distribution. This, in turn, reduces the likelihood of cracking.

(4) Control the amount of residual austenite to avoid structural transformation during gear grinding, which leads to significant structural stresses. Strictly limit residual austenite to 25%, and for crucial gears, it must be controlled within 20%.

(5) The main focus is on controlling the size, quantity, shape and distribution of carbides to obtain a dispersed distribution of fine-grained carbides. This increases the fracture resistance of the material and reduces brittleness.

(6) Control the martensite level to obtain cryptocrystalline and fine needle-shaped martensite, avoiding the formation of thick needle-shaped martensite, thereby reducing the sources of cracks and improving the fracture resistance of the material. The ideal martensite level is 3.

(7) Implement necessary process measures to control heat treatment deformation, reducing grinding tolerance.

2. Technological Measures in Machining

The literature indicates that the average temperature in the contact area between the grinding wheel and the gear surface normally varies from 500 to 800°C, with the temperature at the grinding points reaching up to 1000°C.

Furthermore, more than 80% of this heat is transferred to the gear. The substantial heat generated during gear grinding leads to significant thermal stress and thermally induced expansion and contraction in the grinding area of the gear surface.

If this heat is not controlled effectively, the gear surface can easily develop cracks and burns.

Therefore, the emphasis of machining technological measures will focus on minimizing and controlling the heat generated by grinding.

(1) Reduce the surface roughness during the rough milling phase to control it between Ra3.2 and Ra3.6.

(2) Strictly regulate the remaining nominal size during rough milling; It is not permitted to arbitrarily increase the grinding tolerance.

(3) After heat treatment, adjust strictly according to the prescribed position and allowable range to minimize the error caused by thermal distortion as much as possible.

(4) Before grinding, it is imperative to use roller cutting technology on the hardened tooth surface for tooth scaling. This ensures a uniform grinding tolerance, reducing it as much as possible, thus minimizing grinding heat.

(5) Select and combine cutting quantities rationally. The guiding principles should be higher wheel speeds, faster movements and appropriate feed forward. According to foreign data: the rough grinding phase of the tooth surface is the crucial moment for the formation of grinding cracks. The vast majority of grinding cracks occur at this stage. Special attention must be given during this phase.

(6) Grinding wheel selection is a crucial step in the tooth grinding process. The appropriate choice of grinding wheel has a great impact on the accuracy and efficiency of tooth grinding. Improper choice of grinding wheel hardness, grain size or structure can easily cause surface burns and grinding cracks. Therefore, the wheel must be chosen considering the following aspects:

Abrasive: Red corundum, also known as PA, has a hardness comparable to white corundum (WA), but with better toughness. When grinding high toughness steel with corundum, the efficiency is higher than that of white corundum. The durability of the grinding wheel and the roughness of the ground surface are also superior, hence the preference for PA.

Bond: The material that binds grains of sand together to form a grinding wheel. Currently, the grinding wheel binder used in gear grinding machines is predominantly ceramic adhesive (coded V); It has stable properties, resists water and heat without degradation, adapts to various types of refrigerant rectification, and is economical.

Hardness: The harder the grinding wheel, the lower its porosity. During grinding, the spaces between the grains of the grinding wheel are quickly blocked by the grinding particles. Coupled with the poor self-sharpening of hard discs, the dull grains do not come off easily, which may cause grinding between the disc and the surface of the workpiece. This affects heat dissipation and increases grinding heat, which can easily cause burning and cracking. Softer wheels wear quickly and can directly affect gear accuracy if used incorrectly. Therefore, the principle for selecting the hardness of the grinding wheel is: choose a harder grinding wheel for processing soft materials and a softer grinding wheel for hard materials. When grinding low-carbon hard steel alloys, a K to J grinding wheel should be chosen. (New-old model comparison: K- medium soft 1, J- soft 3).

Organization: Priority is given to grinding wheels with a porous general structure. The organization of grinding wheels is mainly divided into five classes.

Grit size: The smaller the grit size, the more grinding particles participate in grinding per unit area, correspondingly, the cutting force and grinding heat increase, which can easily lead to wear. To ensure the accuracy of gear surface grinding, common grain sizes are between 46# and 60#. For gears with smaller modules, grinding wheels with larger grain sizes should be selected, while for larger gear modules, grinding wheels with smaller grain sizes should be used. (Grain size is represented by a number; the larger the number, the smaller the particle size.)

Shape and size: The gear grinding machine model is Y7163A, using double conical grinding wheel (code PSX1). The dimensions are Ф350×Ф127×32.

The sharpness condition of the diamond in the grinding wheel dresser should not be neglected. Due to the dullness of the diamond tip, the grinding wheel becomes dull after dressing, which leads to a significant increase in grinding heat. Therefore, once the diamond becomes blunt, it must be sharpened immediately to restore its sharp working condition, which is a prerequisite and guarantee for the correct grinding wheel wear.

Coolant plays a critical role in the grinding process and should be given sufficient attention. Gear grinding machines operate based on the principle of grinding generation, where the grinding wheel and the tooth surface make point contact during the grinding process. The resulting grinding heat is carried away by the powerful coolant flowing over the grinding wheel and tooth surface. This grinding method helps prevent cracks from forming, prevents the grinding wheel from becoming clogged, and prevents grinding dust from spreading, resulting in good environmental effects. Therefore, the coolant must be abundant and sprayed directly into the grinding area with a selected flow rate of 40 to 45 L/min and pressure of 0.8 to 1.2 Mpa. It is essential to maintain the purity of the coolant, filter it during circulation and control its temperature, using a radiator if necessary. Special attention must be paid to ensure that the flow rate and force of the coolant sprayed are sufficient. Any changes should require an inspection of the coolant pump filter for blockage. Regular cleaning and inspection of the filter is also necessary.

4. Process measures to eliminate grinding cracks on hardened gear surfaces

When grinding cracks appear on hardened gear surfaces, the causes of these cracks must first be analyzed. Then, based on working conditions, the following treatments should be applied:

1. The effects and application of secondary quenching method

(1) Secondary quenching method

By properly extending the tempering time for carburized hardened parts, increasing the tempering temperature, and increasing the number of tempers, sufficient tempering can be achieved to eliminate and reduce grinding cracks. The specific approach is as follows:

After quenching the gear at 180℃ for at least 16 hours, gear grinding or scraping can be carried out before grinding. In cases of severe grinding cracks, two low temperature temperings can be carried out.

Aging in hot oil between 160°C and 180°C for 12 hours produces even better results.

Due to the simplicity and effectiveness of these methods, they are commonly used to prevent and eliminate grinding cracks.

(2) The effects of sufficient tempering are as follows:

Sufficient tempering significantly reduces the grinding sensitivity of various types of steel.

Sufficient tempering reduces microscopic stress.

Sufficient tempering allows microcracks to weld automatically.

Sufficient tempering leads to better elimination of quenching residual stress.

(3) Methods for Identifying Proper Temper

For already processed hardened and tempered components, we determine the suitability of tempering by observing the surface color of the tempered part. A golden hue indicates sufficient tempering, while a straw yellow color suggests that additional tempering is required.

For hardened and tempered components that have not been processed, sandpaper can be used to polish and shine a specific part of the component's surface until it exhibits a metallic shine. We then determine the suitability of tempering by observing the color of this surface after tempering.

(4) Considerations during secondary tempering

To minimize distortion of components as much as possible, components can be held at a furnace temperature of 100°C for 1 to 2 hours during tempering, then the temperature is raised to 180°C and tempering is carried out for 14 to 15 hours.

For hardened and tempered components that have already been processed, adequate protection must be provided during tempering.

2. Wipe Method for Crack Removal

Cracked tooth surfaces are cleaned by scraping with a hard alloy roller cutter, followed by grinding the teeth. This method is mainly used when:

The thickness of the tooth has sufficient margin.

It is often applied when the depth of the crack is relatively shallow.

3. Grinding method for crack removal

A serious grinding crack occurred in the grinding of a large planetary gear, providing a typical example of the grinding crack removal method, as detailed below:

(1) Brief introduction to the situation of large planetary gears with severe grinding cracks:

Technical parameters of large planetary gear: m=9z=66α=20°f=1, tooth width=60

Material and heat treatment condition: 20CrMnTi with carburizing depth of 1.8 ~ 2.3, surface hardness HRC58 ~ 62

Deformation condition and grinding margin: After carburizing quenching, due to deformation, the actual size after normal line expansion is: 1.25 (mm). Grinding margin: 0.65 (mm); When severe grinding cracks appear, the remaining grinding tolerance is: 0.7 (mm).

Current state of the grinding crack: The most severe grinding crack has 14 parallel cracks on the right side of a tooth, distributed perpendicularly along the width of the tooth to the grinding direction. The length of the crack is close to the height of the tooth and almost all teeth have cracks on the left and right surfaces. The number of cracks varies, with a discontinuous and irregular distribution. The condition of the fissures is extremely serious.

(2) Implement the following measures:

Place gears with severe cracks in hot oil at 180°C for a 12-hour aging process.

Replace the grinding wheel: The original hardness of the grinding wheel was grade K, now replaced by grade J.

Due to prolonged use of the cooling pump without a filter installed, the cooling oil tank became severely contaminated, with a 6 cm thick layer of grinding sediment covering the entire bottom of the tank. Therefore, thoroughly clean the refrigeration oil tank, replace with new oil, ensuring the cooling quality and effectiveness.

In the grinding process to remove grinding cracks, the feed is controlled at 0.025(mm).

In the grinding process to remove cracks, the grinding wheel must be grinded every two full rotations to maintain an accurate working state during operation.