For high power transmissions, the optimum overall performance of the gear material can be achieved by forming a hardfacing layer on softer and more ductile materials. Steel is the ideal material because it can be used in many ways for surface hardening. The most widely used method is surface carburization hardening, which heats the entire workpiece to an austenitic temperature range and is in a high carbon atmosphere so that it A surface carburized layer is obtained, which is then subjected to quenching and tempering to obtain a high surface hardness.
As a heat exchanger for David Brown, we are increasingly concerned about the unpredictability of deformation, as our designers and engineers are increasingly demanding tolerances, especially for large gears that are small in size.
Over the years, through a large number of tests on all types of gears, the permissible values ​​and acceptance criteria for specified deformation and expansion have been obtained, all of which have now been incorporated into our CAD/CAM systems and calculated using CAD/CAM. As part of the design process.
Now, the deformation is a study of some major projects in countries such as the United States, Japan, and the United Kingdom. This article outlines the progress we have made in this area. 2 Deformation and expansion Because gears are one of the most sophisticated engineering parts, tolerances are in microns. As shown in Table 1, deformation and expansion are the decisive factors in manufacturing. Table 1 Typical helical gear tolerances Example of the number of teeth Modular tooth surface width maximum error and m adjacent circumferential section error cumulative weekly error Error shape error tooth The main steps to the direct quenching carburizing cycle, large cross-section changes and high heating rates produce sufficiently high temperature field gradients and stresses within the workpiece, resulting in plastic deformation heating through the austenite transformation zone (from Aci to Ao this transition is accompanied by Volume shrinkage, the size and shape of the change in size depends on the geometry and size of the workpiece.
The part of the furnace directly exposed to high temperature will generate a large temperature gradient. The heat resistance of the workpiece is low at the carburizing temperature. The yield strength of the workpiece is very low. Due to the weight of the workpiece and the weight of the workpiece stacked on it, it is extended by relaxation and creep. Deformation.
Cooling to homogenization temperature and holding the workpiece at the load edge of the furnace by heat radiation can cause large temperature gradient quenching or cooling in the air to slowly cool to room temperature. The deformation can be complicated and varied, depending on the occurrence of austenite. The bulk phase change type, which in turn is determined by the cooling rate.
If this is necessary, the possibility of deformation is increased because it causes the workpiece to transform into austenite again, and then transforms from austenite to martensite or other group to weave a high heat exchange associated with the quenching medium. The coefficient produces the largest temperature gradient and tissue transition gradient throughout the process in the workpiece.
At this stage, the stress in the workpiece will decrease, the martensite structure will change, and a small amount of deformation will occur. These changes are more likely to occur in the small section of the workpiece. 3 Factors affecting deformation and swelling 3.1 Designing all parts The design must consider both the use of the part and the manufacturing process of the part. The design of the carburizing gear needs to be carefully weighed the need for these two aspects. 3.2 Grade and state of the steel. A small amount of carburized steel will cause trouble, causing abnormality. Predicted and uncontrollable deformations 835M15 and S156 (4% Ni, Cr, Mo) are specific examples because of their high alloy content.
The state of the material prior to machining is very important. Normalizing and low temperature annealing is generally considered to be the best low temperature annealing because it is not intended to soften or homogenize the material, but to eliminate internal stresses caused by normalizing.
3.3 Machining a large amount of cutting material, and possibly cutting quickly and selectively, will generate a large residual stress. When it is released during heating to the carburizing temperature, it will cause deformation 3.4 support and the correct placement of the furnace workpiece in the carburizing furnace is a key, otherwise the workpiece will loosen and creep due to its own weight. Reasonable support and distribution of workpieces in the furnace can reduce the temperature gradient between the workpieces, thus reducing the performance difference between the parts. 3.5 Carburizing process parameters The time kept at the carburizing temperature is an important factor, which depends on the depth of the carburizing layer. If the size of the teeth is doubled, the depth of the layer should be doubled, and the carburizing time should be increased by 3 times. The extension of the carburizing time will cause the slack and creep of the workpiece at high temperature. 3.6 Quenching and quenching is mainly to obtain the surface hardened layer. Since the rate of temperature change during quenching is one to two orders of magnitude larger than any other stage in the process, large temperature gradients and tissue transition gradients are created in the workpiece, causing large variations in the workpiece. 4 Typical deformation and expansion. Large typical shapes can be described in several dimensions, including: 4.1 The deformation of the disc gears commonly used for disc gears is often predictive of common disc teeth. The deformed teeth also change their position, arrangement and shape, as shown, and these changes can be unaffected by the overall deformation of the gear. It is well recognized that the change in the helix angle is a potentially serious problem in the deformation of the gear teeth. However, the cumulative error of the weekly section becomes a more serious problem, but it is not covered by other deformations and does not attract sufficient attention.
4.2 Inner ring gear If the inner ring gear is correctly placed in the carburizing furnace, the deformation of such gears is also predictable. Otherwise, the unevenness and the associated ellipticity will have serious consequences. This may include grinding time, Definition of outer 抒/m ring shape / mm plus cutting amount to reduce hard layer thickness or excessive scrap rate 5 Gear deformation and bulging change Usually, the key factor is not the type or average value of the deformation, but The degree of change of deformation The relationship between the taper of the outer diameter of the disc gear and the outer diameter, with a similar relationship between the expansion of the outer diameter, the ellipticity and the outer diameter allows us to establish acceptance criteria. For example, the maximum allowable value of the outer diameter taper is 0. Mi Xi Bu diameter), because we have calculated that for gears with less than 100 teeth, the grinding of the teeth can be completed normally within the above tolerance range. There are no serious problems. Further techniques are being developed to maintain various deformations within the established standard range. The outer diameter taper is a function of the outer diameter. 6 control gear deformation and expansion. 6.1 machining allowance is determined by the geometry of the gear used and
In this way, we have entered the determined machining allowance data into the CAD/CAM software. These software automatically calculate the hobbing size to ensure that the final required size is ground after heat treatment.
6.2 The axial bending of the straight shaft gears varies along their length (as shown), and users generally require their straightness to be within 0.125 mm. In practice, we have been able to ensure that the non-straightness of the entire length after quenching is less than Q75mm/m. Therefore, if necessary, all the shaft gears should be rotated on the dedicated straightening machine with the center hole at both ends for calibration. This straightening machine can be processed in large quantities in a fully automated manner.
6.3 Controlling the deformation 6.3.1 Spiral bevel gears Most of the spiral bevel gear teeth have been ground before carburizing and hardening, and only after grinding and polishing to improve the surface finish.
Therefore, heat treatment deformation control is very important. Bevel gear pairs require strict shape control during heat treatment. The unevenness and ellipticity tolerance of the bevel gear must be small. We have recently redefined and relaxed these tolerances, examples of which are shown.
The inherent shape of the worm may make it the most difficult transmission member for carburizing and quenching. Since the root diameter of the spiral is relatively small, special measures are required. For most worms, it is the most free suspension during heat treatment. Important, despite the current measures, even in most rigorous designs, the full length of the straightness still exceeds 1.25 mm/m. To avoid the risk of radial cracking or cracking of the root during straightening, the goal of straightening Special techniques for controlling deformation over a length of no more than 1.0 mm/m 6.4 These special techniques include: 6.4.1 Modifying the shape to reduce the size and cross-section variation of the gear, after the heat treatment, rather than the heat treatment, to the grinding The size will be better.
Prior to this, the KEVIN system successfully completed two pilot projects. The first test project was conducted from 1989 to 1990. The content of the project was SBB (Switzerland) passenger car air conditioning system (KUR) auxiliary equipment inverter printed circuit. The maintenance of the board was followed by a second pilot project with a comprehensive target for the maintenance of the European urban rail passenger car of SBB (Switzerland), the time of which was 199. The results of the pilot project in 1993 were as follows: obstacle, service time); 40 faults can be eliminated in smaller repair shops (reducing repair time and reducing maintenance costs); one customer purchased the research results, in addition to use at the Olten repair plant, in Basel; in new In the project, such as IC2000 (SBB double-decker), KEVIN is part of the supply within the scope of ABB Henschel (Hen-schel), and has developed plans for other projects. 7 Prospect KEVIN system currently It has been transplanted to IBM-PC and compatible machines according to customer requirements and considering the operator's EDV area. Currently, it is passing the comprehensive planning system (CAPE/C), in the early stage of rolling stock planning. The phase forms application data and its description, and its combination with technical diagnosis of rolling stock is of increasing importance because unnecessary extra expenditure does not occur and the density of data is guaranteed.
In the future, all repair shops and control centers can be connected to each vehicle when needed (for example, using radio), and all relevant LCC data will be called. Wu Yuehua translated Zhang Xiaoren School 6.4.2 De-stressing between processes, roughing, stress removal, and then grinding The cutting process will prevent most of the various types of deformation caused by machining stress. 6.4.3 Special Supporting Tools These tools will be a cost-effective alternative to press quenching, which usually doubles the cost of heat treatment.
6.4.4 Staged heating and controlled heating rates minimize distortion caused by temperature gradients and austenite transformation gradients.
6.4.5 Phase change simulation Phase change during heating and cooling can be simulated and estimated by software, such as AC3 developed by David Brown, to correlate the phase transitions at different cross-sections of the gears and correlate the phase transitions with the observed deformations. It is possible to simulate different geometries of materials and heat-treated workpieces to help select specific methods to reduce deformation. 7 Summary In this paper, for carburized and quenched workpieces of various shapes and sizes, a method for reducing heat treatment deformation is outlined, except for seepage. The instantaneous mechanical properties and stress and strain of carbon steel in the phase transformation have an effect on each phase change. As a function of temperature, it can be determined experimentally. Otherwise, the mathematical model cannot accurately predict deformation and swelling.
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