The steel selected for gears must be strong to prevent tooth breakages. It must be hard to resist the contact stresses, and it must be ductile enough to resist shock loads imposed on the gears, due to any outside influence or dynamics built up in the system. The material selected for gears, solid with the shaft, must also be capable of resisting any stresses imposed along the shaft. Materials and heat treatment are important for the gear motors.
Through hardened pinions should be made approximately 40 BHN harder than their mating wheel to even out the life of the two parts with respect to fatigue and wear. Bar stock may be used for most industrial applications up to 300 mm dia., above this size forgings are usually used. In cases of high stress, it is advisable to purchase forgings as the structure is far superior to the rolled bar. Stepped forgings can also be obtained and may offer a more economic alternative. Cast steel is often used for gear wheels but care must be taken to select a high-quality material, devoid of blowholes, etc.
Steel for gears is usually treated in one of the following ways:
Through hardened (including annealed or normalized)
The material is heat-treated before any machining is carried out. This avoids any heat treatment distortion, but because it has to be machined, there is a limit to the hardness, and therefore strength, to which it is possible to go. Most gear manufacturers dislike machining steel over 350 BHN, as not only does it reduce tool life, it must also have an effect on machine life as well.
The most common steels (to PN-EN 10083-1+A1:1999) in this group is being C40, C45, C50, C55 and C60. The final selection is based on the allowable stress levels and the limiting ruling section involved.
Flame or induction hardened
The gear teeth are first to cut into a gear blank, and then surface hardened. This retains the strong ductile core while giving the tooth flanks a very hard-wearing surface. On small teeth, of 4 modules and under, the depth of hardening from both sides may converge in the middle and therefore make the whole tooth brittle (see Fig. 1). This is quite acceptable providing a slightly lower allowable bending stress is used for calculating the strength of the tooth, usually 80% of the allowable stress value of steel with hardness equal to that, of the root when in the unhardened condition.
Spin hardening, where the component is spun inside an induction coil, has the same effects as above.
Because there is a certain amount of distortion due to the heat treatment, it is usual to leave a grinding allowance on the tooth flanks for grinding after hardening. Hardened gears can be left unground, but because of distortion, a certain amount of hand dressing of the teeth may be required to obtain an acceptable bedding mark when meshed with its mate. As hand dressing is a skilled, laborious job, it is best avoided if at all possible.
Full contour hardening hardens the flank and the root of the tooth, and this avoids the abrupt finish of residual stresses in the critical area as in the case of flank hardened teeth. For flank hardened teeth, use only 70% of the allowable bending stress of steel with the same root hardness in the unhardened condition.
Flame or inductioned, hardened tooth flanks can, depending on the type of steel used, be expected to reach a hardness of 50-55 HRC at the surface and attain case depths of up to 6 mm. It offers a strong tooth, easily hardened, and wheel rims of suitable steel can, with the proper procedure, be welded to mild steel centers.
Bevel gears are not usually induction hardened because of the tapered teeth, and if flame hardened, care must be taken to ensure that the flame does not damage the thin end (toe) of the teeth. Suitable steels for flame or induction hardening include 34Cr4, 41Cr4, and 42CrMo4.
The teeth are finished cut to size in the blanks and are then 11isplac. This is a fairly low-temperature hardening process, and because of this, distortion is kept very low, and there are usually no corrective measures needed. The main disadvantages are:
- The length of time for the process, which is usually a minimum of 80 hrs.
- The case depths obtained after this very long time are only in the region of 0.6 mm maximum, and would not therefore be suitable for heavily loaded large teeth.
Nitriding can give tooth hardness in the region of 68 HRC, which is one of the hardest surfaces available to the gear manufacturer.
Because this process involves subjecting the whole gear to the hardening effects, no further machining, except grinding, can be performed on the gear. Therefore any keyways or holes etc. must be machined into the component before nitriding. It is as well to remember to have threads masked during the process too, or these could become unacceptably brittle. As for any heat treatment process, do not plug holes that could cause expanding air to explode components. Suitable steel will be 31CrMo12 or 31CrMoV9 (to PN-EN 10085:2003). This should be purchased in the hardened and tempered condition, and then stress relieved after roughing out.
Case carburized and hardened
The steel used is usually a strong, low carbon alloy steel, which after cutting the teeth, is subjected to a carbon-rich atmosphere. The carbon is allowed to soak into the skin to a specified depth, and then the gear is hardened, quenched, and tempered. Not only does this hardening affect the case, but it also hardens the core material, giving an extremely strong tooth with a flank hardness of up to 60 HRC and case depth of up to 3 mm.
Because of the high temperatures and long soak times, carburized gears tend to suffer a great deal from distortion unless controlled, and sections should be left “heavy” and symmetrical so as to minimize distortion.
Careful consideration must be given to the manufacturing procedure of carburized gears, as the final hardness prevents any further machining operations except grinding. It is usual to pre-machine pinion shafts from the roughing out stage by turning the outside diameter of the teeth to the size and leaving approximately 5mm (depending upon the required case depth) all over elsewhere. The teeth are then cut leaving a grinding allowance. It is then sent for carburizing and annealing, and in return, the “unwanted” carbon is machined from the soft shaft. Keyways and holes etc. Can also be machined at this stage. The component is then hardened and tempered.
An alternative to machining the carbon from portions to be left soft is to mask the areas using copper paint. The disadvantage is that a small scratch can let carbon seep in and maybe cause trouble at the final machining stage.
The disadvantage is that a small scratch can let carbon seep in and maybe cause trouble at the final machining stage. Threads should not be carburized, as they would become brittle during hardening and could cause a failure.
Wheels and certain shafts can be pre-machined, leaving just a grinding allowance on the sides, teeth, and in the bore. They are then carburized and hardened in one go. The component has just to be ground all over and is then complete. All companies that undertake to carburize would be only too happy to offer advice on the best procedure to adopt.
The steel purchased must be fine grain and in the normalized condition. After any rough machining operation, it should be stress relieved. Common case hardening steels include 18CrMo4, 20MnCr4 and 18CrNiMo7.