Common mistakes in spiral bevel gear design for rock crushers, oilrig rotary tables, thrusters for marine positioning systems and for other large applications.
1. Selecting incorrectly large number of teeth on the pinion.
Increased number of teeth on the gear results in reducing of strength. More teeth mean smaller tooth for the same pitch diameter. A good example of correct design is an automotive drive axle gear set. A common automotive drive axle gear set has a pinion with 11, 12, or 13 teeth. Because of the 35 degree spiral angle and low addendum the area of root undercut on an automotive pinion is very little or non. The face ring gear normally does not have any toot undercut because of the large number of teeth and high face angle. For unrelated gear performance reasons the quantity of teeth in large bevel pinions used on rock crushers, marine thrusters or rotary tables is high. We have seen spiral bevel gear sets with number of teeth on the pinion up to 31 and the most common number is 19. The reason of large number of teeth on the large pinions is inability of the traditional gear manufactures to cut large teeth. The traditional gear manufacturing method known as
Gleason or
Klingelnberg requires different cutting tools for different size of the gear teeth. Because of the Gleason or Klingelnberg bevel cutting tools are expensive the traditional manufacturers are cutting lager and smaller gears with the same cutters. The value of the smaller gears is higher so the manufacturers prefer to have a right tool for the gears that they cut the most. When somebody needs to order a larger gear the traditional Gleason or Klingelnberg manufacturer would convince the customer to increase the number of teeth so the teeth get small enough and they can be cut with the existing cutting blades. In addition to the increased cost of the bigger cutters the larger tooth requires very high power on the cutter spindle is the Gleason or Klingelnberg manufacturing process is used. In practice, the existing Gleason and Klingelnberg machines are not capable for productive and accurate tooth hard cutting of the tooth depth exceeds 2-inch.
However, not all the spiral bevel gear manufacturers are the same. With different spiral bevel gear manufacturing methods the correct number of teeth can be offered in production without increasing the cost of the bevel gears.
Spiral Bevel Co uses advanced non-traditional manufacturing method, which allows reducing the cost of the bevel gears when the size of the tooth is increased. Let’s compare 90-inch pitch diameter gear set produced on Klingelnber or Gleason machine with 90-inch pitch diameter gear produces at Spiral Bevel Co using digital technology. Klingelnber/Gleason pinion would have at least 21 tooth on the pinion because of the machining limits to cut a dipper tooth. The number of teeth on the pinion from Spiral Bevel Co can be a low as 11 or even less, depending on the ratio, spiral angle, addendum modification, and other parameters. In order to keep the same ration the number of teeth on the ring gear from Spiral Bevel Co will be also reduced. Reduction of the tooth count at Spiral Bevel Co results in the following advantages for its customers:
- the manufacturing tome reduces proportionally to reduction of the number of teeth
- the tooth bending strength increases for 20% or more because of the increase tooth size
- pitting resistance increases because of the large tooth is less sensitive for shaft misalignment and as result of increasing of the total area of the tooth contact pattern
- additional reduction of the root stress for 10% or more because of larger fillet radius on the root
- Spiral Bevel Co does not need to relax the gear after the hard cut because of the novel process does not leave residual tension stress on the surface due to the novel method of tooth hard cutting
In general, the 90-inch spiral bevel gear set from Spiral Bevel Co will transmit 30% higher torque compare to the identical size bevel set, which was traditionally manufactured on Gleason or on Klingelnberg machines.
2. Incorrectly small root fillet radius.
The fillet radius on the root of the tooth is critical for the bending strength of the gear. Larger radius provides less stress concentration and the gear lasts longer. The traditionally manufactured bevel gears have incorrectly small fillet radius on the root because of the limitations on the cutter blades and of the manufacturing process that involves Klingelnberg/Gleasson machines and tools.
Spiral Bevel Co offer spiral bevel gears with optimized fillet root radius that is normally used with Gleason or Klingelnberg tools. In addition to the larger fillet radius Spiral Bevel Co offers optimization of the fillet curve spline, which will provide additional strength compare to the traditional radius.
In practice, bevel gears from Spiral Bevel Co have 15% or better reduction of stress concentration on the root fillet.
3. Incorrectly ignored chamfering on the tips of the Toe and on the Heel of the tooth.
Adding the chamfers on the Toe and on the Heel is a common practice in gear design, but it is commonly ignored on large industrial gears such as gear for rock crushers or oilrig rotary tables. The good example of chamfering can be found on automotive drive axle gears on marine bevel gears used on Volvo-Penta stern drives. It is a basic role in spiral bevel gear design that the chamfer on the Heel is formed as an OD of the gear and the chamfer on the Toe is formed as the flat plane of the gear face. The common size of the chamfer extends approximately to the middle of the tooth height. The correctly sized chamfering can improve strength and reduce cost of the traditional gear. It would also make the gear easy to handle and assemble.
Spiral Bevel Co offers optimization of the chamfers for additional improvement. Larger chamfers reduce the active tooth surface, which can potentially reduce the area of the tooth contact and increase the contact stress. Spiral Bevel tooth contact simulation software is capable to model position of the tooth contact so the larger chamfer does not reduce the contact area.
The other advantage of Spiral Bevel manufacturing process is ability to modify the root land in order to mate it with the chamfer on the mating gear of a pinion. The root land on gears from Spiral Bevel Co does not need to repeat the conical shape of the cutter hear, like we see it on Gleason/Klingelnberg gears. Because of the flexibility on the shape of the root land Spiral Bevel Co offers additional 10% increase of the gear strength, which is impossible if to cut the gears on Gleason/Klingelnberg generators.
4. Tooth-by tooth induction hardened bevel gears.
Induction hardening is a low cost alternative to case hardening. The common mistake in induction hardening of large spiral bevel gears is using traditional tooth geometry of the tooth. While tooth-by-tooth induction hardening works well on helical and spur gears, spiral bevel gears have some specifics that need to be taken into account. The normal section of the traditional bevel tooth geometry known as Gleason or Klingelnberg is different along the tooth. The commonly used method of tooth-by-tooth or space-by-space induction hardening does not provide identical clearance between the inductor and the tooth surface because of the tooth profile is different in each any position of the inductor along the tooth. Gleason or Klingelnberg generating manufacturing method does not produce a spiral bevel gear that can not be successfully induction hardened by a movable inductor. While some manufacturing companies are offering tooth-by-tooth induction hardening, the hardness of the case is often inconsistent and results in premature pitting. It is also very difficult, if not impossible to inspect the hardness of each tooth and along the tooth of the not repeatable induction hardening process. As a general rule, the tooth-by-tooth induction hardened gears are, frankly, as good as gear with no surface hardening. While most of the teeth may have sufficient case hardness it is enough to have the only one tooth with lower hardness in order to fail the entire gear set. While some tooth-by-tooth induction hardened Gleason gears may demonstrate increase of durability, probability to have lower hardness area on one or on a few teeth is high.
Case hardened gears demonstrate higher repeatability in prediction of gear failure because of the better reliability of the case hardening process. In simple words, you do not know if the induction hardened gear will fail in one year or after five year because it is impossible to inspect each individual tooth for case hardness and case depth. With case hardened gears you would know that the gears would last for the specified design time so you can plan the time to replace the gears. Because most of the customers do not want to be caught by the surprise of fail gear they prefer to pay little extra for more reliable process of case hardening.
Spiral Bevel Co. offers case hardened gears, induction hardened gear or gears that do not need to be surface hardened because of the advanced design of the tooth. On the case hardened gears Spiral Bevel Co offers advanced finishing operation of gentle hard cutting of the hardened surface in order to achieve higher tooth accuracy after the heat treatment. On the induction hardened gears Spiral Bevel Co offers minor modifications of the tooth profile that allows increasing repeatability of the induction hardening process. Spiral Bevel Co is also offering spiral bevel gears with special tooth form that does not require induction or case hardening.
The are other common errors in design of gears for rock crushers, industrial rotary tables, marine positioning thrusters and on other. Our best advise to the designers of these gear would be to ignore the old gear hand books and look at the modern automotive gear sets that can be simply scaled up to the required larger design. It is well know fact that automotive gear technology is at least 25 years ahead of we can find in aerospace or marine. It may look surprising for somebody but the spiral bevel gears on our JSF $300bill. aircraft from Lockheed or on purchased for 10bill. Chinook helicopters from Boeing are way behind the bevel gears that an average Joe from Alabama drives every day on his $100K 1975 Ferrari.
Large variety of automotive bevel gear designs is available for purchase on e-Bay. Just pick the correct ratio and scale to up to the size of you rock crusher, oil rig rotary table, helicopter or an aircraft. From the point of view of our expertise in bevel gears the correctly scaled and modified existing automotive gear outperform any designs that some companies are still trying to create by themselves. Then call Spiral Bevel Co for manufacturing validation. In addition to the best possible design the gear needs to be manufactured at the lowest possible cost. The cost of manufacturing makes Spiral Bevel Co different from the traditional gear suppliers. We do not claim ourselves to be aerospace, automotive or marine gear suppliers. We have been in all of these areas and we have learned the best out of all that. We claim ourselves to be the best in gears, more specifically in spiral bevel gear design and manufacturing. If you want to make a better bevel gear, just give us a call. Most likely we will give you a fee advice for design corrections and offer a better gear because we know that there is always a better way to design gears.