Reply With QuoteI spent many years work at the manufacturer of the largest gears in Australia. I left a long time ago, but at that time the largest gear made weighed over 200 tonne. Many of the gears were made for large mining equipment - grinding mills, draglines, bucket excavators etc. Some of the grinding mills I have worked on were driven with 4.5 MW motors (there are larger mills of around 7MW, but they have wrap-around motors, not gear drives).
The gear rating standard has been superseded since I last designed any gears, but the previous standard includes factors for gear precision. The new standard will still have factors for precision, but I am not familiar with it.
The gears rating (capacity in torque or power) is increased as the precision is increased.
There is a wide range of gear precision numbers, and the tolerances for profile errors and tooth to tooth pitch errors are incredibly small for the higher numbers and only achievable by finishing the gear on a gear grinding machine.
The gear cutting machines used can cut the gear teeth with camber to compensate for deflection of the pinion shaft between its support bearings, and can also with compensation for torsional twist across the width of the gear.
In the gear design process it is important to check for tooth deflection (bending from the root to the tip) and in some cases the profile has to be modified at the tip to compensate.
Gear tooth strength can be increased by increasing the gear width, within limits.
When the bearings wear in a gearbox, precision goes out the window and gear capacity is dramatically reduced.
But probably the greatest reduction in strength resulting from worn bearings, is due to the change in distribution of load across the width of the gear tooth.
This results in the load being transmitted through one side of the gear (equivalent to reducing the gear width) and leads to teeth breaking on that side.
The grinding mills that I mentioned above, use an array of sensors that continually measure the temperature across the width of the gear, close to the the teeth in mesh. These sensors are part of the control system and are detecting changes in load distribution due to deflection before damage can result.
One of the main things that Land Rover did when they upgraded the LT77 to the R380 was to increase the width of the teeth (made possible by moving reverse gear into the extension housing). I understand they also increased the width of some gears from the first R380 to current version.
In an earlier post in this thread, I explained how tooth load of 5th gear will be higher (for same vehicle load). It should be pointed out that due to the pressure angle of the gear tooth profile, the tooth load is not tangential (it is always directed along the line of the pressure angle). So for a particular tangential load for the required torque, there is a proportional radial (gear separation) force component. And with helical gears, a proportional axial load component.
These tooth forces (tangential, radial and axial components) are transferred to the bearings.
So if you have an increased load on the vehicle, whether due to gear in the back, trailer or hill, if you use use a lower gear than 5th, the gear tooth loads and resulting bearing loads are reduced.
The damage done to gears and bearings are cumulative over their life. High load events may not result in failure at the particular time, but some damage is incurred, affecting the life.
Bearings and gears are designed for a particular finite life. The cumulative damage of both are reflected in Miner's Rule, which is used to determine an equivalent design load (as the load spectrum gets complicated so do the calculations using Miner's Rule). Miner's Rule can tell us (as you would expect) that a high load over a short time does equivalent damage to a lower load over a longer time.
Originally Posted by Larns
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