Posted by Max Clinch on October 20, 2000 at 00:16:48:
In Reply to: Hypoid Gears in Packards posted by Keith Carlson on September 15, 2000 at 17:08:09:
I saw your querry an because of my back ground think I have an answer for you. In the very early days on motor vehicle production straight tooth bevel gears were used. These satisfied the need but were noisy and could only carry limited power.
With more powerful engines a more powerful drive train was required. So spiral bevel gears, just developed fitted this duty. They could carry more load, quieter, and run at higher speeds due to the teeth overlapping. This was very convenient as they would fit into exactly the same housing as the straight tooth bevel gear set. This was a good solution at the time however nothing stands still. As engines developed more power the weak part of the gear train became the bevel pinion. This was because of the ratio used typical 5:1 - 6:1 the pinion becomes very small on spiral bevel gear set. The spiral pinions began to fail under load by teeth breaking off or splitting in half. Remember a tooth on the pinion does 5-6 time more work than a tooth on a gear wheel. Also from what i have seen many of these pinion had bore thru them as aposed to being part of a shaft.
Hence the "Gleason Company" of New York USA ( who invented the bevel gear cutters and are still gear cutting machine tool manufactures) arose to the challenge again and develop the hypoid bevel gear system. The hypoid gear is a cross between a worm wheel and a spiral bevel gear. By offsetting the pinion down they were able to increase the spiral angle of the pinion. When the spiral angle(or helix angle on helical gears) is increased, it increase the pitch diameter of the pinion. Hence a larger pitch diameter means that it can carry more load. The down side was the tooth action. It changed from a combination of rolling and sliding as with spiral bevels to a tooth action of pure sliding between similar metal parts. In a worm and wheel set this friction problem is over come by using two different materials ie case harden steel, ground tooth surfaces on the worm and bronze on the wheel.
In hypoids for load carrying capacity it was necessary to maintain the use of case hardened material for both members. Therefore other methods had to used to reduce friction. The first was by Gleason in that special machining methods were developed to produce a very good finish on the tooth surface. Automotive gears are very accurate and can not be easily reproduced in a jobbing workshop. The next solution to this problem came from the oil companies with the development of the Extreme Pressure oils (ie EP oils such as EP90 etc etc). These oils were developed specifically for diff gears sets so that there would be no frictional welding of metal when the teeth were meshing.
Hence the reality is the the lowering of the hump in the car is a by product of the above development of bevel gears. All this took place from 1900 thru to 1940. After that process and manufacturing procedures where refined to the point were now it is very rare to be overhauling differentials.
I hope this has given you some insight into your queries.
Albion Gear Company P/L
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