I suspect it may be related to the crossed-axis helical geometry resulting in high contact stresses.

That, and the relative cost and difficulty of replacing a distributor gear vs. a cam.

Mike Halloran
Pembroke Pines, FL, USA

the idea of using two different materials  is based on the fact that one part (the cheapest one to replace) is wearing faster then the more expensive part - hence a steel crankshaft and softer metal journalbearings (with a steel back). in the latter case the bearings can be replaced when worn whereas the crankshaft only needs some minor regrinding to a somewhat smaller diametre.

with gears however, you have two careful mating surfaces, and simly using a softer material will not help - it will wear faster alright, but when worn, you would still have to change both gears.

different materials are sometimes used (a resin based gear and a steel pinion) but in those cases lower running noise is the aim, not easy overhaul.

another point to remember is that all automotive transmissions are designed for a specific time span - and that all the gears available should total the total life expectancy of the vehicle. that also means that for example the backward gear has a design life of say only about an hour at full load - and that is sufficient for the gearbox to last say about 300000 km - since you do not go backwards a long distance and certainly not at maximum torque. in the same transmission the highest gear will last a lot longer under maximum torque conditions, since the car builder knows that it will be used that way.

there is a difference between typical automotive gears and industrial gears in this respect - industrial gears are usually designed for prolonged use (20 or more years at the rated torque) and this is reflected in the dimensions of industrial gears compared to automotive designs.

Especially the cost of a steel roller cam.

Regards

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90deg cross axis helical gears, by nature, operate with relatively high contact stress.  This is made worse in the case of a typical domestic OHV distributor/oil pump drive, by the fact that the gear mesh is likely a 1:1 ratio.  So the same tooth on one gear is always bearing against the same tooth on the mating gear, amplifying the wear related issues due to geometry errors.  As romke suggested, it's much easier and cheaper to replace a (removable) distributor gear than it is to replace an entire camshaft.

As for transmission gears or rear end gears, they are made from different materials, use different lubricants (sometimes), and have different heat treatments.  The automotive business is very cost conscious.  The engineers at the automotive OEMs spend a lot of time and effort to get the optimum performance from the lowest cost component.  They would not hesitate to use a different alloy and heat treatment for each gear in a transmission if it would save money.  

As a mechanical engineer working in aerospace, I'm always amazed that an auto company can build and sell something as complex and reliable as modern automobile for the price that they do. It's truly impressive.

As a mechanical engineer working in automotive, I'm always amazed that an aerospace company can keep something as complex and reliable as a modern aircraft in the air. It's truly impressive.

grins.

Cheers

Greg Locock

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They sell for a much higher price than costs of manufacture, its been like that since inception.
Nowadays its massive overhead that raises costs.

Quote (dicer):

"They sell for a much higher price than costs of manufacture"

Thats generally so that they can afford to stay in business and continue to pay their employees.

Sheesh, it capitalism.