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Understanding Case Hardening of Steel

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Dave Palmer
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Re: Perhaps you could help a U.K. company making small engines...
Dave Palmer   9/5/2014 10:42:55 AM
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@Amclaussen: What you're describing (gear tooth fracture) sounds like bending fatigue, not rolling contact fatigue.  The optimal case depth to prevent bending fatigue is not necessarily the same as the optimal case depth to prevent spalling.  In order to prevent bending fatigue, you need to balance a hard case with a ductile core.

When it comes to spalling resistance, there's no such thing as too deep of a case -- although beyond a certain case depth, there's no additional benefit.  On the other hand, when it comes to bending fatigue resistance, an excessive case depth is actually harmful.

Flame hardening is similar to induction hardening, except that an oxyacetylene flame is used to heat the part, instead of an induction coil.  Generally, flame hardening does not provide as much control over the case depth as induction hardening (although a skilled operator can control the process very well, and there are some automated flame hardening processes that are very good).  Also, while induction coils can be designed to match the contour of gear teeth, most flame hardening processes tend to harden the tips of gear teeth more than the root of the teeth.  This can potentially be a problem, since the highest bending stresses are at the root.

If the failures occur relatively early in the life of the engine, then I'd suspect quench cracks.  These are cracks that form due to the rapid cooling during the quenching of the steel.  On the other hand, if the failures don't occur right away, I'd suspect either excessive case depth, or inadequate hardening of the roots of the gear teeth.  If the failures are a common occurance, then it's possible that the gear is simply underdesigned for the loads it needs to bear.

If you have a broken gear, I'd be glad to take a look at it.  Send me an e-mail (see my bio for the address) and I'll give you my address.

Amclaussen
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Platinum
Perhaps you could help a U.K. company making small engines...
Amclaussen   9/4/2014 5:55:16 PM
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Dave:  Maybe your expertise could be of interest to a small company that makes (among other things) small internal combustion engines used in not so small model airplanes!

RCV Engines Ltd. are based in Dorset, UK., and they designed a very clever, quite distinct geometry that solves the old problem of installing a Four-Stroke I.C. engine inside a fully cowled model airplane without the cylinder projecting outside the fuselage... RCV, by the way, means: Rotating Cylinder Valve.

As you may have seen, 99% of the flying scale models show some or all of the cylinder because the cylinder is placed at 90 degrees to the crankshaft, thus, unless the model has a large diameter nose, the model engine will be very very visible, destroying the illusion of a real 100% scale model.

RCV designed its unique engine with the cylinder placed alongside the propeller shaft direction, therefore the cylinder is placed lenghwise and can be installed entirely inside a tight cowl, occult from sight.  The actual crankshaft is placed vertically, and the complete cylinder rotates around the piston at half the crankshaft speed. The problem is that the crankshaft has to drive the cylinder sleeve through a pinion and gear machined in the edge of the cylinder sleeve, thus receiving the full impact of every piston explosion on the same tooth.  The gear teeth are "Flame-Hardened" to help in keeping the teeth from breaking and wearing down, but the heavy stresses from the Internal Combustion process are too harsh and from time to time, a tooth fails catastrophically destroying the engine (and sometimes the model!).  When reading your article, I remembered the Flame-Hardening mentioned in their information, and seeing that some aeromodelershave had some failures, I am suggesting you to contact them. Maybe your expertise could improve their engines by a better understanding of the needs of the material heat treatment best used for their application.

Their info is:

RCV Engines Limited
4 Telford Road
Ferndown Industrial Estate
Wimborne, Dorset

BH21 7QL
UK

Best Wishes. Amclaussen, Mexico City.

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