My old Pinto had a toothed belt. I guess Ford was not aware that this was prohibited by your sources.
In a non-interferance engine (The pistons will not contact open valves) there is no problem.
In an interferance engine, damage will be done if the piston contacts an open valve. For this reason, a manufacturer may choose to go with a chain or gear drive for greater reliability.
P.s. The old pinto went over 200,000 miles. The belt lost it's teeth once when running, but no damage was done. I replaced the belt and drove the car for more years.
Jesel makes belt drives for OHV engines and claims that their belts are stronger and more reliable than chain or gear drives. Granted, they are in the business of selling belt drives so take that with a grain of salt, but they do make a very good product. You can find more info about their belt drives here:
Your source is wrong, I have seen all three, in both cam in block and overhead cam applications. Granted this covers OEM, aftermarket kits and industrial diesels, but such a sweeping statement just isn't correct.
Hmm, only chains and belts? I have seen older engines with shaft drive overhead cams. Sloppy, poor reliability, but probably better than the old chains, and multiple spur gears would be hard to maintain (engines prior to the mid 1950's).
Franz
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I've read about an old engine with cams driven by eccentric lobes connected by connecting rods. I can't figure out how you'd get a 2:1 speed ratio that way, so they must have been crank-speed cams.
Toothed belts work. If the center to center distance is less than a foot you can omit the tensioner if you space the shaft centers correctly. (Belts would rather not be flexed in both directions.) Usually belts are external and require oil seals on the cam shafts and additional covers as a result but Honda has made a vertical crank lawnmower engine that has a belt running inside the engine where it is coated with oil - something that is not ordinarily recommended for belts.
Chains love oil. The folk designing chain guides have gotten really good at it of late and have helped to make chains the preferred automotive drive for OHCs. Find out all you can about guide design before you begin laying out the chain route.
The connecting rod approach (4.5 liter Bentley and others) required a 2:1 gear. As with shaft driven OHCs, temperature compensation was a problem. Shafts with bevel gears usually have some kind of axial adjustment for the gear position at the head.
Okay, a pop quiz: The Crosley OHC engine had a shaft drive with right angle bevel gear assemblies at the crank and the cam. The shaft between the engine and the cam spun at 2/3rds engine speed. What was the gear ratio of the bevel set drivng the cam?
EngJW - I've only seen the cast iron engine in service and most of those were the 4 cyl. units made later for outboards. I think the sheet metal engine was killed off early because the attempts to use the then new science of arc welding didn't work well. All the blocks leaked somewhere. The only sheet metal block that I've seen is on display at the museum in Auburn, IN.
ivymike
I remember vaguely seeing a system such as you mentioned.
I think it was a Honda motorcycle. somewhere there was a 2:1 gear reduction I think on the bottom end. I do remember the drive because it was so unique. Each shaft had three eccentrics about 1 1/2 inch in diameter. They were ofset 120 degrees. The connecting rods were flat metal about 3/16 inch thick. When it went together, one set of eccentrics were fixed to the shafts, and the flat plate con-rod placed on them. Then the second set was put in place, displaced 120 degrees. Then the last set. As I remember, the con-rod plates were in contact with each other and a lot of lubrication was involved.
I seem to remember my friend spending a lot of time in the shop with his nearly new bike and I saw the cam drive arrangement quite a few times.
yours
