Mechanical design - cam timing chain versus gear-driven

[pnorm91]

mechanical design -- cam timing chain versus gear-driven

hey all,

I am a mech-e student, but I am still in the infancy of my degree, so I haven't really had any courses pertaining to this sort of thing, but having work with cars for a number of years, and having what I consider a strong mechanical inclination, I have a knack at identifying an area where I could see something being done a different way. That being said, I obviously can't know without seeing the rest of the engine, what other factors might affect the design of the valve body. But, I was wondering, looking at this picture, can anyone say what might be the advantages or disadvantages for using a solid gear which contacts the other two directly, rather than via a timing chain? is there any advantage to using a time chain in regards to how it handles the torque? it seems to me that the tight fit of a gear on gear mechanism, provided they fit together well, would be much more mechanically sound. Am I missing something?

I look forward to discussing it!

 

[SteamKing]

"Am I missing something?"

Yeah. How do you transmit torque to turn the camshafts from the crankshaft? Building a geartrain embedded inside the engine block may not be feasible and it would certainly be very difficult to replace any gears which might become damaged. At least with a chain, you can take a link out and replace the chain without having to make a two-piece engine block.

I'm guessing this head is for a motorcycle engine or maybe a small auto engine. Most auto engines drive the camshafts and other engine auxiliaries from the front of the engine. For mass-produced OHC engines, the cams are driven by belts or chains. With proper care, the chains will last the life of the engine, but belts must be changed periodically to ensure that they don't fail, possibly wrecking the engine as a result. Belts are cheaper and thus more common. Both systems are relatively quiet in operation.

If cost and noise are not considerations, then a total gear train can be used for cam drives, a la the old Cosworth V-8 F1 engines:

 

[pnorm91]

that is fascinating, I have never seen anything like that. Thanks for sharing!

so then the real considerations are regarding the space limitations, and the ease of repair if I read that correctly. If that were not the case, then is gear on gear a better set up than a chain? just in general, isn't a gear on a gear going to be much more stable over long periods than chain on a gear? What I am considering is the fact that there is less slack in a gear/gear system than a gear/chain system. of course this isn't doesn't seem like such a big issue if the system doesn't experience high torque. aside from the obvious space limitation preventing it from using a chain, I could see this being a reason for starters to contact the flywheel gear directly. Am I even close in my reasoning here?

thanks again!

 

[SteamKing]

All mechanical design is a series of tradeoffs. Cost, ease of construction, and ease of repair figure into every design. Sure, gears are probably less prone to developing slack than a chain or a belt, but they are also more expensive to make. However, gears are also prone to losing teeth, and this could disrupt valve timing.

In your motorcycle engine, the small package of the engine means that small gears must be used, and small gears can only transmit so much torque and still have a long service life. The central location of the cam drives make it difficult to install a series of gears without having a two piece block or two banks of cylinders, all of which adds to the cost of manufacture and maintenance.

The F1 Cosworth engine in the picture is a handmade unit which probably sold for more than $50,000 when new. Racing engines run only a few races before they are scrapped, and in between races, they are torn down, inspected, and re-built. Using a lot of gears is OK because of the amount of inspection and maintenance these engines receive. For everyday use, this would be unacceptable in terms of high cost and inconvenience.

Starter motors engage the flywheel because of the mechanical advantage to having a high speed, small, low torque motor use the large gear ratio between the motor pinion and the flywheel gear to step down the RPM of the starter motor and multiply the torque turning over the engine. A direct drive starter would have to be much larger than starters now used, and then placed in the drive train somewhere, not to mention designing a slow turning electric motor.

 

[AlephZero]

What steamking said, plus, gears and chains need lubrication. Timing belts do not. Deleting oil feed passages, oil seals, etc all saves cost and weight - which is why most mass produced car engines now use belts.

With proper care, the chains will last the life of the engine,

Hm... I once had one that diidn't. One failed chain link took out all the pistons and valves in the engine!

aside from the obvious space limitation preventing it from using a chain, I could see this being a reason for starters to contact the flywheel gear directly.

You want the starter drive to disengage when it's not in use. That's easy to do with a gear drive by moving the gears apart, but a chain drive would be operating all the time. (Remember the starter motor only turns the engine at about 100 RPM - at engine's red line speed, the starter motor's RPM would be insane if the drive was still engaged.

isnt a gear on a gear going to be much more stable over long periods than chain on a gear?

Not with modern belt materials, which don't stretch (or at worst, stretch in a very repeatable way). On a smaller scale, you might consider that the computer printers, scanners, etc use toothed belts, not gears, for the very high accuracy positioning of the print heads etc.

 

[SteamKing]

All mechanical parts are subject to failure. I own a car with a chain driven OHC engine and am quite satisfied with it. Even the latest belt driven engines I believe still recommend that the belts be replaced after 50-60,000 miles.

I believe chains are used in current M-B V-type engines in lieu of belts.

 

[pnorm91]

thanks for all the input! I definitely did not know as much as I thought I did. But I always enjoy learning more about engine parts.

 

[Baluncore]

There is a subtle advantage of chain (or belt) over gears in four stroke engines.

Because there is a 2:1 ratio required, gears will always have one tooth on the crank gear meeting the same two teeth on the cam gear. That will result in uneven wear. When dismantled and reassembled the timing marks on the gears will line up again and return the teeth to their old mates. That system may become very noisy.

A chain or belt will be designed to have a number of links that shares no common factor with either gear. That is a hunting tooth system that will spread wear evenly to all teeth on both gears, through their contact with all links on the chain, (or steps on the belt). That system will continue to run quietly as it wears evenly.

A gear train between the crank and cam must cross the cylinder head gasket at some point where there may be a gear tooth clearance problem. A chain or belt requires a tensioner which sometimes gives trouble, but it crosses the engine block to head gap without problems.

It would take many gears to connect the crank shaft to an overhead cam shaft. Gears are better suited where the cam shaft lies in the block close to the crank shaft, push rods are then needed to reach the rocker shaft.

 

[Mykacanada]

Wasn't the 79,1980 Mazda RX7 a rotary engine ? You could look at those examples.

 

[Travis_King]

Not that this adds much to the main discussion, but as to what Mykacanada said:
Old Suzuki motorcycle that used a wankel engine: http://en.wikipedia.org/wiki/Suzuki_RE5, as did a few others.

 

[SteamKing]

Wasn't the 79,1980 Mazda RX7 a rotary engine ? You could look at those examples.

It's not clear what you mean. The Wankel engine doesn't have valves and therefore no need for a camshaft.

 

[AlephZero]

Because there is a 2:1 ratio required, gears will always have one tooth on the crank gear meeting the same two teeth on the cam gear.

That is not the case if you use a train of more than two gears. You would probably need more than two gears to span the distance between the crank and the cam, unless the gears were unfeasibly large. See the Cosworth V8 picture.

 

[Baluncore]

AlephZero. You are correct. An idler gear is analogous to a belt or chain.
But then the idler gears in a train should not have common factors with their immediate contacts.

 

[Spirou]

"What steamking said, plus, gears and chains need lubrication. Timing belts do not. Deleting oil feed passages, oil seals, etc all saves cost and weight - which is why most mass produced car engines now use belts."

Hi, I would argue that the trend right now is towards chains, moving away from belts. Also, I think some newer belt driven engines uses oil lubrication to prolong the lifespan of the belt.

Here is a good list, at least for the European market.

http://www.honestjohn.co.uk/faq/chain-cam/

 

[Ranger Mike]

There is nothing wrong with the belt timing chain to drive gears in a street engine. Go racing and the thing stretches. The up grade has been to a true roller chain. Lasted longer. Gear drives have been used and are great until you lose a gear. This is rare. The main problem is the parasitic drag associated with the whole drive. Big time loss of horsepower. This is why the NASCAR boys went to timing belt about 20 years ago when kelvar was used. The main reason was the belt cut way down on harmonics of the drive train which was a big contributor to drive line failures. The belt was light, strong and had the least of all drive mechanisms when dynoed for parasitic drag and over all performance. I have run all three and gears are cool when you pull into the local drive in for a root beer but man does it have noise!

 

[Spirou]

There is nothing wrong with the belt timing chain to drive gears in a street engine. Go racing and the thing stretches. The up grade has been to a true roller chain. Lasted longer. Gear drives have been used and are great until you lose a gear. This is rare. The main problem is the parasitic drag associated with the whole drive. Big time loss of horsepower. This is why the NASCAR boys went to timing belt about 20 years ago when kelvar was used. The main reason was the belt cut way down on harmonics of the drive train which was a big contributor to drive line failures. The belt was light, strong and had the least of all drive mechanisms when dynoed for parasitic drag and over all performance. I have run all three and gears are cool when you pull into the local drive in for a root beer but man does it have noise!

Do you mean cam chain also is parasitic to power and fuel consumption?

 

[Ranger Mike]

Ahttp://www.summitracing.com/search/part-type/timing-chains turns the http://www.summitracing.com/search/part-type/camshafts in concert with the http://www.summitracing.com/search/part-type/crankshafts and drives the oil pump, possibly the mechanical fuel pump, maybe the distributor.

A timing chain also acts as a vibration damper. It absorbs a certain amount of crankshaft vibration and shock before it is transmitted to the camshaft. This vibration can shorten the life of an OE-style timing chain. High valve spring pressure can also increase harmonic vibration through the camshaft, further stressing the timing chain.

Back in V8 engine days most OEM-style timing chains didn’t last long. They would wobble and stretch rapidly; many even came with out-of-round sprockets that would throw off cam timing and wear quickly. You had to check the sprockets carefully before installation, then inspect the chain religiously. The only way to advance or retard cam timing was with offset drive pin bushings (and enlarged cam mount bolt holes to allow for movement) or with offset crankshaft keys. Adding insult to injury, the OE-style lower crank gears were of the press-molded or sintered iron variety. Needless to say, reliability at high rpm wasn’t a design priority with these setups.

That changed when aftermarket large-pin timing chains were introduced. These chains have larger-than-stock pins, which actually roll as the chain and sprocket teeth contact. Most have heat-treated gear teeth, and some have keyways in the crank gear that let you advance or retard the camshaft (usually by four degrees). This type of chain is stronger and lighter than the roller-style chains. That reduces operating tension and wear, so this type of chain lasts longer.

Roller Timing Chains

http://www.summitracing.com/search/part-type/timing-chains/timing-chain-style/single-roller have been available since the 1960s. The chains have a semi-floating tube that covers the chain links, allowing the chain to literally roll over the gear teeth. This helps reduce friction and chain stretch.

There are two types of roller timing chains available. The http://www.summitracing.com/search/part-type/timing-chains/timing-chain-style/single-roller is much like the chain on a bicycle—the links or side plates of the chain pass over and around a single set of teeth in each gear. A http://www.summitracing.com/search/part-type/timing-chains/timing-chain-style/double-roller is exactly what the name implies—gears with dual sets of teeth with a chain link for each set. Many late model OE engines, like GM’s LS-series V8s, have a single roller timing chain. A single roller is also ideal for most high performance street engines. When you get way up in the horsepower and rpm strata where big lift cams and very high valve spring pressures come into play, a double roller chain can better control harmonic vibration at the cost of a little additional weight. That means more stable cam timing in the engine where even a little deviation can cost horsepower.

A variant of the roller chain is thehttp://www.summitracing.com/search/brand/cloyes-gear/part-type/timing-chain-and-gear-sets?GroupBy=ProductName&keyword=True%20Roller This chain has pins or rollers that actually spin as they ride over the gear teeth. This further reduces friction compared to standard roller chains with rollers or pins that are fixed in place. You can get a true roller chain in single or double roller versions.

Some roller timing chain sets are manufactured with larger than stock pins. These are typically found in standard roller chains. This design yields a stronger, more durable chain, but adds more friction than a true roller.

Roller timing sets with iron cam gears have wear issues on iron engine blocks. Most engine builders machine the backside of the cam gear or the block area surrounding the cam nose to accept a bushing or a Torrington bearing. This prevents galling between the cam gear and the block. Many roller chain sets have steel gears to eliminate the problem altogether.

Another issue with a roller chain setup is the “chordal action,” a whipping motion that happens when the chain turns on the gear teeth. That whipping motion can literally turn the chain into an “S” shape. Chordal action can weaken the chain and also negatively affect valve timing, ignition timing, and mechanical oil and fuel pumps. That’s why it’s important to check the timing chain on regularly. If it’s tired, replace it.

The following was done at http://www.hotrod.com/how-to/engine/ccrp-0510-camshaft-drive-comparison/

Summary in my words - true roller chain drive = 1

Gear drive with fixed idler gear = 2

Gear drive with dual floater gear idlers = 3

True belt drive = 4

Not a lot of difference. Nascar runs belts for a reason. Drag Racers love gear drives. Pick your poison!

Cam-Drive Dyno Results Tested at Westech Performance Group SuperFlow 902 engine dyno All drives at 103 installed centerline Averages calculated from 3,100-6,600 rpmFixed Dual Drive Chain (1) GD (2) Idler (3) Jesel (4)

Peak HP 633.5 636.3 634.0 633.0
Peak TQ 595.4 596.3 596.3 594.2
Average HP 511.7 511.2 511.0 510.7
Average TQ 552.8 552.1 552.1 551.9http://www.hotrod.com/how-to/engine/ccrp-0510-camshaft-drive-comparison/#ixzz3eTCfN6zd

 

[Spirou]

@Ranger Mike, I guess it could be interesting for comparison that one of the worlds most fuel efficient petrol engine is a chain driven engine called 1,8 HSD found in some Toyota models. On the other hand the Volkswagen group have recently moved from timing chain to belt (e.g. 1,4 TSI), claiming it saves fuel.