Geared Crankshafts in IC engines 2

[factsb4pride]

I have been working on a project to develop an IC engine that uses gears to translate reciprocating motion to rotary motion. I have heard many posts here, as well as information posted elsewhere, that says current gear technology is not strong enough to allow gear teeth to survive combustion pulses without damage or undue wear in the long run.

However, another member recently posted info in a thread about Neander motors, which has 2 counter-rotating "crankshafts" that are directly geared to one another. Then power is taken from one crank by a CHAIN drive to the tranny. So combustion pulses are definitely being transferred across the gear tooth faces and through the gear teeth. AND this is a diesel engine, NOT a gas motor, so the forces the gear tooth must handle are very high.

So I guess my question is, do current gear tooth profiles and fabrication materials allow for this now? Are objections to this design of engine based on current realities, or outdated facts and opinions?

Yes, I understand no current engines use this method (except the neander motor), and that it is not a common practice. But my question is: Is it a viable research path to take, and if not, why not. Please be specific.

 

[factsb4pride]

Ah, I think I see it now Mike, and I wonder if you couldn't substitute drive chains for belts to connect the pulleys in your idea, and make it durable enough for engine use...

just a thought, not sure about it at all.

 

[MikeHalloran]

Yes, you could use chains; I've done that, too, but for manual positioners. Sorry, I have no photos of either type.

I'll try a word picture.
Stationary timing belt pulley, size 2N.
Arm rotates around its center, length L.
Tip of arm carries another timing belt pulley, size N,
free to rotate relative to arm.
Second timing belt pulley carries another arm, same length as the primary one, typically more delicate.
A belt of appropriate length connects the two pulleys.

As the primary arm rotates around the larger pulley, the tip of the secondary arm moves in a straight line.



Mike Halloran
Pembroke Pines, FL, USA

 

[factsb4pride]

I was able to visualize it earlier, and it may hold some promise.

If the neander motor can pass the diesel combustion pulses from 2 large cylinders through a single chain and crank sprocket, surely a similar type of chain could handle those same loads for a single cylinder in the configuration we are currently discussing. You could even have 2 chains per cylinder, one on each side of the mechanism, to further distribute the dynamic combustion loads that must be absorbed.

Anyone have any info on chain design and dynamic load capacities?

 

[BrianPetersen]

You'll find that commercially available roller chains have load and speed ratings that are probably dismal for what you are trying to do. Since there is almost no cushioning of the impact loads from combustion forces, the service factor is going to be high.

http://ustsubaki.com/general_cd/data/pdf/section_a/pages/a_bsdin_selection.pdf

Motorcycle drive chains operate well outside these rating tables.

Keep in mind that chains and belts have some compliance to them which is not present with gears.

 

[factsb4pride]

Well, after further research, I discovered the central crankshaft chain drive on on the neander motor is to run the camshafts, not to the tranny. One of the guys working with me misinterpreted a graphic he was studying.

Power is sent to the tranny through the main crankshaft gears. Which is bad and good. Bad that chains have not been used in this application, which is supported by the post before this one. Good in that it illustrates that gears can transmit these kinds of loads.

I will be contacting neander directly to see if I can get more details from them.

Question: do internal ring gear & pinion sets with a 2:1 ratio have stronger gear-to-gear interaction than an external gear of the same size would have with another external gear half its diameter? It occurred to me that the pinion gear might "nestle" into the ring gear more so than regular external gears would, and subsequently engage more teeth at the same time in the process. The mazda rotary engine drive gears do this nicely, but with a 3:2 gear ratio. Would this effect also apply to 2:1 ring gear/pinion gear sets?
And if so, by what factor increase (or decrease) in dynamic load bearing strength over external gear sets?

 

[factsb4pride]

Neander is not responding to my emails, but sorry anyway for the confusion on that front.

I am now looking at alternatives to involute gear tooth profiles. Double circular arc gear designs are reported to have 1.5 to 4.5 times the load bearing capacity of involute gear teeth. So I am taking a look at them to see if they could make a direct gear drive viable. Involute gears were close to making the cut already, so if the gains in strength that are claimed for double arc circular gears are valid, then its very possible they will work out in a watt/cardan style gear drive for combustion engines.

I have numerous papers and studies I am reading about this, but if anyone has any recent info on this, please post.

 

[factsb4pride]

Any thoughts from anyone on using double circular arc gears in this application? For or against?

 

[factsb4pride]

Sorry about the extended delay in posting, but I have been waiting to hear back from Neander Motors. While they did not reveal any details, which is understandable, they DID say that the large gears on their engine cranks DO in fact engage directly with gears in the tranny. So that means they have gear teeth directly absorbing diesel combustion pulses.

This is encouraging to me in that it shows you can absorb diesel combustion loads with gear teeth, and perhaps that method can be used in alternative engine crank designs.

Additionally, I have been researching alternative gear tooth profiles, and believe that an improved form of W-N circular arc tooth profiles would offer additional strength and increased hertzian contact loads over a standard involute gear tooth profile.

No one has posted in awhile, so curious if anyone is still following this, or has anything to comment...

 

[tbuelna]

factsb4pride,

Thanks for the link to Neander Motors.

Coincidentally, a few years back, I worked on a diesel engine that had an identical crank and conrod arrangement. Except that this engine was an opposed piston two stroke instead of a four stroke, and it had four cranks instead of just two. So, besides having gears connecting each pair of adjacent cranks, it also had a geartrain connecting the cranks at opposite ends of the engine.

This engine was a very high firing pressure (3000 psi) single cylinder diesel. We did a few hundred hours of testing and never had trouble with the gears. The gears were all involute spur gears, made from case hardened 9310 and finish ground with a slight crown profile.

With regards to your question back on Jan. 11 about the load capacity of an internal ring and pinion versus two spur gears having the same ratio, the ring and pinion combination would tend to have better load capacity due to its greater contact ratio. Of course, there are also other practical considerations between the two choices, such as direction of input/output rotations and mounting requirements.

Hope that helps.
Terry

 

[factsb4pride]

Hey Terry,
Appreciate your post! The setup you describe almost sounds like the research Achates has been engaged in. I am intrigued by your report of using case hardened gears with involute tooth profiles in an engine application as we have been discussing. That is the kind of info I am looking for here. Do you have any further info to share about that project, where I could perhaps take a look at a patent or further data? For example, in the engine you mention, did the same teeth in the mesh always take the combustion load pulses, or did different teeth do so with each combustion cycle?

Also, on Neander Motors website they have a cutaway of their engine that shows it also is using what appear (maybe) to be involute gears on the cranks meshing directly with the input gear of the tranny. I will attach it to this post. Those guys have created a real jewel of an engine!

Appreciate your comments as well about contact ratio being greater with ring gears & pinion than with 2 external gears. I have been unable to locate a formula to calculate those values with ring gears & pinions though, so if you have it, please post.

So it seems that it might be possible to build a cardan/watt style internal combustion engine, using either spark or compression ignition, based on Neander's motor and also the experiences of Terry. I am encouraged!!!

 

[factsb4pride]

Oops, forgot to point out that the gears in the Neander motor are SPUR gears, yet no mention of excessive gear noise is mentioned in any review I have read of the Neander Motorcycle. That is amazing, and very welcome news.

 

[factsb4pride]

Terry, I found the internal ring gear & pinion formula for calculating contact ratio a second ago.

 

[tbuelna]

factsb4pride,

I looked at the cutaway picture of the Neander engine. If that picture is accurate those guys don't understand gear design.

If you look closely at that picture, you'll notice that the crank gear that meshes with the tranny input gear has the same face width as the other crank gear. If properly sized, this middle gear should have a much wider face width than the other crank gear for a couple of reasons.

First, the middle crank gear is an idler. So it's teeth are subject to reverse bending loads, and typically should have a wider face to equalize the bending fatigue life with its mating gears.

Second, the middle crank gear is subject to both combined crank gear loads where it meshes with the tranny input gear, while the outer crank gear only sees about half of that load.

Regards,
Terry

 

[factsb4pride]

Could be its just for illustration, and they don't care to show what their methodology is in that kind of detail. Or maybe there is another input gear on the other side of the engine that is not shown in that view. Good eye though!

From my research, and as you pointed out, face width of the internal ring gear and pinion (or external gears such as the Neander Motor) should be the most easily adjustable variable to increase gear train strength in an engine application such as this. That is also assuming you use the largest tooth size that is practical, and don't get too carried away with cylinder bore size and subsequent gas loads. I would also incorporate an appropriate lubrication system.

 

[factsb4pride]

Here is another cutaway view of the Neander motor. Not possible to discern all the details from it however.

 

[BrianPetersen]

It's worth noting that spur gears are conventional design practice in motorcycle gearboxes, and in the primary drive from crankshaft to clutch basket on a motorcycle engine - they're all like that, the Neander engine is no different. Most of them do have slight gearbox howl.

The crank gears on the Neander engine don't have to deal with the full cylinder pressure. The kinetic energy transfers between the pistons and crankshafts without going through the gears. Since both crankshafts operate together but in opposite directions, both crankshafts accelerate and decelerate together, so the gears only have to transmit the net output torque to the clutch basket. Same as any other motorcycle engine.

 

[factsb4pride]

@Brian Peterson: What you say about the crank gears is true except for the gear(s) engaged with the tranny input gear(s). Those crank gears MUST pass all of the forces generated by the engine through their mesh with the tranny input gear(s).

This confused me at first as well, but if ANY of the crank gears are meshed with tranny gears, in that mesh gear teeth are handling direct combustion loads.

As far as the NVH thing, from what I had read I expected spur gears to generate too much NVH to be used in this application, but that may indeed be incorrect. No reviewer mentioned any objections to gear or tranny noise/vibration whatsoever in all of the Neander Motorcycle articles I have read (about a dozen).

 

[BrianPetersen]

The crankshaft on a motorcycle engine (including the Neander engine) is meshed with the drive gear for the clutch basket. There is a vibration damper between the drive gear for the clutch basket, and the clutch basket itself. This takes out most of the nastiest torque pulsations and averages them out. The crankshaft has by far higher inertia than that drive gear. The crank-to-clutch-basket gears are not handling combustion loads, only the average drive torque plus whatever small component of the inertial / combustion-related pulsations is related to accelerating or decelerating the (low-inertia) clutch basket drive gear.

This is what that clutch basket looks like from the backside ... the gear drives the basket itself through those springs, which take up all the shock loading ...

http://www.rdmotorcycles.co.uk/CLUTCH BASKET OUTER 002.jpg

 

[factsb4pride]

I understand the that most clutch systems have the ability to absorb and even out combustion pulses in most engines. That being the case, does that reduce the bending loads on gear teeth in that configuration? My research shows that gear teeth contact forces are manageable in this application, it is the dynamic tooth bending loads that are on the borderline with regard to involute tooth profiles.

I guess my next question would be: how effectively does the clutch shock absorber react and absorb gear tooth bending stresses vs. gear tooth contact stresses? If the clutch can handle both those forces in a dynamic way, then I am unsure as to why a watt/cardan style combustion engine has not been pursued by anyone in the past. If the clutch can absorb all of those forces with no problem or side effects to the engine, why have so many researchers walked away from this type of crankshaft design, considering all its advantages?

 

[BrianPetersen]

You're missing the point.

The SPRINGS in that clutch basket take up the shock loads. The gear accelerates and decelerates with the crankshaft and only the average torque is transmitted to the clutch and that means - neglecting the inertia of the drive gear, which is small compared to the inertia of the crankshaft - only the average torque is handled by the gear teeth, NOT the instantaneous combustion loads.

If you use a Watt gear setup, you cannot have a vibration damper between the piston and the gears! The combustion load goes straight through to the gears and THEN to the inertia of the crankshaft.

Do you see the difference here?