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 Are objections to this design of engine based on current realities, or outdated facts and opinions?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.

 

[tbuelna]

factsb4pride,

While you haven't given exact details of your proposed "hypocycloid" gear crank drive, if it's something like the ones described in the link below then you will have one very big issue to consider. With a 4 stroke engine, I believe you will have load reversals at the gear mesh. Since involute gears need to have some amount of backlash to operate, the point of load reversal in the cycle will create very high impact loads on the gear teeth as the backlash is taken up.

Take a look here:

http://www.google.com/url?sa=t&source=web&cd=6&ved=0CDgQFjAF&url=https://

http://www.doria.fi/bitstream/handl...sg=AFQjCNFvYoUXgTmjJK6S8E_Fo4mbUw705w&cad=rja

Hope that helps.
Terry

 

[factsb4pride]

I have not been forthcoming about all the details of the design I am working on because my employer would frown on that. Sorry if I have been too cryptic.

The goal is too reduce parasitic friction between piston and cylinder walls without adding TOO much complexity or weight or cost. SOME increase of those items is allowable, but within reason. The fact that most Stirling engine use hypocycloidal crank designs is encouraging, but the design requires modification for IC engines, which is what I am working on due to higher gas loads on the piston.

@tbuelna: The link didn't work that you posted, but I think it is the same paper I posted earlier in the thread.

The subject of gear backlash is covered in another paper. They conclude involute gears are the best to use, as long as backlash is kept very tight for the very reasons you mention.

https://oa.doria.fi/bitstream/handle/10024/36120/isbn9789522145345.pdf

Is anyone aware of a gear tooth profile that has sliding AND rolling at the pitch diameter contact point? Seems "rolling only" contact between the involute gear teeth at that point reduces the lubrication layer of the oil on the gear teeth. I am also open to alternative gear tooth profile designs, if anyone has any to suggest that might work better than involute gear teeth in this application.

 

[factsb4pride]

By the way, does anyone know what type gear tooth profile is used on mazda rotary engines crank gears?

 

[factsb4pride]

@PJGD: re: hunting tooth suggestion

Unfortunately, the "hunting tooth" concept cannot be applied to applications where an exact 2:1 ratio is required. While I understand why it seemed a good idea, my research shows it is not viable here specifically. If it were, my task would be greatly simplified!

 

[potteryshard]

I too have been thinking about geared IC crank assemblies, although I certainly can claim no professionalism or even the barest competency in the area, following are some of my conclusions:

1. If possible, enough flywheel effect should be built into the "inner" rotating masses rather than in the composite crank as a whole or even externally so that the "inner" flywheel effect carrries through to minimize torque reversal effects on the teeth.

2. Gearsets should be arranged if possible to that marinally adjustable idlers can be used to lightly preload the mesh. Thermal expansion effects will need to be examined.

3. If possible, the application should be designed so that mounting arrangements provide a bit of compliance and so that peak loads tend to drive the teeth deeper into mesh, rather than apart.

4. Application of gear lubricant exactly at the point of maximum load might possibly be used to flood the teeth at that point to buffer peak loads.

5. If helical gears are chosen to prevent cogging effects, it may be beneficial to use the helical gears in opposite-handed pairs to create composite herringbone gears sets to elminate end thrust.

6. Gears need not be of constant thickness around their full circumference; it may be desirable to allow full thickness only at maximum load and or wear points and thin the remainder of the gear to optimize rotating mass or balances.

7. Because the gear set is phase-locked, it may likewise be desirable to use different tooth profiles or pitches at different parts of the pattern; e.g. large buttress teeth at max pressure, and smaller more closely spaced teeth at torque reversal. Yes; a machining nightmare, but maybe a point-by-point optimization that would allow the use of less expensive sintered or precision cast materials.

8. Consider direct cushioning of the peak gas loads... Maybe the rod big end bearing could float slightly in an oil cushion or an elastomeric insert to spread out the peak, or a bit of compliance could be built in to the rod itself.


Obviously, there are countless other possibilities that might even better facilitate a cost-effective design; when you are trying to find 'subtle', you don't want to avoid it's cousin 'outlandish'. They tend to hang around together.

 

[tbuelna]

By the way, does anyone know what type gear tooth profile is used on mazda rotary engines crank gears?

factsb4pride,

The rotor phasing gear in a Mazda rotary has involute geometry. However, this gear's function is likely different from what you're proposing. The gearset in the Mazda rotary only serves to keep the rotor faces properly oriented as the rotor orbits within the housing. Thus the gear mesh is fairly lightly loaded without reversals. The combustion forces acting on the rotor faces are taken out thru the crankpin.

Terry

 

[factsb4pride]

Thanks for your continuing comments Terry. But I can create a geared crank system that does the same for a piston engine that is done by the geared crank system in a rotary engine. Problem is, the crank pin does not take up all the combustion pulse forces. The gear teeth, in both the mazda rotary, and in hypocycloidal piston motors, must absorb loads.

The gears in the rotary are more easily able to handle the loads presented to them due to the fact there is only a 3:1 drive gear ratio in a mazda rotary, while there is a 2:1 drive gear ratio in piston motors with a hypocycloidal crank system. So given equal volume combustion chambers, at peak combustion pressure the mazda rotary drive gear will see 33% LESS tangential gear tooth loading than a piston engine with a 2:1 drive gear ratio.

 

[factsb4pride]

OOOPS, I posted the incorrect ratio for rotary engines, when I should have posted 3:2. Sorry about that.

 

[patprimmer]

Well that kinda also throws a completely different light on the argument and your maths to support it.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[factsb4pride]

@ Pat: please be more specific. WHAT throws a completely different light? What maths?

 

[factsb4pride]

I would like to see the "maths" that show how a single gear sprocket and chain can take all the diesel combustion loads off of the geared crankshaft neander engine. I am still trying to understand how THAT is workable, but gears taking those forces isn't.

 

[patprimmer]

Ummm it is real obvious. If you can't see it there is no hope.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[tbuelna]

factsb4pride,

With regards to phasing gear tooth loads in the Mazda rotary, you can find a good explanation in Kenichi Yamamoto's book "Rotary Engine". It's in chapter 3.3.2 (page 25). Figure 3.17 gives a max ring gear load of about 480kg at 8000 rpm and WOT (I'm assuming the load is referenced at the gear pitch line).

Hope that helps.

Terry

 

[factsb4pride]

Thanks Terry, its nice to encounter engineers here who understand how to provide evidence to back up their opinions...

I will do some calculating and reading on the loads in the rotary engine drive gears and get back to you.

One thing I missed in the first paper I quoted: they suggest the face width of the gearing should be equal to the pinion pitch radius. This is different than the other paper I posted, which suggests the gearing width should be 1/3 the pinion gear radius.

 

[factsb4pride]

As far as the chain drive being able to handle more than a gear drive, I think I have a handle on that now. A chain spreads forces to many sprocket teeth tooth at a time, unlike gears, which usually apply force to one, or maybe 2 teeth at a time.

 

[BrianGar]

Ive always wondered if something like this could be made large scale, you'll notice it exerts no forces on the piston skirt and bore wall.
I have to think about it some more yet though, every time I look at it I get mesmerised...

http://www.youtube.com/watch?v=JYlBe7VkC6k

BG

 

[MikeHalloran]

That's the standard Watt gear setup.
Should be scalable, but the gears get heavy.

I've built an equivalent linkage using timing pulleys with levers affixed. With timing pulleys and a timing belt, you also don't need an internal gear. You just arrange for a small pulley to orbit a big one, and put a lever on the small pulley. Works very nicely for light duty; I wouldn't attempt it in an engine.



Mike Halloran
Pembroke Pines, FL, USA

 

[1gibson]

What about gear noise and knock sensors? (As if there aren't enough challenges already.)

 

[BrianGar]

Mike, thats a way out of the internal gear that had not come to me on first thoughts, very good.

BG

 

[factsb4pride]

@MikeH: Got any photos of your linkage design? I am sure we would all love to see it. I am having difficulty picturing what you described. I am working on rendering my current project in Solidworks, and will post it when it is finished.

My current design is a V4 configuration, utilizing zero counterweights, yet still providing a perfectly balanced engine.