When a Driven gear becomes the Driver 1

[Crank Man]

Greetings, Everyone! Thanks in advance for any responses!
First time posting here, so here goes:

In a gear drive, there's usually a gear that is loading the mating gear so that the faces of the gears are engaged. However, let's consider a basic example to get my point across. The cam drive on a typical single cylinder lawnmower-type engine causes the crank gear to drive the cam gear. Only one side of each gear tooth is loaded, especially as a cam lobe meets resistance as it begins to compress a valve spring. As the cam lobe reaches the highest point of lift and goes over the nose of the lobe, the compressed valve spring is forcing down on the trailing flank of the lobe, and in turn is causing the cam gear to load the crank gear. In doing so, the spring force has caused the cam gear to now drive the crank gear. The gear backlash has now been taken up and the gear teeth are now loaded on the back side of each gear. There is a brief instance where the driving face of the crank gear loses contact with the mating cam gear face and the cam gear rotates until it is now driving the cam gear.

I'm designing a gear drive that reverses that power flow every rotation of a crankshaft, and a design engineer friend of mine is saying that that slight "click-click" of the gear backlash will, over a very short time, destroy the gear teeth because gears like to be driven in one direction only. The gears are heavily loaded, but I argue that there isn't an immediate reverse loading as the there is actually a transition where the shock loading isn't immediate, but transitional. Is the answer to have some sort of anti-backlash gearing that loads both sides of a gear set?

As I said, I appreciate any insight I can get! I have a patent on a crankshaft design, and I want to build the next prototype without fear of having the gears crater. I'm happy to share the design with anyone interested.

Crank Man

 

[Compositepro]

See thread406-437206

 

[BrianPetersen]

Bear in mind:

1. The lawnmower engine that you use as an example probably doesn't have a very long design life, and noise reduction probably isn't a significant factor.
2. Other engines that use gear cam drives and which have 6 cylinders or more and with a single camshaft (exhaust and intake lobes are on the same shaft) probably have little in the way of torque reversals on the camshaft. (example, Cummins diesel engine, I believe the Duramax V8 also uses gear driven camshaft)
3. If the backlash is kept to a minimum, the impact component of the backlash will be minimized.
4. Oil film between gear teeth acts as a cushion.
5. The remaining backlash and impact loading can be factored into the design life of the gear set.

 

[GregLocock]

One way to control backlash is a scissor gear, basically two halfwidth gear wheels with a spring between them. Honda OHCs (might be a tautology there) often use them.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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

 

[BigClive]

I think some twin-cam Toyotas use the spring-loaded gears between the cams as well. I would be interested to know what the invention is.

 

[RodRico]

You may find this article enlightening...

"The load reversal factor is discussed in Clause 16.2 of ANSI/AGMA 2101-D04. The standard states that for fully reversing loading conditions on a gear, where the critical point on the fillet sees both a full tensile and nearly equivalent full compressive load in the same rotation cycle, the allowable bending stress number is adjusted to only 70 percent of its value. This is equivalent to a load reversal factor of 0.70 for fully reversing loading conditions."

http://gearsolutions.com/features/n...f-agma-load-reversal-and-reliability-factors/

 

[TugboatEng]

On the topic of spring loaded gears, Falk marine reversing gears have a spring loaded "detuner" on the astern pinion. It's a small gear about 2" wide on a 20 inch wide gear. It's purpose is to keep the astern pinion loaded against the bull gear in one direction so it doesn't rattle while the vessel is traveling ahead and the astern pinion is idling. In practice, the springs break and take out the gear set so many operators have removed them.

With that out there, I would be hesitant to use a similar setup on a gear that sees frequent reversal of load as it puts a lot of cycllic stress on the springs which can lead to fatigue and failure.

 

[BrianGar]

Common enough in heads with multi valves too, and also used in boxes to cut down idle chatter as mentioned.

Some use springs like a clutch disc, others use a spring like in video below,

https://www.youtube.com/watch?v=gq8q0wV1Avk

You could also design it like a VVT Pulley where oil pressure loads up each section via compartments but that might be a bit involved.

I do believe any type of scissor gear design will suck a bit of power - not much however.

Brian,

Edit - have you thought about chains? A lot of cams are coupled with chains, with a tensioner on the slack side - oil plunger, or sprung, or both.

 

[Crank Man]

Gentlemen,

Sorry for the delay in responding and thanking everyone for the replies. I've had a death in the family, so my attention has been elsewhere. I also wanted to clarify something: I used the camshaft analogy to illustrate the gear issue, but the application of this is not for a camshaft but for a crankshaft. For those interested in looking at what I'm trying to overcome, take a look at my patent, which is US patent number 7185557, titled, "Epitrochoidal Crankshaft Mechanism and Method". In the simplest terms, on the upstroke of the piston (compression or exhaust strokes), the crankshaft is driving the piston upward, but there's an eccentric lobe between the crankpin and the big end of the connecting rod. The positioning of the eccentric lobe is controlled by the gears. On the downstroke (power or intake, but the power stroke has me concerned), the positioning of the lobe is still loading the gears, but in the opposite direction. Also, I assume the spring tension of the scissor gears has to be matched with the anticipated load so that there isn't any shock loading to the gear teeth. Is my logic correct?

Again, thanks for any replies!

Crank Man (Mark)

 

[TugboatEng]

What are you trying to achieve with such a mechanism?

 

[Crank Man]

I just returned from my father-in-law's funeral service, so I do apologize for the delay in responding. Also, sorry for the length of the reply!

The crankshaft design is rooted in the formula that Work = Force X Distance. The design allows the piston within any IC engine to travel approximately the entire stroke length before releasing the trapped cylinder pressure which is why the gearing is involved. Engines with conventional crankshafts (both 2- and 4-stroke engines) must vent their cylinder pressure while the piston is still traveling down the cylinder bore to scavenge and then recharge the cylinder for the next power stroke. Time is involved, so the timing of the valve or porting comes into play and we get into a ‘rob Peter to pay Paul’ scenario. Once we let the cylinder pressure out, the remaining downward piston travel doesn’t contribute to making further power since the piston no longer has cylinder pressure acting upon it. My design allows the cylinder pressure to escape only after pushing the piston much further down in the bore (Distance) where the piston then dwells for a significant portion of crankshaft rotation (approximately 30 degrees). Using stock valve timing (or port timing in 2-strokes), the piston is significantly further down in the cylinder when the exhaust valve opens, and more of the available cylinder pressure (Force) has been utilized before letting it escape the confines of the cylinder. The piston then dwells at the bottom of the stroke motionless, allowing adequate time for the exhaust gases to vacate the cylinder before the piston begins to move upward. I never tried to quantify the ‘negative’ work of expelling the exhaust gases, but the cylinder pressure will be lower than before (due to the greater expansion), and with the piston motionless, most of the pressure relief will occur without the piston having to fight to climb back to the top of the cylinder. The Work performed on the piston increases the engine output without increasing the displacement or rotational speed. The cylinder pressure can continue pressing on the piston crown longer, capturing more of its energy than is used in an engine fitted with a conventional crankshaft. Overall engine efficiency is realized by using all the available energy in the cylinder pressure to produce Work on the piston.

The real benefit in my eyes is performance enhancement. In the racing world, most classes are displacement limited. The real gains in this design occur during the intake phase of operation. Again, using stock cam/valve timing with my crank design, there is a tremendous improvement to the volume/mass of the air/fuel mixture that can be trapped in the cylinder. In my last prototype (a 4-stroke), this amounted to 28% more combustible mass per intake cycle and, since horsepower is directly related to the mass of fuel the engine can ingest, the math works out to 28% more power at any RPM point. My calculations are based on static volumes using stock cam specs, so no telling how much better this could get with a cam designed to complement the piston motion characteristics. In my 2-cycle prototype (based on a 25cc weed eater), the intake volume was 17% greater than the stock volume after adjusting the porting to achieve the stock time/area port relationships. I merely duplicated the factory settings, so I wouldn’t be cheating. That engine had instant throttle response and torque that twisted my arm like no conventional weed eater!

While additional air/fuel is good, another benefit becomes evident involving compression ratios. That volume of combustible material must be compressed before making power but attempting to compress the mixture in the stock head volume would quickly lead the engine into detonation. Remember, there are 2 types of compression ratios: One is based on the swept volume of the piston and the combustion space, and the other is based on the actual trapped volume once all the valves and ports are sealed and then compressed into the combustion space. So, one is theoretical, and the other is actual. Since I can now trap more mixture, I lower the static compression ratio by making the head volume larger. In doing so, I retain the actual trapped compression ratio (it’s all about the ratio of compression, not the volumes), so the initial pressure within the fuel charge is unchanged from stock, and it keeps the detonation in check. Now, as the piston begins the power stroke, the rate at which the cylinder pressure changes/falls due to the changing cylinder volume is less than in a stock cylinder head. This results in retaining a higher cylinder pressure for any given piston position compared to the stock cylinder pressure. The initial pressure in both cases is the same, but the rate of cylinder pressure loss is not as bad as it is in a stock engine. Imagine an engine with a very high compression ratio (measured full stroke). The high numerical ratio means that the cylinder head has a small volume. As the piston recedes from TDC, the rate of volume change occurs quickly, so the pressure must fall to compensate for the increase in volume. The force acting on the piston crown is falling rapidly. In the larger combustion space required to compress my design’s additional air/fuel mixture, the same piston motion will result in less of a pressure drop, so pressure remains higher, longer. Since the design incorporates the gearing which was the reason for the initial post, there is a longer moment arm during the power stroke. Combining the longer arm with greater pressure to act upon it resulted in the 28% improvement without increasing the displacement of the engine and without increasing the RPM. Granted, fuel consumption is increased, but we are shooting for the most power we can get in a displacement-limited racing class.

Sorry for the long-winded explanation, but thanks to anyone who’s read to this point!

Crank Man (Mark)

 

[swimfar]

That's a really cool idea! (no pun intended) Thanks for sharing the details with us.

 

[gruntguru]

"The real benefit in my eyes is performance enhancement. In the racing world, most classes are displacement limited. The real gains in this design occur during the intake phase of operation. Again, using stock cam/valve timing with my crank design, there is a tremendous improvement to the volume/mass of the air/fuel mixture that can be trapped in the cylinder. In my last prototype (a 4-stroke), this amounted to 28% more combustible mass per intake cycle and, since horsepower is directly related to the mass of fuel the engine can ingest, the math works out to 28% more power at any RPM point."

You should do some research into what is currently possible using wave tuning. VE can be >110% at certain rpms and near 100% over a wide range. Variable length intake systems and variable valve timing can extend both even further.

Conventional crank/rod engines dwell for longer near BDC than TDC and tuners do change l/r ratio to adjust this effect. I am not hearing any tuners seeking more dwell at BDC than what is currently possible.

Best not to talk in terms of "longer arms" and pressure vs leverage. Everything needed to describe the work output of the cylinder is contained in the P-V diagram.

je suis charlie