Quarter Scale Merlin V-12

[ddmckee54]

Looking good Terry. You've got oil leaking from where you want it to leak, and you've stopped the oil leaking from where you didn't want it to leak.

Out of idle curiosity, what's the Merlin's all up weight so far?

Don

 

[gbritnell]

The extra work on the carb pieces is icing on the cake. Well done!
gbritnell

 

[kvom]

It's hard to believe it's been 2 years since post#1. The finish line is in sight. :thumbup:

 

[DICKEYBIRD]

It's hard to believe it's been 2 years since post#1.

What's mind boggling to me is the truly prodigious amount of painstakingly researched & documented work of the highest caliber he's completed in those 2 years.

 

[mayhugh1]

I spent several days working on the design of a display/running stand for the Quarter Scale before I realized that I really needed the final design of the electric starter. Since it will have to be shoehorned into a crowded area at the rear of the engine, it will likely affect the design of the stand. Even though I've always considered an electric starter to be a nice-to-have option that could be added later, I've continued my background search for a suitable motor.

Back in post #423, just after I had located my first candidate dc brush motor, Naiveambition suggested that I take a look at motors actually designed for starter applications in small gasoline engines such as those used in yard equipment, etc. Following his advice, I located some small 12V permanent magnet brush motors being sold as replacement parts for some popular go-kart and scooter engines. Some of these were being offered by Chinese suppliers on eBay for just over $20 plus free shipping, but I wasn't able to find any electrical or dimensional specs on any of the ones that looked interesting. After some procrastination, I ordered a couple different units hoping that I might at least be able to appropriate some useful components from them.

I've included a photo of the two motors that I received which shows them lying along side the Nichibo that I already had. The new motors are physically a bit larger than the Nichibo, but either could be made to fit into the available space if a custom offset gear box is used to adapt them to the wheel case. These motors are some 30% heavier than the Nichibo, and while playing around with them I discovered that I needed to be careful and tightly hold onto them when they're energized.

One of the motors has a built-in 6:1 speed reducer and a measured current draw of some 10 amps at its 2300 no-load reduced rpm. The motor without a speed reducer draws 12 amps at an unloaded 16 krpm. In comparison, the Nichibo's current draw at its unloaded 16 krpm is only 2 amps. It's likely that these motors were designed for some serious torque - probably as much as five or six times more output than the Nichibo. After less than a minute of running the housings of both motors become too warm to hold while, in comparison, the Nichibo with its internal fan seems capable of continuous operation.

At first, I was enthusiastic about designing a starter around one of these new motors, but after some thought I began having misgivings about their high outputs. If blindly installed in the Quarter Scale, they could be capable of breaking parts in the engine's seemingly fragile starting system. The problem that I'm dealing with is that I need a motor with just the right amount of torque plus just a little more, but I really don't know what that right amount is. Eventually, I realized that I may have to iterate the starter's design in order to determine its real requirements, and so I decided to begin with a conservative design based on the Nichibo. An alternative approach would be to start with one of the high torque motors and to use an electronic speed controller to adjust its output. I actually don't have a technically sound reason for starting out with the Nichibo, and I may end up changing course after getting further along into the starter's development. Just in case, though, I've ordered a 40 amp speed controller.

Most of the complexity in the Quarter Scale's starter will lie in the offset gearbox required even by the Nichibo to move the motor off the axis of the wheel case starter input shaft so it can clear the engine's supercharger and coolant pump. In order to begin tying down some of the design variables, I selected a 30:1 gear reduction ratio which was determined earlier to be nearly ideal for the Nichibo and, hopefully, not unreasonable for the other two motors. I also chose 48 DP for the gears not only because I needed to achieve a rather large reduction ratio in a relatively small area, but because I already had a nearly complete set of 48 DP gear cutters.

I would have liked to have started with a 12 tooth pinion on the motor shaft in order to achieve the target reduction with the smallest diameter gears. However, the Nichibo's shaft is .157" in diameter, and it has a half-diameter milled flat over half its length. I had some concern about the strength of a minimum diameter pinion fitted to this particular shaft, and so the gear reduction was begun with a 16 tooth pinion. Unfortunately, the pinions are integral to the shafts in the other two motors, and they also appear to be metric.

The envelope of the gearbox was determined by trial-and-error using SolidWorks patterns printed out full-scale and then cut and trial-fitted to the engine. I previously designed the wheel case starter input shaft to use an Oldham coupler, and so the location and form factor of the gearbox output shaft had already been defined. After some thought as to how the gearbox might be assembled, I designed its complex top cover as a separately attached component to the wheel case, and I made it independent of the gear train. The gear train, itself, is a standalone testable assembly on a sub-plate which doubles as the bottom cover of the gearbox, and it includes the mounting details for the motor(s). The gearbox will be lightly packed with grease, and bronze sleeve bearings will be used on all the shafts.

The next step is machine all these parts and assemble them into a first-pass starter. - Terry

 

[Ken I]

Just to chip in with some observations on electric motors / starters.

Starter motors are not 100% rated and burn out if run continuously - they are generally rated to "kill" the battery.

What you can get out of a motor is limited by the torque / saturation of the iron core rotor - so go for more revs and gear down.

I've used those Nichibo motors for odd jobs (like a slotcar tyre grinder) and have run a 9V up to 24V - WTH a D.C. motor just spins faster. But to get one of those to crank your monster (I'm estimating 420cc) you'll need to rev the beans out of it and gear down (seems to be your thinking).

Those motors are frequently used on cordless screwdrivers etc. with a convenient double planetary drive reduction system which might save you on reinventing a gearbox.

Your power supply should be able to establish a current limit and therefore a driving torque limit.

I'm guessing there is already some sort of engage / disengage mechanism or roller clutch in the hand crank drive etc. so what follows is probably redundant.

You will need an override roller clutch (sprague) so that the engine cannot drive the starter motor - it will over-rev and explode. Even short "hunting" of the motor can apply enough overdrive torque to do damage (bent teeth and shafts).
If your gear down ratio is large your efficiency can fall below 50% and the gearbox is non-reversible (self locking) and the engine cannot overdrive it (might lock up your hand crank if directly linked) - it can only start to break things - the sprague becomes mandatory.

Most starters these days use planetary gear drives to gear down and a solenoid / stirrup lever to engage the pinnion / sprague (via a splined shaft).

Older inertial types used a helical spline which "threw" the pinnion into engagement - overdrive by the engine simply returned it.

One problem was that hunting of the engine during start up would throw the pinnion out of engagement and led to that annoying grind, crank, fire, wheee, grind cycle of a failed start up on a cantankerous engine.

A cure for this (a Bendix patent I think) was the addition of a spring loaded radial detent pin which locked the pinnion in the engaged position - this only disengaged centrifugally one the pinnion's rmp's were about double the starter's nominal (overrunning on the sprague it's not overruning the motor). You hear this as a distinct whine and spin down after the motor starts.

The downside was if the motor failed to start the pinnion remained in engagement and made push starting that more difficult - almost locking up the engine. You would struggle (typically locking the wheels when you "dump" the clutch) and then you would hear the "whee-clunk" as you got it to disengage - thereafter it was back to normal.

Can't wait to hear this run - awesome work.

Regards,
Ken

 

[tornitore45]

https://www.pololu.com/category/22/motors-and-gearboxes

Just a point of reference.

Mauro

 

[tornitore45]

If your gear down ratio is large your efficiency can fall below 50% and the gearbox is non-reversible

Can you explain that? I know is true because I have experienced that with very high compounded ratios, typical is a cordless drill where you can tighten the chuck without the motor turning backward. However, intuitively you would think that 50% efficiency holds both way and if it works as reduction should at least turn as multiplier, even accepting the fact that the frictional torque is multiplied.
I suppose the statement is not just a practical rule but a fact of physics, which escapes me.

 

[Ken I]

I suppose the statement is not just a practical rule but a fact of physics, which escapes me.

Yes it is a rule of physics - to do with friction but the proof is complicated.
Perhaps easier to consider a small angle wedge lifting a heavy load - it doesn't matter how heavy the load it won't dislodge the wedge until the wedge angle matches the friction angle - at that point if you do the calculations you will find the system efficiency is 50%

Regards,
Ken

 

[mayhugh1]

Ken,
Thanks very much for the information. The comment sbout an excessive gear ratio also caught my attention. There is a sprag clutch between the starter and the crankshaft, but both the electric starter and the manual drill starter shaft will be on the same side of it. This means that when the drill starter is being used, the gear reducer will need to turn. Do you think my 30:1 reducer will be an issue? -Terry

 

[mayhugh1]

I'm afraid I was able to answer my own question. It turns out that a rule of thumb used by designers is to allow 10% efficiency loss per gear contact, and I have four gear contacts in my design. I cobbled up a four-contact gear motor with parts from a junked water softener timer. It had a reduction ratio of only 15, and I could barely spin it from its output shaft.
This isn't a problem that I had considered but it's serious enough to scrap my original design. I spent yesterday preparing the blanks with their pressed-in bronze bearings so I could begin machining the parts today, and so the timing of all this is pretty good.
Adding a second sprag clutch is an option, but now using one of the other motors with an electronic speed controller is beginning to look attractive. I have some more thinking to do. Again, Ken, thanks for your helpful comments. - Terry

 

[tornitore45]

Thank you Ken, the inclined plane explanation makes sense.

 

[editor123]

You can limit the torque by limiting the current to the motor (or volts if you prefer).

 

[Ken I]

Terry,
I thought that might be the case and I was concerned you might have "missed a trick" (been there - done that).

Straight cut spur gears typically give 94-98% efficiency but that's for ground gears mounted on ball / roller bearings - slightly less for helical because of the end thrust.

That 50% efficiency limit is something I have to live with every day in my line of business - robotics - we use high ratios with high inertial loads - but if you fall below 50% the gearbox self locks under deceleration and you cannot control it with the servo drives - the gearbox self destructs in short order. This most commonly occurs because the customer used the wrong grease in a 200:1 cyclo drive - dropping its 70% down to 40% - destruction follows in as little as 3 days.

Home made gears on plain bearings might just go to 90% and it compounds.

0.9^4 = 65%
0.85^4 = 52%

You are probably sailing too close to the wind with 4 gears.

The additional sprague is probably the best option to unload the the hand crank shaft (external starter) otherwise its likely to impose more load on the hand crank than the engine itself.

Glad to have been of some help - or as Thomas Edison said - "another useful piece of negative information".

Regards,
Ken

 

[mayhugh1]

While re-thinking the starter design, I realized that I had forgotten about the pair of gears inside the wheel case that interconnect the manual and electrical starter shafts. This beveled gear set is a 2:1 reducer. That is, when the manual shaft is driven, the electrical shaft follows at twice the rpm. These gears divide by two the load seen by the electrical starter compared to that seen by the manual shaft, but from the manual shaft's perspective they are yet another gear set that will contribute to its lock-up. It now seems that any gear reducer used in conjunction with the electrical starter should be isolated with a one-way clutch.

Before further complicating my already complex Nichibo design with the addition of a such a clutch, I decided to take a serious look at the high torque starter motors I've been recently collecting. I have no electrical data on any of them, and I really hate blindly lashing electrical stuff together. But, I realły don't know the starting torque requirements of the Quarter Scale well enough to be sure the Nichibo is actually capable of starting the engine even while running at its ideal operating point. It loks like some experimenting is going to be required after all.

I just received the third and last eBay motor that I had ordered earlier. This one is a component in a starter for a John Deere JS-30 (6.75 hp 90 cc) lawn mower, and it was another suggestion from Naiveambition. This motor looks similar to the Nichibo, and its measured unloaded current draw and rpm are also similar; and so I'm not sure it fits my current definition of 'high torque.' The starter includes a 6:1 gear reducer. The motor has a 12 tooth 30 DP pinion gear pressed onto its shaft, but for some reason its 72 tooth driven gear is a non-matching 32 DP.

I thought my next approach to a starter might be to simply power one of the high torque motors with an electronic PWM controller and completely discard the troublesome mechanical gear reducer. My reasoning was as follows.

A typical gasoline engine requires a cranking speed of 100-200 rpm in order to reliably start. The small engine starters based upon the permanent magnet motors that I'm currently looking at typically use a 6:1 gear reducer. This means that these motors are capable of supplying the torque needed to start their engines while spinning between 600 and 1200 rpm. Of course, this assumes there is no additional gear reduction between the starter and the engine.

The Quarter Scale should have similar rpm starting requirements. Although its 350cc displacement is considerably larger than the engines these starters were designed for, the Quarter Scale's displacement is distributed over a number of small cylinders with small compression bumps rather than one or two large ones. Inside the Quarter Scale wheel case there is a built-in 10:1 gear reduction between the electrical starter shaft and the crankshaft. Therefore, the Quarter Scale's electrical starter shaft will spin need to spin at least between 1000 and 2000 rpm while cranking the engine at its starting rpm.

Hopefully, at these rpms, one of the fit-for-purpose starter motors will be capable of supplying at least enough torque to start the Quarter Scale. The downside of using a motor with the potential of supplying even more torque than required is that the engine's starting system must be stout enough to handle the starting power including the occasional abuse of an inadvertent fault such as a temporarily blocked prop. With some modification, a PWM controller should be capable of limiting the available torque to just what is required and at the same time provide a soft start to reduce shock to the gear train.

A new housing was designed. With no gear reducer, the need for an offset went away, and the motor was centered on the axis of the starter shaft. The diameter of the new housing was selected to fill the available space and included a notch to clear the pesky coolant pump. I designed the complex upper portion of the housing to attach to the wheel case independently of the particular motor used. The bottom plate of the housing, which is also the mount for the motor, was designed around the John Deere motor with the intention of modifying it for one of the other motors should testing show it to be inadequate.

An issue arose with the motor's pressed-on pinion gear. I didn't want to pull the gear from the shaft for fear of damaging the motor should it need to be replaced later. The steel Oldham shaft had to be blindly splined so it could be driven by the pinion. I didn't need to perfectly match the splines to the gear teeth, but they had to be stout enough to handle the torque that the resulting socket would be required to handle.

The only way I could think of cutting the splines in my shop was to mill them. I laid out the tooth profile of the pinion gear and then approximated a socket around it using a circular pattern of twelve .075" diameter drilled holes arranged around a plunge-milled center hole. Standard small diameter end mills typicalły don't have the half-inch flute length needed to remove the material left among the holes. But, in my collection of eBay carbide circuit board cutters, I found one that did. A .075" diameter cutter running at 0.5 ipm took a while to do the job, but the result turned out great. When completed, the pinion was a perfect slip fit inside the socket.

It was satisfying to finally start machining the components of the housing since I'd been drawing and re-drawing them for a couple weeks now. The housing and its bottom plate were machined from aluminum, and the splined Oldham shaft was machined from 12L14. The Oldham coupler was turned from 1144 Stressproof.

A CCM9NW 40 amp PWM controller, available everywhere including Amazon:
https://www.amazon.com/dp/B00RFDFL54/?tag=skimlinks_replacement-20
was used to control the motor from a 12 volt UPS battery. I selected this particular controller since its PWM frequency is switch selectable for 240 Hz, 2.2 kHz, or 22 kHz, and without some testing I didn't know the best frequency to use. An external pot is used to control the duty cycle of the waveform, and therefore the power, applied to the motor.

The first test involved spinning the crankshaft at 150 rpm without the spark plugs installed. I began with a PWM frequency of 22 kHz. The measured voltage at the terminals of the motor was 2.1 volts indicating a PWM duty cycle of about 18%. The average current draw from the battery measured 5 amps which, under an 18% duty cycle, indicated a peak current of 28 amps.

I then gripped the prop shaft as tightly as I could with a gloved hand to add some additional load to the starter. The motor easily overcame my grip, and at 150 rpm the voltage rose to 2.8 volts and the average current to 13 amps. At the new 23% duty cycle the peak current had risen to 56 amps which was actually beyond the recommended 40 peak amp maximum of the PWM controller. Assuming a motor efficiency at this operating point of, say 40%, I estimated (.4 x 2.8 x 13 x 5252 / 746 / 150) the torque delivered to the crankshaft (after the wheel case 10:1 gear reduction) to be about 0.7 ft-lbs which is considerably less than my earlier guesstimate of 10 ft-lbs at this point needed to eventually start the engine.

I performed similar tests using the other two available PWM frequencies, but 22 kHz worked best by far. The available torque decreased with decreasing frequency, and the duty cycle had to be considerably increased to obtain the same rpm. Switching noise emanating from the motor was very audible at 2.2 kHz, and at 240 Hz the available torque was so low that I could easily stall the prop shaft with my hand.

The next step was to repeat the tests with the spark plugs installed to provide a more realistic load on the starter. I re-ran the cranking test as I installed the plugs one by one. I could detect trouble as soon as the first plug was installed. The motor baulked at turning over as soon as that cylinder hit its compression stroke. Running the PWM wide open wasn't sufficient to get the starter to turn the engine over with just two plugs installed. Fortunately, my battery powered drill, driving the manual input, could still easily turn the engine over under the same conditions.

After a bit of actual hands-on experience, I'm gaining some appreciation for the torque levels I'll need to be dealing with on this starter. My concern about breaking parts inside the wheel case has also gone up a few degrees. I now better understand why John Ramm starts his engine by hand slapping the prop. - Terry

 

[stevehuckss396]

Holy smokes everything looks good. How close are you to a test fire? It looks about 90% done so i'm guessing your right about half way. ;D

 

[mayhugh1]

Well, there's still a running stand, control panel, gages, oil tank, coolant tank, fuel tank and pump, radiator, and of course this #@$% starter. -Terry

 

[kvom]

Might be late to the party, but can you not get an idea of the needed torque from a torque wrench attached to the shaft?

 

[mayhugh1]

Might be late to the party, but can you not get an idea of the needed torque from a torque wrench attached to the shaft?

Kvom,
Yes, you can. That's how I came up with the estimate I'm currently using. The problem is that you need to spin the crank with the torque wrench at the same speed you intend to crank the engine when starting it. The cylinders' compression creates the lion's share of the load that needs to be driven, and because the cylinders leak down during cranking, the load is rpm sensitive. - Terry

 

[Ken I]

Terry,
There is just so much "wow factor" to this engine already - but how about a scale Coffman starter. Some Merlins were equipped with such. Start it with a bang !

The RC guys use similar frame size motors in higher power D.C. and PM rotor three phase - all of which crank out serious power - might be of use to you.

Regards,
Ken