The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => Lucas, Ignition, Charging, Electrical => Topic started by: RDfella on 16.03. 2021 20:22
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Couple (two for the price of one) of queries for the electrical wizards amongst us.
First, distributor advance springs. Am using a Lucas DKX2 distributor on the project (should I call it a special now it's running and registered?). The advance has two springs, both .730" measured inside the loops with 9 coils and wire thickness of .029". Measuring the advance curve using a strobe and the machine's rev counter, I got half advance around 1700rpm and full a shade under 3,000. I'd like to get the advance in sooner, especially the first half. So does anyone remember the 60's car distributors which had a light and a strong spring - where the lighter was in tension at rest but the stronger one slack, to pick up its load later? More to the point, where I might get some?
Secondly, am slowly progressing with the starter design, mainly held up waiting for parts like bearings etc. But my main concern is that as I'm using a ratchet instead of a spragg clutch, it may engage with a bang that could be destructive. Now some diesel engines have a clever design of starter whereby only two field coils are energised until the bendix is fully engaged, whereupon the other two are connected and the full 1,000A does its job. No room on the diminutive motorcycle starters for that, I'm afraid, but would like nevertheless to create a 'soft start'. I'm thinking of maybe having a separate button that supplies, say, 3 or 4 volts to the starter to engage with the engine, after which I'd hit the usual start button. I'm using a Firestorm starter (known to start a short-stroke 1,000cc high-compression bike) together with a 200A CCA 12V battery. Could I achieve what I want with a resistance and, if so, what Ohms would be suitable?
Thanks in advance for any assistance,
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Hi RD,
I can only recommend you contact the distributor doctor to ask about the springs,, http://www.distributordoctor.com/
As to your starter issues, here is a random thought.. I do not know if the starter has a pre engaged or inertia gear??
Now pre engaged solenoids usually have two coils built in, one is in series with the starter winding so it is "bypassed" once the starter is fully engaged , the second coil holds the engagement in place
in other words the starter does not spin until the gears are engaged
John
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Ref the starter question. Digital controllers are the modern method of controlling a motor.
This explains the method better than I can:
https://en.m.wikipedia.org/wiki/Electronic_speed_control
I've been playing with Arduino processors over the last few months. They are very small so easy to house on a bike. On your machine I'm wondering about using one of these devices to send a signal to a digital motor controller. The effect should be an immediate engagement of the drive followed by a steady rise in speed.
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Tell me you once had a Spectrum GB *smile*
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Tell me you once had a Spectrum GB *smile*
Amiga
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This sketch shows how it works. The (full) current is applied in pulses that can be lengthened or shortened
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Thanks for the input, folks.
Re the starter - I'm familiar with reduced-power on starters before engagement, as on diesel motors where usually only two field coils are energised with the remaining two making contact once the pinion is fully home. Unfortunately there is no room to modify a motorcycle starter motor thus. BTW, the starter motor will be fully engaged at all times, the only engine engagement / disengagement being the ratchet mechanism on the crank, the life of which which I'm hoping to prolong by a soft start.
The use of a speed controller is interesting - have them on my model boats - but an overkill regarding what I'm looking for. I just need a second or two of reduced power to soften the ratchet engagement. As I don't have a bendix engagement, it won't be feasible to incorporate that in the motor, even if that were physically possible. I was presuming a separate circuit (eg incorporating a resistance) to supply reduced volatage which could be manually activated before hitting the start button proper.
Re the distributor, I contacted the distributor doctor, but his solution was to send him the distributor and he'd rebuild it to whatever advance curve I wanted. Sounded a bit long-winded and potentially not cheap. All I want is some springs. Failing that, I'll make some.
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Hi RD
How is the ratchet gear engaged / disengaged?
Could it be done with a solenoid that could be powered then after short time delay the starter motor is powered
Any resistance would need to be rated in the kilowatt range if connected in series with the starter
If the starter is 4 brush, could it be split into two and two which would reduce the initial "kick" ??
John
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Arduino controlled PWM driving FETs or IJBT’s is very modern but unnecessary. I once made a 12v motor soft start by adding an in-series relay with a time delay switching past a low value resistor, so it started for a second at half volts then clicked to full volts. Is that what you mean or have I got the wrong end of this?
If it is… mine was more trivial as 15 amps, but the maths is the same: The resistance is just ohms law V=I*R so 6v (drop) at 200A = 0.03 Ohm. The resistor’s power rating = IxIxR = 200x200x0.03 = 1200W. If the starter motor is 1200 watts then its 0.06 Ohms and 600W. Still sounds brutal but I doubt the current would be that high at 6V or the resistance that low - use some nicrome heater element wire (off an old bar heater) and experiment. This wire is often 4 Ohms per metre, or thereabouts. For the ‘switch’ use another starter relay (in series with the first) and ‘time’ it with a resistor (100 Ohm?) in-line with a capacitor (220 microfarad?) that is across this relay. These values are wild guesses as I don’t know the current draw of the relay. If the relay current is super high then drive that with a smaller third relay. Does that make sense? If so – start with higher resistance values and lower it to see what works.
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damn - posted itself before I'd finished. Ignore, see post below
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John - the ratchet is spring-loaded. It will engage when the starter turns but disengage when the motor starts and the pawls are free (centrifugally overcoming the springs) at 800rpm. What bothers me is the fact that, depending where the motor comes to rest, the ratchet could have 30* of rotation before the pawls engage the notches. As such, it could engage with a hefty kick, possibly damaging the ratchet. What I'd like to do is gently motor the starter over to get pawls into contact first before hitting the start button. Starter is 4 brush and, as you say, feeding two coils initially is a well-oroven way of achieving a soft(er) start. However, space is damned tight in there and I doubt I could physically achieve that. Was thinking more along the lines of a separate button to achieve initial engagement, possibly through a series resistor, which would then be released ready for the starter button to be used.
Damned nuisance I couldn't find a suitable one-way sprag bearing. Would have made life so much easier!
Mikeb - looks like you have the solution, very helpful. With enough time I have been known to work through electronics (eg radar / autopilot problem tracing) but it makes my brain hurt. Not my forte.
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Hi RD
For testing purposes you could use a six volt battery that would give 25% of the power of a 12v battery
sequential solenoids with the 6volt engaging for enough time to engage the ratchet then the 12v taking over????
John
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John idea sounds good to me. Momentarily a small voltage just to get the ratchet to engage then apply full power.
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I have some nichrome wire. It's quite fine guage. 32 and 28. I'd be happy to send some off to RD but it may be too fine. MikeB, do you have any idea how thick the wire should be?
Mike, that picture is not in focus, do you think you could try again?
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That diagram doesn't look right to me. Surely the low impedance of the second starter coil will prevent the capacitor from charging via 100 ohm. What about a two position switch, first position energises the main relay, and the second position energises the the resistor cutout. Much like a heat position on a diesel.
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If I'm seeing this sketch correctly the nichrome wire/custom resistor is 0.1 Ohms.
I've been testing the nichrome wire that I have.
Using my 34 guage wire about 1mm equates to 0.1 Ohms
Using my 28 guage wire about 5mm equates to 0.1 Ohms
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turns out a few problems with my idea. as BSAreg notes the r-c won't cut it on a starter relay (whereas it will on a small low power relay). i tested a starter relay (off a GS550) that was in the shed and it draws 3 amps on the primary. so that needs better control than a simple RC. GB i've rescanned an updated pic see below - now uses an extra small relay and a transistor. this bit could be made to be tiny.
i made the circuit and tested it on the bench with an old XS750 starter and a 100A battery. the only nichrome i could find was very thin so I used 4 strands in parallel, about 4cm in length which made about 0.15 Ohm. see the video here:
https://youtu.be/Pqnqqve0vco (https://youtu.be/Pqnqqve0vco)
if you listen you can hear the motor starts slowly then kicks in. next problem the nichrome glows a hot orange even in that half second. probably don't want that on your bike RD unless its got sprinklers.
So how to electrically dissipate ?600W is a significant problem.
i have some thicker nichrome somewhere but can't yet find it.
RD would it be feasible to rewire the internals of your starter motor to separate the pairs of brushes and power initially only one pair then both? I think that's a better option. Then delay the second pair of brushes using either a timer/relay as above or maybe simpler BSAreg's idea of a 2-stage switch - either of which drives a second starter relay.
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EBay has cheap 20A DC speed controllers. They use PWM. They're usually about 100 x 50mm and fully adjustable.
I motorised the travel on my mill table using one. Works like a charm.
You could perhaps use one of these to provide the low voltage feed to the starter.
Another alternative to the resistor is a string of diodes in series. Diodes create a voltage drop of about .9volt. I'm not sure how much heat they put out or how they handle current.
cheers
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It seems RD's concern is the force imparted when the ratchet engages. A PWM system could give a slow start but will give full power straight away.
RD, have you considered redesigning the engagement method?
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Firstly, with regards to Neil's suggestion I redesign the engagement method - no chance! I spent many hours designing an engagement method because a sprag clutch - the obvious and easy method - was not possible unless I was prepared to overspeed that by 300%. I reckoned that was too risky and so came up with the present ratchet mechanism. The pawls are spring steel but the rest is mild steel. Of course, it may not work. The gearbox drive mechanism may not work either but if that comes to pass I'm certainly not going to spend any more time designing & machining. I'll just unbolt it and sling it in the bin. For the last three weeks I've done nothing but measure, calculate & machine parts for the starter in my workshop. Remember, too, that this includes a complete re-design of the primary drive outer cover. That in itself has taken a couple of days and is not even halfway there.
There is an old saying: 'everything in my favour is against me' and I'm beginning to live that.
I've posted a pic of the ratchet for Neil to ponder, but the pic below illustrates my frustration. At point A there's a welded-on spacer behind the gearbox to enable the output shaft bearing to recess deeper and keep the bevel gear B as close as possible to the rear. If only. I discovered that this spacer was in the way of the support bolt behind the gearbox, the latter resisting the leverage the chain drive to the crank has on the gearbox. And so the spacer had to go, meaning bevel B is now further away from the rear. That in turn means input shaft C is further forward and consequently there is now no room for the motor driven gear D.
At that point, and because the new rear wheel bearing had arrived at last, I turned my attention to fitting the rear hub complete with newly-attached 49t sprocket. First of all, I cut a bearing seal from two thicknesses of 1/8 gasket cork, given the felt seal was missing and is no longer available. Then I forced the wheel aside (can't get the wheel out without tilting the bike on its side and it's currently strapped to my lift) to fit the sprocket hub. Oh dear. No adjustment because the bigger sprocket fouls the rear chainguard bracket. So next job is to vandalise that bracket. Whilst I figure out how to re-arrange the starter / input shaft connundrum. I'll likely have to order different gears, meaning another three week delay waiting for delivery. Could cut them myself, but frankly - and given my luck recently - I can't be arsed to start making involute cutters and cutting gears.
Maybe I should fit a sidecar and carry a generator and roller starter with me for whenever the bike needs a re-start? Or maybe not. Drove a combination once - that was enough.
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If I'm reading the ratchet design correctly the pawls will be on the engine side and so when the engine is running the pawls will swing away from the stepped ring by virtue of their counterweight 'tail'. Is that correct?
What about cutting a lot more slots for the pawls to engage with; surely that would at least give less time for the motor to have reached full torque before the pawls bite? In fact, how about a saw-tooth all around round the inner edge of the ring and give the pawls matching serrations. The current large cut-outs are a possible weakness of the ring. A saw-tooth design could be shallower.
Getting an accurate number of notches around the ring could be a worry. Is that something a dividing head could be used for?
If one were hand cutting the notches and the final ones did not quit meet accurately with the start it shouldn't really matter if they were cut back so the pawl missed that area; after all, there are three pawls so one bridging a small gap should not matter.
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"If I'm reading the ratchet design correctly the pawls will be on the engine side and so when the engine is running the pawls will swing away from the stepped ring by virtue of their counterweight 'tail'. Is that correct?" Spot on, Neil.
You're right also in that more notches would cut down the 'lash' before engagement, but even doubling the number would still leave appreciable slack depending on where in relation to the engagement the motor stopped. I had originally toyed with the idea of using an internal gear with alternate teeth removed (bit like the saw-tooth you mentioned) but, like everything else in engineering, a compromise must be made. In this case, mindful of the fact it takes 3bhp to turn this motor over to start it, I opted for strength. Hence the relatively deep notches. More notches would mean less depth of engagement and less strength. I look at it as a reasonable compromise. Also, one can't afford to have a pawl not engaging, as that would cause a possibly destructive side-loading whilst potentially overloading the remaining pawls / pivots. This isn't a Briggs & Stratton lawnmower! A sprag bearing would be the ideal answer, but sadly the size I would need have an over-running speed limit of under 3,000rpm. This engine revs happily to 7,000 and should be safe to 9,000.
But who knows? The ratchet may fail, the gearbox might destruct, the reduction I've chosen could be way out or it simply might not turn the engine over sufficiently to start. As things are I'm now facing a 3 week delay whilst another driven spur gear arrives as a result of having to move the output shaft bevel gear. Last week I decided to machine the output shaft chain sprocket I'd received a couple of weeks ago (I'm mounting that by threading it to the shaft rather than a key and grubscrew). Put in in the lathe to open out the pilot bore and promptly blunted the drill. The sprocket was hard. What the hell is the use of a pilot-bored gear / sprocket if it can't be machined? Tried annealing it without success, so I'm now waithing on another one of those.
When I first sketched out this starter I never foresaw this much hassle. But then after wasting months trying to sort Mikunis before finally giving up and changing to Amals, maybe I should have known......
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Not missing completely, just bridging a small gap. If the job could be done accurately there would not be a gap. I've never used a dividing head but I think I know what they do.
The snotche's don't have to be small. I was just thinking about reducing the distance for the pawl to travel before it engages.
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Hi RD,
A worry I have about your design is that when the engine is stopping the panels will re engage and try to drive the starter??
John