HOME and how to join › Forum › Open Area › General Scott topics › Scott Crank Failures item in ‘Yowl’
Deleted User.
Deleted UserOk! I give up!
Despite having spent half a working lifetime making something of drunken-spider-climbing-out-of-an-inkwell ‘drawings’ scratched on the back of fag packets I cannot seem to get a grip on this one in the George Silk item “Scott Crank Failures” (bottom of page 262 in the February 2010 issue of Yowl): “In the lathe with a white paper backing we ran the cranks at 3,000 rpm. 98% of the oil went out the back 2% through the big-end feed……If you tip a trough up at one end etc, so we machined the groove .30” out of centre. The re-test showed 98% through the big-end hole, 2% out of the back.
It’s the “tipping the trough” bit that’s getting me, does that mean the oil gathering recess in the crank disc is re-machined offset by 0.03” relative to the crank axis or is it deepening it, can it only be done on a new crank disc or . . .
Can someone put me out of my misery – please!
Graham ParkerSorry I cant help – but I was also pondering what this meant.
The mod certainly seems to have a dramatic impact and would seem to be well worth doing. Tantalisingly (or may be dangerously?) he suggests “You could easily apply this feature to a Scott crank“.
I just need to know what this “Feature” is…?
Graham ParkerSorry I cant help – but I was also pondering what this meant.
The mod certainly seems to have a dramatic impact and would seem to be well worth doing. Tantalisingly (or may be dangerously?) he suggests “You could easily apply this feature to a Scott crank“.
I just need to know what this “Feature” is…?
Graham Parkeroops – sorry messed this one up and couldn’t work out how to delete the posting
Deleted UserOh! I’m so glad I’m not alone; I was beginning to think my little grey cells had finally gone phut!
Because the crank is rotating it is creating its own “gravity” so making the oil lip eccentric to the axis of rotation has the effect of “tipping the trough”, at least that is how I’ve rationalized it.
So we want the bottom of the “trough” to be coincident with the big-end oil hole. In other words the part of the now eccentric lip describing the greatest radius needs to be next to the big-end.
However I see a problem; if we were to machine an existing crank then the lip would be moved away from the big-end oil hole negating any benefit. An alternative would be to manufacture and insert an eccentric dummy lip in the existing one thus keeping the low point of the “trough” next to the oil hole.
As it happens loose bits going round fast make me kinda nervous but with a bit of clever machining the existing oil lip, being angled in, can be used to advantage as it should be possible to spring the addition in and a few screws should make it quite solid.
One thing that has occurred to me regarding the reported test is that the big-end oil holes are quite small so if the amount of oil delivered to the lip exceeds the quantity that can escape through the oil hole then the residue will spill over the lip thus appearing to be “lost”.
This is in no way intended to be a criticism of the experiment, just that further investigation is indicated, indeed it would make a nice 3rd year project for an engineering student.
What is surprising is that it hasn’t been done before, (or has it?), but that is the nature of cleaver ideas, so darned obvious once someone else has thought of it! And to my mind it is a good idea!
Deleted UserI have the greatest respect for the opinions of others, but believe there are several “difficulties” with design details that were introduced after Alfred left the company.
I understand that George meant that if he made the undercut eccentric, then any oil caught would migrate to the point of greatest radius.
This is OK if you are making cranks, as George was in EN24T and turning in these features. It is not possible to convert hardened Scott cranks.
Next, step back from the problem and take a wider view.
Ask yourself why Alfred saw no need to use undercuts and oil holes in the original two speeders. Answer, because they were not chasing big end failures to the same degree. When others stretched the engine in the 20’s and decided to make the big end wider, they started to have more problems.
My opinion is that all the big end problems come from the rod wagging about at the top. I have seen enough Scott engines in distress to go straight for the alignment of the little end as the culprit and sure enough the errors are in proportion to the damage. When you put a big end together, it should run round smoothly. There is very little resistance to motion. If this is so, then just where do you imagine the communicated effort comes from to rotate the crankpin bush on the crankpin. I have seen lots of these and again the connection with rod little end errors is hard to deny. We have a rod standing vertically on rollers. If in use, alignment errors in the little end cause the rod to want to skate over one way and reverse on the second half of the stroke, in short, to “wag”, then it will try and stand on the edge of the rollers. This does two things, it wears the rollers barrel shaped and cam locks the crankpin bush.
We must remember that the circumference of the inside of the rod big end is 5.89″ and the circum. of the crankpin bush is 3.534″ Now the poor old rollers can skid about and cope with this as long as there is clearence and an oil film. You take the clearence away by having the rod over at an angle and the roller can not cope with two different speeds at once so a considerable turning force is applied to the crankpin bush.
I know it will upset a few folks, but I honestly believe that the wrong hare has been chased for years. The engine works just fine without any crank weakening oil drillings, as long as the rod stays upright.
In truth, I build my engines with the little end of the rod shimmed in the pistons, so they can not “wag” and I hve absolutely no problems with big ends. If you do not do this, as I realise that I am a bit of an engineering purist, then at least be sure that if you clamp the inside of the rod big end to a precision angle plate, put a gudgeon pin through the little end, you should not get more than 0.0015″ difference between one side and another. After this, the degree of error just shortens the effective life of an engine, until, say an error of 0.006″ will give you the definitive 1000 mile engine if you are lucky. This is why I always recommend a prospective first time Scott buyer to be sure the object of their desire has done at least 1000 miles without expiring. Now you all might disagree, as you have every right to do, but, my friend, it works for me and in the end, that is what we need. An engine that is smooth, powerful and durable. The basic design is excellent and if we just do a few minor modifications it can be much better still, but one thing it needs before all else is the quality of accuracy. Alfreds memory deserves we do the job properly!
Deleted UserI am entirely in agreement with what Roger says: When he said “wagging” in para.2 I immediately thought “spacers” and then lo, in para.4 there it was!
The ability of the current undercut lip really should be sufficient to gather enough oil, after all most 2-strokes work quite happily on the merest whiff of the stuff.
Douglas KephartI figured the oil groove was eccentric when I read it, and thought what a rather clever idea.
With the oil groove concentric, the pressure of the oil in the bottom of the groove is equal all the way around. So it is only the depth of the groove that provides the ‘head’ above and beyond that provided by centrifugal force. Most of that centrifugal force is being wasted pressing statically against the bottom of the groove. If there were no hole to bleed it off the oil in the groove would just lay there (while the engine was running.) But make the groove eccentric, and the amount of eccentricity is added to the effective depth of the groove resulting in more oil being directed to the oilway leading to the crankpin. That is assuming the extra amount of oil can pass through the oil way. Otherwise it will just spill over the groove wall, because at any given point the height of the groove wall is no greater than before. The practical test carried out by George Silk with the lathe and paper to capture the pattern of the oil being slung off the crank suggest indeed the oilway can pass the additional oil directed to it that would otherwise be wasted. It should be clarified that more oil is not being introduced to the system (via the main bearings), just that less is being lost overboard, resulting in more getting to the oilway.
Could these modifications be made to existing cranks. Well why not? I know original Scott cranks are heat-treated and tempered, but are they hard? Harder than plain steel yes, but uncuttable? My understanding is (and correct me if I have this wrong because I never thought to test mine) that they are not case-hardened. I presume this since they have demountable races that are dead hard. Therefore the crank can be tempered for maximum toughness, not hardness. Now 300M, 9310, 4340 steels heat treated but not case hardened (indeed you should not case harden 4340) can be cut in the tempered state with carbide tooling. Not particularly long tool life, but doable. Actually fairly easily with Cubic Boron Nitrite tipped tools (even steels up to 72Rhc, I have done it.) Now I do not know if CBN tipped tools were available in the Silk era, but certainly carbide was. The cheapest way to experiment would be to grind up an carbide tipped undercutting lathe tool (or buy a solid carbide radius grooving tool for about $50), offset mount mount in a chuck, and have a go. But practical experience suggests that though the carbide may be hard enough to cut the tempered steel, you may get a lot of chatter because you have too much cutting edge engaged continuously in the cut. In a softer steel you might get away with it. In these cases I have had better luck (less chatter) with milling, and rotating the part with a rotary table. You can readily buy half round milling cutters (sort of like a Woodruff key seat cutter, but with a full radius) in solid carbide for about $80.
Another point brought up with modifying existing cranks is the oilway leading to the crankpin will need to be re-drilled. Why? Granted the opening for the oilway will be above the new ‘floor’ of the groove. But you do not need to drill a new hole, just elongate the entrance to the oilway (with a Dremel moto-tool and a small grindstone) till it is flush with the bottom of the groove. The oil will flow sideways into the aperture, and flow into the original drilling. Right?
Anyway, I though it sounded intriguing and viable. Anyone want to try it? 😀
-Doug
Deleted UserWhile my sympathies tend to lie with Roger’s views on this subject if someone did want to experiment why not machine up a “crescent moon” insert rather than remove metal from a part that seems to have marginal strength anyway?
This insert could be sprung into the existing groove, the horns of the moon placed adjacent to the oil-way leaving it ideally placed to take advantage of any additional oil pressure/flow. In order not to compromise balance it would probably be wise to make it of something light and it would also need to be positively locked in place.
On the approach of “first do no harm” at least any “interfering” with a sound crank would be kept to a minimum!
AnonymousRoger mentioned this, and I did an article in the last edition of Yowl on the physics of the oil pressure/flow and how the groove geometry affects it.
Scott cranks are case hardened, but I do not know the steel grade(s) used. Silk were EN24 so you can post machine Silk cranks, but not Scott ones.
For the comment in the previous post about a crescent moon type thing, you can simpl use a round section (wire) circlip, but from my article as long as you have enough oil heading into the groove it won’t make any difference.
Scott cranks are fragile enough, I’d never recommend anyone machining through the case hardening especially near the crankpin as you are just inviting disaster by introducing machining stresses and/or grinding/machining paths which all just promote cracking. I’d also point out that TT Rep and some other cranks have no oil groove and from my running provided good quality rings are used, with high grade rollers and correct clearances/tolerances you don’t need an oil groove at all. At least that is my comment, others may differ.
Cheers,
Rich
Deleted UserMade from 3% nickel carburised, refined and hardened. Case depth target 1/32″ Case hardness Rockwell C 60/62 This is similar to the old EN36.
It should be remembered that when they were made as modern consumer products, it was not envisaged that they would still be in use 75 or so years later. You can all have your own guess at how long the factory thought the service life would be. An interesting project would be to calculate the number of Scotts that have survived as a percentage of total production and then compare this with data from other quality manufacturers.
Their longevity certainly put a limit on selling new models, well, that and the failure to update the design to any extent until Matt and then George had a go. That reminds me of the descendant of the late unlamented Ned Ludd, who pronounced with such clarity and passion “The only Proper Scotts were made in Shiperley” Alfred, who was dedicated to continually improving the bikes and going forward, would turn in his grave!
I am sure his spirit would approve of the Scott based experimental 4 cylinder Phased Transfer experimental engine. That reminds me, Bob Collett now needs a frame and it is too big to go in a featherbed frame.
Thoughts turned to a Dragonfly frame. If anyone knows of a Dragonfly frame, or something similar, please give Bob Collett a ring on 01455 272 458 as I will be out of circulation till July 7th (Sighs of relief all round!)
