Knife Edges for Crank Balancing

[TechNoir]

Hello folks, I have decided to finally get around to making some knife edges for static crank balancing. Its something that I have wanted to get around to for a while but have been putting it off because of higher priorities.

Their intended use will be for vee twins and singles. Parallel twins, triples, fours and mores will be farmed out to be dynamically balanced.

I am pondering on the best way to construct them. Obviously only one corner can be at a static height and the other three will need to be adjustable. However, for them to be of use on a wide variety of engines I was wondering on the benefits of trying to make the width between the knife edges adjustable. There must a wide variety of physical dimensions of crank shafts so it would be good to try to design the apparatus to be of use on as wide a range of cranks as possible.

So I was wondering if those people who have this type of thing in their own workshop could share their opinions and experiences. I am looking for dimensions such as width (between knives), height and length. Plus how have you designed the adjustment mechanism to get them level? And finally are yours adjustable for width or are they fixed?

Also, what did you use for the actual knives?

I have done a fair bit of googling and have read a few books but I am keen to hear the experience of the folks on here.

I did find a you tube video showing the late George Yarocki balancing an indian crank and his rig looks nice and simple but it is fixed width which I guess is a reflection of his specialising in Indian Scouts.

Thanks,

John

[BoschZEV]

I have a set of Crown balancing wheels of sensitivity 1 gram-cm I use for this, but maybe some of my experience might be helpful. I set these wheels on an Al holder I made that raises the centers above the table by ~14.5".

As you already know, they have to be adjustable for height and width. Having balancing wheels rather than knife edges makes leveling a bit less critical, but I set them on a surface plate whose surface is level so it isn't a problem anyway.

I recently balanced a 1928 Ariel crankshaft, having a 1" shaft on one side and 7/8" on the other. Instead of adjusting for the difference in height I was lucky in that inexpensive 2" OD bearings are made with both of those IDs. Slipping those on the shafts gave me identical heights for the balancing wheels.

Actually, even if tried to adjust the heights I would have had the problem that the bases of my balancing wheels are 1" wide so the centers are inboard by 1/2" and a spline on one of the shafts extends closer to the flywheel than that. Because of that spline, using the wheels directly would have been impossible so I would have had to machine an adapter for one of the shafts had those bearings not been available. No matter what, I had to machine a new spacer for the Al holder that moved the centers closer together than the other two spacers I have for it (if I were to make a new bracket I would design it differently to allow variable spacing).

For what it's worth, on that Ariel crank a difference of 8 grams hanging from the connecting rod makes a 1% difference in the balance factor. That is, if you want to achieve this, ahem, level of accuracy you will have to be able to level your knife edges well enough that you can tell whether the crank is "perfectly" balanced to within ~8 g (and, ideally, better than that). Others with direct experience with knife edges should speak up about this, but I would think that, at a minimum, a machinist's level like a Starrett 98 (good to 0.005"/ft.) would be required. That, plus gage blocks to use along with the level when the edges have to be at different heights.

[T. Cotten]

John!

I have conjured a few knife-edge contraptions, but if I were to do it all over again, I would use 3/4" or larger ground round stock instead.

Somewhere in "Essentials of Speed" or maybe it was "Uncle Frank's" there is a picture. Its too simple to put into words.

....Cotten

[TechNoir]

Hi Bosch, thanks for the detailed reply, I had wondered as to what rig you are using for the Ariel (I have a alter ego over on another site where you are describing your Ariel build) . I have seen quite a few pictures of the type of rig that you describe and if I could find the wheels only it would be a simple job to build the rest of the apparatus. I will see what I can find.

Tom, thanks for confirming about the ground bar. I think I may have seen the picture that you are referring to. (also you have reminded me that I must get a copy of Uncle Franks Q&A book)

Going with Bosch's KISS principle then ground bar on a simple base would be the easiest to build and also (I think) the most sensitive as long as they were both straight and level.

Referring back to Bosch's post, I had thought that a machinists level would be the best tool to level a knife edge. I have heard to expect it to take much longer to level the apparatus than actually use it.

If I do go with a knife edge/ground bar type of arrangement then how long would you go for? One revolution of a 1 inch shaft is obviously 3.142 inches of travel along the edges and I guess that you only really need room two or three revolutions which equates to a little under 10 inches. I was thinking 18 to 24 inches, Tom what length would you go for?

I am off work now for a couple of weeks and tomorrow is errand day. I will ponder what approach I am going to take with this tonight and tomorrow I can feel a trip to the steel stockist coming on. I will let you know how I get on.

John

[Tommo]

Here's mine.
The knife edges are levelled by eccentrics.
It mounts on my mill table overhanging the side to give con-rod clearance.
Straight edges are just over 600mm long and mounted on an angle to achieve knife edges.
Knife edges are 165mm apart and the bottom cross pieces are 150mm long
Has stops at either end to prevent the crank rolling off
The bottom cross piece is mounted so you get feet at each corner.
I thought that I might need to widen it at some time so the cross pieces are bolted in so wider ones can be made and fitted easily.
The only pain is having to make ground rings to fit those assemblies that have mainshafts of differing sizes.

[tfburke3]

I use my homemade truing stand,flipped 90 deg.and clamped over the bench edge.Seems to work pretty good.
Tom

[TechNoir]

Tommo, many thanks for posting the pictures of your setup, that gives me more food for thought. I have two HSS plane blades about 600mm long that I acquired years ago thinking I could use the HSS to make tooling from because the sharp side of them has a few chunks missing although I have not actually done anything with them yet. Maybe I can use the other side of them if they are tilted at an angle? I will put that thought into the mixing pot with the other ideas and see what comes out.

Bosch, I found this on google, I think it is is like your jig?



Tom, I remembered about the picture of the ground bar. It was actually Tommo who provided it to me when I first asked about balancing some time ago. Here is the picture, I assume its the same one that you were referring to?




I found this picture on an aussie site here:




Similar to Bosch's setup but it looks homemade. I guess the key is concentric and accurately machined disks plus sensitive bearings. It would have a smaller footprint but would take longer to make and, maybe, not as sensitive as knife edges/bars.

All good food for thought, I will sleep on it and decide how to attack this tomorrow.

Thanks to you all once again,

John

[TechNoir]

I use my homemade truing stand,flipped 90 deg.and clamped over the bench edge.Seems to work pretty good.
Tom

Thanks Tom, I have a truing stand but its not the greatest and wouldnt want to use it so whatever I use will be a "new build".

John

[T. Cotten]

Yes, John!

That drawing sums it up, although I don't remember the arbor confusion.
(And I would drill vertically through the ends of the rods, so then they could be leveled with threaded rod and nuts.)

So keeping it as simple as possible, a "truing stand" and "knife-edges" are different animals.

You can true wheels between any machine centers with enough throw.

Knife-edges are pretty much for balancing only.

(But no doubt, a reasonable balancing could be achieved on centers, if great care is taken.)

....Cotten

[BoschZEV]

Bosch, I found this on google, I think it is is like your jig?

I guess the key is concentric and accurately machined disks plus sensitive bearings.

Yes, that Crown picture you found is one just like mine, except mine only say "Crown" without the "20 in." The stand I machined has flats for them to sit on with vertical tabs through which I use bolts to secure them using the holes otherwise used for the rods in the photo.

The homemade version shown in another photo shows no evidence the wheels in it had been balanced. Each of the Crown wheels has several shallow balancing holes drilled in it. When I went to the garage to check how they compared with the other photo I gave one of the wheels a flick with my finger and it spun for 35 sec. I've never measured their runout but given everything else about them I'd bet it's pretty small. Just as you wrote, the keys are good disks and sensitive bearings.

Having recently been fiddling with my Ariel's crank makes me appreciate the properties of these wheels all the more. The fact the crank stays in one location, rather than simultaneously rolling and turning, is a definite advantage both when the imbalance is large as well as when getting down to the final few grams. Not having to spend the time to precisely level it is another definite advantage of wheels over knife edges.

I was lucky in finding my set on eBay some years ago and having to pay relatively little to get it (I don't remember how much). Sometime later I came across the retail price; again, I don't remember what it was, but I do remember it was a pretty significant amount.

[BoschZEV]

Similar to Bosch's setup but it looks homemade. ...would take longer to make and, maybe, not as sensitive as knife edges/bars.

In case you meant your "maybe" uncertainty to apply to balancing wheels in general, not just to a homemade version, my Crown wheels easily do better than 1 g on my Ariel crankshaft. Since ~8 g corresponds to 1% in the balance factor on this crankshaft (e.g. 60+/-1%) this means being able to reach at least 0.1% in precision (e.g. 60.0 +/-0.1%). Given the time it takes to hang weights on any type of balancing rig, take the flywheel off to drill holes (and the imprecision of the depth of those holes), and hang weights again to check the result (if it was checked), I suspect that even the former, 1%, is better than the factories did it.

[Shaky Jake]

You guys have some nice tools. Here's a link to a video of what I've used. Feel free to poke fun at it:

https://youtu.be/JSW2ckp0DvY


It's just a slightly modified wheel truing stand, but it did give me repeatable results. Using this set-up and an accurate scale I was able to determine that the stock balance factor was 58%. Replacing the iron pistons with aluminum ones changed it to a touch over 79%. I didn't feel like drilling my 100 year old wheels, so I left it at 79%, put it together, and rode it 4,000 miles. It didn't shake any more than expected. After 4,000 miles I tore it down. The main bearings look and measure just like they did when I put it together. Keep in mind this engine only turns at about 2000 RPM at normal cruising speed. I'm thinking about balancing it this time, just to compare. I don't expect to notice much difference though, my PowerPlus felt as smooth as the other ones.

People don't like to hear about this. I even see that someone gave me a thumbs-down on YouTube. :-) Balancing is a big effort, and people like to feel like they get big benefits for their efforts. This isn't the first engine I've balanced, but it is by far the oldest and slowest. Balancing is much more important on a high speed engine. Feel free to disagree, I respect your right to do so.



Merry Christmas,

Kevin


.

[BoschZEV]

Balancing is a big effort

+1

Balancing is much more important on a high speed engine.

I agree. Since the oscillating imbalance force goes as the square of rpm the same weight imbalance on the flywheels would result in 2.25x more shaking at my hope-for 3000 rpm as at 2000 rpm. If I have to rev it to 4000 rpm at times, such as having to drop back a gear but still maintain a reasonable speed on a road in the Rockies, that shaking increases to 4x

In my case no one had any idea what the original balance factor of my engine was, the engine had a different piston than when it was new, and I had good reason to have lost confidence in the skill of the previous rebuilder. If it shakes itself apart after I rebalance it to what I've now determined the factory spec to be I'll know it's the fault of the designer, not because I took a shortcut in the rebuild.

[TechNoir]

Kevin, Your stand and your results give me even more food for thought.

Bosch's point on engine speed is also pertinent. A modern speedway single cylinder engine must be more prone to vibration than, say, Bosch's Ariel. I believe that chassis design is also a factor.

My triumph is a parallel twin and when it was first put together I had the engine work done by someone else, including static balancing. It was un-rideable above 55 mph even thought I have letters from the original owner dated in the 1940's stating that he had had it up to over 90 mph and occasionally touched 100mph downhill. I then had it dynamically balanced and it was transformed.

Once again, thanks to everyone for their input on this.

John

PS. I forgot to mention, the Triumph sludge trap arrangement must mean that as the engine ages the balance factor constantly changes. I wonder if the original designers thought about this?

[BoschZEV]

I believe that chassis design is also a factor.

In 'Tuning for Speed' Phil Irving wrote:

"There are in fact so many considerations involved that it is impossible to quote one figure as being ideal, since it varies with every type of engine, and even for the same engine in differently-equipped frames. The only source of reliable information is the parent factory who have certainly done a lot of experimentation. The M.O.V. Velocette factor is as high as 85 per cent, while some engines have been under 50 per cent, but failing any reliable information 66 per cent is a good starting-off point."