Knife Edges for Crank Balancing

[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.

[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

[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

[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.

[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