Knife Edges for Crank Balancing

[T. Cotten]

...So my question is, assuming I can verify how straight these actually are, what is an acceptable level of straightness?

Also, when placing a, say, 20 or 30 pound crankshaft assembly on knife edges then how much flex do you get in them anyway? I can return these to the vendor and swap them for higher quality/more expensive items but before I do I want to gauge what level of straightness I am aiming for?

John!

I had the edge of my planer blades ground flat as a pair to assure they were matched and "straight"
And yes, a flywheel assembly definitely can make rails sag. I have to place the assembly quite close to supports, even with 1 1/16" rails, and would suggest that the supports be no more than six inches apart.

....Cotten

[TechNoir]

Hi Bosch, thanks for taking the time to reply, I know that you are very busy at the moment so your words are very much appreciated.

Since posting my last post I have had a think and a chat to a friend. My lathe bed is a vee way and so whilst it is an old lathe with relatively worn ways the portion of the front way just behind the vee is relatively unworn as the saddle does not touch it. Using this as a comparator I think most of the problem is with one of the straight edges rather than a combination of both.. My friend has a lathe which has had relatively little use since its beds were professionally reground so I am going to compare the straight edges against his lathe bed also. I shall see how they all compare and if the results are consistently the same then I should be able to determine exactly where the high spots on the straight edges are. Then I will decide if I am going to return them or address the high spots myself.

I still might invest in a precision straight edge as per your suggestion, I am sure I will get some use from it more than just checking these knife edges.

Thanks very much for the calculation, Your conclusion:

What this means is if your parallels are bent or tilted by this amount you wouldn't be able to balance your flywheels better than a negligible 0.2 g because you wouldn't know if they were rolling because of weight imbalance or because of tilt of the straight edges. Assuming I haven't made a mistake in my calculation

Seems to say that 0.2g is an acceptable tolerance when balancing these old engines. The balance I have ordered is accurate to within plus or minus 0.1g so if I could reduce the error in the straight edges to, say, 0.001"" then I think we are good to go.

You were also kind enough to point me in the direction of calculating the sag in the "beams"

This also can be calculated. Or, we can again cheat by using an on-line beam calculator. However, I'll leave that as an exercise for the reader because you'll have to enter the dimensions of your straight edge and look up Young's modulus for steel to get a result. Once such calculator is at:

http://www.engineeringcalculator.net...alculator.html

This I had thought about a bit more (maybe because I work in Civil Engineering and there is someone talking about this sort of stuff everyday in the office even if I am not a Civil Engineer myself). I already had plans to beef up the straight edges by sandwiching them between some larger section steel bars. However your first calculation, giving an indication of the effect of slope on the balancing operation, will enable me to determine just how much sag is acceptable and how beefy the sandwich needs to be or maybe just shorten the beam a bit.

I will post an update once I have advanced things a bit further although it might be a week or two due to some seasonal distractions.

Thanks very much once again your thoughts and comments are very much appreciated.

John

[BoschZEV]

I don't have a surface plate (it is something that I want/need to get but my small workshop is already at greater than 100% capacity so I have been holding off on getting one)

The problem with surface plates is they weigh so much that you can't just put them out of the way when they're not being used. Since Nature abhors vacuums and uncluttered spaces they will fill up with items "temporarily" placed on them. But, I digress. With a surface plate determining how straight your straight edges are is, ahem, straightforward. Without it the least expensive alternative I can think of is a precision straight edge, and those aren't all that cheap.

what is an acceptable level of straightness?

That can be calculated. To arrive at a good estimate, assume you have a straight edge that tilt downward by 0.002" over a distance of 1 ft. This makes it an "inclined plane," for which equations aren't needed because we can cheat using an on-line calculator such as the one here:

https://www.engineeringtoolbox.com/i...es-d_1305.html

To see where this leads, assuming there is no friction, and plugging 10 kg (22 lbs) into the calculator along with an angle of arctan(0.002"/12") = 0.01 deg., a force of 0.0017 Newtons is required to pull that weight up the plane. The same 0.0017 N is trying to get it to slide/roll down that plane. Plugging in the acceleration of gravity, this is 0.17 grams. That is, an imbalance of only 0.17 g will cause your flywheel to roll down that slope. But, put another way, if you have 0.18 g on the other side of the flywheel, it will roll up the slope. What this means is if your parallels are bent or tilted by this amount you wouldn't be able to balance your flywheels better than a negligible 0.2 g because you wouldn't know if they were rolling because of weight imbalance or because of tilt of the straight edges. Assuming I haven't made a mistake in my calculation...

Also, when placing a, say, 20 or 30 pound crankshaft assembly on knife edges then how much flex do you get in them anyway?

This also can be calculated. Or, we can again cheat by using an on-line beam calculator. However, I'll leave that as an exercise for the reader because you'll have to enter the dimensions of your straight edge and look up Young's modulus for steel to get a result. Once such calculator is at:

http://www.engineeringcalculator.net...alculator.html

[TechNoir]

Hello Folks,

A pair of "straight" edges turned up yesterday but I need some advice. They are 750mm, say 2' 6", long so I would need to shorten them to my intended 2' long knife edges but before I use them I want to make sure that they are straight. I don't have a surface plate (it is something that I want/need to get but my small workshop is already at greater than 100% capacity so I have been holding off on getting one) so the obvious thing to do was to compare them against each other. Resting the two beveled edges together (see picture) there is a gap in the middle. Using feeler gauges as a guide I can get a 0.002" gauge between them for about 6" to 8" but not a 0.003". The gap then tapers out over the next 6" or so at both sides.

Now I am aware that this is not definitive, they could be much worse than this if one is convex and the other concave but I don't have a known straight edge to hand to compare at the moment.

So my question is, assuming I can verify how straight these actually are, what is an acceptable level of straightness?

Also, when placing a, say, 20 or 30 pound crankshaft assembly on knife edges then how much flex do you get in them anyway? I can return these to the vendor and swap them for higher quality/more expensive items but before I do I want to gauge what level of straightness I am aiming for?

Thanks






John

[T. Cotten]

I did not disagree with the 'stationary' statement, Folks...

My 'hold and release' technique achieves the same thing.

What bugs me is the drawing. Showing it level would make too much sense.

And the countermass must have bubbles.

....Cotten
PS: Tried to watch the U-tube, but it just sits and spins for me.
Is that George Yarocki?

[TechNoir]

Food for thought.

Jake, thanks for asking that question, I had pondered the same thing. The youtube video that I mentioned at the start of the thread is here

https://www.youtube.com/watch?v=HuyYAEIXvqI

And shows the same thing at about 3 mins and 40 seconds.

John

[Shaky Jake]

The Q&A illustration of the flywheels on 'rails' still intrigues me, Folks!

Note that the center of the crankpin is not level with the center of the mainshaft, suggesting the countermass is still heavy, yet the holes are drilled symmetrically level to the mainshaft.

....

....Cotten

Cotton, I've been thinking on this, and I'm not sure if I agree with your conclusion. Or perhaps the scotch whiskey is clouding my mind, and I don't understand what you're saying. Note that the document itself says "when properly balanced, the wheels will remain stationary in any position on the parallels."

Here is a link to a video that illustrates that:

https://youtu.be/iNY-wvHfZ9g



Food for thought.




Kevin


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[Shaky Jake]

I shouldn't trust my memory, Kevin,

Wish I could find my copy, but I think the later Military Chief manual uses just one piston, one pin, and one set of rings hanging on the rods, which back-calculates around 64%.
Can you back-calculate the percentage produced by the earlier method? It would seem quite a bit higher!

....Cotten

I think you're right Tom. Your memory is better than mine. If I recall correctly, the PowerPlus back calculated to 58% when done that way.

I feel like it is a waste of time to put too much effort into being accurate when balancing these old engines. They're rugged and slow. The old way is as good as any way.





Kevin


.

[T. Cotten]

The Q&A illustration of the flywheels on 'rails' still intrigues me, Folks!

Note that the center of the crankpin is not level with the center of the mainshaft, suggesting the countermass is still heavy, yet the holes are drilled symmetrically level to the mainshaft.

(When checking an assembly on edges, I gently hold the wheels to where the crank and mains are level, and then observe which way the crank moves when released.)

The page 177 illustration makes me wonder if wood might deaden the action of the wheels on the edges.
Ideally, a balanced assembly could rock.

At any rate, methods that eliminated a scale for weighing hardware were needed for "field repairs".
The enigma of the Indian Military manual is that it went into balancing at all!

....Cotten

[TechNoir]

I shouldn't trust my memory, Kevin,
....Cotten

When I first dipped my toe into the subject of balancng I posted this thread

http://www.antiquemotorcycle.org/bbo...in!&highlight=

Tommo was kind enough to reply and post some extracts from the Q&A book which are there. I am not sure if this is the text that you are referring to but it does talk about balancing using various combinations of pistons, pins and rings.

John

[T. Cotten]

...The traditional way to balance early Indian twins is to do it with both rods, one piston, both pins, and all of the rings suspended from the crankpin....

I shouldn't trust my memory, Kevin,

Wish I could find my copy, but I think the later Military Chief manual uses just one piston, one pin, and one set of rings hanging on the rods, which back-calculates around 64%.
Can you back-calculate the percentage produced by the earlier method? It would seem quite a bit higher!

....Cotten

[Shaky Jake]

You'd need 1/2 the stroke plus the rod center to center length plus the compression height of the piston plus a little more for good measure.




Kevin


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[T. Cotten]

Hi Cotten, thanks for posting the picture of your rig.

You have confirmed what I had intended to do which was make my rig high enough to hang a rod and piston under it. Tommo had mentioned something similar previously so thats what I am going with.

From the axle to the end of the rod on my 20F is about 12 inches and I would obviously need a bit more to allow for a piston. I have a 1950's Matchless single that will need attention in the foreseeable future but that crank/rod assembly is a couple of inches less than the F. I am guessing (without checking the engine specs) that a JD would be a bit more than the J Harleys? I would also assume that a Big X or an Indian would be similar dimensions to a J or JD and that all of those would be more than most British singles? So, if you don't mind me asking, what height would you go for from the top of your base to the top of the knives if you were to do it again?

John

Good question John!

Taking a tape measure to a Chief crank for a model, it looks like the single-piston/rods assembly that Kevin suggests, at ninety degrees balanced position would need at least ten inches for comfort. (A full travel beneath the edges isn't necessary, just workable clearance at 'level'.)

I err'd on the side of caution, and built mine far taller than I can now explain the reasoning, at 15.5".
But that's better than too short.

....Cotten
PS: I think bearings could work fine, Kevin!
Or even just between centers if your lathe is big enough.

[Shaky Jake]

Folks,

Without getting into the quagmire of factor and rpm yet,
I found that it is most practical to determine an existing factor of an assembly when the 'edges' are suspended high enough to accept installed rods, and perhaps a piston.
(It saves a lot of math!)



.....Cotten

Yes. Exactly. The same is true if you are using bearings rather than edges, of course.

The traditional way to balance early Indian twins is to do it with both rods, one piston, both pins, and all of the rings suspended from the crankpin.




Kevin


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[BoschZEV]

[continued from previous post because it was 35 words too long]

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Sidebar: The difference in weight of the bronze in an original 1" ID bushing and the current 13/16" reducing bushing in the small end:

Density of steel = 7.75-8.05 gram/cm³
Density of bronze = 8.7 grams/cm³
Width of connecting rod = 0.87"
Width of current reducing bush = 1.065" (tapered)
OD of bush = 1.1875"
ID of original bush = 1.000"
ID of current bush = 0.8125"
From this, the excess weight of the small end over that with the stock bush = 7.4 g
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