Knife Edges for Crank Balancing

[TechNoir]

Today I rechecked all of my measurements of yesterday. One thing I noticed was that the rods had a slight amount of stiction so, taking a leaf out of BoschZEV's book, I mounted it between centres in my lathe, lubricated it and spun it for a few minutes by hand in both directions.

Then I cleaned it really well and rechecked everything. First i noticed that the 90 degree imbalance was much less, less than a gram. I think there had been some oil on the flywheel previously that was having a very small influence on the balance.

I have concluded that by using the new pistons that I have had made the balance factor is altered by a bit less than 1%. I reread this entire thread and also a previous thread that I posted on the same subject and I have decided to leave things as they are.

Whilst it might seem a lot of effort to end up doing nothing, I am happy because now I know that I have decided to do nothing based on facts rather than based on hunches.

So I will now continue things over on my 20F build thread.

Many thanks to everyone who has helped me on this topic. I have another engine (a Matchless single) that will need balancing in the not too distant future and I am sure a few more in future too so I am sure I will get plenty more use from it.

John

[TechNoir]

A milling machine perhaps?

Actually the mill just needs a few relatively small parts painting but in principle you are right.

The other thing that will need paint is a lathe. Its off having a repair done at the moment and then its off to have a full bed, saddle & cross slide regrind. It would be a shame not to give it a new coat of paint before re-assembling it.

I love the fact I'm not the only person whose heirs will have to write ads to try to get rid of heavy masses of metal whose function is a complete mystery to them.

A few grams isn't much, but since you went to all that trouble to vex your heirs, and as a result you now know there's an imbalance at 90-deg., why would you not want to deal with it? That's the problem with having instruments to measure things. Had you not measured it you could have convinced yourself the crankshaft was fine as-is. And it well might be fine. But, now you know it's a full 2-3 grams not-fine so it is something of concern.

Bosch, I have a big thank you for you. From your comments in this thread and from following your Ariel build you have reminded me that I remember most of my science and the mechanics I did at school and university.

As we know Kinetic Energy = 1/2 x Mass x velocity² so I am aware that you don't need much mass to create a lot of energy if you move it fast and the energy increases with the square of the velocity. Now I am also aware that my 20F is not going to rev to 14.000 rpm like my Yamaha does but I am know that if I want/need to address this issue then nows the time.

John

[BoschZEV]

Is a 2 to 3 gram imbalance at 90 degrees to the crank pin bad or is it acceptable?

I love the fact I'm not the only person whose heirs will have to write ads to try to get rid of heavy masses of metal whose function is a complete mystery to them.

A few grams isn't much, but since you went to all that trouble to vex your heirs, and as a result you now know there's an imbalance at 90-deg., why would you not want to deal with it? That's the problem with having instruments to measure things. Had you not measured it you could have convinced yourself the crankshaft was fine as-is. And it well might be fine. But, now you know it's a full 2-3 grams not-fine so it is something of concern.

[Peter Cooke]

Looks good John, good job.

"I need to paint a few things with some machinery enamel...."

A milling machine perhaps?

[TechNoir]

Hello Folks, its been quite a while since I did any updates, I will explain why when I start updating my 20F thread in the not too distant future.

I finally managed to get around to building some knife edges. Here they are, I decided to keep it simple but go quite strong with the beams. The knife edges are epoxied into a rebate milled into some 50mm x 20mm steel bar:






The main base has leveling screws to get the edges roughly level using a builders boat level. This gets them within the range of an engineers level so they can be set up using the 3 leveling screws just under the ends of the beams. I used a reasonably fine thread for these screws, 6mm x 0.75mm which gives approx 34 tpi which I am glad about because when leveling the knife edges it was good to have a reasonable means of fine adjustment.

They are just in primer at the moment. When the weather gets above freezing I need to paint a few things with some machinery enamel so I will paint these knife edges at the same time.


However, due to some unexpected distractions, I only managed to make some measurements today. Hopefully tomorrow I wont be distracted and I can get on with getting this engine back together.

Here is my 20F crank on the edges. I have a question.

In the picture below where the big end is to the left of center it tends to want to return to the top. If I balance it so that it doesn't do that then when I move the big end just to to the right side of center it tends to want to go down to the 3 o' clock position. We are only talking 2 to 3 grams to change the bias. The crank has a slight imbalance at 90 degrees to the pin. Is a 2 to 3 gram imbalance at 90 degrees to the crank pin bad or is it acceptable?






To answer a question that Cotten asked me a long time ago, from my initial observations the crank had a balance factor of 55% with the pistons that were fitted when I got it.

However I cant say if these are the original ones. They are certainly of an original type but the cylinders had been re-sleeved some time before 1936 so its impossible to say if they are the actual original ones. They are 3.2975" in diameter at the top above the top ring. The manual states that the original size is 3.297" at the top and the skirt is 3.305" so they could be the original ones as they seem to be about the right size.

John

[TechNoir]

Seasons greetings to everyone, I hope you (for those who celebrate it) had a great Christmas. The last of my guests left earlier today and so I have less distractions although it was nice to have them, it is also nice to have some peace.

Thanks to BoschZEV for taking me on a trip down memory lane, following Bosch's post I had a look at some options for my knife edges. The trip down memory lane was because I did a year of Structural Mechanics in my first year of University in 1996.

Because I have a few different ideas I put it into Excel so I could compare the various amounts of sag for the different knife edges I have in my head. The first thing I noticed was that one of my options gave an angle of slope similar to the theoretical example that Bosch posted about.

.....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.

However my maths gave a result for the mass of the imbalance needed to move the 10kg flywheel to be a factor of 10 greater than the example. I double checked the on-line calculator that was posted and I think Bosch made a typo in his original example. I think it should be 1.7g rather than 0.17g. What do you think?

Anyway, my plans for the knife edges have settled on 300mm between supports and using the beveled straight edge with a stiffening bar, probably the thicker of the two in my table below. (note the straight edges are approx max 38mm x 5mm with the bevel starting 8mm from the edge) In order to get the maximum stiffness in the edges I will need to fix the straight edges to the bars. I plan to use something like Loctite Epoxy Adhesive rather than mechanical fixings although I may mill a small "shelf" into the support bar so that the load is also physically bearing onto the bar rather than being transferred to the bar 100% by the adhesive in shear.

I only have bathroom scales for weighing the crank assemblies but I am sure they are accurate enough for this estimation. My 20F crankshaft assembly and 2 rods weigh 12kg's and a 1953 Matchless single crankshaft and rod came in at 12.6kg's. So I based my calculations on a weight of 13kg's.

The results (assuming I have not made some stupid error) are summarised as follows:




If I were to go with round bar for the edges then to get the sag minimised I would need quite large diameter bars. I would then wonder if the width of the bar would cause issues when trying to fit the crank on them?

The final decision is a couple of weeks away at least because I have some, essential, distractions to take care of. I have a small mill that I am rebuilding, it is not far away from being functional and I want to get it to that stage before I look at these knife edges.

Also, I bought a new to me Lathe (an upgrade from my current one) which is being delivered next week. It will need some attention before I can use it because it is 3 phase so I need to wire it to retain it as 3 phase but power it through a vfd. Also, remember my workshop is at greater than 100% capacity, in parallel I need to sell my current lathe.

John

[TechNoir]

Hi Cotten, thanks for your input again. More food for thought. I now have several ideas for my edges and a few days to ponder my plans.

Thanks to everyone who have contributed to both this thread but also everyone on the AMCA forum for your help and words of advice over the past year.

Seasons greetings to everyone.

John

[T. Cotten]

My apparatus is flawed, Folks,...

But it illustrates the pitfalls of attempting to clamp to planer blades, as a firmly set set screw screws them.

(Horizontal against the blade in the attachment.)

Ideally, nothing should need to be retained by anything but gravity.
Round stock is quite secure in v-blocks, especially with the weight of the flywheel assembly upon it.
The trick is getting the v-blocks up high, rock solid.

And a steady hand placing the wheels on them!

...Cotten

[Peter Cooke]

No problem John,
I have to admit I've not done it myself. I was reading through the thread and it wasn't until you mentioned sandwiching your straight edges between flat bar that it occurred to me. I'll be needing to balance my VL engine somewhere down the track so am interested in the topic. There are many ways to skin the cat...

[TechNoir]

What about milling a slot in a length of ground round bar and clamping your straight edges into the slot. Then, if you wanted, you could rotate 180 degrees and use the ground round bar edge instead. You get extra rigidity and 2 sets of edges to choose from.

Peter, that is a really good idea. The one drawback (for me) is that I don't have a functional mill at the moment although I am hoping to remedy that in the near future. If I do then I will give it some serious consideration.

Thanks,

John

[Peter Cooke]

At the moment I am planning on sandwiching the straight edges between flat bar....

What about milling a slot in a length of ground round bar and clamping your straight edges into the slot. Then, if you wanted, you could rotate 180 degrees and use the ground round bar edge instead. You get extra rigidity and 2 sets of edges to choose from.

[TechNoir]

I would use ground round rod, and now I must add shorter length.
Cotten

I did take note of thhose comments and my planned setup would not be difficult to adapt to rods or to shortening the length

(Please note the mirror behind my edges in my previous attachment. It is valuable especially when placing the flywheel assembly squarely on the rails, as it is as if you stood back a few feet for perspective.)

Cotten

I saw the mirror and guessed that was its purpose.

An ideal fixture would be monolithic,.. cut, ground, and polished from granite, with perhaps agate 'edges'.

(*sigh*)

Cotten

Oh for an ideal world, but thats a whole other conversation!

I didn't mean to imply that. I actually think you can get by with less sensitivity than that, i.e. 1 g or even worse. More serious issues might be sag and/or the tendency of the relatively wide and thin bars to twist under the weight.

I have pondered the possible distortion of the knife edges and have some ideas for variations on the basic principles. At the moment I am planning on sandwiching the straight edges between flat bar however the outside of the sandwich could be substituted for angle or tee section to increase rigidity.



John

[BoschZEV]

Seems to say that 0.2g is an acceptable tolerance when balancing these old engines.

I didn't mean to imply that. I actually think you can get by with less sensitivity than that, i.e. 1 g or even worse. More serious issues might be sag and/or the tendency of the relatively wide and thin bars to twist under the weight.

[T. Cotten]

I learn too, John!

My apparatus taught me that planer blades were not a great idea, although I use smaller ones on a wheelhub, rim, and tire poise.
Somewhere I posted that if I were to do it all over, I would use ground round rod, and now I must add shorter length.

There's complications in holding the blades, as even setscrew tension warped them in the fixture, and can only hold them 'finger tight'.
(Please note the mirror behind my edges in my previous attachment. It is valuable especially when placing the flywheel assembly squarely on the rails, as it is as if you stood back a few feet for perspective.)

An ideal fixture would be monolithic,.. cut, ground, and polished from granite, with perhaps agate 'edges'.

(*sigh*)

Cotten

[TechNoir]

and would suggest that the supports be no more than six inches apart.
....Cotten

Hi Cotten,

Thanks for your input. I had planned supports further apart but given the weight of these old crankshaft assemblies I can see why you recommend 6". I will weigh my 20F and my Matchless and see what level of sag my proposed apparatus calculates as with these weights and adjust the length to suit.

I have 2 straight edges, each 2' 6" long. If my "good" one turns out to be good then if I were to half it I would still have two good edges each 15" long. This would seem more than adequate given your 6" recommendation.

Many thanks to you too for your help with this, I am learning a lot.

John