Knife Edges for Crank Balancing

[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

[Peter Cooke]

Looks good John, good job.

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

A milling machine perhaps?

[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.

[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

[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