Balancing a crankshaft?

sosam117 Wed Nov 03, 2021 6:20 pm

For years I have wondered how does anyone balance a crankshaft?

I have a 1.3cc Aurora Mills diesel that vibrates really bad!
I would like to know how to check the balance of the piston, connecting rod and crank to stop the bad vibration?

Any help here please?

ffkiwi Thu Nov 04, 2021 3:44 am

The general rule of thumb is you balance half the reciprocating weight...anything more is virtually impossible in a single cylinder engine...unless you start resorting to exotic materials like tungsten inserts (the Cox Conquest uses these..)

In the case of diesels-many have no counterbalance at all-just a circular crankweb-yet don't seem to suffer that much from vibration. In the early days (1950's) one of the tricks employed was to use an unbalanced prop-and put the heaviest blade opposite the piston when mounting the prop to the engine. This seemed to help a bit. Diesels -if ferrous technology-invariably have heavier pistons (and often rods as well) than a comparable displacement glow....so you are up against it a bit.....in turn this leads to some fairly automatic responses by engine tuners...lightening the piston being one of the first options. You really only have two ways of doing this-depending on machine tools available-either by milling out the inside of the piston (taking care not to reduce the wall thickness where the gudgeon pin fits or alternatively-buy drilling a ring of holes vertically in the piston wall to reduce the weight somewhat [this was done in the famous Oliver Tiger-in the factory tuned version-with a ring of 1/16" diameter holes drilled into the piston wall]

The Indian Mills uses the same tooling as the original-and these use a counterbalanced crank-with cutouts on the crankweb either side of the crankpin...so not a lot of option for increasing the counterbalance aspect-you run out of material to grind off the crank. So that really leaves lightening the piston..or the unbalanced prop opposite the piston as options...

ChrisM
'ffkiwi'

sosam117 Thu Nov 04, 2021 6:00 am

ffkiwi,
I've been told I could drill holes in the counterweight on the crank and then fill the holes with lead.
I have some special lead that is denser than the lead any person could purchase.

The lead is from my dad who worked at a nuclear power plant and it is leftover lead from relining the reactor core.
I have drilled the holes in the counter weight and made a mold to hole the lead in place when I pour it into the holes in the crank but I want to know after I fill the holes with the lead how do I balance it or see how close I am to having it balanced?

Thanks

getback Thu Nov 04, 2021 6:20 am

Check this out sosam117 and think smaller

gkamysz Thu Nov 04, 2021 8:49 am

Tungsten is normally used. And in cranks with little volume to work with, will be more effective. Tiny components are tough to weigh. I've calculated for a few engines I made models of in CAD.

The last bit in the video about what is right for a given application is the hard part. It basically comes doe to an initial guess and trial and error.

sosam117 Thu Nov 04, 2021 9:00 am

gkamysz wrote:Tungsten is normally used. And in cranks with little volume to work with, will be more effective. Tiny components are tough to weigh. I've calculated for a few engines I made models of in CAD.

The last bit in the video about what is right for a given application is the hard part. It basically comes doe to an initial guess and trial and error.

Thanks Greg,
I don't have any Tungsten, nor could I heat it up to pour into the drilled out (ground out) holes.
Just have the basic tools at home (Dremel tool, ladle to melt the lead and a torch with MAP gas).

aspeed Thu Nov 04, 2021 10:03 pm

Some of the guys were adding lead to the counter balance of the Fox .35 stunt motors because they are out of wack. Just a slot milled near the OD opposite the crank pin. It faces the backplate. It is heated up and solder put in. It would not need to be too hot or the hardness of the pin would go away. Don't let it go anywhere past a straw colour. Blue or grey makes a scrap crank. I think the Bees are a bit light on the counterbalance. I have heard the .051 pistons shake more than the .049 piston would. I think lightening the piston is a better plan, but suppose whatever works is fine. I have released plane with a ferrous piston OS .25 or .35 with an 8"-6" prop that made my hand tingle for a few minutes. The ABC ones never did that.

sosam117 Fri Nov 05, 2021 6:47 am

I posted the same question in RC Groups and a gent from England showed me how to balance the Aurora Mills crank.
He uses a Dubro prop balancer.
He has the piston and con rod on the crank, then he places the entire assembly on the Dubro prop balancer.
He adds on lead weight to the counterbalance on the crank until the assemble hangs level.
The piston/con rod at 3:00 o'clock and the counter balance at 9:00 o'clock.

He says that is approximately how much lead I'll need to balance the crank.
Next would be to drill maybe 3 small holes in the counterbalance and pour the melted lead into the holes.
Then recheck the balance as before.

Thanks all for your help!

Surfer_kris Fri Nov 05, 2021 7:25 am

The trick is to measure the needed weight beforehand using the balancing test, you can then calculate how much material that you need to replace, based on the density of the crank material and your filler material.

You'll likely find that you need to replace quite a lot of material if the replacement material is only slightly denser than the crank material, as is the case with lead compared to steel. That is why most people turn to tungsten instead, you cannot melt that in place but you can make a retainer for pressed in weights, like they did on the Cox conquest engine.

For DIY stuff it is perhaps easier to work on the piston weight and reduce the thickness of the piston skirt. That's what most would do on the .049 cox engines, for instance.

A good example of the importance of piston weight is the OS 10FSR and the 10FP. These engines have the same crank but the FP runs so much smoother due to the aluminium piston (ABN technology).

ffkiwi Fri Nov 05, 2021 2:20 pm

Sosam is flogging a dead horse on this particular exercise-the problem is the Mills 1.3 crankshaft has such a thin web-at 2.7mm thick-measured off an actual spare shaft of mine-that any lead you could introduce is going to be insignificant..and that's before you try drilling holes in a hardened crankweb in the first place-let alone how you would retain the lead in position-the front face of the web abuts the crankcase bearing area, the rear face is swept by the conrod-so the lead slugs would have to be flush with both faces-so there is nothing to physically keep then in position during operation....and with differential expansion....I doubt they'd be staying in position very long.

The only practical option n this specific case is reducing piston weight and/or using a slightly unbalanced prop and experimenting with the blade position relative to the engine till you chance on one which gives a minimum vibration position-advice given sosam on another forum...

It is no accident that crankwebs have got much thicker on modern engines...and pistons much lighter, in a continual drive to reduce vibration levels...

ChrisM
'ffkiwi'

Yabby Fri Nov 05, 2021 4:34 pm

the suggestions of ffkiwi seem to make good sense here as a possible solution. I understand the importance of crank balance but have never done it myself! I was fortunate my TZ race bike cranks were rebuilt for free by a fellow who was an absolute guru of doing those cranks. I watched him do it at times and it was truly beyond me!! Anyway the idea of using a slightly out of balance prop to balance the entire rotating component seems to make good sense, and if you get it wrong you can try again with no harm done. Is it possible to add to/remove from the prop plate as a means of also getting some balance? again that can be removed if it doesnt work out and replaced.

I will be interested to hear how it works out with whatever method you use. I certainly havent done it, and it is beyond my skill set, but I am genuinely interested to understand better. Especially if you find some level of solution.

All the best with it. sunny

Yabby

Surfer_kris Sat Nov 06, 2021 5:14 am

Yabby wrote: Is it possible to add to/remove from the prop plate as a means of also getting some balance? again that can be removed if it doesnt work out and replaced.

OS engines used to have that, you'll find it on the old .15 max III for instance. Not sure how much difference it made as the prop-driver is made from aluminium.

aspeed Sat Nov 06, 2021 8:44 am

I don't remember which motor had it, but it was a Russian one. That vicinity anyway. There was an offset in the counterweight and/or a bit of a cutout in the front of the crank to account for the intake window. Pretty sure anything that runs under 25,000 rpm this would not matter.

Surfer_kris Sat Nov 06, 2021 12:35 pm

The OS .10FSR will vibrate a lot already around 15000rpm. I once lost control of the elevator (broken linkage) on an RC model due to excessive vibrations....

Swapped out the engine for a .10FP instead and the problem was solved, the FP is such a smooth runner. Smile

ffkiwi Sat Nov 06, 2021 4:29 pm

aspeed wrote:I don't remember which motor had it, but it was a Russian one. That vicinity anyway. There was an offset in the counterweight and/or a bit of a cutout in the front of the crank to account for the intake window. Pretty sure anything that runs under 25,000 rpm this would not matter.

The Russians leave few stones unturned in engineering their engines...back in the early 90s a few people in the UK started importing Russian 'Typhoon' combat engines....these were 'rough as guts' training engines you were supposed to use for practising and preserve your good engines for competition. They were dirt cheap at about 20 pounds apiece which was about $60 NZ at the time and quite a few came into the country-about a dozen or more. We found them excellent-and I even used them in FF F1C...mine turned out to be the best of the lot-just by sheer chance-a good 1000rpm over the rest of them. We decided to tear it down to see why it was so good [it turned out to be an exceptionally round liner-round to less than 2 microns..] but the really interesting discovery was finding that the crankshaft had a semi circle of holes drilled down the axis from the front of the large shaft diameter portion...OPPOSITE the shaft port, in an attempt to balance the induction cutout.

Now if they went to this amount of effort on a cheap training motor-one can only imagine what went into a top line engine..

ChrisM
'ffkiwi'

TD ABUSER Wed Jan 04, 2023 9:21 pm

sosam117 wrote:ffkiwi,
how do I balance it or see how close I am to having it balanced?

Thanks

Fire up the engine, rev it up and if it runs smoother you know you are working in the right direction with crank weight / piston lightening.
I think a little bit of piston lightening is something that is easy to do with little risk..tape up the OD of the piston with duct tape if you are clumsy. Some engines might gain power by removing the portion of the piston skirt that blocks the flow of gas that is pushed up the transfer port when the piston reaches BDC.
A notch in the piston skirt [that matches the transfer port] might make sense after you remove the backplate and take a look for yourself.
Not all engines look restrictive when the piston is at BDC..but some do.

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