[dynosor]

How did you calculate what weight was needed?

Hi Ken

When studying the counter weighting applied to Craig's crankshaft it is apparent that the engine was considered as four 90 degree V-twins that happen to have their crankshafts connected. The advantage of doing it this way is that the loads trying to bend the crankshaft across the neighboring throws are greatly reduced. This advantage is probably more useful when considering the joints between the segments of a composite crank than it would be for one cut from a single piece of steel.

Another aspect that is evident is that the counterweights retain all of the original disc material opposite the crankpin from the line through the center of rotation. While material that close to the axis of rotation does not add much counter-balancing weight (hence the tungsten weights), this extra "flange" material increases the effective "flywheel" mass at each crankpin throw. In full size racing engines this extra flywheel mass would be cut away to improve throttle response, but in the case of a model engine with a composite crank, the extra flywheel mass at each crankpin throw usefully damps out torsional vibration that would tend to stress the joints between crankthrow components.

As for the total rotational inertia of the crankshaft, the additional inertia that results from the extra material at each crankpin throw flange can be subtracted from the flywheel proper, if desired. As Craig's engine will power a boat, building revs instantly would not seem to be a top priority. A slightly "heavy" flywheel will allow the engine to run smoothly at a lower speed than if the total combined crankshaft and flywheel inertia were to be reduced.

Consider that Steve Huck's original engine design retains the full crank flange discs and therefore the total effective flywheel inertia would be higher than for Craig's design interpretation (taking into account the 1.5:1 scale). Also, if Steve's engine runs smoothly enough without special balancing, then Craig's design can only make it smoother; especially at higher revs. Then again, Craig’s engine is larger and so are the loads that would benefit from balancing at any given rotational speed.

As far as the method for calculating counter balancing weights required is concerned, the link provided by Bluejets explains it rather well: http://modelenginenews.org/etw/etw_bal/p2.html

[cforcht]

Dynosor, Thank you for the detailed explanation. Had I attempted explaining, I don't think anyone would have understood anything other than I clearly didn't know what I was talking about.

[cforcht]

here's a picture of the lower end assembled. the rings are not installed yet. I still have to make the fixture for them and get them heat treated. maybe I will see if I can whip out the fixture today despite feeling a little under the weather.

[dynosor]

Awsome Craig!

Have you turned over the engine by hand or under power to confirm that it rotates freely? I would oil all the bearings and pistons generously with something like breakfree before doing this and not run too long before re-oiling.

Also, if you were going to run at significant speed you probably should install the second bearing cap screws and torque them all to "spec".

[cforcht]

Yes it does turn over by hand. of course with the pistons in the cylinders there is more resistance but still easily turned by hand. I have not turned it over under "power" the other engine has the starter on it not this one. I still have to put the weights in the other engine and install the pistons and sleeves. I have done nothing at all today. not feeling well. which is unusual for me.

[dynosor]

If you are not feeling well your body is trying to tell you to take a break. Listen to it.

[Ken_Shea]

Dynosor
Thanks much, very good read and very understandable.

[cforcht]

Dynosor,
I have been taking it easy this weekend. had the boys out in the shop getting the other end of the building ready for moving everything down there. had some misc junk cluttering up the area. nothing major aside from a couple derelict cars belonging to my eldest son. those I just picked up and moved with the forklift. so nothing real strenuous on my part.

[bluejets]

Just a thought on counterweight. It appears there may have been room to make the webs larger and therefore remove the need for "plugs".Should have been able to pick up an extra oz or two there I would have thought.

[cforcht]

whos knows, maybe it was possible. at this point though I am satisfied with what I have. once its running maybe I will change my mind and seek a more balanced crank.

[dynosor]

It appears there may have been room to make the webs larger and therefore remove the need for "plugs".

The block does appear to clear the crank webs by perhaps 1/4" radially. Starting with material nearly 1/4" larger in radius than the existing crank's largest diameter would have made a huge difference in the equation:

Not only would there be more weight opposite each crankpin, that weight would be added a larger than crankpin radius: it is not just the weight that matters as much as the weight x its distance from the centerline perpendicular to the crankpin radius.

Or one can start with a smaller diameter rod blank and add tungsten weights... Remember that the parent design did not consider dynamic balancing and that most dimensions were simply scaled from that.

[cforcht]

As Dynosor pointed out the parent design never had any consideration for balancing. Also its primary use was not something that would be run more than a few minutes at most at shows and for the entertainment of friends. So balancing was not a real concern. I however have decided to put my engine to use for something other than a display. because of that I sought to balance the crank. yes I suppose I could have gone with a bigger blank to achieve a full 100% balance but when I decided to look at the balancing aspect there were other things already in place. such as the material for the crank was already in hand. so I chose to balance the crank as well as possible with the given design.

[RomanLini]

You must get a kick out of seeing the bottom end all thogther like that?

I've re-built plenty of engines over the years, but have never crafted an engine from scratch out of chunks of metal... It must be an awesome feeling when it all comes together, especially on such an elaborate and good looking build!

[cforcht]

You must get a kick out of seeing the bottom end all thogther like that?

I have to say yes to that. I always get a thrill when I make all these different pieces according to a print or CAD drawing and then it actually assembles like its supposed to.

[ebobby35]

steve hy my name is bobby im new here and the reson im here is becuase i came across a littel motor done some surching and it came to u if,when,and you would i would like to make a littel v8 but of my own dont need plans just alittel infow if u could help i would be completly greatful thanks Bobby

[cforcht]

ebobby35 the guy you are looking for (Steve) is the owner of these two threads

http://www.cnczone.com/forums/i_c_en..._v8_blown.html

http://www.cnczone.com/forums/i_c_en..._required.html

[bluejets]

steve hy my name is bobby im new here and the reson im here is becuase i came across a littel motor done some surching and it came to u if,when,and you would i would like to make a littel v8 but of my own dont need plans just alittel infow if u could help i would be completly greatful thanks Bobby

Bobby,
Better to start with a simple single cylinder engine if you have never built an engine before.Poppet valve is a good example. The examples you see here are from many years of experience and using expensive machinery. Find a simple set of plans from a well known design, that way you can be sure of some success. There is a lot more to the engines you see here than meets the eye.

[cforcht]

Bluejets, well said. and good advice.

[bluejets]

You must get a kick out of seeing the bottom end all thogther like that?

I've re-built plenty of engines over the years, but have never crafted an engine from scratch out of chunks of metal... It must be an awesome feeling when it all comes together, especially on such an elaborate and good looking build!

I'll second that...... also add there is a great feeling when a project fires up for the first time. I have a couple of simpler engines that sit on one end of the workbench ready to run, and when people visit or I just happen to have a spare minute, I fire them up just to hear them run. Great stuff.

Must say also I have an understanding of the amount of work and experience that has gone into each and every piece of the engine here and have to say, good work once again.

[cforcht]

Thanks, When I started this project I knew it would be lots of machining. but it was even more than I anticipated. I dont mind. I just didn't evaluate it to the extent it really required. but its been fun to see it take shape. granted its not done yet but every little thing I check off the list is one step closer to firing it up. I have to say its one of the most complex things I've built in a very long time.