Engine Balancing

[Jim Allen]

Marty, I think if you Google his name there will be some information. If you can find a year 1968 HP 61 engine like the brand new, still in the box, never run, engine model #456 that was given to me by Don Jellick, you would be very surprised how advanced this thing was. Don Jellick & Herb Stockton are the only people in the world to beat the Russians two times at Kiev. Their Paul Bugal .15 size diesels turned 38,000 RPM with no problems using a bell valve that was bolted to the crank pin. This engine had a special hanger piston that can be seen in my photo album. The 61 size engine had a balanced bell valve that could survive 50,000 RPM. Look at the photo & you can see the bell valve & slanted carburetor.

If you Google"The FMV Story" you will see technologies used in their 1978 team race engines. Detailed explanations of the steel front end design that I use on my .90 cu in engine, except mine is hardened & ground; their mathmatical calculation of crankshaft bending told me where to place the seal to prevent rubbing inside the housing; their explanation of how much heat is generated in a bushed connecting rod; their design of why the piston should be barrel shaped; their hanger piston design to give a 2 to 1, rod to stroke ratio; etc; etc. These guys were modelers who learned how to "build" not just modify engines.

Jim Allen

 

[Tom Foley]

Tom, its all done with the crank outside of the engine, bob weight hanging on the crank pin.

Yes , but the actual weight that the bob weight represents must be the weight of the rod/piston ., where does the addition/ removal of weight go in the crank ? Talk about a run - on - sentence !LOL

 

[Marty Davis]

Tom:

The calculations that have been discussed are to determine the weight of the Bobb Weight. Once you determine that, you make a weight that is the diameter of the bottom of the rod that has a hole in it that allows it to slip over the crank pin. You then make the weight the correct weight as determined by the various formulas that have been discussed. Once you get the bobb weight the correct weight, you place the weight on the crank pin and the crank on a balancer and balance the crank perfectly. You may have to remove weight from the heavy side of the crank on the balancer, or you may have to add a piece of sintered tungsten (Mallory) to actually add weight to the counterweight. Once you do that, you may have to remove small amount on the counterweight to complete the balance. You need to finish by having a crank with bobb weight on it not stop at the same place on the blalancer (just like balancing props).

Hope this helps.....

 

[Tom Foley]

Tom:

The calculations that have been discussed are to determine the weight of the Bobb Weight. Once you determine that, you make a weight that is the diameter of the bottom of the rod that has a hole in it that allows it to slip over the crank pin. You then make the weight the correct weight as determined by the various formulas that have been discussed. Once you get the bobb weight the correct weight, you place the weight on the crank pin and the crank on a balancer and balance the crank perfectly. You may have to remove weight from the heavy side of the crank on the balancer, or you may have to add a piece of sintered tungsten (Mallory) to actually add weight to the counterweight. Once you do that, you may have to remove small amount on the counterweight to complete the balance. You need to finish by having a crank with bobb weight on it not stop at the same place on the blalancer (just like balancing props).

Hope this helps.....

Thanks !

 

[Kez]

A trick that I learned from RC flying back in the 80's is to line the propeller up in the 12 o'clock position with the piston at TDC. With the heavy blade in the 6 o'clock position, it did help smooth out an engine with insufficient counter balance weight. I have been wondering if we can use the same trick on a boat engine by removing material on the flywheel inline with the crank pin.

 

[Jim Allen]

Kez,

With front intake engines, such as the Nelson .45 long stroke, that have very large diameter crankshafts, typically .7500" OD, there is a balanced air prop driver for speed planes & a balanced flywheel for boat use. Both pieces have orientation marks to insure the correct mounting positions. This 30,000 RPM engine is one of the smoothest & most powerful engines I have tested. Please notice the cylinder head bolts that pass through the liners lip, not outside the liners lip. Also notice the additional mounting lugs on the front end to stabilize this area. Some tether car engines have both features.

Jim Allen

 

[Mike Rappold]

If an engine needs more counterweight, what is the best way to drill a hole through the crank? Seems like the crank is harderned.

Also what is a good source for a counterweight material?

Mike

 

[Andy Brown]

Mike, The cranks are normaly around 60RC. Jim could tell you the best way to drill it. Tungsten slugs are normally used.

Before adding weight it is best to have lightened the pin side of the crank as much as safely possible along with the piston and rod. Go for the slug/s as a last resort.

It's a trade off. You can get it smooth in the vertical axis, but if you do it all with the counter balance it will vibrate in the horizonal axis.

 

[Jim Allen]

Mike, The cranks are normaly around 60RC. Jim could tell you the best way to drill it. Tungsten slugs are normally used.

Before adding weight it is best to have lightened the pin side of the crank as much as safely possible along with the piston and rod. Go for the slug/s as a last resort.

It's a trade off. You can get it smooth in the vertical axis, but if you do it all with the counter balance it will vibrate in the horizonal axis.

Right on Andy! To much weight on the crank counterbalance will really make things shake. You can drill any crankshaft with a C-2, carbide, four faceted point drill. One piece crankshafts are usually made of 8620 or 9310 case hardened steel. No coolant or lubrication is required if the drill is sharp. Mallory metal (machinable carbide) is easier to work with, but I have used carbide slugs. Machine the pieces a little smaller in width than the width of the crankshaft counterbalance area. After inserting with a press fit, peen the outer edges of the holes on both sides. You don't want those pieces coming out when the engine is running.

As Andy & others have stated, getting the reciprocating down as low as possible is better than adding weight to the crankshaft counterbalance. Another option is to lengthen the connecting rod to the maximum by removing the bushing in the top end of the rod (hardened steel rod) & reduce the height of the piston bosses.

Jim Allen

 

[Mike Rappold]

I have question with regards to the different balancing formulas presented. I was playing around with the weights for the piston, rod, wrist pin, etc. for a 90 size engine using the three methods presented in previous posts. Using Rod Geraghty formula I calculate a bob weight of 15.7g, using Jack O'Donnell formula I get 34.9g, and using Dave Marles formula I get 21.5g. A pretty big range between formulas.

I am not trying to start a who is right, who is wrong discussion but was wondering if one formula is better for certain sizes of engines, targeted rpm range, steel vs aluminum con rod, etc.?. I have never tried to balance an engine so this is learning curve. Is it better to start lighter and work heavier or vise versa?

Mike

 

[Andy Brown]

I have question with regards to the different balancing formulas presented. I was playing around with the weights for the piston, rod, wrist pin, etc. for a 90 size engine using the three methods presented in previous posts. Using Rod Geraghty formula I calculate a bob weight of 15.7g, using Jack O'Donnell formula I get 34.9g, and using Dave Marles formula I get 21.5g. A pretty big range between formulas.

I am not trying to start a who is right, who is wrong discussion but was wondering if one formula is better for certain sizes of engines, targeted rpm range, steel vs aluminum con rod, etc.?. I have never tried to balance an engine so this is learning curve. Is it better to start lighter and work heavier or vise versa?

Mike

Mike, What .90 engine? Some are smooth running in stock form and some are real shakers. I'm just wondering which one needs all of that weight?

 

[Jim Allen]

I have question with regards to the different balancing formulas presented. I was playing around with the weights for the piston, rod, wrist pin, etc. for a 90 size engine using the three methods presented in previous posts. Using Rod Geraghty formula I calculate a bob weight of 15.7g, using Jack O'Donnell formula I get 34.9g, and using Dave Marles formula I get 21.5g. A pretty big range between formulas.

I am not trying to start a who is right, who is wrong discussion but was wondering if one formula is better for certain sizes of engines, targeted rpm range, steel vs aluminum con rod, etc.?. I have never tried to balance an engine so this is learning curve. Is it better to start lighter and work heavier or vise versa?

Mike

Every engine will be different according to what that reciprocating weight is. Larger amounts of counterbalance will be required when using steel rods & lower amounts for aluminum rods. A lot of things come in to play here, longer rods should have higher reciprocating weights but they may not if the piston bosses are shorter & there is no bushing in the top of the connecting rod. That "strobe" will tell you many things about what is going on when the engine is running at WOT.

Jim Allen

 

[Mike Rappold]

If the motor has a drum, how does that factor into the engine balance? Seems like it would need to be balanced itself not to influece the crank. Thoughts?

Mike

 

[Jim Allen]

If the motor has a drum, how does that factor into the engine balance? Seems like it would need to be balanced itself not to influece the crank. Thoughts?

Mike

It is difficult to make a very good static balance on a standard or inverted drum valve. Rotary valves & bell valves can be staticly balanced extremely well. I believe Jack O'Donnell uses is dynamically balanced bell valve.

 

[Andy Brown]

I balance my cranks and drums in th most "prehistoric" way (as in 'Flintstones' LOL), but it works. I won't bore the rocket scientists with my "methods" LOL

 

[RodneyPierce]

Andy, then please do bore us "non rocket scientists" with them if you will... Im interested.

 

[Tom Foley]

Ready to be bored as well .

 

[Joe Warren]

If the motor has a drum, how does that factor into the engine balance? Seems like it would need to be balanced itself not to influece the crank. Thoughts?

Mike

Most Drums are Balanced to a certain degree

it would be nice to balance the Drum and Crank at the same time but this would be quite

difficult without and require an elborate fixture.

How do you figure that most drums are balanced?? when 1/2 of the drum is missing on one side of the drum valve for the intake port opening??

 

[Joe Warren]

Crankshafts are balanced correctly after you have weighed all of the parts that are swinging on the crank pin and established a bob weight in grams to put onto the crank pin. Then you can find out what the counter weight needs. More or less weight?? After the bob weight has been determined It would be wise to install all of the rotating parts while spinning the crankshaft for balancing to see if they need attention too.

 

[Andy Brown]

Andy, then please do bore us "non rocket scientists" with them if you will... Im interested.

Nawwww...my crank balancing method is really Barbaric! Besides, there's not much point in putting a bunch of math calculations into an IMPOSSIBLE situation.