Balancing Drums

[ids987]

The way I see it (in my own mental picture), the crankshaft and the drum are both rotating masses. I don't think they can be considered part of the same rotating mass, as the drum floats on the crankpin. The drum is therefore a standalone rotating mass, and should ideally be balanced as such (this is obviously challenging or maybe next to impossible where there is a ramp - although some have used delrin ramps). The crankshaft is a rotating mass which is alternately driven by, and driving, a large reciprocating mass. In my view, this should therefore be balanced as a system in its own right (separate from the drum) - where the crankshaft is deliberately imbalanced in an attempt to compensate for the reciprocating mass. If the flywheel was rigidly attached to the crankshaft (and preferably keyed), I would think you could regard it as part of the same rotating mass as the crankshaft, but if you deliberately imbalance the flywheel to balance the system, the resultant forces will obviously act on, and be transmitted along the crankshaft.

As most of us probably know, and as Jim pointed out, you can not truly balance a single cylinder engine - simply because the effect of the reciprocating mass changes according to the crank angle. Various formulas (like those already quoted) exist to find the best compromise for the desired RPM band.

I'm gonna stick my head down now and duck !

Ian

 

[NITROSNIFER]

Can someone show a pics of this in action. I always wondered what type of aparatus I'd need to balance a crank and where to put the tungsten

Thanks in advance.

 

[Don Ferrette]

The way I see it (in my own mental picture), the crankshaft and the drum are both rotating masses. I don't think they can be considered part of the same rotating mass, as the drum floats on the crankpin.

I will agree on both being rotating masses but I would think the drum cannot be considered to "float" as it is being driven by the crank & is constanting engaged and under load by said crank's rotational force. There has to be some load placed on the crank by this action that would relate to the balance of the pieces together I would think. :blink:

 

[ids987]

The way I see it (in my own mental picture), the crankshaft and the drum are both rotating masses. I don't think they can be considered part of the same rotating mass, as the drum floats on the crankpin.

I will agree on both being rotating masses but I would think the drum cannot be considered to "float" as it is being driven by the crank & is constanting engaged and under load by said crank's rotational force. There has to be some load placed on the crank by this action that would relate to the balance of the pieces together I would think. :blink:

I'm thinking that there is "float" - both rotationally, and radially (as the crankpin sits in a slot in the drum). The two masses will definitely influence each other, but I don't think that this is in the same way as a single rotating mass. Although the crank drives the drum when it is being driven by the piston, the two are not fixed together in any way. I wouldn't say that it is constantly engaged and under load by the crank's rotational force either. The drum has it's own inertia, and as the crank slows down - due to compression etc, I believe there will be a point in the stroke where they disengage, then the drum will try to drive the crank. I'm thinking that if the drum is unbalanced, it will wobble and chatter on the crankpin - rather than fully coupling the energy to the crankshaft.

Again, just the way my "mind's eye" sees it.

Ian

 

[Don Ferrette]

Good thoughts Ian. I wonder if anyone has any actual physical data on how a drum interacts with the balance or imbalance of the motor? Jim? Rod? Marty? :unsure:

 

[nws]

With the drum not balance, check the rounds of the bushing. In the OS-Picco_Rossi back housing they will be out of round..

I had a Picco with the the OS back housing on it. the drum was not balance.

The motor would not pull a big prop off the turns , check the housing and it was out of round bad .

That is when I started balancing all of my rotors.

It made big difference. Had to put a T bushing in housing.

I turn a piece of alum. Turn the OD. and bore the ID. so the rotor will slide in,then balance the rotor.

It worked for me.

Norris

 

[Jim Allen]

Good thoughts Ian. I wonder if anyone has any actual physical data on how a drum interacts with the balance or imbalance of the motor? Jim? Rod? Marty? :unsure:

I think Ian has the right idea about what is taking place in our toy motors. When developing the bell valve, which has an internal clearance of .0001, an attempt was made at using a slightly unbalanced valve with an external weight added to a balanced flywheel. This did not solve the problem of an unbalanced valve & it did create a problem with the unbalanced flywheel. A static balanced valve runs without any of the problems that result from running unbalanced valves. Of course a concentric flywheel is necessary (+-.0001 runout) also.

Jack O'Donnell is presently using a dynamic balanced bell valve in his small tether car engine that is similar to the one in my boat engine. He reports significant gains & no rubbing at the .0001 clearance, even with the very high G force that a tether car experiences. He is presently making dynamic balanced bell valves for larger engines.

As a side note, there is another problem with induction valves; standard drums, inverted drums, rotary valves, hoffman valves or bell valves which DO NOT have some means to control their movement forward & aft as the engine is running. An attempt to correct this rubbing problem & the resulting mechanical noise problem can be found in late model .90 size K motors. Some engine builders feel that a front intake engine is the ultimate system to eliminate all of the problems previously mentioned.

Jim

 

[Kim J]

I made atleast one mistake in my first reply regarding the rollers ( big end conrod ) should be included when measuring the weight of the reciprocating mass.

Also only using 50 % of the weight of the bottom part of the conrod makes sense.

When making imbalance on the flyweel on a K& B 3,5 OB posision is important. Whith piston abdc the small diameter hole should face towards the cylinderhead. This was described accurately in an article by John Oian about 20 years ago in Model Airplane News. Using this method is nevertheless a quick fix.

Any part that is not connected firmly to the crank should be balanced neutral. A drum has a elongated hole and are connected loose to the crankpin. No one would use a out of balance propeller to improve the balance of the engine, even if its connected to the crank with a cable.

Different manufacturers have different ideas regarding balancing. Novarossi uses two tungsten pins. CMB uses none on the new 21, 45 and 91 Hydro ( Valvola ). Also they have a slot in counterbalance part of the crank, making it lighter and impossible to place a tungsten pin.

Check: http://en.wikipedia.org/wiki/Engine_balance

Kim

 

[steve speas]

Are you going to keep that information a secret? Check a Rossi inverted drum & get another surprise.

No, sorry. I didn't think anyone would care.

The MAC drum is out of a VERY strong running motor and the CMB drum is out of a box stock new motor. The MAC drum has the ramp in it and the CMB dose not as it has the stationary slug. The MAC drum was WAY out of balance and the CMB was pretty close to being balanced. I guess this shouldn't be too surprising as the MAC drum has the heavy ramp cast in the drum.

-Buck-

Hmmm, here's a twist on that.....

Can the out of balance drum do anything to offset the out of balance state of the crank based on the points of imbalance of both pieces? Just thinking out loud here....... :blink:

HEY DON, YOU REMEMBER 98 NATS AND MY OPS 45 twincraft WITH THE K&B ROTOR. BALANCED[darn caps button] the hell out of it. 15 sec. laps back then.55% nitro. won a couple of things with that motor.

you cant balance a single cyl. motor but you can true the rotor & flywheel

steve

 

[SayMikey]

Are you going to keep that information a secret? Check a Rossi inverted drum & get another surprise.

No, sorry. I didn't think anyone would care.

The MAC drum is out of a VERY strong running motor and the CMB drum is out of a box stock new motor. The MAC drum has the ramp in it and the CMB dose not as it has the stationary slug. The MAC drum was WAY out of balance and the CMB was pretty close to being balanced. I guess this shouldn't be too surprising as the MAC drum has the heavy ramp cast in the drum.

-Buck-

Hmmm, here's a twist on that.....

Can the out of balance drum do anything to offset the out of balance state of the crank based on the points of imbalance of both pieces? Just thinking out loud here....... :blink:

HEY DON, YOU REMEMBER 98 NATS AND MY OPS 45 twincraft WITH THE K&B ROTOR. BALANCED[darn caps button] the hell out of it. 15 sec. laps back then.55% nitro. won a couple of things with that motor.

you cant balance a single cyl. motor but you can true the rotor & flywheel

steve

No OFFENSE but do you guys really think you can better the balance of modern motors?? Think back to the days when a picco ops or k/b shook so bad it spit bolts at you? With todays technology and CNC id say they are plenty close enough. I have cmb67 greenys over 3 years old ,all thats been changed is bearings. Now thats technology .......Mike

 

[Don Ferrette]

HEY DON, YOU REMEMBER 98 NATS AND MY OPS 45 twincraft WITH THE K&B ROTOR. BALANCED[darn caps button] the hell out of it. 15 sec. laps back then.55% nitro. won a couple of things with that motor.you cant balance a single cyl. motor but you can true the rotor & flywheel

steve

Yup, sure do. That was one freaky fast OPS 45. B)

 

[Jack ODonnell]

Are you going to keep that information a secret? Check a Rossi inverted drum & get another surprise.

No, sorry. I didn't think anyone would care.

The MAC drum is out of a VERY strong running motor and the CMB drum is out of a box stock new motor. The MAC drum has the ramp in it and the CMB dose not as it has the stationary slug. The MAC drum was WAY out of balance and the CMB was pretty close to being balanced. I guess this shouldn't be too surprising as the MAC drum has the heavy ramp cast in the drum.

-Buck-

Hmmm, here's a twist on that.....

Can the out of balance drum do anything to offset the out of balance state of the crank based on the points of imbalance of both pieces? Just thinking out loud here....... :blink:

HEY DON, YOU REMEMBER 98 NATS AND MY OPS 45 twincraft WITH THE K&B ROTOR. BALANCED[darn caps button] the hell out of it. 15 sec. laps back then.55% nitro. won a couple of things with that motor.

you cant balance a single cyl. motor but you can true the rotor & flywheel

steve

No OFFENSE but do you guys really think you can better the balance of modern motors?? Think back to the days when a picco ops or k/b shook so bad it spit bolts at you? With todays technology and CNC id say they are plenty close enough. I have cmb67 greenys over 3 years old ,all thats been changed is bearings. Now thats technology .......Mike

ABSOLUTELY !! Cant get them perfect, but you can get them a lot better than what the factory is doing. J. ODonnell

 

[Mike Rappold]

It was mentioned earlier in the topic but can someone explain how they are holding the drums and balancing them? Also what is being used as the balancing equipment?

Thanks in advance.

Mike Rappold

 

[Tom David]

I am sorry guys, but I am having a hard time walking away without makingone more point.

Earlier, Kim made a good point how someone used the flywheel

On a K&B 3.5 outboard (to work in conjunction with the crank)

as part of the rotating mass and counter the reciprocating mass.

The flywheel is CONNECTED to the flywheel in the same axes

spinning 1:1.

Unless the flywheel is indexed to the crank and on the crank during the balancing process it will be void in the picture. If the flywheel is balanced as well as the drum than the work will be in the crank counter weights in relation to the rod, pin, clips, and piston.

-Buck-

Sorry hit wrong button on last post.

Buck, would you agree that the drum or disc is considered a

rotating mass?

Rotating mass..Yes

Regards

-Buck-

Would you agree that (if assembled correctly) the drum or disc

Is TIMED with the crank and is spinning in same axes 1:1?

Spinning in the same axis at 1:1 yes.

I don't understand your term TIMED as the drum will only work correctly one-way.

Think this...

Drum spinning in it's housing by itself. If it is balanced things are good.

Now spin the flywheel by itself. If it is balanced things are good.

O.K. If the rod, pin, clips, and piston are balanced than things are great right? Well how do you balance the rod, pin, clips, and piston? Counter weight on the crankshaft that it is attached to.

Now this means that the drum and flywheel should be balanced independently in my view.

Please help me see your view.

Regards

-Buck-

Hello Buck,

I want to take time this Saturday morning and express my point of view on this subject.

First, before I go on, I would like to say that I am not contesting the results that Jack and

Jim have accomplished. My hat is off to them for the research and development of improving

the performance of their engines.

Earlier, a web site that was posted by Kim J introduces an entry level of discussion on the

Topic of Engine Balance. If you page down to the bottom you will see a

General index. Click on Overview of balance, related to the straight 4.

Now I know we are comparing apples with oranges, but I am going to

use the intro of balance shafts here to prove a point. As you read the

article you will see that engineers decided to introduce rotating

shafts (that are driven by the crankshaft) that would help counter balance the

Secondary vibration of this engine. I would point out that they rotated

twice the rpm of the crank on this 4 stoke engine.

The old 2-stroke Detroit Diesel’s also used this technology to help in the balance

process and they rotated the same rpm as the crank.

Now, back to our discussion.

Lets say that my rotor in a NIB .67 engine is not static balance neutral.

And we agree that it is driven by the crank with its axes parallel with

the crank Then I believe that it is possible that the imbalance of the rotor

can work with the imbalance of the crankshaft to HELP offset the dynamic

forces of the reciprocating mass.

I believe it is ill relative that the drum runs in a bushing. If this was the case then

A crankshaft’s counter balance would have no effect if it was running in bushings.

There are still many R/C airplane entry level engines being built with bushings to support the crank.

The important fact is that rotating drum’s axes is inline with the crank and is turning 90 degrees to

the reciprocating mass just as a balance shaft.

Do all engine manufacturers have this balancing act perfect. Not at all.

Can improvements be made on out of box engines. Yes they can.

As Jim and Jack pointed out, It will NEVER be perfect.

My point is this. Just because your rotor is not static balance neutral, does not

mean that the over all balance of the masses that are rotating in your engine

Is incorrect.

Regards.

Tom David

 

[steve speas]

It was mentioned earlier in the topic but can someone explain how they are holding the drums and balancing them? Also what is being used as the balancing equipment?
Thanks in advance.

Mike Rappold

mike, i made a hollow tube of alum. put it in the rotor then put it on a airplane prop balancer, i think i can dig up a photo if you need one

steve

 

[Mike Rappold]

It was mentioned earlier in the topic but can someone explain how they are holding the drums and balancing them? Also what is being used as the balancing equipment?
Thanks in advance.

Mike Rappold

mike, i made a hollow tube of alum. put it in the rotor then put it on a airplane prop balancer, i think i can dig up a photo if you need one

steve

Steve,

Pictures always help. Thanks in advance.

Mike