Originally Posted By: ISO55000
Hi all,
Let me give you a brief primer in layman’s English here on what “balancing and blueprinting” is, what it delivers and a quick run-down of the process. Mainly because when I train people at plants in vibration analysis and balancing this subject almost always comes up because we like playing with our engines here in the South.
It is also made known through these discussions on breaks that apparently a lot of people sell “balancing and blueprinting” parts and services that are either completely wrong ( even fraudulent) deliberately or innocently leading people to think or believe they want or need something they don’t. There must be a lot of money in selling this “stuff”. I get the impression from these conversations that there must be some kind of “air of secrecy” in the engine world about this and if I believe what the guys discuss with us is that most people in the “B&B” business for engines don’t have a clue as to what they are talking about or doing much less have the equipment to do it even if they do know how.
The truth is that this is a very well-known and understood science practiced thousands of times every day on all types of equipment globally and every ASNT Level-II Vibration Analyst can tell you the exact same thing I am. There is no difference between balancing a turbine, fan, tire or car engine. The goal and the process is the same.
Just like any other firm in our business, we globally do vibration/ balancing on any rotating mass there is to the G1 standard and then certify it. We also train/certify individuals in the same. Granted we have never done a car but that’s because nobody ever asked us to but give us a PO and we will be on it this week.
Anyway, at least anyone reading this can make an informed and intelligent choice as to whether they want to do this or not based on whatever benefit they are after weighed against the effort and cost.
We have a saying in the vibe world: “If it’s shaking- it’s breaking”. Thus is the need for balancing.
* This short primer is done in layman’s English for the average reader who wants a fundamental introduction to the technology, not a “how to” guide so a lot of technical accuracy will be sacrificed and a lot left out for the sake of readability so any vibe guys out there please don’t sharp shoot me. This is at best a skeleton outline.
What is balancing?
In the most basal definition balancing is defined as the center of mass and the center of geometry rotating at the same center in all planes and ranges. “Perfect” balance is impossible in this universe so we get as close to the mathematic value as possible. Also, balancing is tied directly to a desired RPM (cycles per second) and load so a given rotor may have to have a ramp up and/or ramp down to normalize. There is no such thing as a universal balance that works in all loads and speeds.
What is blueprinting?
This term is unique to the automotive industry because as a rule engines do not ( in their standard design) consider balancing a major factor so there is no up-front “blueprint” that lists critical tolerances, standards, finishes, alignment tolerances other than the standard builders print stuff. In industry (for machines and components requiring balancing as a part of the lifecycle) the “blueprinting” is done during the FEED (Front End Engineering and Design) and certified in the FAT (Factory Acceptance Testing) and comes with the equipment from the OEM. (In cases where it’s a unique situation we have to build our own to match actual usage conditions but the process is the same). So, in the case of engines the “blueprint” will be the final result of all the work put into the engine and serve as the guide for any future work.
What does balancing actually do? (Assuming a machine is not so far out of balance that it is shaking itself apart at start up)
We have the first and second laws. Balancing does not “add” anything related to additional performance or in any way make a machine better, stronger, faster etc. et al. If the capability is not built into it- balancing will not make it happen. What balancing will do (at a given load and RPM) reduce internal stressors which in turn reduce heat, reduce component stress, reduce misalignment, reduce wear (and the list goes on) allowing the maximum potential to be utilized at that given load/RPM and as a result will increase overall lifecycle usage. This is where it gets fuzzy because these benefits are percentages of the defined parameters for a given machine already known (not something new or stand-alone) so it may be difficult to draw a line where the benefit of “precision balancing” begins relative to the performance already experienced. Then you need to decide if you want to focus on performance, reliability, longevity, cost of ownership or whatever because there is no “one size fits all” measurement.
Basic Considerations of Balancing
Balancing is the working relationship between 4 major inputs relative to that center of mass and geometry.
Mass- The mass of each body (part) in motion relative to the internal and external stresses which will act upon it.
Rigidity- The resistance to react to forces ( bending or flexing) and normalize relative to the RPM and load in terms of elastic and plastic displacement
Geometry- The true centerline axis that everything is orbiting and how well that orbit is maintained in all degrees of freedom
Influences- Accounting in the above for things such as thermal growth, internal densities, forces and so forth (can be a very long list depending on what is being balanced) Basically anything that can affect the 3 above so they can be factored into the equation.
Basic Tenets of Balancing
There are basically 4 steps to the balancing operation which must be assessed before you can even begin.
Housing- The housing (block) and any supporting whatever must be of sufficient mass, proper metallurgy and normalized (heat and cold treatments as required) to withstand about 5 times the expected operational loads and temperatures. This is the baseline anchor point to which all things are indexed so if this is in any way inadequate then simply stop because all further efforts are futile.
Static Trueness ( parts)- Every single part involved starting at the shaft ( crank and cam in an engine) to the end of the given train MUST be precision machined and ground to be absolutely true in all dimensions to its sister parts.
Static Weight (parts)- this is important (emphasis on the word static) but not as critical as most people think but you have to start out with all like components weighing ( both in total weight and geometric weight relative to stress centers) as possible. Many people mistakenly believe this is “balancing” but that is not even close to being true. It might be better stated that this is the best possible starting line for the balancing operation. Reason being, a static balance is all well and good but a static machine is not spinning and under load (which is where everything matters) and all those forces will affect everything (often randomly) and no known modeling technique can find or anticipate all the variables. A “bi-lateral” physical weight balance from centerline has little bearing to a normalized body under influence because you are going to have to add or remove additional weight anyway during the balancing operation.
Dynamic Forces- You need to calculate (WAG, SWAG or Engineering Equation) all expected forces in all planes to be encountered. (Knowing good and well that all you will ever get is in the ball park- thus the need for the final field balance anyway)
Basic Steps to correct fundamental factors in the Balancing Process ( what one needs to pay attention to and get done- in industry this is all done at the FEED but if you don’t have it or know it, you need to check and do it)
Castings- Must be in true plane, all shaft axes centerlined to less than .0005” overall concentricity, Treated and normalized to remove heat affected zones, machining stresses and so forth and secondary machining ( adding or removing mass) as required to meet the requirements of whatever stress you calculated so the casting can maintain all those critical tolerances and alignments.
Machining- Grinding (dimensioning) ALL RELATIVE TO THOSE AXIS CENTERLINES [ example- the heads cannot just be true and flat, they MUST be true and flat relative to the crank centerline], polishing (finish for movement and lubricity) hardening/tempering/normalizing (stress removal) and then matching all those precision components to the individual orbit train and their place along the shaft and block. (This is why everything is numbered)
Precision Assembly- Dialing in components, pinning, precision alignment, dimensional torqueing (tension and dimension) and so forth
Now the actual balancing
Now the engine is on the balancing bed (can’t use a stand for this because the average rebuild stand will act like a tuning fork- you need a bed with 5 or better times the entire mass of the engine)
Laaadies and gentlemen…..
Index your shaft, calculate your journal and lobe frequencies and get the stobes and accelerometers and start youuuuuuuur balancing……………
1) Engine turning at RPM (an electric motor itself balanced doing the turning so you don’t get much transient vibration) measure each journal and lobe and review the plots
2) For each one based on the spectral data- add/remove gram weight as required to smooth them and bring them in specification
3) Spin and confirm (tweak as necessary)
4) Button it up; throw it in the car and drive.
Summary
That’s balancing in a nutshell. There’s a lot of engineering and math required and a good amount of highly specialized equipment required.
Personally, unless the engine is going to be used in an application like aviation (crashes are deadly), combat or racing (for profit), I don’t believe balancing benefits outweigh the investment for the average car owner but that’s just one person’s opinion and worth just that.
It also makes for a good project so have at it if you want to experiment.
Few end points
Don’t worry about buying all these “balanced parts” and paying extra because if the entire train is balanced (and machined/normalized to maintain that balance during operation) you just wasted your money.
Rebuilds- If a shop does not have the equipment, metrology and precision machining capability (and a certified treatment facility under contract) then they ain’t balancing to begin with regardless of what reputation or song they sing.
Static parts weighing is just that- that’s not even close to dynamic balancing and all but meaningless during operation. (I say all but because there is a positive benefit- just that it’s negligible unless you go all the way)
Now maybe people realize why these high end engines cost so much- there’s a lot of science involved.
Anyway, I hope that helps you guys understand a little more about the concept of balancing and what it entails.
Outstanding!!! Thank you for your post!!!
Ed