How to balance rotating assembly?

[TheNeedForZ]

Hi, all.

 

I am looking for instructions on the process of balancing an engine's rotating assembly(L28E, L28ET or any other type). Some books talk about this but they only inform the readers about this process without giving any real instructions. Since the process is uaually very long and labour intensive(I ASSUME it's very expensive to have a machine shop do this), I would like to be able to do this at home if possible.

 

I seem to like this type of work (shaving metal and measuring them), even though I get dog-tired afterwards. I just finished polishing 1 L24 rod with my dremel. I did it as per "race rod prep" instructions in the "how to modify your datsun..." book. Man I love the new appearance of the rod compared to stock. This type of qualitative work is easy but now I want to take a step up and do actual balancing.

 

Here is the list of tools I have :

 

3-in-1 machine (lathe-mill-drill, plus some machining tools and measuring devices)

 

Hand-held Dremel rotary tool

 

Electronic balance (one of my ammo-reloading equipment, accurate to +/- 0.1 grain, can convert between grain and gram)

 

Plus typical garage/household tools (big hammer, bench vise, files, angle grinder, .45 handgun, computer with internet connection, membership in a forum called HybridZ.org...)

 

 

Info or links are all appreciated.

[dr_hunt]

With what you have you can balance the pistons and rods, you have to weigh the big end and the small end of the rods separately. They make a tool that will hold the big end and small end respectively, level, so you can weigh the other end on a scale. You weigh one set of rings and rod bearings, the pistons (start with the lightest, make them all the same as the lightest one), and then you have to spin the crank and determine where weight needs to be added/removed as appropriate. You don't have that equipment from your list, so you'll have to take the weight information to your machinist so he can spin the crank with the bobweights on the journals and balance the crank. Your balancer may want the harmonic balancer and flywheel, depends on who's doing it and how sharp they are.

[jt1]

Usually a balancing job runs $250 -$300, so it's not a huge expense, but a significant one. There's no way you can balance the crank, since that takes a specific and expensive machine. But, you can save some money by doing the rods and pistons yourself, then let a machine shop do the crank. Weigh all the pistons, pick the lightest weight, then reduce the weight of the others to the weight of the lightest. Take metal off the underside of the piston, and don't do it all in one place, try to be pretty equal with the amount removed. When you get them all within 0.5 gram you're done.

 

To balance the rods, you need a fixture to hold the rod horizontal so you can weigh one end at a time. The fixture supports one end, the other end rests on the scales. the reason for this is that all the small end weights must be equal, and all the big end weights must be equal. The fixture lets you weigh one end at a time. For your results to be worth anything, the fixture must hold the rod horizontal, and it must be consistent. One you get the fixture worked out, weigh all the small ends, remove metal from around the pin boss to get them equal, then repeat with the big end.

 

If you're going to polish beams, resize, change bolts etc, do that before you start balancing the rods.

 

After all this, take everything to a shop and get them to balance the crank. You will have learned a lot and saved some money.

 

John

[SHO-Z]

Here is an "OLD WAY". My dad did this back in the late 40s on his V8 60 flat head Midget race car. He set up a jig with each end of the crank setting on a razor blade. Installed the rods, pistons and rings and balanced the assembly. He would turn the crank a couple of degrees and see if it stayed in that position. If it moved he would figure out grind where needed.

 

From his stories it was time consuming doing the grinding with a bench grinder and all. But it was how all of the other racers did it.

[PeterZ]

V motors are balance differently than in-line engines. From my memory of my machine shop days in line cranks don't need the bobweights, only V motors. You still balance rods, pistons etc. as above but they don't add the weights to the crank for balancing. That was very late 70's technology. Anyway there is a lot of info available if you do the searching. Good luck.

[TheNeedForZ]

Thanks for the info dudes.

 

I've done some internet searching. Most of the hits are shops advertising their service. Some information about balancing the rods,pistons and that's it, nothing about crank balancing, no explaination to how the balancing machine works. But anyway, I've found something interesting : I read about "overbalance" and "underbalance" in one of the pages. It is said an overbalanced assembly reduces vibration at high rpm but increases it at low rpm. Underbalanced assembly does the opposite. I thought a perfectly balanced assembly will have the least vibration thruout the range but I guess I knew too little. So why do over/underbalance affect vibration at different rpm?

[rudypoochris]

Here is an "OLD WAY". My dad did this back in the late 40s on his V8 60 flat head Midget race car. He set up a jig with each end of the crank setting on a razor blade. Installed the rods' date=' pistons and rings and balanced the assembly. He would turn the crank a couple of degrees and see if it stayed in that position. If it moved he would figure out grind where needed.

 

From his stories it was time consuming doing the grinding with a bench grinder and all. But it was how all of the other racers did it.[/quote']

 

Im not a genius or very sharp in this subject at all, but I think what your dad did is static balancing. I believe dynamic balancing must also be done for the rotating assembly to spin correctly. Not positive but thats what I have read.