connecting rods weighing question

[ZIGGY]

However, trust, but verify.

I wondered where Reagan heard that...

[engineguyBill]

The small end of the rod is reciprocating weight and the big end is rotating weight.

* very little of the rod mass is pure reciprocating or pure rotating weight.

This is true, it is not physically possible to seperate the rod into two distinct parts - one part reciprocating and one part rotating. But, in terms of simplicity, the small end is typically considered seperately from the big end. This is why the ends of the rod are weighed seperately, instead of just weighing the whole rod as one unit.

[Wolfplace]

The small end of the rod is reciprocating weight and the big end is rotating weight.

But... it's not. The separation of motion paths* isn't related to how much weight is on either end, but on what a molecule in any position is doing.

* very little of the rod mass is pure reciprocating or pure rotating weight.

Agreed but sometimes a simple solution like weighing each end is both what we can easily do & what appears to work reasonably well

Do you have another suggestion for choosing a bobweight to balance a crankshaft other than this simplistic approach?

[Belgian1979]

FWIW, I've never needed to touch an Oliver to the grinder. They are always within spec. However, trust, but verify.

jack vines

absolutely but I do not have an accurate enough scale and the mandrels to mount the big, small end. I tried to do it as good as I could but the measurements were all over the place.

[hodge]

weighing on a scale that does not do tenths is scarey, one gram could be 1.9 or 1gram or .6 could be 1 gram. what is it

[gunt]

since yee are on the topic , i have made up several versions of the set up for measuring rod weights and i can never ever get repeatable results , i've read a lot set it up several different ways , could be the light engines im working with , but as in the pic in the forum , then the big end is on the scale , and i have a roller bearing set up it is still not enough for the rod to center itself on it , any advice is welcome

[strokersix]

In the pic, free end is suspended from a string and the scale end is on a pin.

If you suspend both ends from a string I suspect your trouble will diminish. Setup a little more complex but I think it would work to improve accuracy.

I have not done this, thought experiment only.

[gunt]

thats a thought , i will try it , and ya i have the other end suspended

[travis]

https://www.linkedin.com/pulse/use-disc ... ika-goyal/

Looks like another spammer got in the door...

[racear2865]

Ive played with balancing for several years now http://abrobalancing.com/
Several things comes to the forefront in this discussion. A couple years back, I made a fixture that does repeat. I shared it on another forum someplace. If yall are interested, I will find it and repost. But lets get back to balancing. The 50% rule is just that a rule. It was not derived from a mathematical equation. It is not written in stone. It has been used simply because no one has found any thing better. It seems to work "MOST" of the time, so we have stuck with it. The amount of oil is a hypothetical guess at best. It does makes since that a BBC holds more than a SBC. Again I say its makes since. Getting under 2 grams is a waste of time unless, you have proven in a particular situation, that it warrants it. BUT prove it to me and then show me how you repeat it. If the machine is not grouted into the floor and the floor separated from the rest of the shop and your program does not have damn good filters to filter out all noise except what you want to measure, you "aint" gonna git there. And if your chasing zero, you "aint" gonna git there either. Its unobtainable. As the machine approaches zero, the software will begin to hunt and bounce all over the locations.
reed

[BillK]

thats a thought , i will try it , and ya i have the other end suspended

How about a picture of your setup ? I don't know how you would suspend the end of the rod that is on the scale with a string ? Almost impossible. You need to make sure the rod is as close to level as possible when you are weighing it.

A good way to check your method is to add the two weights together. They should add up to the total weight of the rod. If not you don't have it level.

[PackardV8]

We have two independent variables at work here. Making all the big ends, small ends, pins, pistons, et al, weigh the same statically within one gram should be achievable every time. Getting to the same weight has a benefit and will smooth the running independent of the crankshaft balance.

Getting (crankshaft balance) under 2 grams is a waste of time unless you have proven in a particular situation that it warrants it. BUT prove it to me and then show me how you repeat it. . . . And if you're chasing zero, you "aint" gonna git there either. Its unobtainable. As the machine approaches zero, the software will begin to hunt and bounce all over the locations.

Agree, trying to spin up a crankshaft with bobweights derived from this equalized static weight is another thing. As mentioned above, once we get down toward two grams, it's very difficult to get the same machine to repeat. We just went through this testing a nearly new ProBal machine. It came with the test mandrel; an accurate a piece as will ever be put on the machine, but the machine was just sitting on the floor.

If the machine is not grouted into the floor and the floor separated from the rest of the shop and your program does not have damn good filters to filter out all noise except what you want to measure, you "aint" gonna git there.

Repeatedly spinning up the perfectly balanced mandrel, the readout was close enough, but it was always slightly different, including the location to drill to correct. I shudder to think about drilling a crank and guaranteeing to get it repeat within less than one gram. It's nearly impossible to replace the bobweights in exactly the same location and orientation.

[BillK]

We just went through this testing a nearly new ProBal machine. It came with the test mandrel

Jack,
Thats interesting. My 25 year old ProBal will repeat every time with the calibration arbor on it. Same with anything else you spin.

Heres my take on it. When I first got the machine the first balance job I did was for one of my drag racing friends. He was there for the learning experience. No matter what I did I could not get that darn thing to repeat two spins in a row. I tried different oil, wicks to hold oil you name it. So I called Bob at Pro Bal, this was before T&S bought them. He told me I needed to get the roller bearing setup instead of the V-blocks.

I bought the roller bearings and never looked back. Contrary to what everyone says they do not mark the crankshaft hardly at all and they repeat 100% every time.

If for some reason I purchased a new balancer I would make sure it came with roller bearings instead of v-blocks.

[PackardV8]

Hi, Bill,

The ProBal we just sold came with all the goodies, roller and V-blocks, test arbor, flywheel arbor, angle protractors, bobweights.

When I say it wouldn't repeat, as mentioned, the mandrel would repeat to under one gram, but exact left/right and the angle to drill was slightly different each time. When one is trying to claim a crankshaft is balanced to under a gram, I'd like to see him then remove the bobweights, remove the crank, zero the machine, install the crank and bobweights and have it indicate under a gram the next spin; most won't.

they repeat 100% every time.

Since yours repeats, probably it's the precision installation and the operator being intimately familiar with the machine.

[Dave Koehler]

V blocks.
If your design allows it reduce the contact patch by machining or grinding the nylon or whatever to a V shape.
Keep a bit of oil on it also.