Low Cost DIY Rod Weighing Setup

[nm9stheham]

FWIW…. Pishta and I have been setting up low cost, home-shop weighing equipment for big and small end rod weights. This is a summary of one setup that is simple to build and repeatable. It is not the ultimate in accuracy but will measure end weights to a few grams tolerance, which is around 10 times tighter tolerance than the standard factory balance tolerance. And with a small expenditure…..

BTW, if you want to see it ‘in action’, skip 3 posts down to get past the theory and build info.

BACKGROUND OF ROD WEIGHING TECHNIQUES:
The standard methods of rod weighing divide the rod weight along the beam axis (the long axis) on either side of the rod's center-of-gravity. The usual devices hang the rod so that the plane of the big end is vertical. You can see these jigs all over the internet, like this:
Connecting Rod Balancing Stand

The rod's big end plane being vertical is done so that the equipment is adjustable and is easy to use, but there is nothing requiring the plane of the big end be vertical; the plane of the big end can be horizontal or at any angle, as long as the beam axis of the rod is accurately held horizontal.

The jig described here is one that holds the rod horizontal on a flat plate, but it works on the same exact principle: support each end on a line across the exact center of each end, with the beam axis level, and then weigh one end at a time. This jig generally achieves a +/- 2 gram repeatability, but is not as fast and convenient to use as purchased hanging setup. So it’s not suited for a pro shop, but for the price a handful of scrap materials, IMHO it works to expectations for a home shop, and gives the accuracy needed for a low-mid range hot-rodder.

NOTE on the scale used: If you are using a cheapo scale, they have ‘issues’ for this type of use regardless of the type of jig or rod orientation. Their readings will be thrown off with miniscule side loads or tilt loads, due to their cheap design (which do not have adequate numbers of electronic load cells to compensate for these loads).

Work-arounds are described for these cheapo scale problems for this flat plate jig, which only creates side loads on the scale, not tilt loads. The ‘hanging rod’ type of jigs put tilt loads on the scale, and that does not appear to be manageable with the cheapo scales.

 

[nm9stheham]

Layout:
Below is a pix of the layout of the jig plate. The jig is a flat aluminum plate with one support at the scale end and 2 pivot supports at the unweighed end. The general design is as follows:

• Flat aluminum plate, about 10"x 5". I used .060” thick, half-hard aluminum which I had as scrap. This can be thicker; more weight introduces more potential for inaccuracy if the layout is off. The plate should be as flat as you can get it, so that when laid on either side on a perfectly flat surface, it does not rock.
• Corners should be perfect 90 degrees, and edges very straight. It’s necessary to make the plate as squared-up and symmetrical as possible, as any assymetry around the center axis puts a bias in the weight results.
• Imagine dividing the plate exactly in half (see black dashed line in pix). Any hole on one side of this center axis has to be matched by the same sized hole on the opposite side, even if the matching hole has nothing in it.
• The single pivot pin over the scale is fixed, short length, and has a smooth rounded point on it to allow it to slide easily.
• The 2 pivot pins at the opposite side are taller, threaded, and have sharp points at the bottom. These are also the height adjustments to level the plate.

The 2 red dashed lines in the pix are the lines on which the pivots are located. The lines need to be exactly parallel to each other and parallel to the left and right ends of the plate. Their distance apart is the rod’s center-to-center length. (6.123” for a SBM rod.) They need to be placed exactly the same distance from the center axis (and so are also the exact same distance from each end). Care in the layout of these lines and the hole locations pays dividends in final accuracy.

The 2 registration circles are laid out around the center of the scale pin as precisely as possible, and are what I use to visually center the rod’s big or small end around the scale pin for weighing. There is probably a better system for this, but these circles were quick and easy.

The next pix is an underside view so you can see how the pins mount in the plate. The scale pin is just a small round headed bolt, with a smoothly rounded tip. The 2 pivots pins are straight pieces of all-thread with the lower ends sharpened, and locked in with nuts. All are ¼”, but they can be bigger. These pivot pins need to mount so that they are perpendicular to the plate, and so that their lower tips are parallel to each other, so the whole plate can pivot evenly on these pins’ tips.

There are 2 small spacer plates used to keep the rod’s beam axis horizontal when laid on the plate.

 

[nm9stheham]

Setup:

Start with a very level and smooth work surface. I use a small machinist’s flat, but something like a chunk of flat granite or very flat formica or Corian counter top can be used. This needs to be leveled first. I set mine on a thick cloth to allow some slight give to the mount, and used large flat washers to level it.

Next put the scale in place and then place the assembled jig on the work surface and scale. Use the threaded pivot pins to level the jig plate, both in length and across the width.

In the side view pix, you can see a small jar lid inverted on the scale with the jig’s scale pin resting inside this lid. I put light oil or WD-40 under this lid to help make things move smoothly over the scale surface. This lid may not be necessary in all cases, but is part of the need to relieve side loads if you are going to use a cheapo scale and get past their issues. A thin piece of smooth Teflon would work too.

The last piece of the setup is to place one of the spacer plates. These are seen in the last 2 pix below at one end or the other. These go under the big end of an SBM rod and are simply used to make the rod rest level, despite the rod’s big end being narrower than the small end. I used .090” aluminum, and these have to be cut in nice, square-cornered rectangles and holes drilled in the exact center so they rest symmetrically on the weighed or unweighed end, and don’t effect weight at the other end. Without these spacer plates, the big end weights will tend to weigh up about 3 grams heavy due to the rod not being horizontal.

Once set up, then zero out the weight of the jig on the scale. If you are using a cheapo scale, then first tap lightly several times on the work surface, using a small wooden block to tap. This will ‘jiggle’ the setup and relieve any side forces on the cheapo scale; you’ll see the scale reading ‘settle in’ and not change with further tapping. Then zero the scale. Whenever you place or remove the spacer over the scale pin, then the scale needs to be re-zeroed.

Initial test is to place a weight on the unweighed end of the plate, right on the line between the 2 pivot pins. If things are right, there will be no weight change on the scale. This is shown with a 65 gram socket placed on the line of the 2 pivot pins by eyeball, and the scale still shows zero. This was done 3 times with the same 0 result. Next pix, the socket has been moved off to the left and the scale shows -6 grams. This illustrates how any asymmetry in the plate will throw off accuracy.

 

[nm9stheham]

Weighing a rod:

The next pix’s show actual weighing of both end and the rod total. Note that a spacer plate is always under the big end of the rod to keep the rod level. For each weight:

· Place the spacer plate, tap the work surface to relieve the side loads if a cheapo scale is used, and re-zero the scale.

· Then place the rod, using the registration circles around the scale pin to locate the weighed end of the rod as precisely as you can, and the center hole at the unweighed end to align that end of the rod.

· Then tap the work surface to relieve the side loads if a cheapo scale is used, and once ‘settled in’, take the weight reading.

· As check on your work, take off the jig, re-zero the scale, and take a total weight reading. (Note that even a cheapo scale will NOT suffer from side-load or tilt errors when just the rod is placed on the scale by itself.)

The BIG test of how well this set-up works is to weigh both ends, and then check to see if those 2 weights add up to the total rod weight. I usually get up anywhere from 0 to +/-2 grams error in this check. If it exceeds +/- 3 grams error in the check, then I re-weigh. And if I want ‘final’ numbers, I’ll take each weight 3 times and average the readings.

 

[nm9stheham]

Observations on the scales:

· The issue of side-loads throwing off the scale readings is unique to the scale, not to the jig. The tapping on the supporting work surface with each change is to ‘jiggle’ things, and let them move the tiny amount needed to relieve the miniscule side loads that result from even the tiniest movement of the jig or rod vs the scale.

· I have not yet sprung the $$ for a better scale, like the ones from Summit, to see if they are better for this problem. Without a better scale, I can’t separate out the sources of error: the jig itself, my use of the jig, or the cheapo scale.

· IMHO, springing for a good scale is a higher priority than an expensive rod hanging jig.

· The cheapo scales seem to come pretty well calibrated.

· DON”T EVER PUSH THE CAL BUTTON ON THESE SCLAES UNLESS YOU HAVE THE CORRECT CALIBRATION WEIGHTS on hand. If you do, you are hosed!

· As your own calibration source, buy a 1 or 2 cup plastic measuring cup and weigh it with the scale when new. Mark the cup weight on the cup and keep it. If you want to check the scale, then you can:

o Weigh the cup to see if the reading is the same

o Then add cold water with the same burette you use for measuring head chamber volumes. Each cc weighs almost precisely 1 gram per cc, so if you add 100 cc’s of water into the cup, the weight on the scale should go up exactly 100 grams.

 

[67Dart273]

I'm certainly no expert on rod weighing specifics but it seems to me that having so much weight into the jig itself would cause inaccuracy simply because it starts to become more and more of the total combined rod and jig weight. I'd think a "skeleton" jig for lack of better word that is much lighter would then allow the small change in rod weight (between rods, etc) to then be a larger overall change in comparison.

"Extremes" Let's say the jig is about equal to or double the rod weight. You could reach a point where a change in rod weight that you might desire correction would be so small in comparison to the total weight that the resolution of the scale simply will not show it up

On the other hand if you could get the jig down to say, 1/8 of the rod weight, then any change obviously becomes a larger percentage change

I give you guys credit.........You are always thinkin' !!!

 

[nm9stheham]

I'm certainly no expert on rod weighing specifics but it seems to me that having so much weight into the jig itself would cause inaccuracy simply because it starts to become more and more of the total combined rod and jig weight. I'd think a "skeleton" jig for lack of better word that is much lighter would then allow the small change in rod weight (between rods, etc) to then be a larger overall change in comparison.

"Extremes" Let's say the jig is about equal to or double the rod weight. You could reach a point where a change in rod weight that you might desire correction would be so small in comparison to the total weight that the resolution of the scale simply will not show it up

On the other hand if you could get the jig down to say, 1/8 of the rod weight, then any change obviously becomes a larger percentage change

I give you guys credit.........You are always thinkin' !!!

It's not about the total weight of the plate in this design... it is about how well the symmetry of the plates layout is maintained.

Tnx Del,
If the jig's symmetrical design is maintained in the construction, then:

• the jig weight on each end will be identical
• there will be no leverage of uneven jig weights from one end effecting the other end
• thus, all the weight of the jig will correctly zero out at both end of the rod.

The above can be shown via 1st level static mechanics. (Free body diagrams and all that stuff....)

Now if the jig gets heavy, then inaccuracies in layout and build can lead to assymetrical weights in the jig plate, and that can lead to biases in the weighing. If that happened, the big and small end weight totals would show a bias above or below the total rod weight. So, yes, you are better off keeping the plate light, so that any layout inaccuracies have very little effect on the measurement results.

FYI, this total jig weight is 173 grams, including the bottle cap 'slider' on the scale. That is 24% of the lighter stock SBM rods, and around 28% of the lightweight aftermarket I-beam rods.

 

[MOPAROFFICIAL]

Always swung them sideways, never flat.

 

[moper]

Always swung them sideways, never flat.

That's how I used to always do them too. But "technically" they could be in any position, as long as the centers of the big and small ends are aligned. It's just easiest to use a couple shafts on adjustable mounts to do it as far as I'm concerned.

 

[nm9stheham]

The one thing this jig will do is allow you get by with a $20-50 scale and obtain usable results, since this jig cannot put a tilting load on the scale. I did not realize all this going in about the cheapo scales..... it just worked out that way.

I've tried the conventional sideways hanging method with the cheapo scale, and the results were all over the map, and changed hugely with just a tiny change in the angle of how the rod set on the scale. Pishta can comment, but is sounds like he ran into the same problem.

Always swung them sideways, never flat.

Yes, that is the conventional method. But seriously, it does not matter as long at the long axis (beam axis) of the rod is level, and your support points are on lines across the centers of the ends, perpendicular to the long axis. That is what allows you to correctly separate out the 2 end weights per the standard methods for balance work, and is exactly how the conventional jigs work. That was all figured out before even going down this road.

I've just about decided to keep the SCAT I-beams I have been admiring in their box and, if so, will weigh them all with this, and we can see how close it comes to their marked weights on the box.

I'd be curious as to the brands and models of scales that the pro shops use here.

 

[pishta]

...having so much weight into the jig itself would cause inaccuracy simply because it starts to become more and more of the total combined rod and jig weight...
On the other hand if you could get the jig down to say, 1/8 of the rod weight, then any change obviously becomes a larger percentage change

I give you guys credit.........You are always thinkin' !!!

These load cell scales are most accurate at 75% of their rated load. If you tare (zero out) the weight of the jig, then anything on top of that will be the actual weight. So summation of jig plus rod weight will only increase accuracy up to the rated weight. Most load cells can handle 300% of their rated load, but the rated load is zone where they are most accurate. I believe industrial grade scales have 3-4 load cells in planar series to reject angular force irregularities. Im gonna give the flat plate design a try, already got the plate cut out of some lightweight 22G aluminum from the bottom cover of a cassette deck.

 

[pishta]

Didnt work for me. How are you centering the rod on the plate? Are you using the part line of the cap onto a reference line or something? I would make a mandrel on top of that spacer plate so the big end is always centered on it. I tried to make concentric centering rings on both ends so I could flip the rod without disturbing the plate on the scale. You have to account for the fatter little end so it ends up looking like half a donut (big end riser and centering ring) around a piston pin on both sides. F'n cheap *** single sensor scales.....

 

[nm9stheham]

Uh-oh.. an unhappy customer, dang LOL. For registration circles, I just carefully scribed some circles exactly around the center of the scale pin, at the ID of each rod end. Is your 'spacer donut' arrangement perfectly circular and centered? Seems like it ought to work, but in thinking about those before, they cannot be tight, as if they are off a tiny bit, they could tip a bit and their contact point on the plate would not center on either line. Maybe set that aside and just try a small rectangular spacer plate for now.

If that is 22 ga AL, that is only .025" thick, correct? Look to check if it is bowing any. That might throw things off if it does, as the contact points may move outward by different amounts at each end, and the rod ends would no longer be supported at their centers. Is the scale pin a round ball end so it can slide easily on the scale surface?

And are you re-zeroing the scale as you change from one end to the other and moving the spacer? It might not be clear enough in the write up, but I HAVE to tap on the work surface to get to my cheapo Ohaus scale to settle in after ANY change, and the 'settling in' tapping ritual has to be done done before zeroing out the scale to get rid of the jig weight. Even if I slightly re-position the rod, I have to then tap the work surface a half dozen times to get the side-loads to go away for those cheapo scales.

If I get time, I will do more with the cheapo Horrible Freight scale like yours; my 1st test of that seemed to work A-OK but I did not spend much time with it.

 

[pishta]

OK, reproduced your jig as best as I could using a bearing instead of the 3rd pointed thingy. My pan looks to be about as thick as yours, no deflection at all under weight. I F'd with this thing for 10 minutes then moved the entire thing to the very level concrete floor of the garage. WOW, what a different. It was looking alot better on the rigid surface as I didnt have that chunk of metal you had as a base. Il upload a video of all the drama, kinda boring if your not into this stuff but just goes to show you what the most minute positional changes can do to these scales. funny thing is this scale is incredibly repeatable when you lay something on it, but these loads off a fulcrum really send it to ****. Under the big end center is a small hole that the bearing sits in so as to eliminate any side load and distance variation. Pardon the upside down video, dont know how to rotate it.

 

[RustyRatRod]

Cool stuff, yall!

 

[pishta]

just noticed a flaw on my part. See the cork riser for the big end? Its not on centerline...wonder if that is an issue? Ill put them on center tomorrow. Neither ends add up to 752.....

 

[pishta]

Rusty, go to bed! It must be tuesday morn already in GA!!!

 

[RustyRatRod]

Rusty, go to bed! It must be tuesday morn already in GA!!!

It is, but I have gone to a four day work week because of health issues, so I have Mon, Tue and Wed off. Yay me.

 

[gliderider06]

Excellent read! Should be a sticky.

 

[nm9stheham]

just noticed a flaw on my part. See the cork riser for the big end? Its not on centerline...wonder if that is an issue? Ill put them on center tomorrow. Neither ends add up to 752.....

Yes, everything needs to be symmetrical from one end of the plate to the other. In that way, any weights associated with the plate will divide exactly evenly between the 2 ends. And, yes, I put my spacers for the big end right on the 2 lines, and made them symmetrical around one end line or the other; in that way, nothing on the plate will create any leverage on the other end.

The BB seems to work really well! The single point contact on the scale seems to be the key to make the cheapo scale behave reasonably well.

OK, on the concrete floor; that is a good change. I suspect you removed a variable that was allowing the scale itself to move a minute amount. Next time you look at this, try tapping the concrete floor beside the scale with a block of wood or a dead blow hammer and see if the readings 'settle in' to a consistent number. The minute vibrations seem to relieve any residual side or tilt loads that effect the scale.

As for the end weights not adding up.... the big end weight you got looks totally consistent with what I am getting for the stock heavy rods. The small end weight looks quite a bit off; I ma seeing 240 +/- grams consistently. It is almost like the jig weight was not completely calibrated out for the small end measurements....?? That is where tapping on the floor to 'settle' things may make the zero-ing out of the jig weight better.

These cheapo scales sure take a lot of 'finageling' to work with for this...

 

[nm9stheham]

Excellent read! Should be a sticky.

Hopefully, things like this contribute to folks becoming more educated about balancing and less intimidated by the balance process. It is all set up to be a very cut-and-dried procedure. IMHO, gearheads who are at a '3rd level' of motor-building, like those who have good tool skills and who have advanced to things like setting valve spring heights, and who are comfortable with doing some basic math, have the level of skills to do a lot of this (if they want to). The rod/piston balance part, and computing the resulting bobweight to which the crank is to be balanced, is one that can be done in a home shop to a very good degree of accuracy, if the tools and procedures and care in work are there.

Now, the crank balance needs the proper machinery so that stays at the machine shop. (Or a wildly good static setup LOL).

 

[pishta]

if we could get very fast reporting via networked load cells, it could be set up in such a way that the entire crank V-block fixture could be on these load cells and reporting to a microprocessor in tandem with a magnetic pickup degree wheel to report when these events occur in degrees during a spin up, you could build a radial balance fixture. Back to the balance fixture.....Mark, did your ends weights add up? I feel were getting very close to a cheap balance fixture design.

 

[dartfreak75]

Very cool thread. Good info! Thanks for this. Someone should make this a sticky.

 

[nm9stheham]

if we could get very fast reporting via networked load cells, it could be set up in such a way that the entire crank V-block fixture could be on these load cells and reporting to a microprocessor in tandem with a magnetic pickup degree wheel to report when these events occur in degrees during a spin up, you could build a radial balance fixture. Back to the balance fixture.....Mark, did your ends weights add up? I feel were getting very close to a cheap balance fixture design.

Yes, you can see it in the 3 pix in post #4; the rod total was 1 gram different from the sum of the 2 separate end weights; that 1 gram difference is beyond the ability of this scale to resolve. I have been doing all 3 weights on each rod and checking the sum of big and small ends vs the trod total as a check on measurement confidence.

If I get to + or - 4 grams difference between the rod weight vs the sum of the big and small end weights, then I re-weigh. This is what IMHO is a weakness in just weighing the big end and the rod, and then subtracting to compute the small end; the big end weight can be way off and you would never know it if you don't do the complete set of 3 weights on each rod. (Or, you spend some big $$$ on professional setup like the CWT system.)

 

[pishta]

no love with the small .1g scale. Revised table with risers inline with centerlines did not help, although it lowered the little end weight from 265 to 223! That was expected as the big end was supported closer to the little end. Big end was more difficult to weigh I got a 10g variance but Im thinking its closer to 499 than anything...but the total rod weight is unchanged at 753? Will try the 1g scale again. Who needs .1g resolution anyway! Ill spare the boring video, although the little end was highly repeatable within .4g. My hunch is the bearing support socket is causing side load again. Gonna run a full roller next....and the beat goes on...