Lathes and dynamic balancing

[mbensema]

Thanks for picking up my omisson gamachinst, I should have mentioned that if the counter weight is drilled and tapped that weight will always need to be added. Adding it on the compensating weights is a much nicer way to do it. It's nice balancing round machines, not so much to take into account [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]
How does your machine work, do you do several runs with trial weights at different locations or is it a one shot deal with a trim run? I used to work for a centrifuge manufacturer that built their balancing machine and could balance a 2 ton part in one or two trials.

[art]

hi there i was given a bal machine.im going to be bal. singles high rpm up above 16000. thay run a needle bearing on the bottom of rod with a press in pin .to bal them i need to scale the top end stuff ,as per top pin piston top bearing and rod wt of top end at 90 deg right . then i need to scale the rotating or btm end . as per big end of rod and caged needle bearing at 90 edg on the scale. made a fixture to hold the rod when i get the wht. of the rod ends . you say to use 65 percent for 10000 to 12000 how about 12000 to 16000 rpm.

so now i have the top end whts .and the btm end whts now how do i figure the bob whts. does it mater if the bob wht is put on as in where its installed .my cranks are round with offset pin pressed in . jim

[gamachinist]

Art,you'll have to take a guess at the % on the higher rpm.Since the chart I have jumps 5% for each 2000 rpm,I'd say go to 75% on the recip weight.
It looks like you've got the idea as to weighing the rods out.Use 100% of the big end of the rod and the bearing.As per above guess,use 75% of the recip weight(piston,pin,locks,bearing,rings,and the small end of the rod) added to the rotating weight.Most bobweights are aluminum V blocks that are bolted together with washers added to get the right weight.I'm going to pick up a scanner in the next few days and can scan the instruction sheets so you can see a picture.PS weigh in grams.BTW what brand of machine is it?Did you get any paperwork or instructions with it?

mbensema,I very seldom get it right in two or three tries.Sometimes it falls into place but most of my jobs involve major changes in weight of parts so they almost never go easy.The strobe gives you a reference point to add or remove weight.Sometimes you do what it tells you and you have to undo 50% of what it said or it will sometimes throw off the other end or some other curve ball.It isn't one of my favorite jobs to say the least.I always try before I drill for sure but even then it doesn't always go as planned.Often,shifting the weight 10 or 15 degrees will make it better.
Robert.

[Hackasaw]

Single cylinder engines are inherently imbalanced.They cannot be brought into perfect balance but:use 100% rotating and
55-58% for 7500-8000RPMs
60% for 8000-10,000 RPMs
65% for 10,000-12,000 RPMs
Now onto Motorcycles etc.
This says for single cylinder engines use 61% recip and 100% rotating


does this imply that single cylinder motorcycle engines are balanced to over 8000 rpm? I do know of some ultra low rpm singles that use this 61% but what are these 10,000 rpm engines? two stroke racers?

what about a briggs that never sees 3000?

I find the specs somewhat odd from this respect

[Butch]

Visit your local Kart racing track. You'll see some Briggs turning more than 3000. Those modified briggs sound like flat track mc when then wind them up.

[gamachinist]

I don't know Hackasaw.As for the Briggs,I would look in a go-cart racing manual for info.I've never balanced a single cyl engine so I can't tell you anything from personal expeirence.I never tached a two stroke but 7500 RPM doesn't sound out of line.I don't think most large bore four stroke singles would be comfortable at those speeds(at least not the older ones).
As for finding the specs odd,I agree.It's like porting and polishing.Everyone has a different idea about what is best and has a good explaination that contradicts the last thing I read on the subject.
For instance,look at the difference in recip % on a Ford 3.8 vs. a Buick 3.8.They have basicly the same configuration but the recomended % are different.Why does the BSA twin have a different % from the rest of the twins?Does a Triumph or Norton use the same as a BSA or the other figure?Beats the hell out of me General.For more detailed info the "man" is Gary at Automotive Balance Service(714-671-0728).He is the one I called about a Corvair as I had never done one.Perhaps a visit to an automotive website would give you more info than my expeirences(if you can get past the Ford vs Chevy vs Dodge etc ad nausium).
I'll try to help if you have anymore questions.Robert.

[gamachinist]

Butch I've done a little work on racing Briggs engines but never assembled one or heard it run.One I did last year was a stock appearing class that the owner wanted things done that weren't possible.IE boring out a carb throat larger than the outside diam.Got impatient with me after I made a torque plate(a waste of time imho) and was almost done when he pulled the job.
I took one in from someone I knew a little better that turned out a little better.Sleeve an alum bore block,bore a NIB sleeved block and touch up the valve seats.I thought about asking about balancing it but he didn't ask so I let it slide.
I don't know what they turn up but I think it is over 6000 rpm.
Robert.

[Hackasaw]

THe norton commando uses a much higher factor........ it's engine, transmission, and swingarm are mounted in "isolastics" shimmed away from the rest of the bike anywhere from 2 to 10 thou..... I've set mine on the closer side and live the the vibes but too close nets you a broken frame even with the third isolastic unit upgrade.

Small two strokes often see over 10 grand but the larger a two stroke gets....... the slower it will typically rev and yes...... especially the older ones whose very long strokes pushed the limits of piston speeds at even moderate revs. I think a YZ or KX 80 will buzz up to maybe 14 grand but the needles tend to skate on the rods so goodbye crankpin hello new rod......... the little punk shouldn't have missed the shift [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]

A counter balanced four valve 650cc Honda single will comfortably rev over 7 grand and I don't remember how far but I think I had my 500cc Ascott kissing 8ish grand before it resembled the exxon valdez carnage.

Alchy burning briggs see 6500 max unless people have been lying to me

Just weird the two different specs for motorcycles and then just plain singles...

oh..... you never told me how fast your dynamic spin balancer wound up during proper use........

[Hackasaw]

oh........ model airplane engines spin up pretty quick too......

[stephen thomas]

Robert-

thanks for taking the time to make this list! I have printed it out and put it in one of my motorcycle engine books. Might help be inspiration to get around to building a bike out of the Routt kit and new crank bought 20 years ago for the Bonny that broke one. Or maybe finishing the cases on the triple that tore up the gearbox a dozen years ago. Families come, and time flies by. But don't guess I'd have it any other way. again, thanks. smt

[gamachinist]

Stephen,are you refering to a Trident or Rocket 3? I may have a spare five speed left side shift(I think the gears etc are the same on the five speeds).Robert.

[gamachinist]

Most of the time the balance RPM for V-8's is 450 to 550.The heavier the assembly the slower the speed.A single will probably balance at about 700-800 RPM(just a guess).I've got a Briggs shaft lying around that I might set up for funsies to see.If I find a bunch of spare time I'll break down a Triumph twin junker to check it out too.It'll be a while before I can dig the twin out as I may be going up to Clevland Ohio to pick up a mill I bought on eBay.
Robert.