Slow rpm Chang fa?

[mobile_bob]

Sirpedrosa

i am not much for "an eye for an eye" and all that
and i am ok with turning the other cheek, however

slap me once, i will turn the other cheek
slap me again, and all bets are off!



i don't believe anyone answered to the name "glort" in this thread, so maybe we have nothing to be concerned with.

i just don't see the need to be aggressive with ones response, and i also realize there are cultural differences around the world and with the members of forums, this one being no different.

having said that, one should check himself to make sure he is countering a point of view, not the one who presents that point of view.  when it starts to look personal, it is much like a former supreme court justice said when asked "what is pornography?" his response
"i don't know how to describe it but i know it when i see it"  and i think that philosophy applies hear.

if i have learned anything from life, it is simply this,  "if it don't feel right, it probably isn't right?"

which follows "if it walks like a duck, quacks like a duck" what do you suppose it is?

perhaps we should all take a step back and remember what we type may not be read in the spirit it was written in.

bob g

[38ac]

A Blackstone MP is a single cylinder 31HP diesel engine rated at 850 RPM. I used to own one.  It had a rivited plate next to the governor housing that boldly stated DO NOT OPERATE THIS ENGINE UNDER 750 RPM. It was also in the manual, it didn't go on about it with a long winded explanation, just said don't do it. Why? I don't know?
Just adding this since the one poster stated he had never seen or heard of it.

[BruceM]

https://www.youtube.com/watch?v=xkBKXB--els

Awesome low speed, old iron engine, that Blackstone MP.  Thanks 38ac.

[mobile_bob]

another example of critical speeds i witnessed first hand back in the late 80's while working in a truck shop, they had a "on truck" wheel balancer, that would allow for balancing the complete rotating assemble, hub, drum, wheel and tire.

now i understand that this is a balance issue, but it was very interesting that the critical speed was not a single speed but sometimes as many as 3-4.

lifting the front axle off the ground, just off center, so that one tire was still in contact, but most all the weight off of it, we would spin up the lifted tire with a 10hp 3phase 3450 rpm motor with a friction drive. 

it was interesting to see the doors while closed on the truck, literally hammer themselves in the door jambs to the point that it would chip paint! 

as the wheel began to slow down, the doors would stop pounding, and slow a bit more they would start back pounding again, then quit when it slowed a bit more, etc etc

these were examples of critical speeds that could be correctly rather easily with lead weights,

it was pretty easy to tell, that uncorrected one would not want to drive the truck at any of those speeds, because while the doors probably wouldn't start beating around, it is likely one would feel the vibration even though dampened and decoupled by tire to road contact.

btw, that blackstone is a monster!  and i love it!

bob g

[38ac]

 Bob, as has happened to several other of my diesel treasures a person came along who had a larger desire to own that Blackstone than I had desire to keep it.

As it pertains to engines, broad generalities can almost always be dismissed. As my MP Blackstone proves to say any engine can be operated under rated RPM is simply untrue. However is is true that most engines are happier when not run balls to the wall, maximum RPM and maximum load.  The bulk of the time an engine will let an attentive operator know when it isn't happy. My 1115 Changfa type has a balance shaft as most do and at varying RPMs the gear clatter in serious. Obviously, it will last longer if operated at an RPM where the gears are not being tortured. Single cylinder engines are much more susceptible to this behavior for reasons that should be obvious thus to compare multi cylinder gasoline engine and say what is operationally good for one of those must be OK for a single cylinder diesel just doesnt work. 

[Fred8]

Fred8,

Here is some stuff that might be of interest. It shows some of the info your requesting.

Thanks for proving exactly what I have been saying.

Kubota EB series engines:
https://www.sv-zanshin.com/r/manuals/kubotaebseriesbrochure.pdf

The documents show an engine speed RANGE of 1400 to 2800 RPM on one of these engines and a similar RANGE on the others.  It also shows what the power output is at those lower than max rated power speeds. I don't see anything that says don't operate below this speed or only operate above this speed. 

For my application 2600 rpm’s works best with my EB300 genset.

And for another lower load application 1800 May have been perfectly fine and does not mean the engine is going to self destruct as you have implied previously.  YOUR application is not necessarily the same as everyone else's.

As the graphs show and as I was saying, the output is different at different RPM.  If your engine needs full or close to max RPM for the load on it so it does not Lug, then obviously it will be happier and that's the place to run it.  If the load on the engine is equivalent to what it will put out at a lower RPM, Then Running it at that speed is not going to destroy it because the power  output at that speed is sufficient.

Honda GX390 Tech Manual:
https://www.trictools.com/wp-content/uploads/2015/07/HONDA-GX-390-TECH-MANUAL.pdf
Pages 3 and 4 shows recommended engine speeds.

SPEEDS being the operative word.
Here we see an operating RANGE of 2000 to 3600 RPM.  That is neither " extremely slow"  nor is operating the thing at any speed above 2000 going to cause problems providing the output of the engine at that sped is sufficient for the load.

I also see a note: " Operate the engine at not more than 90% of the maximum horsepower available at a given rpm".  Again what I have been saying, the output changes with engine speed.  And again, it would seem obvious not to lug an engine which will occur then there is too much load for the power produced at a Given RPM. I would have thought people playing in this game would also realise there is no point to running an engine at a speed where it is say producing 10 Hp when the load is only going to generate a max of 5 HP.
Slowing the engine down in this case will save fuel, noise and wear and tear. But it does not seem to agree with the forum agreed position.

Page 10 Resonance Check (critical speed)

This is more to do with the equipment attached than the engine itself. It talks about with the equipment and the engine loaded.

On this modern engine and the Kubot,a There is no referral to Minimum engine speed or speeds to avoid.
I did not look at the first example, there was a warning on my browser about the file so I did not download it. Being a Military engine It would be more than conceivable it had certain and critical parameters.  Same with the old engine mentioned. Obviously if the engines have this specified one would be smart to observe it but how many MODERN engines that one can get now have this?
On the modern engines given as examples, there is proof they CAN be operated at lower than max recommended RPM and I see no suggestion let alone proof that running them outside of the max rated power speed will damage them.

Obviously if there is some specialised engine or something old that has a Min RPM for whatever reason that is different but I would say that does NOT apply to the majority most people are going to use here.

 

I was surprised that a Honda could have such bad vibration at the speeds mentioned.

Engine isn't going to be perfectly balanced at every speed and I note the range you mention is far from the  " extremely slow" You have repeatedly mentioned but also repeatedly failed and obviously avoided to specify what that is and who has said that most likely because you are the only one saying it. 

Clearly there is a forum position that is the accepted one on this and none other will be considered.
If people want to run their engines at full rated speed even if not needed, all good by me.  Shame there is not more emphasis on fact than going with the group mentality even when proven by the manufacturers documents shown it's not the hard and fast rule it's made out to be.

Carry on!   

[guest18]

Deleted

[mobile_bob]

seems to me we have two issues at hand

1.  perhaps glort has a "man crush" on hwew?  and...

2. just because one has never educated himself on things like critical speeds, has never witnessed them, doesn't understand the millions of dollars spent on research to either mask them, or move them outside the operating range of the engine and driven units, and doesn't believe in them "does not" mean they don't exist

i have heard there are those that are "flat earthers" maybe we have one here?



bob g

[guest18]

Deleted

[mobile_bob]

hwew

you should be deeply flattered my friend, it is generally only movie star hero's that have man crushes!

i can feel the warm glow all the way here!

bob g

you know its ok to be flattered, we are an all inclusive, non discriminatory group of folks here, we won't think anything less of you.

[guest18]

Deleted

[mikenash]

seems to me we have two issues at hand

1.  perhaps glort has a "man crush" on hwew?  and...

2. just because one has never educated himself on things like critical speeds, has never witnessed them, doesn't understand the millions of dollars spent on research to either mask them, or move them outside the operating range of the engine and driven units, and doesn't believe in them "does not" mean they don't exist

i have heard there are those that are "flat earthers" maybe we have one here?



bob g

Hey Bob the critical speeds are certainly a "thing" - especially when you have rotating masses at different speeds in different planes

In many of the sites i work on we have "lineshaft" pumps - a series of vertical shafts down a well driving a pump at the bottom in the water, and a 6-cyl John Deere engine (horizontal crankshaft) providing the drive, and, obviously, a right-angle drive in between.  All of these have had critical speed ranges that need to be avoided established by trial and error and both written on the pumpshed wall and programmed into the engine controller

So the combination of the lineshaft & the JD might be fine from idle to 1800 RPM then prone to shaking itself to bits from 1850 - 2200, then OK again from 2200 - 2450

It's certainly a well-recognised phenomenon

[38ac]

Would critical speeds be the correct term for the RPM ranges where the gear train on my Changfa type 1115 sounds like it is crushing rocks? It doesn't have lots of hours, maybe 400 total.  The gear noise hasn't gotten any worse but it isn't operated in the speed ranges where it was bad when new.

[mobile_bob]

i think "any" speed where there is undue vibration is a critical speed
and should be avoided for longevity purposes.

doesn't mean you can run at that speed, but you accept some level of risk in doing so.

every rotating part has its own resonance, right down to the drive belts

no one designs a drive system to run where the belts are hard to keep from jumping all around, when either an increase or decrease in speed can easily control the whipping around, and if you can't change the speed because of frequency control (as in generator use) then you have to use other means to control the resonance/whip.

i noted speeds on my s195 where the balance shafts vibrated and raised hell, now in my case i could change the rpm and stay well away from that speed.  had i not been able to there are ways of at least dampening or basically covering up the problem, that doesn't mean it goes away, or doesn't still exist, just that it might be reduced to an acceptable level.

the gear cover of a changfa has enough overlap on an s195/1100 and maybe the 1115
to allow for drilling/threading and installing an oil return from a bypass filter to admit oil directly onto and into the gear train at the balance shafts.

the gear train normally only gets a misting of oil, enough to keep things lubricated and maybe cooled enough, but not enough to provide damping of those straight cut gears.

years ago i had an old worn atlas 12" lathe, and when running in back gears the gear train was horribly noisy,  a dribble of oil would quiet the gear train right down dramatically.  the same could be done with the changfa's.  we all know they have enough oil capacity to allow for some to be used for this purpose.

back on topic

a few nights ago, while watching youtube, i came across a clip of the american straight 8 engine's

buick among others made straight 8 engine's, the earlier versions only had 3 main brgs, which was ok so long as you didn't exceed some rpm where the resonance/critical speed would allow for enough crankshaft whip to allow the rod big end to contact the lower part of the crankcase, knocking out a hole and you know the rest.

the obvious fix was to add a couple more main brgs, this did not mean that there was no more resonance/critical speed, it was just that it was constrained.

so you either stiffen things up to raise the resonance to a place you are not going to be, or you mask it so it can't be felt, or if you can't do either you make note  in the engineering spec's not to run at that speed.

and no you won't read much about this issue in the common repair manuals, but this stuff is supplied by the oem's engineering dept to those that are incorporating this engine into a piece of equipment.  the equipment engineering dept are sure aware enough to inquire about critical speeds and you can bet they ask and get that information.

i mentioned a while back on this post about a two volume set of internal combustion engine engineering books published by MIT. they have examples of critical speeds and a section devoted to understanding and dealing with the issue.  if i had to advise anyone here about such things, i would certainly favor the published works of engineers with decades of experience over any faceless unpublished character on an internet forum.

bottom line, if you have issues with a project and you are wondering about this sort of thing,  don't take my word for it! and don't take the word of anyone else that claims such concerns are unfounded,  go to the library and do a little reading, it is quite fascinating.

[guest18]

Bob, I know you remember the Honda GX390 generator project that I pulled the plug on.

I remember us discussing how the engine was using a huge amount of gas in an hour running near 3000 rpm. The engine was blowing out black smoke and were scratching our heads for a while until we figured what was going on. We first thought it could be the drive pulley out of balance so the pulley was removed. The engine was ran again and it still had vibration problem from 2600-2800 rpm. But, when the speed was brought up to near 3000 rpm the shaking got so intense that it numbed my hand. I’ve never experienced such vibration on a Honda engine like the GX390. It turned out that the vibration caused the needle valve in the carb not to seat properly and in turn too much fuel was being fed to the carb. I would of never guessed that this phenomenon could happen. The real kicker was the engine seemed to run fine at 3600 rpm. The engine still had vibration but, not as intense.

I have no idea how the Honda engine was assembled in the factory. I could be wrong but I don’t think every engine is balanced separately during assembly. This engine could of had a poor tolerance stack on parts.