Multiple power heads

[cgarman]

I've heard a lot of debate as to the merits of running 2 power heads on a mill. I'd still like to know if anybody has done this, and actually compared the results to a single power head. I assume it's not 2x as fast, but would expect something like 1.3x, etc.

In the meantime, to those of you who say it can't be done, and that it would run worse than a single power head, I offer this...

Technical facts

There's a small mention of how the ignition point of each engine is 'random' making the timing smoother. That makes me wonder if a 2-head setup where the engines aren't synched (every setup basically, since no two saws are identical) would eventually settle to a point where the ignition points of the saws cooperate. Would they fire alternately? If so, the would both contribute to the power output. Even if one saw is running 5% faster than the other, that would mean they fire independently 19 out of 20 times.

I'm an electrical engineer, so I'm curious how the math works out, but I don't have the engine experience to know. Any mech engineers out there?

 

[gemniii]

Have you tried that unique feature called "search"?

There's been several threads about it but start another one.

I might. then again might not, post some links, but I don't have time now.

 

[BobL]

There's a small mention of how the ignition point of each engine is 'random' making the timing smoother. That makes me wonder if a 2-head setup where the engines aren't synched (every setup basically, since no two saws are identical) would eventually settle to a point where the ignition points of the saws cooperate. Would they fire alternately? If so, the would both contribute to the power output. Even if one saw is running 5% faster than the other, that would mean they fire independently 19 out of 20 times.

I'm surprised an engineer couldn't figure this out. Given the engines are connected by a chain under tension, they absolutely MUST turn at the same RPM, otherwise the chain would stretch like an elastic band on one side and bunch up on the other side bar. Now Whether they fire in synch or not depends on where the pistons are in their respective cycle relative to each other when the chain is put on the sprocket. Once the chain is on, the difference between the ignitions is fixed and will not change until the chain is removed.

 

[grandpatractor]

I'm surprised an engineer couldn't figure this out. Given the engines are connected by a chain under tension, they absolutely MUST turn at the same RPM, otherwise the chain would stretch like an elastic band on one side and bunch up on the other side bar. Now Whether they fire in synch or not depends on where the pistons are in their respective cycle relative to each other when the chain is put on the sprocket. Once the chain is on, the difference between the ignitions is fixed and will not change until the chain is removed.

Except for some clutch slippage. That will change things.:msp_huh:

 

[bobt]

This is a thread that I have no business posting in, but here goes.

I would think that getting the two powerheads perfectly in sincroninzation would be the biggest problem. Like BobL stated one side of the chain might get too much tension, while the other side would go into compression. I would expect problems to occur in that condition, and it would actually be worse than using one powerhead.

Allow an analogy:

Years ago, the LS&I RR Co. had a problem at an Iron Mine where they needed to haul more cars of ore out of the mine faster. The problem was a steep grade not far from the mine. They could only pull maybe 40 cars at a time over the hill, because the drawbars would separate under the extreme tension produced by the addition of more cars to the train.

They had an idea to add a "pusher" engine to the train, and therefore double the lengths of the trains being brought out of the mine. Here's what happened.

If the pusher engine was not pushing hard enough, it was worse than before as the engine added more drag to the pulling engine. Drawbars continued to snap.

On the other hand, if the pusher engine was pushing too hard, it pushed the cars together, and the cars bunched up, and derailed. Not what they wanted either.

The experiment was soon discontinued, and a long siding was constructed instead, where the short trains could be stored and coupled together. The siding was of course placed beyond the top of the hill.

Tension and compression ruled the day for the LS&I, and I fear it would be similar for coupling powerheads on a milling setup.

Bob

 

[BobL]

Except for some clutch slippage. That will change things.:msp_huh:

That's true, forgot about that, So the timing difference gap will then indeed be random. Once the clutches engage they will still have to turn at the same RPM.

 

[BobL]

This is something that was recently posted in the Aussie milling forum in a discussion of the engineering and Physics of twin powerhead setups.
I think it's an interesting post especially for those besotted with twin power heads to think about.

My experience with the twin head setup of 090's - the physics of it is mute in the real world usage.

Thats because in our Jarrah hardwoods and the lengths of logs we would slab at 4.2 meters and upward - means that you run out of fuel in one or the other saws long before you cut one full slab off.

No real problem just refuel & re oil the saws, and keep cutting - right?

Ahh - no!

because both saws drive in the same direction, you have to fit one head upside down too the other to get them both pulling the chain in the same direction, which means that the refuelling and oiling fillers are upside down on at least ONE saw!

So,

You take the saw out of the cut which is a laborious proposition when you are on your first cut and have your ladder or rails fixed too the top of the log along which your saw guide runs too get you a flat cut.

Taking the saw out - filling one tank - the flipping the whole rig over too fill the other tank, (and same with the oil) then put the whole shebang back together into the cut and refix the guide rails and try to start 2 saws - and re commence cutting, means that its actually slower to use a two ended saw than it is too just use a one head saw and refuel it in position and keep on cutting!

The downtime, in removing the whole rig to refuel so often soon overtakes any benefit in increased cutting speed from the second head!

So - the whole things a bust and a total waste of time and effort - it looks good in theory but in practice will drive you nuts.

The whole two heads thing is moot - anyone with any experience soon works out its a sheer waste of time for the w a n k factor and that if you have to do it too make a living - you'll soon see sense and go back to just one drive head and the right chain and get the job done in a timely fashion.

Most anyone with any experience - eventually gives away the whole chain saw slabbing idea and buys slabbing attachment for their lewis saw, driven by the 20 hp 4 stroke motor and just whips off slab after slab while the 090 twin head guys are spending all day twisting spanners to keep fuel up too their saws thru the crazy refueling procedure with one saw head upside down.

 

[betterbuilt]

I was asking a client that I have who designs gears and such about this. He thought about it for a while and said it wouldn't be much faster or more efficient. He said the two powerheads would fight each other just like BobL and bobt said. I'm gonna give it a try in the spring anyhow.

 

[BobL]

I was asking a client that I have who designs gears and such about this. He thought about it for a while and said it wouldn't be much faster or more efficient. He said the two powerheads would fight each other just like BobL and bobt said. I'm gonna give it a try in the spring anyhow.

I don't recall saying this. I don't think they would fight each other any more than one piston fights another in a multicylinder engine.

 

[betterbuilt]

I don't recall saying this. I don't think they would fight each other any more than one piston fights another in a multicylinder engine.

True, I mixed you up with someone else. I can't figure out where I read that.

 

[bobt]

I don't recall saying this. I don't think they would fight each other any more than one piston fights another in a multicylinder engine.

Hmm, why would you say that? In a multi cylinder engine, the cylinders are fixed to the same crankshaft and they work together. But then if they are not in perfect timing, the engine's performance suffers. Consider two cylinder engines that have two sets of points. You have to time the engine for max rpm for each cylinder seperately from each other.

In a multiple powerhead milling situation, they are separate, and unless the powerheads are EXACTLY throttled to provide the same power even though they are turning at the same rpm I figure that there will be loping and galloping between those powerheads. That situation can't be advantagious. Am I wrong???

Maybe I am way off on this.

Of course your post about the powerhead needing to be mounted upside down pretty well nullifies this whole argument.

Edit: Um, thinking this over, both powerheads need to be oriented the same way,,,,no?

Bob

 

[PhilB]

Am I missing something? (one power head having to be upside down). Consider the power head in the normal (one head CSM) position, looking down on it, it turns ccw. Add the second power head to the other end of the bar (rotated 180 deg) but not upside down. It too turns ccw. Both pull the chain. The normal head is pulling the chain cutting the slab while the second head is also pulling the chain but not cutting (the return side of the bar). It seems to me that the chain would have to be kept tight to work properly. A new chain with its stretching could be a problem.

I'm considering making a two power head mill and am interested in further discussion.

 

[PhilB]

Another way to picture this: Imagine you're freehand milling. The chain is pulling towards you (normal sawing). Now walk to the other side of the cant and finish the cut. The saw is cutting on the back side of the bar pushing away from you, if you didn't turn the saw upside down. The chain is cutting the cant in the same direction.

Phil

 

[glennschumann]

Redux

We visit this topic from time to time, and it is worthy of discussion for the new folks. The search engine isn't always the best for finding the information, so below are my thoughts:

A double ended mill does work, and no, you don't have to turn one engine "upside down".

The two saws do not need to be perfectly synched... if one saw runs 12k at WOT, out of the wood, and one runs at 11k at WOT out of the wood, when the mill is set in the wood, both saws are turning at, say, 9k, excactly at the same speed. Neither saw will know that the other one is attached, as each one is "trying" to get back up to its max rpm by chewing through the wood.

I also agree that a double ended set up is heavy and difficult to use, expecially without a helper. If, however, you have two 066's and a 6' log, a double ended bar is probaby the way to go. Those slabs you cut are going to require a couple of people to move around anyway.

Will a double ended set up be twice as fast? No. Will it make an impossible job possible? Probably. I'm just waiting for the right log to justify the investment. I probably would not use a double ended set up on anything that I could get an 066 to do by itself, however

 

[BobL]

Am I missing something? (one power head having to be upside down). Consider the power head in the normal (one head CSM) position, looking down on it, it turns ccw. Add the second power head to the other end of the bar (rotated 180 deg) but not upside down. It too turns ccw. Both pull the chain. The normal head is pulling the chain cutting the slab while the second head is also pulling the chain but not cutting (the return side of the bar).

You are right - methinks the aussie poster is spinning a bit of a yarn!

Saws like 090 and 075/76 are still a PITA to refuel mid-log

 

[rarefish383]

An other point to the refueling problem. If I were going to run a rig that big I'd bypass the factory fuel tank and mount a bigger tank in the middle of the rig some where and run fuel lines straight to the carbs.

I do have a pair of 100cc power heads, but I don't want to hook them together. One is on the alaskan, and I'm making a heavy duty mini mill to make a bit of a monster beam cutting rig with a 24" bar.

The problem I see with two power heads is your chain still has the same size teeth and you still operate at a max RPM, so you will only cut so fast. Granted, the extra power head should help you maintain your max RPM under load. They make bars for twin heads, and if they didn't work at least half way well, people wouldn't buy them and they'd quit making them.

As you can see, I really don't know anything about twin heads, I just haven't made a post in a while, Joe.

 

[cgarman]

I'm surprised an engineer couldn't figure this out. Given the engines are connected by a chain under tension, they absolutely MUST turn at the same RPM, otherwise the chain would stretch like an elastic band on one side and bunch up on the other side bar. Now Whether they fire in synch or not depends on where the pistons are in their respective cycle relative to each other when the chain is put on the sprocket. Once the chain is on, the difference between the ignitions is fixed and will not change until the chain is removed.

Why is it that every time I say I'm an engineer people think I'm trying to 'pull rank'? I was just looking for an answer, and didn't want people to shy away from throwing some math my way.

Wow guys, thanks for jumping my $#!+. This engineer already worked out the clutch slippage idea and the fact that the saws would both produce a torque vector in the +Z direction (you know, up) without flipping one over.

Also, thanks for suggesting a web search. I have, and I keep getting people who say it doesn't work for the above reasons. Meaning they haven't tried it - otherwise they would have realized these 'problems' don't exist.

So has anyone actually tried it? It SEEMS to me like you'd be able to keep the chain speed up without the loading. Meaning you'd be able to (and have to) push on the saws to get a faster cut. This might work well with an inclined setup like BobL uses.

 

[Timberframed]

I had recently posted a video of two 090's going at a 52" or so Ash log last month that I taped myself on site 5 years ago. Definitely faster but I won't consider the investment. Doing just fine with a single head setup. The ox is slow but the Earth is patient.

 

[betterbuilt]

You know the more I think about the dual head thing one thing does come clear. The guy who thinks that one saws gas tank has to be down obviously hasn't tried it. If he actually tried it that way, I can only imagine what might have happen. Not to mention the chain would be on backwards on one saw.

Got this from this thread.
http://www.arboristsite.com/milling-saw-mills/104534.htm#post1637673

 

[cgarman]

You know the more I think about the dual head thing one thing does come clear. The guy who thinks that one saws gas tank has to be down obviously hasn't tried it. If he actually tried it that way, I can only imagine what might have happen. Not to mention the chain would be on backwards on one saw.

Got this from this thread.
http://www.arboristsite.com/milling-saw-mills/104534.htm#post1637673

That was kind of my point - lots of armchair experts around.

Then again - maybe the guy who says it's inefficient was running one saw backwards. In that case, I guess he's right.... 090 Contra - 076AVE = 029?