Math/physics question on twin drive Stihl 090

[gerhard]

Hi all,

in the thread "How good is the 090 Stihl really?" some marvellous contributions whizzed by. Especially the tandem drive milling saw featuring two 090 units (adding up to 17 hp in one machine) in Sawchain's post was very intriguing.

When it comes to steering threads off track with farfetched examples and lengthy yarns, i'm guilty as hell. I only mean well and try to steer back every time, but this time i got more flak than usual, so i decided to put my question before you in a new thread.

The question itself is introduced in the first attachment. The second attachment is about the source where i got the torque statistics from.

There must be some math and physics savvy forum members around to come up with the right answer. My brain bogged down while trying to.

thanks in advance and greetings

gerhard

[BobL]

My intuitive answer to your question is the chain will distribute the back tension produced by the wood being cut between the two bearings.

[dai sensei]

I'd agree Bob, equal, based on Newton's law.

[Travis Edwards]

Thats an interesting one gerhard and honestly I guess it was one met by the guys who were using the big twin power head crosscut saws long before the twin 090 setup came about. The twin power head is something that has been around for a LONG time, well compared to what we now define as chainsaws anyway. My understanding of how it would work would be that the chain in the cut (C to D) would load up under the force that is required to cut, meaning that power head 1 would start to lose revs, which in turn means that within milliseconds the chain between A and B would pull tight so the saws would reach equilibrium, this said however the load on the CHAIN between C and D would always be higher due to the force required to pull the chain through the cut, and the fact that there would be 8.5 hp or 4.6 NM of torque pulling between A and B but this would convert to the full 17 hp and 9.2 NM (minus a bit for friction etc.) pulling between The entry of the cut on the C side and the sprocket of power head 1 on the D side. This would cause any slack in the chain to be on the C side of the number 2 power head hence chain tension would be somewhat critical so as not to allow the chain to jag or bite on the exit of the sprocket or the entry of the bar on power head 2. This is one of the reasons I do not like cutting with the top side of the bar with a chainsaw mill. It is like trying to run the feed of a conveyor in reverse which takes any slack from the bottom side to the top side and then causes idlers etc. which have been designed to take the force in one direction to collapse. (believe me it happens.) A lot of the older crosscut saws I have seen actually had a much smaller power head on the number 2 end, and was basically there as a helper rather than an equal. I have also seen this configuration on alaskan mills of the 70s and such where they would have a big saw (090 or the like) as power head 1 and a smaller helper (such as an 031) as power head 2, however this may have been due to the fact that these were the saws that the owners had.

I once saw a saw in a timber museum when I was a kid on a trip to new zealand which on the main drive end had what would have been approx a 9 hp four stroke and the other end would have been approx 3.5 to 5 hp also four stroke. The chain that this saw ran was nothing like what we know as saw chain and was in fact very much like having the teeth of an old crosscut saw attached to a chain if you can imagine what I mean, and was sharpened in a similar means and appeared to even be set like a crosscut saw.

Sorry I cant answer the mathematical side of the question (my patience for mathematics is wearing thinner and thinner as my age increases.) Hopefully I have been of some use in the matter though.

[Travis Edwards]

Actually this leads me to another question if you were using 2 power heads would it not actually be like connecting batteries in series or parallel.

I.E. if the 2 power heads were connected by a common crankshaft you would achieve the 17 hp but connected in the way described would it not be that you simply have 2 lots of 8.5 hp with a higher combined load capacity?

see the link to describe what I mean. Connecting Batteries in Series or Parallel

[pjt]

This is how I see it....highest tension in chain to lowest

1...highest in section D due to load from cutting and friction
2...next is section B based on output from saw2, only marginally higher than section A
3....section A is only a little bit less than B due to friction losses
4....lowest tension in the chain would be in C
Therefore add A and D equalls load on saw 1 sprocket bearing
and add B and C equalls load on saw 2 sprocket bearing,
so saw 1 takes to lions share of the load and saw 2 somewhat less.
This argument is based on both saws having somewhat equal output.

Pete

[dai sensei]

....so saw 1 takes to lions share of the load and saw 2 somewhat less. This argument is based on both saws having somewhat equal output.

You may be right. But this assumes slack in the chain, probably true to some extent, just not sure of impact and real answer would also depend on sharpness & heat of chain I think.

Rather than maths, as there are too many variables I tend to go with experience, and prefer to accept Travis's explanation

[Travis Edwards]

You may be right. But this assumes slack in the chain, probably true to some extent, just not sure of impact and real answer would also depend on sharpness & heat of chain I think.

Rather than maths, as there are too many variables I tend to go with experience, and prefer to accept Travis's explanation

Glad my jargon made sense to someone then.

[pjt]

You may be right. But this assumes slack in the chain, probably true to some extent, just not sure of impact and real answer would also depend on sharpness & heat of chain I think.

Rather than maths, as there are too many variables I tend to go with experience, and prefer to accept Travis's explanation

Travis said the same as I said it, just with more explanation with regard to machine performance

Even if the chain was tight, say tighter than any of us would normally adjust it, the chain would still have the lowest tension in section C, the actual value of tension in section C would be higher by the "extra" chain tightness but also this would be the case for all sections.

If we look at the drawing and then draw on load/tension/force arrows without assigning any values to the arrows but with correct direction ( A is the same as A chain rotation, B is the opposite of B chain rotation, C is the same as C chain rotation and D is the opposite of D chain rotation) A and D both pull on sprocket of saw1 and C and B both pull on sprocket on saw 2, I think we all agree that the highest tension in the chain is in D and the lowest in C, then if we assume that A and B are equal in force (they wouldn't due to friction and differences in saw output) therefore load on saw1 sprocket bearing equalls A and D and load on saw2 equalls B and C.

The max actual value of tension in the chain is a function of available tourque and the radius of the drive link contact point on the sprocket F=T/r when F=force in Newtons T=tourque in Nm and r=radius in metres.

Pete

[Timeless Timber]

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.

Those who have done a bit of it will tell you that - using cross cutting chain on a saw that's long cutting, overloads the saw/s and bogs it/them down slows the chain speed too a point it doesn't cut effectively in hardwoods or clear the long shavings.

But if you remove every second tooth on each alternate side so you have a skip-chain it drives about twice as fast cuts much cleaner and easier - so much so that you can get away with using one 090 and you can keep it all in place while you refuel.

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 ho 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.

Next thing is that when you cut wide hardwood slabs anywhere near approaching the heart - you get both radial and tangential grain within the one slab - and there's about a 5% difference in expansion and contraction rates between the radial grain within the slab and the tangential grain - even when kiln dried down too a 12% EMC (Equilibrium Moisture Content).

The result is that all of these slabs Will split in from each end about a meter or so as they dry and later when in use as a slab dining table - more so if you affix them too a cross timber support without the ability to expand and shrink thru a sliding mortice fitting!

People who pay a $ squillion for a slab table out of one piece get mighty pizzed when the end splits let me tell you from practical experience.

In the end, we just produced from the about 250 - 300 tonnes of slab grade jarrah logs a year that we milled for 20 years, 1 slab per log about 1/3rd of the way down the tree!

To do that - we would mill the log in the traditional sense (well not really we sawed our wood with the wayne edge ON it - because it makes a nice feature in furniture if you know what your doing, but I digress) until we got down to about one third the way (or 2/3rds if there wasn't a clear wood slab quality in the top half) and when we located the flat face that we wanted too slab - we'd pull the log back OUT from under the mill with the forks on a FEL tractor, put another one in and mill down normally until we located another potential slab face - THEN - we'd raise the mill back UP - keeping both ends equal in number of turns of the raising winder handles, until we could flip the 2/3 or 1/3rd milled previous log - flip it over until the flat side was down,and place it back on top of the second log - and start milling again from the top until a 50 or 60mm slab was left!

Pull your slab out and do the same next time - this way we got the best possible slab out of every log.

Any log that didn't measure up due too defects, just mill the whole thing in the traditional way.

Just saying that the whoile still 090 double ended chain saws for slabbing is something for hobbyists who don;t value their time - there's easier faster better ways to get slabs these days - heck - here in WA theres a few horizontal bandsaw mills - Harvey CALM wood utilization center used to have one that would slab your logs for you - Ian Bax down Busso way used too have one - who ever bought that would likely do the same - why would anyone waste their time and effort with a double ended stihl 090 - they went the way of the dinosaur if you asked me - I remember I had Baxy slab about a 7 tonne Jarrah burl for me once - and the saw kerfs combined wouldn't have added up too 2 chainsaw kerfs - you lose too much valuable wood for starters.

Its a good question if you've never had any experience with a double ended chainsaw milling slabs, but once you have you'd realize it aint worth the suffering unless its a once off event for a hobby maybe and you don't value your time and effort.

Once you've seen a slab produced any other way - you'd never bother again with a chainsaw for slabbing - not even a double ended 090 on an alaskan mill arrangement.

Bin there n done that and didn't enjoy the ride - there's far etter ways too skin a cat!

Cheers

[gerhard]

Hi all,

accompanying story follows...pc has hickups...managed to upload the pics...c u soon!

[gerhard]

Hi all,

pc's running again, here's the story to gowith the pics above.

First of all, thank you all very very much for your input! Special thanks to Travis and Timeless Timber for putting quite a bit of effort into it! I value that very much; brilliant stuff to read, truly enlighting and very entertaining! Travis formulated the very thoughts that went trough my mind. By the way, like with the batteries in series to have more voltage, you get indeed double the hp when coupling the crank shafts. Two 4-cylinder blocks will make an 8-cylinder engine that way.The French also applied this principle in some of their four-current-type electric express train locomotives by building two armatures on one pinion shaft and shoving two field and brush arrays in one motor housing of double length. Using the standard components of two motors, one powerful motor was gained, with the added advantage of connecting the two systems either in series or in parallel for use on the medium voltage networks of Belgium and Italy (3000 V DC) or Holland (1500V DC). For high voltages (15000V AC in Germany/Austria and 25000V AC in France) an additonal transformer and rectifier were used.

Back to the pictures and question. Multimotor drives exist in many heavy applications. Tunnel boring machines (present cutting face diameter record stands at 15.43 meters) can have as many as 14 electric or hydrostatic motors. Reasons for using multipinion drives are already mentioned in the pics above. All these systems have one thing in common; they have rigid gears. Torque is added by the motors to the main gear and is absorbed in another section, where it is converted into work.There is little flexibility in such systems. Sections with higher stress levels in the steel will be measurable around gear teeth and in unsymmetrical bearings loads, but on average the forces are relatively well spread. On all drive pinions the loads will be equal to a high degree, but it's a different story for the bearing system around the main gear's axle. This is a sort of pivot point, around which high strains in odd vectors are exerted. It's like a helicopter's rotor array; the entire fuselage weight is carried through the air hung on its rotor hub bearings. When the two traction motors in the first pic drive on of the locomotive's axles, the trains is in fact pushed forward through the axle bearings and the two pinions, with both pinions sharing the force 50/50 because they also provide the torque in a 50/50 way. Ideally, all pinions must be spread evenly around the main gear (this is so in a TBM and in many pile auger drill motors), but is often not possible (like with the bucket excavator wheel or in train bogies).

As i see it (and Travis agrees in his explanation) the chain array is very different from a fixed gear array. There is no pivot or leverage point as provided by a main gear's axle. The two sprockets are the only active drive and leverage parts present. So there must be much more strain on the left sprocket's bearings, since they must cope with the entire pulling forces and friction caused by the sawing workload, with the additional tug of the right sprocket of the second engine. The question mark is indeed the interesting spot. How much pull is left in that section? Still the combined pull of both engines? I just can't imagine that. I find Travis's explanation the most satisfying one.

Thank you all again, i think discussions like these are brilliant! I love them, haha!

greetings

gerhard

[BobL]

Good post timeless but me thinks you are having a lend of us.

Did you really even run a dual powerhead set up - the reason I say this as there is no need to flip one saw over. In fact if you flip one saw over one saw will be fighting the other - so no wonder you had problems?

Go back and look at the picture posted in the the first post of this thread and check out the chain directions - the saw is not turned over it is just turned around so there is no need to tip the mill over to refuel.

Despite that, saws like the 090/076 etc are still a PITA to fuel mid log - I find I can only get about 1/2 a tank of fuel into the saw. It's usually enough to finish the cut. Some day I'm going to machine myself up an elbow to the fuel cap which will avoid this problem. There is/was a plastic one available for the 076 apparently made for the concrete saw version of the 076 but I have not been able to find one.

It reminds me of a motor bike ride from Albany to Perth some 25 years ago. About ten km out of Albany I was overtaken by a low flying Harley chopper with half a peanut for a petrol tank. When I got to Mount Barker there he was at a petrol station fueling up. About 20 minutes later he over takes me again but at Kojonup there he is again fueling up. This happens all the way along Albany Highway till a few km from the Yule Doo petrol station where he is stopped by the side of the road looking somewhat forlorn - he had obviously run out of petrol. I did wave.

[Travis Edwards]

it is an interesting debate that has started here and on the arborist site forums. It almost seems like we are looking at something completely new and wonderful with the twin head mill, But in reality it was being done before I was born and the same issues we face today were faced by the sawyers back then. Then as now chainsaw milling filled a need and a lot of folk used them but as was stated back then, they waste a lot in kerf, they are slow hard work, the niche they fill is for the people who cannot afford a swing mill or a bandsaw or for those who need to breakdown logs too big for their bandsaws or such.

Sperber used to make and sell double end mills as did GB and granberg, you could in the 70s buy a sperber mill direct from the manufacturer with two 090 or two 075 powerheads as part of the deal.

Personally I rarely have anyone to help me so I prefer the single head option, as it would be a bit hard to reach the throttles on both ends of a 6 ft bar.

At the end of the day however it is something that works and works fairly well but I wouldn't even look at using a twin head mill for anything that could be done with a single powerhead. I have used my 660 on a 42 inch bar and it drives it BobL runs his 880 on a 60 inch mill no worries so you would need to be cutting some darned big logs to warrant the need for 2 090s, at least 60 inches or larger, however by the time you buy the setup for this you would be up for similar dollars as buying a lucas dedicated slabber. I know which I would prefer to operate and which would require less maintenance use less fuel and in the long run be more economical and all round more productive.

[dakotax3]

I enjoy alternative thinking during problem solving but let's not get too far away from what has to be done; cut wood. There is only one working part in the process and that is in the chain transtitioning section C-D, everything else is only contributing toward this. The inverted saw is wasted and is better put to use elsewhere as chains generally don't like to be pushed.

To remind those not to get TOO carried away in theory, may I offer the Spherical Cow metaphor to bring everyone's feet back to ground.

...Milk production at a dairy farm was low so the farmer wrote to the local university, asking for help from academia. A multidisciplinary team of professors was assembled, headed by a theoretical physicist, and two weeks of intensive on-site investigation took place. The scholars then returned to the university, notebooks crammed with data, where the task of writing the report was left to the team leader in academia land. Shortly thereafter the physicist returned to the farm, saying to the farmer:

"I have the solution, but it only works for spherical cows in a vacuum."


Spherical cow - Wikipedia, the free encyclopedia