Why do longer bars require more power?

[brages]

It takes HP to move the chain, easy 1/2 a horse just for the accelerations to make the chain go round at full speed. Then add friction, the longer the bar the more friction there is [snip]

I hate to be a newbie arguing with a master, but...

Technically, no power is going into accelerating the chain once it has reached a steady speed. Think of it in terms of energy - the sum of the kinetic energy of the entire chain when it is running at a certain speed is a constant - no energy needs to be put into the chain to maintain speed, except for friction. Friction occurs between the links and the bar, and between each link.

Is the :newbie: right?

 

[bump_r]

let think about this your saying if you have a 20 tooth chain or a 200 tooth
chain the speed is the same .the moore teeth the slower the speed.
i will put any amount of money you guys wish to wager.Mark a tooth an
a 16'' bar and count the number of times you turn the sprocket until it gets back to the point were you starte now do this with a 32'' bar. now we know
the saw operates at the same rpm with the 2 bars:monkey: :monkey:

It's only moving the same number of teeth per crankshaft revolution. Teeth per rev, not "loop". It does not matter one whit how many teeth there are, just how many get moved versus crank.

 

[Modifiedmark]

let think about this your saying if you have a 20 tooth chain or a 200 tooth
chain the speed is the same .the moore teeth the slower the speed.
i will put any amount of money you guys wish to wager.Mark a tooth an
a 16'' bar and count the number of times you turn the sprocket until it gets back to the point were you starte now do this with a 32'' bar. now we know
the saw operates at the same rpm with the 2 bars:monkey: :monkey:

Your confused for sure, it's not that hard to figure out. Same rpm's, same sprocket, a 20 tooth chain and a 200 tooth chain run the same speed!

 

[timberwolf]

To make chain go around once

16 inch chain 60 drivers 7 pin sprocket = 8.52 revolutions

32 inch chain 105 drivers 7 pin sprocket = 15 revolutions

Both cases though 105 cutters pass any given point and they go the same speed.

 

[scotclayshooter]

I hate to be a newbie arguing with a master, but...

Technically, no power is going into accelerating the chain once it has reached a steady speed. Think of it in terms of energy - the sum of the kinetic energy of the entire chain when it is running at a certain speed is a constant - no energy needs to be put into the chain to maintain speed, except for friction. Friction occurs between the links and the bar, and between each link.

Is the :newbie: right?

I think you are underestimating how much the friction of a longer bar affects the saw!

 

[bigjake]

anothrter thing you guys are forgeting is the sproket and gear are different sizes ??????? .Remember we are talking chain speed how many reveloutions
it takes for the chain to make one complete revolution as to the engine rpm??I gues the term ratio wasnt quit right.But you still loose distance covered by the chain per crank revolution.

 

[timberwolf]

I hate to be a newbie arguing with a master, but...

Technically, no power is going into accelerating the chain once it has reached a steady speed. Think of it in terms of energy - the sum of the kinetic energy of the entire chain when it is running at a certain speed is a constant - no energy needs to be put into the chain to maintain speed, except for friction. Friction occurs between the links and the bar, and between each link.

Hold on there grasshoppa

Every time the chain changes direction it is accelerated, at the tip and the rim the chain goes from one direction to 180 deg in the other, this takes power, also each drive link accelerates 4 times on it's own axis once when it enters the tip where each link has zero rotation and then is set into rotation then at the end of the tip wher it must be accelerated from rotating to going straight, same thing then entering and leaving the rim.

Crazy but couple years ago I did up calculations just to figgure out just how much power was lost to acceleration.

 

[bigjake]

To make chain go around once

16 inch chain 60 drivers 7 pin sprocket = 8.52 revolutions

32 inch chain 105 drivers 7 pin sprocket = 15 revolutions

Both cases though 105 cutters pass any given point and they go the same speed.

you just proved what i am saying rpms are time .how can they be the same speed.your saw runs at a given rpm .the smaller chain passes through its whole cycle more in one minute now how can that be the same.

 

[timberwolf]

anothrter thing you guys are forgeting is the sproket and gear are different sizes ???????

The sprocket tip is just an idler, if its say 11 tooth it will tun 7/11 the RPM of the drive geat if it was a 7 pin gear. But still the surface speed is the same.

What if we used a hard nose bar, then what? See look at it that way the tip has nothing to do with the chain speed. However a hard nose bar will have more friction and a slower chain speed as the engine will be loaded a little more.

Either way 105 cutters pass a given point in a given period of time and the cutters are a fixed distance apart, speed must be the same if RPM is the same. The fact with a 16 inch chain the chain loop goes around 1.75 times for 15 revolutions of the crank, with a 32 it goes around once. Either way 105 cutters pass any point and if the time is the same then given the rpm is the same the chain speed is the same.

 

[stihl only]

In my experience, a longer bar provides more elbow room, and more area for the girls to dance so they don't fall off.

 

[Modifiedmark]

The sprocket tip is just an idler, if its say 11 tooth it will tun 7/11 the RPM of the drive geat if it was a 7 pin gear. But still the surface speed is the same.

What if we used a hard nose bar, then what? See look at it that way the tip has nothing to do with the chain speed. However a hard nose bar will have more friction and a slower chain speed as the engine will be loaded a little more.

Either way 105 cutters pass a given point in a given period of time and the cutters are a fixed distance apart, speed must be the same if RPM is the same. The fact with a 16 inch chain the chain loop goes around 1.75 times for 15 revolutions of the crank, with a 32 it goes around once. Either way 105 cutters pass any point and if the time is the same then given the rpm is the same the chain speed is the same.

I'm glad you took that one, I don't like to type that much.

 

[bigjake]

I stand corrected . i must admit when i am wrong i was confusing cycle time with cutter speed .FK im still confused for every revalution the chain only moves so far .i guess i never gave the friction that much thought.now there
is another theroy the slower cycle time in relation to the friction.Wow
I may have to start drinking early oh well its my birthday.

 

[timberwolf]

Happy B-day. We'll let you off the hook.

Every one of those drive links is in contact with metal and they are all going the same speed so right off the bat your looking at 1.75 times the friction with 32" over 16" given the same RPM. But compound that as a 32 inch chain must have more tention on it to keep it in the rails.

 

[danrclem]

It's like a race car going around a 1 mile track vs. a 2 mile track. If a car goes exactly 150 mph around each track it will take twice as long to go around a 2 mile track even though the car is traveling at the exact same speed.

The original question would have a variable answer to it even though there has to be a difference with any change even though that change might not be noticeable. It would depend on the power of the saw, the amount of length change in the bar and the size wood that would be cut. If you went from a 16" to a 20" bar on a 372 and were cutting 12" wood each time I doubt if the change would be noticeable. I haven't tried this but I'm just going by logic.

 

[scotclayshooter]

Happy B-day. We'll let you off the hook.

Every one of those drive links is in contact with metal and they are all going the same speed so right off the bat your looking at 1.75 times the friction with 32" over 16" given the same RPM. But compound that as a 32 inch chain must have more tention on it to keep it in the rails.

That 32" chain would be double the weight as well

 

[timberwolf]

Race track... Good way of looking at it.

That change from 60 to 72 drivers will show up on the stop watch, might not notice blocking fire wood though.

16 inch chain will dull quicker than a 32 as each cutter cuts 1.75 times more wood, but then its a lot faster to sharpen too.

The total chain weight only shows up in initial acceleration. The directional acceleration at tip and tail is the same short bar or long bar as the chain is making these accelerations at ~ the same speed no matter the length.

In making race chain getting the weight down is fairly importaint, esp on smaller stock saws, but also when chain speeds get up over 100 MPH and accelerations go up exponentially. It's easy to put several HP into just making the chain go around when you're talking 404 or 1/2 chain on bike saw.

 

[TreePointer]

Chain is not a gear, it moves in a straight line where it contacts the wood. if you look at number of revolutions the whole chain loop makes yes, a longer chain makes fewer full revolutions every min. How ever the liniar speed is the same. Well actually, it is slower due to increased friction. Prove it to yourself, tune a saw with a 16 inch bar and new chain measure the RPM make some cuts and time them, then with a 32 inch do the same (edit without adjusting the carb). The smaller the saw the bigger the difference will be, an 088 is going to be less effected as the friction/acceleration losses are a smaller part of total power. 1/2 a horse to an 088 is much less difference at the wood than with an 026.

Wrong, the chain loop is not a gear, it is drivin by the gear. The drive sprocket is the only thing that determines the speed of the chain relitive to crankshaft rpm's


Sorry Timberwolf typed faster and is correct.

That makes sense. Roughly, I was thinking of gear diameter ratios in ideal gear systems, but I made the mistake of assuming that the relevant chain diameter changes when changing loop sizes.

Since the sprocket diameter (akin to gear diameter) doesn't change and neither does the diameter of the portion of the chain contacting the sprocket, then the ratios and mechanical advantage of the quasi-gear system doesn't change, regardless of loop size. Then neither would there be a change in linear chain speed, as you stated. This also assumes the saw has enough guts to render frictional, inertial (from larger chain weight), and possibly other differences inconsequential.

Thanks for setting me straight!

 

[brages]

Hold on there grasshoppa

Every time the chain changes direction it is accelerated, at the tip and the rim the chain goes from one direction to 180 deg in the other, this takes power, also each drive link accelerates 4 times on it's own axis once when it enters the tip where each link has zero rotation and then is set into rotation then at the end of the tip wher it must be accelerated from rotating to going straight, same thing then entering and leaving the rim.

Consider these arguments:

First argument:
If you change the direction of a link, but its speed remains the same, its kinetic energy is the same. You haven't put any more energy into it. If you expended energy, it went into friction, not into accelerating the link.

Second argument:
Even if you disagree with the first argument, the rate (links per second) at which links are going around the ends of the bar and being accelerated is dependant on chain speed (and tip radius?), but independant of bar length. A 16" inch bar has the same number of links per second getting accelerated (I am referring to a change in direction at constant speed as acceleration) around the tip as a 32" bar for a given chain speed.

(Edit: I see that this is what you said in post #36 while I was typing this)


(I'm not trying to say that longer bars don't waste power...)

 

[Crofter]

http://www.arboristsite.com/showthread.php?t=6920&highlight=chain+weight

Here is a link to the 4 page thread on the effect of chain weight on horsepower loss where TW did his original calculations.

 

[timberwolf]

The link is moving at the same speed, but to change the direction of any moving object without affecting it's speed force must be applied. Applying force will take energy from somwhere even if it ultimatly shows up as friction in the case of a bar and chain.

Look at a single link, as it moves out from the saw it is moving in a straight line at a given speed and has a given energy. then when it hits the end of the straight away it must make a corner, to do this energy energy must be imparted. But also look at a single drive link, not only does the whole line enter a corner, but the link also comences to rotate on an axis centered between the rivets. this accelerating happens very quick in the short transition between going straight and entering the radius of tip or tail. Then just like any rotating object it takes force to stop the rotation at the end of the turn.

In one revolution of the loop each link undergoes 8 accelerations. 1 from circular motion to liniar motion leaving the saw plus 1 of driver rotation to zero rotation at the end of the rim, then 1 from liniar motion to circular motion at the tip plus 1 of zero drive rotation to rotation, then 4 more just the same returning to the saw.

Larger sprockets, larger tips and eliptical bar shapes all help reduce these acceleration losses. You make a good point on that.