# How much Hp for a chainsaw mill



## muddstopper (Oct 27, 2015)

Been thinking about building a saw mill for a while now. Was thinking more along the lines of a bandsaw mill, but saw a few Utube vids where folks used old lawn mower engines to power a chainsaw. I know most of the wood I would saw most likely would be under 24in dia, but I want the capacity to cut 48in or wider wood. Since i dont already own a saw suitable for really large dia wood, and old riding mowers are a dime a dozen, I am thinking just build a chainsaw mill using a mower engine. Now i know mower engines dont turn the rpms of a chainsaw, but saws dont cut buy rpms, they cut by ft/min, or ft/sec. So using the right size sprocket I should be able to get the chain speed I need, but just how long a chain can I pull with a small engine. The large Huskey and Sthil saws are only 8.5 hp, so would using a 14hp mower engine with appropriate sprockets pull a 48in chain saw chain.


----------



## olyman (Oct 27, 2015)

muddstopper said:


> Been thinking about building a saw mill for a while now. Was thinking more along the lines of a bandsaw mill, but saw a few Utube vids where folks used old lawn mower engines to power a chainsaw. I know most of the wood I would saw most likely would be under 24in dia, but I want the capacity to cut 48in or wider wood. Since i dont already own a saw suitable for really large dia wood, and old riding mowers are a dime a dozen, I am thinking just build a chainsaw mill using a mower engine. Now i know mower engines dont turn the rpms of a chainsaw, but saws dont cut buy rpms, they cut by ft/min, or ft/sec. So using the right size sprocket I should be able to get the chain speed I need, but just how long a chain can I pull with a small engine. The large Huskey and Sthil saws are only 8.5 hp, so would using a 14hp mower engine with appropriate sprockets pull a 48in chain saw chain.


 I don't remember his name,,but there was such a gent that did just that who used to be on this forum, and no, I don't remember his name. and it worked very well. the only drawback, is the weight........


----------



## muddstopper (Oct 27, 2015)

I cant see how weight would be an issue, unless you are trying to hold the setup like a chainsaw. The ones I have seen on youtube have all been mounted on a frame and a track, like a bandsaw mill. Set it, crank it, and push it.


----------



## olyman (Oct 28, 2015)

muddstopper said:


> I cant see how weight would be an issue, unless you are trying to hold the setup like a chainsaw. The ones I have seen on youtube have all been mounted on a frame and a track, like a bandsaw mill. Set it, crank it, and push it.


 his was set up like a cs mill.....


----------



## muddstopper (Oct 28, 2015)

I dont guess you remember how big a motor or saw bar he was using?


----------



## olyman (Oct 28, 2015)

muddstopper said:


> I dont guess you remember how big a motor or saw bar he was using?


 thinking around 10 hp....been a while....


----------



## muddstopper (Oct 28, 2015)

I'm going to think on this project for a while before I decide what I want to do. I have a 14hp engine, but carb is messed up. I am pretty sure i can pick up a pretty good engine for the price of a carb for the engine I have. I guess I'll just have to see what pops up and then make a decision.


----------



## babybart (Oct 29, 2015)

I have the same project in the corner of the shed I had to put on the back burner for now. I was going to use a 8 hp vertical shaft Briggs. I planned on using a jackshaft with sprockets that would give me a rpm increase of 3-1. My thinking is to get the 3600 rpm motor to about 9-10,000 rpm at the chain drive sprocket. I figured the chain speed would offset the lack of hp. I got the sled welded up and life got in the way.. .


----------



## BobL (Oct 29, 2015)

If the motor has the torque, 10,000 rpm is not needed to cut wood with a chain because to some extent lack of RPMs can be made up for by increasing the raker angle (dropping the rakers) 
eg a 090 operating at lower RPM with rakers set at 9º will out cut a lower torqued saw saw that turns at higher RPM but has to use higher rakers or it will stall. 
The downside is increased Vibe and maybe some maybe some increased wear and tear.


----------



## muddstopper (Oct 29, 2015)

Its been a while since I looked all this up, so somebody correct me if I write something wrong. A chainsaw chain speed is not measured in rpms. It is measured in ftpersec. The average chains saw chain will run about 88ftpersec. While rpms is a factor to determining chains speed, chain pitch as well as tooth count on the sprocket will play a equal part. A motor turning a certain rpms pulling a .404 chain with the same tooth sprocket as a 3/8 chain will provide a faster chain speed than a bar equipped with the 3/8 chain. This is due to the bigger pitch of the chain and the sprocket having to be a bigger dia to accommodate the same number of teeth. To get a 3600 rpm gas motor to spin the chain at or near the same speed as a 9000 rpm saw motor, you would only need to increase the tooth count on the sprocket. A factory Husqvarna 395 xp turning 9600rpms will run a 7 tooth sprocket with a 3/8 chain and spin the chain at 70ftsec. The lawn mower engine turning 3600rpms and running a 14tooth sprocket will run the chain about 52ftsec. Still a pretty big difference in chain speed. The 395xp is about 7hp at 9600 rpms, because of the doubling of sprocket size for the hp gas engine, hp would also be reduced to about half. Even with fancy chain filing, you would still need a bigger engine than 14hp (the engine I have on hand) to match the 395 in cut speed.


----------



## babybart (Oct 29, 2015)

Dunno. Dumb redneck(me) knows rpms based on tooth count and sprocket diameter from racing chain driven items, i.e... 4 wheelers and motorcycles. Don't understand the physics discussed here.


----------



## muddstopper (Oct 29, 2015)

babybart said:


> Dunno. Dumb redneck(me) knows rpms based on tooth count and sprocket diameter from racing chain driven items, i.e... 4 wheelers and motorcycles. Don't understand the physics discussed here.


didnt know I knew physics


----------



## olyman (Oct 30, 2015)

muddstopper said:


> didnt know I knew physics


 me thinks, your forgetting about the torque of the 4 cycle 14 hp. the cs,,aint got NEAR the torque....


----------



## muddstopper (Oct 30, 2015)

I didnt forget the torque, just dont know the torque numbers for the engine I have. I did the calculations once for building a hydraulic chainsaw, with the size motor I was calculating, I had something like 3 times the hp and 4 times the torque of a 3120 CS. I can only guess that the numbers would be similar for a lawn mower engine. That also brings up the question of just how much torque and hp a chainsaw chain can take. With enough rpms, speed could also become a factor, but trying to get the speed required with a mower engine is hard enough to even start thinking about over spinning the chain. I was once told, but havent seen it officially anywhere, that a 3/8 chain could hold up to around 50hp. Since hp is a factor of torque and speed, I guess one could reverse the math to see just how much torque the chain could withstand.


----------



## BobL (Oct 31, 2015)

muddstopper said:


> . The lawn mower engine turning 3600rpms and running a 14tooth sprocket will run the chain about 52ftsec. Still a pretty big difference in chain speed. The 395xp is about 7hp at 9600 rpms, because of the doubling of sprocket size for the hp gas engine, hp would also be reduced to about half. .



Sounds like you are a bit confused between HP/Torque and Force

The output HP and torque of a engine/motor/powerhead is independent of the sprocket/flywheel/pulley used. 
I have a HP/torque measurement test rig and have measured this in practice multiple times. 

HP is a measure of the Work, while Torque is a measure of the turning force, an engine can perform.
Torque = Force/distance
When the sprocket size (distance) is doubled the force at outside of sprocket is halved, but the torque remains constant.

Work(HP) = force x distance.
As we see above if the sprocket is doubled in size the amount of force on its outer edge drops by 2 but the distance covered by the sprocket now doubles so the work done (HP) by the motor stays the same.
Another way of thinking about it is when a sprocket size is doubled the Force on the outside of the driving sprocket is halved but the force is applied twice as quickly

What really determines the amount of cutter penetration into wood is the momentum of of the cutter
This is determined by the force and speed of the cutter.
This is why more torque and more speed increases cutting speed.
BUT
The above all the limitation to penetration of a cutter is determined by cutter and raker geometry.
Its useless to have lots more torque or speed if the cutter/raker geometry restricts wood penetration.
Lots of energy then goes into rotating the chain but real cutting speed is not improved.


----------



## muddstopper (Oct 31, 2015)

BobL said:


> Sounds like you are a bit confused between HP/Torque and Force
> 
> The output HP and torque of a engine/motor/powerhead is independent of the sprocket/flywheel/pulley used.
> I have a HP/torque measurement test rig and have measured this in practice multiple times.
> ...



Not sure I follow. The power, hp, torque, rpms, at the engine would stay the same, but the wouldnt the doubling the size of the sprocket increase the speed of the chain thru gearing, and if I increased the speed of the chain, would that not reduce the effect of the torque applied to the chain. A larger sprocket is gearing up the chain and increasing speed, reducing torque even tho hp would remain the same


----------



## BobL (Oct 31, 2015)

muddstopper said:


> Not sure I follow. The power, hp, torque, rpms, at the engine would stay the same


Well that is not what you said above. , but the wouldnt the doubling the size of the sprocket increase the speed of the chain thru gearing, and if I increased the speed of the chain, would that not reduce the effect of the torque applied to the chain. A larger sprocket is gearing up the chain and increasing speed, reducing torque even tho hp would remain the same[/QUOTE]

Now you are confusing torque and force - torque is not the same as force, this misunderstanding is pretty common. 
The torque stays the same, but the force the cutter can apply to the wood that is reduced.

For two pulleys connected by a belt or a shaft, or a shaft direct driving a sprocket
The torque is the same for both, it's the force that changes depending on the radius of the shaft or pull


----------



## muddstopper (Oct 31, 2015)

Bob, I certainly am not qualified to debate this subject, I dont want to bring a knife to this gun fight. You work with this daily, I only research something like this when I have a project that requires it. I equate changing pulley sizes with changing gearing or pully ratios. Gear something down you reduce speed and increase torque, gear it up and you increase speed. but reduce torque. In and engine, you increase the lenght of the piston rod which decreases rod angle and increases torque. Shorten the rod and you increase rod angle and decreases torque. Changing the pulley size at the end of the shaft would increase stroke, so my question is would this increase or decrease torque??


----------



## abbott295 (Oct 31, 2015)

As in Bobl's illustration, torque doesn't change: torque is measured in pound-feet. You will get the same number of pound-feet whether r=1 foot and F=2 pounds or r= 2 feet and F= 1 pound. Horsepower will be the same also if the rpm stays the same. 

The force available to the cutter will be reduced as you gear up the chain speed, but if you start with a 14 hp engine, should still be more than adequate for a mill. Of course there will be friction losses in the power transmission to the chain cutter.


----------



## muddstopper (Oct 31, 2015)

So increasing sprocket size reduces power, (force), at the saw. Which is what I thought. I may of used the wrong terms, but I knew the results.


----------



## BobL (Oct 31, 2015)

abbott295 said:


> As in Bobl's illustration, torque doesn't change: torque is measured in pound-feet. You will get the same number of pound-feet whether r=1 foot and F=2 pounds or r= 2 feet and F= 1 pound. Horsepower will be the same also if the rpm stays the same.
> 
> The force available to the cutter will be reduced as you gear up the chain speed, but if you start with a 14 hp engine, should still be more than adequate for a mill. Of course there will be friction losses in the power transmission to the chain cutter.



Nailed it!

It's common to confuse force/power/torque especially on the internet


----------



## KiwiBro (Oct 31, 2015)

This is also a useful document, for both your slabber and processor builds, Mr Mudd:

http://www.oregonproducts.com/pdf/harvester/MechanicalHarvesting_2005.pdf

It offers a few reference points that combined with the experience of millers will be worth knowing. I'd also suggest talking with Lucas or Peterson sawmill reps about how they spec and set-up their dedicated slabbers.


----------



## muddstopper (Oct 31, 2015)

Actually, I have already saved the mechanical harvesting pdf file. Good reference material, I have read thru it several times.


----------



## BobL (Oct 31, 2015)

Imteresting to see they rakers are as low as 0.070 pm some of those harvester chains


----------



## muddstopper (Oct 31, 2015)

With those low rakers, you better have some power pulling the chain.


----------



## BobL (Nov 1, 2015)

muddstopper said:


> With those low rakers, you better have some power pulling the chain.



My initial thoughts were 0.070" rakers on 3/4" chain equates to a raker angle 8º which is not that special. I think it was Mtngun (a longtime member of this forum) who was using 9ª raker angles on 20" diameter wood with his 660 at altitude! and I have used 7.5º on Aussie hardwood with my 880 without much problem.

However, 3/4" chain cutter-raker spacing is twice that of 3/8 chain so the average size of the chip generated will be at least 2 maybe 3 times bigger. Unlike what most people think the chips are as much "pulled" as "cut" out of the kerf and it is this that that needs the power.


----------



## olyman (Nov 9, 2015)

muddstopper said:


> So increasing sprocket size reduces power, (force), at the saw. Which is what I thought. I may of used the wrong terms, but I knew the results.


 look in the above posts,,for 820wards,,he has the engine mill I was thinking off..................


----------



## mtfallsmikey (Nov 10, 2015)

BobL said:


> My initial thoughts were 0.070" rakers on 3/4" chain equates to a raker angle 8º which is not that special. I think it was Mtngun (a longtime member of this forum) who was using 9ª raker angles on 20" diameter wood with his 660 at altitude! and I have used 7.5º on Aussie hardwood with my 880 without much problem.
> 
> However, 3/4" chain cutter-raker spacing is twice that of 3/8 chain so the average size of the chip generated will be at least 2 maybe 3 times bigger. Unlike what most people think the chips are as much "pulled" as "cut" out of the kerf and it is this that that needs the power.



Haven't been here for some time. Last time, you had sort of disappeared for a while, glad to see you back, I still apply some of your principles of chain sharpening.


----------

