Four Cycle Engine on Procut Mill questions

[abosely]

I know there's been a good bit of discussion about 4 cycle engines on a chainsaw sawmill, but I have some specific questions I could get some help with.

I live on Big Island HI and will be building a chainsaw sawmill using Procut plans.

Will have 36" bar with 30" cutting width and will be cutting some hard woods and would like to use a 4 cycle vertical shaft engine.

What HP 4 cycle engine would be equal to the Husqvarna 3120 with 8.4 hp?
I understand that a 4 cycle needs to be higher hp than comparable 2 stroke.
But how much more hp?
Want to use an engine that is the correct hp, but not bigger than needed that will overstress things & be heavier than needed. But don't want to get one that's too small and be flogging it to death either.
Not trying to get real fast cutting speed, just comparable to what I would have with a Husky 3120 Chainsaw.

If I understand the setup correctly, the vertical shaft engine will sit in middle of carriage, belt from engine to jack shaft down to sprocket that drives saw chain on bar.
Adjustment on bar to tighten saw chain, an idler to tension drive belt and a centrifugal clutch on engine shaft. I guess it will depend on hp needed if centrifugal clutch will work.

So I guess the actual question is, what hp a 4 cycle vertical shaft engine is needed to match the cutting capability of the Husky 3120 chainsaw?

Cheers, Allen


   



     

[Magicman]

I am no help with your questions but I can Welcome you to the Forestry Forum.  Hang Loose.   

[abosely]

Thanks Magicman! :-)

I'll be cutting some Eucalyptus deglupta, commonly called Rainbow Eucalyptus, for 4x6x20' beams for my sister & her husband and my houses. Mine is mostly all beams with metal roof.
Plus all the dimensional lumber, mostly 1"x3" and some other 1" thick up to 10" wide for the 42' Catamaran I'm building. It's a James Wharram Narai Mk IV Sailing Catamaran.
the stringers are 1"x3" and sheathed with 9mm marine plywood and fiberglass covered.

Cheers, Allen

[Ianab]

The commercial slabbers from Peterson and Lucas use various motors from ~18 to 27hp. But that's for a 5 or 6ft cut.

A 30" cut you could get away with quite a bit less. I think that chainsaw is rated at 10hp? I would expect the 13hp on my mill would be able to run 30" cut chainsaw OK.

[abosely]

Husqvarna lists the 3120 XP as 8.4 hp & max torque as 7.7 Nm at 6000 rpm (5.68 ft/lbs) and up to a 42" bar.

But I don't know what that would translate in 4 cycle hp needed. I'll be using a 36" bar but only using 30" max for cutting.

Cheers, Allen 

[sawguy21]

The limiting factor with 4 cycle is speed. An air cooled is normally limited to 4,000 rpm which means you would need to over speed it to match a 3120, this reduces torque. I honestly believe you would need a fairly large engine with accompanying weight to accomplish what you desire. The power to weight ratio of a smoker really shines here.

[Ianab]

The limiting factor with 4 cycle is speed. An air cooled is normally limited to 4,000 rpm which means you would need to over speed it to match a 3120, this reduces torque. I honestly believe you would need a fairly large engine with accompanying weight to accomplish what you desire. The power to weight ratio of a smoker really shines here.

Correct, but he intends to run a jack shaft and drive belt so it's relatively easy to size the pulleys to get the chain speed you need. A 4-stoke is generally better for torque anyway, so it can handle this.

Yes the power to weight is better on a 2 stoke, but the weight isn't big issue when it's bolted to a sawmill, and the quieter more efficient 4 stroke makes for a better milling set up in this situation.

[Kbeitz]

I'm thinking around 20 hp.
To get up to the 7000 rpm you need your pulleys will need to be 2 to one ratio.
This will cut your HP at least in half. Then the belt drive will rob you some.
Also todays 4 cycle engines don't really put out what they say in hp.
I think they use the flywheel inertia to rate the hp and you need continuous HP.

[bandmiller2]

Unless your in a mega-hurry I would go the other way and slow down your chain speed you can use a smaller more available engine and you bar and chains will last longer not to mention stay sharp longer. To stay light yet have power chainsaw have to turn at high speed which is not the best for chains and bars. Frank C.

[Andre]

If were ever to build a chainsaw mill most likely it would be with a 4 stroke per cycle engine or an electric motor.  I would not use any kind of belt or gearing to increase RPM but instead would just make the sprocket larger to get the chain speed I wanted at the RPM had.
If I could not make the required sprocket myself, I would give this guy a call.
http://www.danzcoinc.com/html/straight_bore.html

[abosely]

I see Lucas Mills uses an 18 hp 4 cycle engine to to run a 60" chainsaw bar. It actually has a 5' cut width, not just a 5' bar.

So I would think 10.5 - 13.5 hp engine should run a 30 bar.

So I'll keep looking. :-)

Cheers, Allen

[Hilltop366]

Welcome!

I used a .404 chain with a harvester sprocket and bar, 16hp briggs opposed twin with the electric clutch from the lawn tractor. The driven pulley was a bit smaller than the engine pulley for some step up from the engine speed. I found a single belt was not enough for the 16-20 inch cuts with full comp chain, if I made another it would have 2 belts. For 10.5 to 13.5 hp one belt may do it.

It is going to need a oiler for the chain as well.

Cheers

[sealark37]

When I put on my engineer hat, my gut feeling says to use 12-13 hp and slow the chain speed.  If it will not maintain cutting speed in the log without lugging, change the set-up or engine as required.  Regards, Clark

[Ianab]

Another thing to consider, the commercial slabbers use a double or super skip chain. Pair of cutters, then 3 or 4+ links. This reduces the power needed, and also the force on the chain and drive.  A 5 foot full comp chain would take a LOT of power and feed force to make it work properly. Each cutter needs a certain amount of power to keep it cutting, and all the total power has to go through the drive sprocket.  Going over a certain  amount of power will just break the chain. Needn't worry about that in the 10-20hp range, but it will limit the max power at some point.

BUT, what it means in this case, if you can run a skip chain, then you need less power than a full comp. It may not slow the cut as much as you think, because your feed force is not spread over less cutters. Means each one is able to get a decent bite and cut properly.

[abosely]

Well I decided to start looking around for a lawn tractor carcass with a 16-18 HP engine to build with.
I'll post up what I find and what I build. I'm sure I'll be asking more questions as it progresses!

Cheers, Allen