Milling with mismatched powerheads.

[deezlfan]

Any obvious reason that a person shouldn't try this if they already owned the saws? I'm just considering a set up like my 288xp on the lead with a 272xp on the trailing side. I know one is a large frame saw and the other is a medium frame. Crank bearing issues? The force on the trailing side must surely be less than the side with the cutters buried so even a set of matched saws must be doing different amounts of work.

 

[BobL]

The force on the trailing side must surely be less than the side with the cutters buried so even a set of matched saws must be doing different amounts of work.

It doesn't matter, the two saws are connected by a crank which is the chain so instead of the combined units producing X HP that produce X plus a bit more . A small saw attached to a bigger one is like adding another cylinder to a multi cylinder motor. Sure it will be smoother if the extra cylinder is added in a specific position but the chain smooths that to some extent and other than a bit more vibe it won't affect the power.

What does need to be similar is the power curve and rev range, you don't want one saw with its peak power being too far in RPMs from the other. e.g. if one saw has its max power at 7000 rpm and the other a 9000 rpm then especially given the peakiness of two stroke motors you wont be adding their max HPs together and you could well be just wasting your time If one has a max RPM of 10K and the other is 13K then that may also leads to probs especially if the lower max RPM saw is the smaller one.

If you out the power curve specs and post these we can comment on their suitability.

 

[deezlfan]

It doesn't matter, the two saws are connected by a crank which is the chain so instead of the combined units producing X HP that produce X plus a bit more .

I'm not sure I agree that the saws are always attached to each other. There is too much going on with the slack in the chain as the chain leaves the sprocket. It is only possible to transmit power under tension as you can't push power through a chain. Not sure I can explain it exactly but one side of this set up has to be easier to pull than the other.

The cut and paste specs from Acres in my example are:

The 272xp is 5.2 hp (3.8 kW) 13,500 rpm after break in, 9,300 rpm under load.
The 288xp is 6.1 hp (4.5 kW) 12,500 rpm after break in, 9,300 rpm under load.

I have no way to dyno any of the saws so I can not supply torque curves.

 

[BobL]

I'm not sure I agree that the saws are always attached to each other. There is too much going on with the slack in the chain as the chain leaves the sprocket. It is only possible to transmit power under tension as you can't push power through a chain. Not sure I can explain it exactly but one side of this set up has to be easier to pull than the other..

I agree, but the outboard engine is not providing power by pushing the chain - its adding its to the power of the inward engine by pulling the chain from around the drive sprocket of the outboard. The net effect is like adding a cylinder to the inboard engine.


The cut and paste specs from Acres in my example are:

The 272xp is 5.2 hp (3.8 kW) 13,500 rpm after break in, 9,300 rpm under load.
The 288xp is 6.1 hp (4.5 kW) 12,500 rpm after break in, 9,300 rpm under load. .[/QUOTE]
The are easily a close enough match.

 

[deezlfan]

Thanks, that's what I was thinking.