Engine RPM?

[sdboers]

G'day,
  New to the forum (as a registered user) - but frequent lurker...

  Planning a sawmill build for the spring and just in the process of laying out all the plans now.  One question I have is on engine RPM.

  From the reading I've done a design which includes a 21hp engine, 19" wheels, 1 1/4" blade and up to a 36" cut should aim for about a 4500fpm blade speed.  Not a formula per se - just some reading and video watching (Thanks Cook Saw...)

  My questions is - should you be running the engine at the speed that delivers maximum torque (2500rpm) or maximum hp (3600rpm).  This will have an effect on the size of the pulleys I choose.

Sean.

[Chuck White]

First, Welcome to the Forestry Forum, sdboers!

I have a 25hp Kohler on my LT40HDG, and the recommended RPM for it is High 3600, Low 1800rpm

Good luck in your sawmill-build!

[sawguy21]

Run it at the maximum recommended speed. That will give the best cooling and ensure the engine stays within the power band under load.

[Crusarius]

I am running 22hp predator with 19" band wheel 14" drive pulley and 4" clutch. (I think the clutch is 4" I may have to double check it)

I am using a gokart clutch. so far it has worked great. I like being able to just let go of the throttle and have it stop. I prefer not to use an electric clutch I feel with the amount of times I engage and disengage thats an awful lot of stress on a bunch of parts. I know it has worked for years on many mills but my preference is not to do that.

[sdboers]

With a 4" clutch and a 14" drive pulley - can I assume you run yours flat out at 3600 rpm?  That'll give you a blade speed of about 5000 fpm.

I also have a 4" centrifugal clutch I will use for the same reason you state.  Releasing the go handle should stop the blade spinning!

Sean.

[Crusarius]

yes. I go full throttle make cut then let go.

[KirkD]

Under Extras above in the Tool Box, Calculator Index, Machinery Calculators there is a band speed calculator.

http://forestryforum.com/members/donp/bandspeedcalc.htm

[sdboers]

Thanks.  Handy reference to have.  I have that all keyed up in my bandsaw spreadsheet as well.

My question is more of a design question though.  Given I haven't built it yet and can choose whatever size pulleys I want to get the end result - where is the sweet spot in the rpm curve for the engine?

Is it easier for the engine to keep the blade speed if it is running at peak torque (2500 rpm) or peak horsepower (3600 rpm).

Seems to me that most folks run their mill flat out achieving peak hp.

Sean.

[ladylake]

If you set your engine up to run 2500 rpm vs 3600 rpm at 4500 fpm blade speed you would need a bigger drive pulley on the engine reducing torque to the band.  I'd set it up for 3500 to 3600 Rpm.  Steve

[sdboers]

So my setup was going to be one of two configurations depending on the RPM you folks recommend:

1) 2500 RPM - 4" clutch on engine, 10.75" driven pulley, 18.75" band wheel.  FPM = 4566
2) 3600 RPM - 4" clutch on engine, 14" driven pulley, 18.75" band wheel.  FPM = 5050

In either scenario - I was going to keep the clutch on the engine at 4".

Sean.