First you should consider the horsepower of your sawmill. I will give some guidelines.
The reason we consider the horsepower first is the fact that we need the torque to maintain a constant band sawmill speed.
Sawing logs is like a heavy loaded truck
pulling up a long tall hill. The truck cannot pull up the hill in the highest gear
that it has or that it could use to run flat out. It has to down shift to a
lower (slower) ratio to get the torque level up to climb the hill. With a band sawmill
it is wise to make the ratio for the hard pull of sawing as if we are pulling
the long hill all the time.
While considering the horsepower, note that if the band sawmill has hydraulics and the
pump is mounted to receive power from the saw engine, that you will loose 5
to 10 horsepower. For example a 25 hp manuel sawmill will cut faster and stronger
than a 25 hp with a hydraulic pump robbing horsepower from the blade.
If I am
making a judgement as to how fast to make a band sawmill I would rather stay in a safe
zone and turn the blade slower and have the higher torque, than to turn too
fast and have low torque.
Another consideration on band sawmills with 30 or more hp, is friction. When the blade
exceeds 5,500 fpm it has to deal with a higher level of friction in the body
of the blade. In the past we thought if we had the hp and torque that we could
rev up the blade and saw faster. We in time realized this is a mistake because
of friction.
On some sawmills we would run as high as 7,000 fpm. This caused some
high friction and when the bandsaw blade ran into some adverse knots or hard places
the blade would cut badly. On the same sawmill we slowed the band to 5,500 and the
problem with sawing disappeared.
You might think it would slow production down.
It actually picked up production and the blades lasted longer, all because we
increased our torque and dropped friction. When friction is dropped the bandsaw blade is more stable.
The horsepower of your sawmill is the greatest factor in deciding how fast to turn
a sawmill. There are other factors such as wheel diameter and wheel roundness.
In the guidelines you will notice the progression of the more hp the faster
the bandsaw blade can go around the band wheels to a certain point and I believe you should
be careful when exceeding the maximum point.
Note: All speeds are given in Feet
Per Minute (fpm) rated for Gas or Diesel engines. When considering electric
motors note that they have about 25 to 30 % more torque than gas/diesel motors.
Here are some guidelines: 8 hp max speed 3,500 fpm, 12 hp max speed 4,000 fpm,
16 to 18 hp max speed 4,500 fpm, 25 hp max speed 5,000 fpm, 30 hp and above max
speed 5,500 fpm.
One thing that does not have relevance to the speed is the bandsaw wheel size, but remember
that wheel roundness does. If wheels are true in roundness they can run at maximum
speeds. I will add that larger wheel have benefits only when they are true round.
Note this: an 18 inch wheels that is true is better than a 30 inch wheel that
is not true.
Does the speed need to change for frozen Logs?
Yes, we have proved that it will
help when a band is turning over 4,800 fpm. It will give best results in sawing
frozen logs to slow the band down to 4,800 fpm or less.
I believe what happens
in frozen logs when the tooth impacts the cut area the fiber is cut along with
the moisture partially thawing under impact and friction and the dust that passes
by the blade gullet will freeze back tight against the blade body and it acts
like a brake pad pressing against the body of the bandsaw blade.
This in turn causes
heat and makes the band stretch and cut wrong. By slowing the bandsaw blade down under
these conditions allows the tooth to get a little larger dust bite and the larger
dust will not pass by the gullet as easy as before.
Every little help we give
ourselves here is positive. Also at the slower speed the band does not have
the friction level to deal with and will not heat up as much, and this will
keep the band stronger, as heated metal become weak.
A formula for figuring bandsaw wheel circumference (length around the wheel per
revolution).
Diameter of bandsaw wheel X 3.14 = _____ then divide by 12 the total is
in ft and inches.
Example 26 inch bandsaw wheel: 26 x 3.14 = 81.64 then divide by 12 is 6.80 feet around
the wheel. So if I want my band to run at 5,500 then I divide 5,500 by 6.80 = 808.
808 rpm is the speed I want to turn the shaft of the drive bandsaw wheel.
In Conclusion: The bandsaw blade needs constant speed with torque more than it needs
speed. If you put the speed to match the horsepower and torque you will have
a great cutting band sawmill.
Until Next Time,
aka... the "Saw Doctor"
Co-Owner CooksSaw.com
Leaders in Bandsaw Technology!
