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Engine Power, Cutting Power & Sheave Size
The cutting capability of your saw is a relationship between engine power (as
reflected in the engine RPM) and the speed (RPM) of the Blade Shaft. The
gasoline engine of the SP-6065 runs at 2800 RPM (full load). If 2800 RPM was a
desirable Blade Shaft speed for the average conditions in which you work, we
would use the same size sheave on the engine shaft and the Blade Shaft; and the
ratio between the two would be 1 : 1.
Fig. 4 — Blade Shaft Ratios
If, however, for your cutting conditions, you need a Blade Shaft speed slower
than the engine speed (and this is usually the case), then we need to INCREASE
the ratio between the two speeds by putting a larger diameter sheave on the
Blade Shaft.
If, for example, you know from experience that you need a Blade Shaft speed of
2000 RPM for the size of blade you normally use (see the Blade RPM vs. SFPM
Chart, below):
2800 (engine RPM) ÷ 2000 (desired BS RPM) = 1.4
This is the ratio 1.4 : 1, which means that for every 1.4 revolutions of the engine,
the Blade Shaft only turns once.
Ratios greater than 1 : 1 also have the beneficial effect of increasing the torque of
the Blade Shaft by the same factor (1.4 in our example) . The manufacturer
advertises that the Deutz BF31011 diesel engine develops 123 ft. lbs. of torque.
To find the theoretical torque of our example saw setup:
123 x 1.4 = 172.2 ft. lbs.
Actual torque of the saw will vary somewhat. In general, more torque means
more cutting power.
1 : 1 1.4 : 1
Blade Shaft
Blade Shaft
Engine
Engine