calculating cycle time

at the end of the calculating cycle time module it is mentioned that 120/rpm is the quick way to get your cycle time. where does the 120 come from? is this a general rule on all motors?

The time period of a wave, is 1 / Frequency. The unit of frequency is cycles/sec (aka Hertz, Hz). To convert an engine crankshaft speed given in revolutions per minute (RPM) to revolutions / sec (Hz), you need to divide by the number of seconds per minute (60). Since it takes two revolutions to make one cycle, we need to multiply the resulting period by 2.

So we end up with 2 / (RPM / 60). This can be re-written as 120 / RPM.

Sometimes it helps to work through a simple example -- an engine running at 600 RPM, the crankshaft turns 600 RPM / 60 seconds/minute = 10 times per second (so the period is 1/10 or 0.1 second = 100 ms) Since each two revolutions equals one cycle, the cycle time would be twice this = 0.2 seconds or 200 ms. That's the same answer we get by dividing 120 / 600 = 0.2