Estimating calculations (Bad memory) must be getting old!

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Auto Engineer

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I've been producing some calculations for frequency and rpm of accessory pulleys on an engine. I started off using the engine idle speed (rpm) 750 - 800 and mathematically calculated a desired Hz range of figures of 12.5 - 13.30. The actual engine frequency is 13.20 Hz.

I moved on and calculated the water pump frequency between 16.35 to 17.43 Hz and the actual frequency is 17.30. I also worked out similar calculations for the AC system rpm etc.

My problem is, when I did this at the beginning of August this year, I now can't remember how I did it!

Any help appreciated to jog my memory, and I promise I will start to write things down so that I can recall them to memory when required. Thank you.
 
Convert rpm to Hz. Divide by 60.
Convert RPM of engine to rpm of accessories.
Crank pulley diameter/accessory diameter * engine rpm
 
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