I was having problems with 2 new Hayden severe duty clutch's not disengaging my engine fan. That's the one that is suggested in our parts book. I've never had problems with their products. And why was there a standard duty, heavy duty, and a severe duty listed in our parts book. The stock GMC fan has 7 blades, and a 2.25 inch pitch. It rotates at a fixed ratio to the crankshaft. So the clutch load is constant to the crank speed. The clutch has 2 operating conditions. Engaged. Or disengaged. Nothing else. And the engaged operation is based on the temperature of the air coming from the radiator to the coil spring on the front side of the unit. This spring tells the clutch what needs to be done for proper engine cooling. So back to my question.

What can the severe duty do the the heavy duty can't. As the unit just turns the fan blade on, or off. Duty cycle shouldn't be the issue. It's a fluid type clutch. And, these units just don't turn on, or off that off ton in our type of service. In my mind. I was missing something. So I called Hayden.

What I found out was that for our size, and number of blades. It goes like this. A 2.00 inch pitch blade would require their standard duty unit. A 2.25 inch pitch blade requires their heavy duty unit. And the 2.50 inch pitch would require their sever duty unit. He stated that if you put a sever duty unit on the 2.25 pitch blade that we have. ( Remember. That sever duty unit was designed for the load of the 2.5 inch pitch blade. ) The sever duty clutch unit can encounter problems having enough load to properly disengage. So to have the fan pull more air. Go to a 2.5 inch pitch blade . And then use the sever duty clutch unit. In short. The heavy duty unit should be use for our fan blades that have the 2.25 inch pitch fan. No others.