I was having problems with 2 new Hayden sever 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 sever 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 sever 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.Bob Dunahugh
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

I agree with all above except that I think there are more conditions than just pure engaged or disengaged as the valve is variable as turned by the bimetalic spring. Once the fluid has redistributed on a cold engine start, the maximum free wheel slip will occur at idle. If you let the vehicle idle you will see how slowly fan is turning. Easily stopped with your hand ( not recommended) Then after engine is idling but at opperating temp, the fan will start to pick up speed, and since at idle (low power transmission) will approach somewhere below water pump speed. However if you rev the engine it will only pickup moderate speed and then slip to a limited speed for that moderate temp. Turn on the AC and after a few mins reving the engine will move more air. There is then the HOT fully engaged scenario in which most power is transmitted with less slip, roar can be heard but still will slip over a certain engine RPM to limit fan tip speed. This max condition usually only happens in high heat after long stop and go traffic, after a few mins of climbing a long grade, or after sudden slow down after long highway road load due to lack of natural road air flow. This has been my observation on many, many air conditioned GM cars over the years. The different dutys of clutch will move the thresholds but also if overspeced will not go to the 'disengaged' state as mentioned In previous post.

OK guys, I retype this one more time. I posted all of this here and more several years ago. Here is the abbreviated version.

I went though this Hayden thing many years back and after testing a total of 6 different Hayden heavy duty and severe duty clutches sent to me by Hayden Engineering, Hayden finally admitted to me that they changed the turn on temperature of their clutches. They actually bragged about the difference of their engagement temperature. The result is the clutch when installed in a GMC seldom turns off.

At that point they asked me to modify the front spring in their clutch to increase the engagement temperature. I had had enough and at Denny Allen's recommendation went to AC Delco heavy duty (not severe duty) clutch. I also talked with AC Delco and supplied them the OEM GM part number. The engineer requested the size and pitch of the fan which I got from Denny. With that information AC Delco engineering recommended the use of the heavy Duty clutch. I installed one and have never had a problem since them.

The major difference between heavy duty and severe duty clutches is the rpm that the clutch spins at when engaged.

Steve did do some clutch testing a few years back but he never tested the OEM spec heavy duty clutch.

I still had those 6 "free" Hayden clutches around here a couple years ago. I do not remember if I trashed them or not.

The heavy duty unit should be use for our fan blades that have the 2.25 inch pitch fan. No others.Bob Dunahugh