Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 30, 2025 ...

Hydraulic fan drives

May 24, 2014, 01:44:50 pm

I ran across a really interesting article complete with drawings of various hydraulic fan drives for coaches, starting with the most basic, on up to very elaborate fuel saving systems.  Drawings are a down scroll of several pages. 

http://www.parker.com/literature/Hydraulics%20Group%20US/Bus%20Fan%20Drive%20Solutions%20Guide.pdf

So, knowing fan rebuild or replacement is in the cards for me one of these days I was wondering if anyone who's been through the fan replacement saga can tell me what the primary issue of fan motor replacement with a standard hydraulic drive fan motor is.  Shaft length/diameter/mounting/space constraints/bolt pattern/bearings/displacement?  I know the existing ones are NLA though can possibly be rebuilt.  I'm wondering if an adapter plate would allow the mounting of a current generation motor.  I'm getting ready to CNC cut some various aluminum parts and considering this as a project. 

Quote Selected

Re: Hydraulic fan drives

Reply #1 – May 24, 2014, 02:53:04 pm

Chuck,
Thanks for the link.  Interesting reading.  I wish I knew more about hydraulics.  The article didn't mentioned it but I have also heard that variable displacement pumps are new way to create more efficient systems - the idea is without flow you don't even have friction losses in the hoses and the pump just directly feeds into fan motor without and control valves.  I think these are usually high dollar systems. http://powersolutions.danfoss.com/stellent/groups/publications/documents/product_literature/520l0824.pdf

If I were to selecting a new motor, I would try to find or measure as many of the basic displacement, flow rate, pressure, rpm, and torque specs as I could on the existing motor to try to match to a new motor.  Finding just the basics might be difficult.  Most of the other things like fan mounting, motor mounting and fittings can be fixed with new mounts - especially with your machining talents.  There is also the option of selecting a different fan blade with more or less blades if you can't match the rpm close enough.  There seems to be lots of variables to designing a cooling fan system.

This place has a lot of interesting items and usually has the basic technical information: Surplus Center
I've found Parker catalogs to be very large, extensive and hard use because I never know if I'm even looking in the right one because there are so many.

Quote Selected

Re: Hydraulic fan drives

Reply #2 – May 24, 2014, 08:45:40 pm

A variable speed fan is different from a variable displacement pump. 
Conventional systems use a fixed displacement pump and motor.  They throttle back the motor by bypassing the flow around the motor using a control valve that may or may not have a variable position control.  Since fluid (verses gas) is incompressible, a fixed displacement pump will always be pumping it's full volume even if it's just returning back to the reservoir via the bypass - it will have very little pressure build up under the bypass condition.  Of course, varying the main engine speed (and thus pump speed) will change the fan speed as well.

P.S. At least that is how I understand it.  I wish I knew more.  Maybe someone else can correct me or add more.

Quote Selected

Re: Hydraulic fan drives

Reply #3 – May 24, 2014, 10:19:12 pm

John,

Yes, a variable displacement pump would be trick as you could potentially run the fan full speed at engine idle rpm.  Of course this would also depend on size of pump and not to be overlooked is the need for the pump to supply power steering as a priority. $$$$ for those swashplate piston pumps. 

It seems that a reasonable modification that would have great benefit would be installing a more sophisticated speed control.  I "think" my coach has a two speed fan, a system that would allow from zero to full fan speed based on engine temps would be an improvement.  As far as going to variable displacement pumps or motors, though,suspect that the present system that sends excess flow to bypass would not be that much of a hp drain.  Looking at the hp/fan speed power curves though, keeping the fan speed to the minimum necessary to keep the engine on the thermostat could save some serious hp.

First priority though is to try to work out an alternate, currently available fan motor.  If anyone has info on the existing dual fan motors including fan operating rpm range I'd be interested in any specs available.  If it's simply a mounting problem that a machined adapter could solve I can produce as many of those as needed.

Quote Selected

Re: Hydraulic fan drives

Reply #4 – May 25, 2014, 09:38:32 am

Hey most everything that has been stated is pretty much spot on. A pressure compensated pump piston pump is very pricey I would think that you would ever recoup the savings in fuel it will fail before you do. Stick with the gear pump and add a electric valve that will slow the speed and add a couple sensors to control it. A gear pump that is bypassing to tank uses very little HP. If you send the data plate on your motor to sun source they can match you up with something equivalent. They are a very big design company. I know ken had a tread on here about his a month or so ago. He bought his motor from fot for big bucks and it came with the fab kit to install. You can fab a mount.I would use a parker motor they seem to last a very long time  in the RR industry that I work in.

Quote Selected

Re: Hydraulic fan drives

Reply #5 – May 26, 2014, 03:09:02 pm

Replace the (seals) and keep driving. 
As for the hoses, if they are a problem (and mine was), replace it.
Take the old one off and locate a source that builds them.

99% of hose issues will be the pressure side of a system.  pc

Quote Selected

Re: Hydraulic fan drives

Reply #6 – May 26, 2014, 09:53:22 pm

Unless you know the design criteria for the whole sys, I would just rebuild what is already there.  An old saying comes to mind "dont fix something that is not broken"  Hope it all works out for you  ----  Fritz

Quote Selected