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Hydraulic Tool Circuit Questions
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Posted by: EJohnson

04/08/2005, 12:22:18

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I have a couple of questions regarding hydraulic tool setups on crew trucks. I have a pair of trucks already equiped, and have to buy 2 more but I am not satisfied with how the ones I have were put together, as the tools get too hot to hold after extended use. So I'm trying to learn more about system design so I can fix the existing, and secofy how the new trucks should be setup.
Basic info: F-450 Superduty trucks, one manual tranny, one auto. Both equiped with 40+ gal fluid reservoirs, one equiped with a fluid cooler as well. The hydraulic pumps are mounted on PTO's for each vehicle.
My questions are about the actual plumbing layout. On the non-cooled truck I have a line going into an adjustable flow regulator (?) which has a relief line plumbed into the return. The working fluid then goes into a valve body that also has a pressure relief built in before going on to the actual tool connection. Is this the correct order of components? It would seem to me that the valving should be first, so the excess pressure and flow get sent back to reservoir sooner to build up less heat. Also, is there a rule as to how much of a size difference there must be between pressure lines and return lines? The feed line seems rather small, and it would be simple to upsize it as the rest of the fittings are a larger diameter. Another possibility would be eliminating the flow regulator entirely and trying to set up the truck RPMs to match the flow demand (I only need 2 different settings 5gpm and 12gpm) as the engine controller has provisions for this.


Eric







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Re: Hydraulic Tool Circuit Questions
Re: Hydraulic Tool Circuit Questions -- EJohnson Post Reply Top of thread Forum
Posted by: maytag

04/08/2005, 21:40:42

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Eric,
Line size is determined by the velocity of the fluid in relation to the pressure-you didn't state what pressure the system is set for now-medium pressure(500-2000 psi)the line should be sized to keep velocity 15-20fps. There are a lot of unknown quantities about your system-has there been any provision to unload the pump(directional control valve that routes the fluid back to tank during the idle time)-what type and brand of pump and DCV are you using. How are you adjusting flow now from 5 to the 12 gal-the flow control valve is a heat generator regardless of where in the system it is. Also your 40 gal.
tank is a little undersized which contributes to your heat(rule of thumb is 5 1/2 times the pump output when not using a heat exchanger. As you probably know when using mineral oil(you didn't say but I'm assuming this is what you are using)mobile equipment will run hotter than one can safely handle the tools(most mobile equipment manuels don't get alarmed unless temp is approaching 160-180 degrees which is a lot hotter than an operator needs to handle or be exposed to the risk of a hose failure.) If you can provide some more details either here or private email I'll try and get you pointed in the right direction-I'm not a salesman so you won't be under going a sales pitch. Maytag






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Re: Hydraulic Tool Circuit Questions
Re: Re: Hydraulic Tool Circuit Questions -- maytag Post Reply Top of thread Forum
Posted by: EJohnson

04/13/2005, 12:46:56

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Hi Maytag, sorry for the delayed response. Ok, let me try to answer your questions in order:
First: Not certain just what the directional control valve would look like. However, the flow control has a line connected to the return, as well as the pressure line going into the on/off valve body. Also, the on/off valve body has an adjustable relief for pressure control.
Type and brand of pump: This truck has a Chelsea PTO w/direct mounted pump. I can't get the exact specifics as the tag is very hard to read, but from what I gather the pump itself is capable of up to 30gpm flow. Rookie question: Would the directional control valve be what I am referring to as the on/off valve body? (still learning the lingo) Flow is being adjusted in a valve block that has a single lever that turns on a 1 to 10 scale. However, I am now wondering if I would be better off deleting that device and simply setting my engine RPM's to handle flow control, as I actually only need 5 and 9 GPM for most tools, with 12 being a rarely used level.
Yes, we have found that the tools get screaming hot, which is what prompted this search. Thanks for your time. Oh, and yes my system design pressure is 2000psi, 5 to 9 (normally) gpm. We run a variety of tools, from saws and jackhammers to impact wrenches and trash pumps.

Eric







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Re: Hydraulic Tool Circuit Questions
Re: Re: Hydraulic Tool Circuit Questions -- EJohnson Post Reply Top of thread Forum
Posted by: maytag

04/13/2005, 18:20:03

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Eric,its just a guess but with two F450 Superduty trucks outfitted like this am I correct that they approach $100,000 for both. This is also speculation-Chelsea makes alot of PTO equipment for farm and ranch trucks-dump truck,winch, etc things that see intermittent use. My guess is that you have a positive displacement pump-probably a gear pump. Positive displacement means(discounting inefficiency at different pressures) for each RPM it will pump X inches cubed of fluid-the only way you can vary the flow is by varying the RPM- a 30 GPM pump and you use only 5-12 GPM-the extra fluid is diverted by to tank either via the extra line form your flow control valve or over the relief valve at that pressure setting-both ways would be extremely ineffient-rule of thumb 1 gal @1500 PSI equals 1 HP- all the wasted HP goes to HEAT. BTW the ON/OFF valve is your DCV(directional control valve). If you bought the trucks new and can get a schematic of the system with a bill of materials I give you a fax number and then make some recommendations-simplest fix for these trucks is thru experimenting lowering the RPM on your engine(reducing the GPM)where so much HP doesn't go to heat-
I would definitely get a different setup for my next truck-more of an industrial style-axial piston pump with pressure compensated contol. Hope this helps some, Maytag






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