rkcarguy wrote:I'd say to be on the safe side I'll need about a 20-22 GPM pump, just a little smaller than yours?
From where did you get that number? 20-22 GPM is much too high for what you are doing. As I said earlier, and will say again, you are going it about all wrong with this focus on GPM.
I initially used a Barnes G25-28 pump with a 1.8 CI/rev displacement because I had modified the Briggs V-Twin to produce a stronger torque curve and figured I could use the higher pump displacement. Testing proved that the loading was a bit too high and I subsequently went with the smaller -23 pump, which is 1.4 CI/rev. Maximum governed engine RPM is 2400, so the theoretical flow at full throttle with this pump is 14.5 GPM, versus 18.7 with the larger -28 pump. Think about that, and also consider that I'm working with more horsepower and torque than you are.
The 100 series motors aren't much different in price than the 50's, so it's not a big deal. Maybe I'll go with the mid sized BMPH80, I think that is the 4.75 cu/in motor you were speaking of. I'm planning a 4000lb limit for gross weight because I suspect I'll be looking at grades in the 3% range, and if you were doing 2x that at those pressures I should be fine.
Before you pick a motor, choose a pump and then select a motor whose displacement will give you the desired amount of reduction. You want to use the largest displacement motor that will support the maximum speed at which you intend to run your locomotive.
What do you figure your oil tank + cooler volume is?
The reservoir's interior volume is approximately 4.85 gallons. A full oil charge is 4.75 gallons, which seems like too much until you consider the oil that is in the filter, lines, pump, motors, etc.
I do not use an oil cooler—none is needed. A properly designed oil reservoir will produce adequate cooling in almost all cases (see below).
- EMD F7 Propulsion Oil Reservoir
As you can see in the illustration, there is a return line filter (10 micron), which is essential to protecting the system from damage. I also have a suction strainer in the outlet on the reservoir, which is the larger diameter one seen in the face view in the drawing. The strainer keeps debris out of the pump.
I'm stuck with the 2nd run of chain and sprockets because I've already built my trucks and designed everything to be built that way unfortunately, but I can size the sprockets however I want. In my experience chain drive has been pretty efficient, as long as you don't go too small on the sprockets.
Chain drive efficiency isn't the issue. Getting the best efficiency from your hydrostatic components is what should be your main concern. That is why I
STRONGLY recommend you develop all of your "gear reduction" in the hydrostatic circuit. Run your chain drive at 1.0:1 and size the hydraulic motors to produce the desired ratio. Using smaller motors and implementing additional reduction in the chain drive forces the motors to run faster, which is less efficient.
As for SAE vs. NPT, I was wondering about that. I know the NPT threads can leak but they are common and easy to work with.
In the fluid power industry, pipe threads are avoided as much as possible; you will seldom find them in aircraft hydraulics, for example. SAE ports use an O-ring seal—no thread sealant is required—and are essentially foolproof. Commercial grade SAE to JIC fittings have a working pressure rating around 5000-6000 PSI, far higher than what is used in hydrostatic propulsion. These fittings are inexpensive and are readily available from many sources, Surplus Center being one of them.
Speaking of fittings, JIC -8 is a good size to use in the high pressure sections of your propulsion system. Avoid elbows and if you have access to a precision tubing bender and a JIC flaring tool, hard-pipe as much of the high pressure system as possible. It will be more efficient than using hose, won't burst and will act as a radiator to control system temperature.
I used type-K 3/4 inch copper pipe to make up the suction and return circuits in my F7. These runs see no pressure, so use of copper is practical and convenient. A large suction line to the pump will prevent damaging cavitation when the oil is hot and the pump is working hard. I used commercial grade acid-core solder to solder the joints, not the lead-free stuff sold at the hardware store. You could also silver-solder the fittings, although that isn't necessary and is somewhat a pain if all you have is a propane torch.
Is there a tried and true product for sealing the threads you've had good luck with?
I have used Loctite's no. 545 anaerobic sealant on hydraulic pipe thread parts for many years and have had minimal problems with it. A less expensive, but not as reliable, substitute is Oatey's Teflon-infused pipe dope, which you can get at most hardware stores. This is the white gooey stuff that comes in a bottle with a brush applicator in the lid. You stir it up real good, liberally apply it to the male threads and immediately wrench the parts. I don't use it in hydraulic work because of its potential to get into the oil flow (plus it's messy), but do use it in compressed air systems.
What is good about using Loctite 545 is the parts don't usually have to be super-tight in order to seal. With the Oatey stuff, you have to crank things down tight to get a reliable seal when the pressure gets into the 1000+ PSI range.
While on the subject of pipe thread sealants, don't even think about using Teflon tape on hydraulic pipe fittings!
Pieces of it often shred off when wrenching the parts and subsequently migrate into the system, plugging up valve ports. Some manufacturers of hydraulic components will not warranty a failure that is traceable to use of Teflon tape as a thread sealant. That is the worst damned stuff ever invented!
Don't use it!