I'm trying to understand how to read the specifications for implement hydraulic flow. The specs say 2 x 20.9 gpm + 16.3 gpm for the 8 ton excavator. Presumably not all of the rated flow will run to the work tool. I'm looking at a hydraulic chainsaw that needs 30-60gpm to run well.
I'm just guessing here, but I think the 2x20.9gpm pumps are for running the travel motors. Probably tandem pumps. Then, the 16.3 gpm is an auxiliary pump running off a power take off to run the bucket/arm/slew drive/implements etc.
You have your normal twin pump delivering 20.9 gallons per minute. The 16.3 gpm pump is probably working a blade or offset arm if fitted. You Definately dont have enough oil flow to run saw, even if you were to join both of your 20.9 gpm pumps together using a flow diverter valve, it still a long way off the maximum flow needed. Smaller saw maybe!
I talked with one of the guys at Caterpillar and he explained the spec sheet to me. It looks like the Primary auxiliary flow will do 33.8gpm and the secondary will do 16.9gpm. That would mean the primary has just barely enough flow to run the saw and the secondary could run the rotator and grapple. Anyone know how to calculate the flow running out the blade? I'm thinking of mounting a wood splitter to the blade, the higher the flow the better.
You need more information. The kind of pumps makes a big difference in the amount of flow you get at a certain pressure. A gear or vane pump will maintain most of the flow all the way up to relief pressure. A variable flow piston pump though will reduce its flow characteristics to the horsepower curve of the engine. I've had multiple occasions where I had to tell people that a 40 gallon a minute pump at free flow might be around 30 gallon a minute at 2,800 PSI and is only 12 gallons a minute at 4,000 PSI. What you need is the graph showing the output curve of the pump before you can decide what attachments you can run.
John C thanks. I was wondering the same thing. Horse Power is just as important as flow. Based on what you said I ran the numbers. 33.8gpm at 4061psi requires 94hp. With 33.8gpm if the rest of the machine were sitting still I'd only be able to get 2500psi from the saw with a 65hp excavator. Shoot...I really want a small machine that can do everything...is that so much to ask?
We usually figure one horse power per gallon per minute per 1,000 PSI. In your case you have 34 gpm times 4 equals 136 ponies. It usually takes a bigger machine before you can split out enough flow for an auxiliary function. What you have to consider is that the machine also has to move something at the same time your attachment is working. So if you are running a mulcher, which takes a lot of flow, you still have to move the machine forward or swing the house and pick up the boom at the same time. So just because the pumps put out 33.8 gallons per minute, it doesn't mean you have all that flow for just one function.