"PhilC" wrote:

Not entirely true. Higher pressure means you can move more with the same size cylinder. The velocity of the oil will depend on the hose size and pump flow rate. If you have a hose that can handle 50gpm and no resistance at the end of the hose then you will get the same oil speed using a 6000psi 10gpm pump as you will with a 1000psi 10gpm pump.

Most hydraulic systems, once filled with oil don't allow that oil to escape except for the valve return to tank. Both the rod and piston end of the cylinder will be filled with oil and will not lose that oil under normal operation except when using the float function and that loss will be minimal. If the hose is full of oil and you pump 5 gallons in one end then 5 gallons will come out the other end no matter the size of the hose.

And much higher velocity of oil. The reason the hose is controlling the speed of the cylinder is due to a restriction in flow not pressure.

Higher pressure does not increase cylinder speed only flow rate increases cylinder speed. They are however both related. For example if you want to lift a certain weight at a certain speed and you calculate you need a given flow rate and pressure to lift the weight and then you double the load but still want the same speed then the flow rate will be the same but the pressure will increase due to the extra resistance.

Yes there is a formula. I think there are a few online calculators that you can use but you need all the info such as pump and valve displacement at a given engine speed, required cylinder speeds and sizes, whether you want the speeds to be the same for extension and retract etc

That's why hydraulic systems have relief valves. The pressure in the pump, valves, cylinders and hoses should not exceed the maximum pressure the relief is set to. Modern valving allows different pressures for each function in addition to the main relief.

Hyd hose is expensive and the bigger you go the more you will pay. I can understand wanting to keep the hoses small. Also smaller hoses are more flexible and require less wires for the same pressure.

"Great Scott! That's allot to take in! This could change the space time continueum!"

"Rome wrote:

"Great Scott! That's allot to take in! This could change the space time continueum!"

something i aleays wanted to know? maybe you can tell me who Scott the Great is???

I have a D4 7U with 4s dozer and no. 44 hydraulic unit. I found that a 3 1/2 x 8 inch cylinder on a 4 bottom plow is very fast acting. Move the control easy when raising and lowering. I used 3/4 hoses to the rear .

"kal" wrote:

I have a D4 7U with 4s dozer and no. 44 hydraulic unit. I found that a 3 1/2 x 8 inch cylinder on a 4 bottom plow is very fast acting. Move the control easy when raising and lowering. I used 3/4 hoses to the rear .

thank you. this is exactly the kind of info i have been looking for

Way back I ignored all the advice similar to what PhilC has given here. From that experience I can say just because the pressure goes up a lot even the speed will decrease with less GPM. Made it about impossible to doze with the combination of pump GPM and ram size I had. Not a big deal with farm implements. I decreased GPM from 50+ to 25+ or so and increase PSI from 700 to 1500.



For older farm implements as long as you can lift them I have never a problem with speed. On a dozer it makes a lot more difference.

"BigAgCat" wrote:

From what I can surmise with this situation, you would have the same pump just the higher pressure pump would have a much smaller hose. Pressure would be the force. Force = mass x acceleration. The liquid travelling through the smaller hose will be travelling at a much higher speed than the liquid travelling in the smaller hose in order to maintain the 10 gallons per minute.

Correct but keep in mind if you use a smaller hose and use pressure to force the fluid through at a higher velocity you will put more load on the pump as it will need to run at a higher pressure to overcome friction losses in the hoses. Older machines don't have variable displacement pumps so the pump dumps to tank when not using a function. As soon as a lever is moved the pump provides whatever pressure is required to move the load. If part of that load is simply forcing fluid through a too smaller hose then it is not optimum.

"bursitis" wrote:

something i aleays wanted to know? maybe you can tell me who Scott the Great is???

en.wikipedia.org/wiki/Great_Scott

"kal" wrote:

I have a D4 7U with 4s dozer and no. 44 hydraulic unit. I found that a 3 1/2 x 8 inch cylinder on a 4 bottom plow is very fast acting. Move the control easy when raising and lowering. I used 3/4 hoses to the rear .

You probably could have gotten away with 1/2" hoses but if it works:)