hydraulics

Dear Allen & Jenny:

connections -

You'd need to review the actual components to create your own schematic. The description may be that the low pressure relief controls the retract speed. Perhaps supplying backpressure. 4 port valves are unsurprising in that they deliver pressure to either extend or retract the cylinder, while connecting the other cylinder port to drain.

David A. Smith

I know it's 'low' pressure, but you might want to reconsider using the adjective 'only' in reference to a pressure of 1000 psi. Some dumb operator might overhear you and figure it's safe to test a pressure leak with a finger.

First, check to see that it's working as intended. No sense trying to hotrod something that's already broken.

No sense trying to hotrod anything in an industrial environment, either. If you need a stronger, faster press, _buy_ a stronger, faster press.

-Mike-

Dear Bud Trinkel:

schematic.

Good points all. However, presses are supposed to travel very consistently. If the pressure in the cylinder drops too low, an air bubble can form, making operation "springy". It is common to provide backpressure in fluid exiting the ram, so that the lowest pressure in the system, the place where offgassing can occur, is in the reservoir. Ultimately, his circuit will need to be investigated.

I have done some "air-over-oil" applications to get the ram down to the workpiece more quickly. But significant changes to the circuit would likely be necessary. Mr. Halloran's suggestion is likely the best one... buy a faster press.

David A. Smith

Hi Mike: Thanks to all who replied.

No such person - only myself and my wife. She is pretty good mechanically but would never attempt to take anything this expensive apart without my help.

It appears to be working as intended but the folks who built it were food people with an idea and a machinist - not engineers. Also, in the food industry ( I did mention this aspect of the problem in the original letter ) none of the inputs are ( pastries ) quite as consistent or predictable as materials like steel or thermoplastics.

The trouble with this idea is that a) we can't afford to spend a lot of money right now b) it took us two years to find this press. Most machines for this operation are hand presses - essentially a clean arbor press with dies for pie shells The company we bought is the only one using hydraulic pressure in their presses as far as we know and we bought the bigger of the two already..........

Re-engineering is the only answer for this one, I am afraid

cheers

allen

Some thoughts - the basics apply here - 1) the relief valves have a pressure setting and a flow capacity range in which they will reliably function - 2) pistons move as fast as the oil supplied to them and as their load allows 3) there is pressure drop across valves, and that drop and the flow determine the power lost as heat in the valve.

this means ---- If you increase the flow across the valves, they may not (or may) "take it" and overheat.

If you want the piston to move faster, you can lighten the load, get more oil per stroke from your pump, or store oil and then release it to add to the usual pump supply (e.g., in many pumps the "piston" puts out a fixed amount of oil each stroke, and in simple circuits such as this one, the pressure is determined by the restriction to the flow and the relief valve, and any flow at that pressure not used to do work on the load is lost across the relief - so during a low flow high pressure cycle time, the resistance to the pump's flow is increased which gets more pressure, with any excess flow above the relief valve setting dumped as lost power at that setting. In the high flow low pressure portion of the cycle, the load is not present, and the cylinder moves as fast as the pump volume delivered. )

Pushing more oil from the pump means getting a larger volume pump.

Using the unused oil volume from the high pressure low flow portion of the cycle means tapping off the flow out of the high pressure relief valve and above a second low relief valve and shunting it into an accumulator to be stored at some pressure and then switched into the cylinder return supply, and thus added to the return stroke flow of the pump - which increases return speed. However, it ain;t simple - You need a separate relief valve for safety, plus some compensated high and low relief valves. Or you can just get a bigger pump for more flow/ two stage pump (same high pressure flow and more low pressure flow), and maybe a flow control valve for the high pressure side.

The low side relief valve presents load to the cylinder so when the cylinder stops, the moving piston mass does not overshoot and pull in air under the piston seals. off the top of my head......

Dear Allen & Jenny:

mechanically

You need to describe its operation (including time down and time up). You need to describe the components you cannot afford to replace. You need to describe what you want it to do (including how fast).

You have to know what you want, and convey it to those that will help. And unless you can bring it in to a local hydraulic shop, you have to be their eyes and hands too.

David A. Smith