CarlDyke.com

Fly Fishing with Bernoulli (Part 1)

admin — Thu, 12 Jun 2008 20:34:37 +0000

As thoughts start to turn towards summer activities (like fishing) and of free and easy days, we bring you some thoughts about how to keep hydraulic flow free and easy as well. It is not uncommon for machine owners to make some changes to the hydraulics over time. Valves get mounted at a new and more convenient location. Hose or pipe runs get moved or replaced as the machine is modified. Perhaps a larger pump is installed in an attempt to increase machine output.

Any of these changes could bring on an increase in system pressure, heat, and energy consumption if hose fittings and diameters as well as tubing bends aren’t taken into account.

Wikipedia.org refers to him as a Swiss guy while clearly documenting that he was in fact born in the Netherlands. In any case, consider a man named Daniel Bernoulli, an 18th century mathematician. His famed ‘Bernoulli’s principle’ states that an increase in the speed of the fluid occurs simultaneously with a decrease in pressure.
In hydraulic systems this translates approximately as "increasing the fluid velocity (fps, feet per second is typical) results in lower fluid pressure." If you can imagine that a gauge mounted on a hydraulic hose shows the pressure going down as the flow velocity increases, then you’ve got the idea.

Some people find this principle very hard to believe so lets switch to pneumatics for a moment. If you use an air nozzle gun on the end of your shop air hose to blow dust off of equipment, then you’ve probably observed how the needle on pressure gauge for the air system drops as you squeeze the nozzle lever (increasing fluid velocity). The needle comes back up as you let go of the nozzle lever (decreasing the fluid velocity). Yes, Bernoulli’s principle holds true. If you study the law of conservation of energy, then it has to hold true.

If you’re thinking; "Hey man, if I switch to a smaller diameter hydraulic hose I’m pretty sure that the pressure will rise", then you are probably right. Switching to a smaller hose in a given hydraulic system will increase fluid velocity (fps) but will also cause greater fluid friction as more hydraulic oil molecules have to come in contact with the wall of the hose or pipe. This increase in friction will result in a load on the system and will result in heat and higher pressure.

For this reason, let’s consider a pipe that is correctly sized for the maximum fluid flow of the system. The hose or pipe diameter is large enough to impose no noticeable load on the system. Lets say that the only source of system pressure is coming from the loaded cylinder or hydraulic motor that is doing work. When the fluid that is being moved in this system at a certain number of gallons per minute (gpm) then enters a larger diameter chamber, it must surely loose some of its linear velocity (fps) and slow down.

This makes sense doesn’t it? The wider highway can deliver the same total flow volume (gpm’s) at a lower velocity (fps). Less velocity means we’ve lost kinetic energy - the energy of a mass in motion. The law of conservation of energy says that we must have converted it to another form. Yes we have indeed. The lost kinetic energy is now potential energy (increased pressure)

Now imagine what happens if the inside wall of the hose isn’t smooth. Imagine what happens if your ninety degree fittings or tubing bends have a bend radius that is too small. This is where the fly fishing comes into the picture. (I know you were waiting for the fly fishing.) This is also the part where Bernoulli’s principle starts to backfire!

Think about this a little and next week we’ll take you fly fishing in a fast moving river. We’ll find out what happens as fluid flow is directed around ninety degree bends that are too sharp!

Electrical Faults (Part 1)

admin — Thu, 22 May 2008 19:28:24 +0000

Troubleshooting an Electrical ‘Open’ Fault - Part I

This ‘newsletter that teaches’ like most of our newsletters, is about logic! This time we’re covering an electrical control troubleshooting topic. The mental tools needed to troubleshoot a hydraulic circuit are the same ones needed for an electrical circuit.

When an electrical control circuit malfunctions and a device (load) fails to energize, the fault may be an open point in the circuit. Whether the electrical load is a lamp, a relay coil, a valve solenoid, a heating element or some other resistive device, it is popular to immediately look at the electrical supply wiring.

Confirming that the necessary voltage has arrived at the hot (AC) or positive (DC) terminal for the device in question makes sense as a quick logical test. The test is easily conducted with a simple voltmeter for either an AC or DC circuit.

Follow Safe Procedures!

We’ll assume that you know how to configure and use your test meter correctly. Usually the black meter lead is connected to the negative (DC) or common (AC - often the enclosure ground terminal is best choice) supply line and the red meter lead is then used to check for the correct voltage on the positive or hot side of the device in question.

If the correct voltage is present at the positive terminal of your device (load), what do we assume?

Often a troubleshooter who is in a hurry now assumes that the load has malfunctioned and has become electrically open. This may be a correct assumption. After all, if supply voltage is present on the positive terminal of the relay coil, lamp or valve solenoid, then it must be burned out or it would function normally! Yes? No?

Well you might try a new lamp or relay in place of the old one, or if it’s a difficult device to replace, you might disconnect its leads from the circuit and check the resistance of the device with an ohms test. We would need to know the correct resistance value and the whole procedure is going to take some time. But if the resistance value checks out, or a new device doesn’t function when placed in the circuit, then what have we missed?

If we go back to the voltage tests we did earlier we could have added one more quick step to that fast, efficient sequence. If the red meter lead detected the correct voltage at the positive terminal of the load, we might have then chosen to move that red meter lead across the load to the negative or common supply terminal. Is it possible that the meter could read supply voltage on this common or negative terminal?

Yes, it is very possible. What does it mean? It means that the wiring that completes the common or negative side of the circuit contains the ‘open’ point in the circuit. The fault is on this sometimes overlooked side of the circuit. A terminal strip screw may be loose or perhaps a wire nut is not holding all leads together. Perhaps a crimped wire terminal is not holding crimp.

The other approach to completing the test with a voltmeter would have called for the black meter lead to move right up to the negative terminal of the load. In other words, your meter might have registered the presence of supply voltage with the red meter lead on the positive terminal of the load while the black meter lead was still touching the main circuit ground or negative.

If, however, the meter reads zero volts when the black meter lead is moved up to the negative terminal of the load itself, then once again you know that the wiring on the common or negative side of the circuit contains an ‘open’ fault.

May 1 - Cylinder Myths, Part 2

admin — Wed, 14 May 2008 16:15:32 +0000

Thanks to those of you who wrote in to let us know that you found the previous newsletter helpful! One smelter customer told us that the lesson really "hit the spot". We’re also pleased to hear that so many of our sawmill customers were able to get value from the lesson. (If you missed the last newsletter, we left a copy for you to read at www.carldyke.com/newsletters2008.)

In this issue we’ll conclude the lesson regarding cylinders that are mounted vertically or are subjected to load under the influence of gravity.

Grab a coffee! This one’s long!

Last time we looked at single rod cylinders that are oriented with their extending rod pointed upwards.
We found that with the directional valve closed to both cylinder ports, the only way for the cylinder to creep downward is to allow for the loss of fluid to the outside of the cylinder. We found that the rod and piston cannot otherwise creep down
because a cylinder full of fluid has no additional room for the volume of steel that is the rod.

Lets finish up this week with a look at cylinders that are mounted in the inverted position. The rod of such a cylinder extends downward.

Again, it is a fairly common misconception
that a worn piston seal will allow fluid to pass from the rod side of the
piston to the barrel side, thereby allowing the rod to creep downward.

We’ll start with the directional valve closed
(we’ll put closed ball valves on each cylinder port just to make it
very clear), and the rod fully retracted. Let’s also assume that the
cylinder is full of hydraulic fluid.

If the piston seal is absent and we have a load on the cylinder, will the rod creep downward?

If the cylinder was full of steel (the piston and rod) and liquid (a largely non-compressible fluid), then what would happen if the rod did creep downward into extension?

If the rod extends fully, the full and sealed cylinder will lose that amount of occupying volume as a consequence. What fills the void that is created inside the cylinder?

A large force (heavy load) pulling on the end of the rod could cause some extension, but the total movement would be very small. This is because the vacuum value would become very great within a tiny
fraction of an inch.

In order for any noticeable movement to take place, a make- up volume of fluid (shown below in light blue) would have to replace the volume of cylinder rod steel that has moved out into the atmosphere.
Remember that the cylinder ports are closed by ball valves. This is the same as if a directional valve’s ‘A’ and ‘B’ work ports are closed.

A damaged or missing piston seal on its own cannot be blamed for a creeping cylinder
rod; the rod end gland seal must also be damaged and leaky at the
same time.

If the gland seal is damaged,
then as the rod creeps downward under gravity and a heavy load, make up fluid can be sucked in to the void being created as the rod leaves the cylinder and moves into the open atmosphere. It is the presence of this make-up fluid that allows creep. Without it, the settling rod would create a vacuum that would hold it in place.

In this case, the make-up fluid is air. Air is, in fact, a perfectly useful fluid (not a liquid) for use in cylinders, especially if you don’t mind spongy, non-positive motion, and you don’t need to work with high pressures.

Whether the rod is pointing up or down, we’ve learned that a missing piston seal on a single rod cylinder with blocked fluid ports will not allow cylinder drift or creeping unless the gland seal is also compromised.

If you know that your cylinder is in fact creeping when the ports are apparently closed at the directional valve, and yet the rod end gland seal is not allowing leakage, where do you look for the real cause?

The directional valve itself is most often the cause. It may not be closing the ‘A’ and ‘B’ cylinder work ports completely. The valve may be worn or it may be failing to return to its center closed position. The cause of valve problems will be covered in future newsletters.

(Click here to return to Newsletters 2008)

April 11 - Cylinder Myths, Part 1

admin — Wed, 14 May 2008 15:07:00 +0000

Dear Maintenance/Service Professional,

This is the first in a series of newsletters that teach! We will be addressing real issues primarily in the fields of hydraulics and electro-hydraulic motion control.

Troubleshooting a hydraulic system problem is rarely easy. Its impossible to get inside the system, and placing test instruments such as flowmeters and extra pressure gauges is nowhere near as easy as making current and voltage tests in an electrical circuit.

One of the common problems we hear about is how an ordinary single rod cylinder that is mounted in the vertical or near vertical position drifts slowly downward after the directional valve has closed.

Gravity and a load that has been hoisted up are the obvious forces that drive the cylinder down. But if the ports are blocked by closed valves, where is the flow path?

It is very popular to assume that the piston seal inside the cylinder has failed and that the drifting motion is occurring because the gravity pressurized fluid is being forced across the piston. But is this possible?

Can a single rod cylinder really behave in this way when the piston seal fails?

To answer these questions we have to study some basic principles of volume.

Imagine a two foot long, 6" diameter pipe sealed at one end and sitting upright on a surface. Imagine that the pipe is already filled with liquid. Then, you come along and drop a length of 2" diameter solid steel rod into that pipe from the open end at the top.

What happens? Liquid overflows and spills out because the pipe was already full. There was no room left in the pipe for the volume of steel you placed into it and as a result, liquid was displaced.

Now consider a vertical cylinder with its rod end facing upward. The rod end gland and seal are in good shape.

Lets start with the rod fully extended and assume that there is no air in the liquid below the piston. The cylinder is lifting a heavy load. Both the bottom (barrel end) and top (rod end) ports are closed by ball valves.

If we now remove the piston seal so that there is a noticeable gap between the piston and the cylinder wall; what will happen?

Will the rod and piston assembly drift down or will it stay in place?

(Click here to return to Newsletters 2008)