After teaching a Troubleshooting Proportional Valves course recently a student told of a problem that occurred at his sawmill — a problem that ended up costing $96,000! He said that the bed lift cylinder on the Chip-N-Saw infeed had stopped operating. He then replaced the proportional valve with exactly the same part number as the original valve on the machine. When that didn't solve the problem several other components were changed in the system. He said, "If I would have known six months ago what I learned today, I could have saved my plant eight hours of downtime. The cost of our downtime is $12,000 per hour."

What he had learned that day was that on the proportional valve, the small threaded plugs had to be removed before installing the valve on the machine. The valve in question is known as a two-stage type valve (Figure 1). The valve requires both electrical power and hydraulic pilot pressure in order to operate. The purpose of the pilot valve (located on top) is to direct pilot pressure to the main valve spool. The pilot pressure can come from an internal passage inside the valve or from an external source.

Approximately 118PSI is required to overcome the springs in the valve housing to shift the main valve spool. In the schematic shown in Figure 2 notice that the pilot ptessure plug is located between the inlet line of the main valve and the pressure port of the pilot valve. When the plug is installed, pilot pressure must come from an external source and be supplied into the "X" port. Once the pilot valve spool is shifted into the straight arrows ("A" position) or crossed arrows ("B" position), oil will be directed to one end of the main valve spool. When valves are externally piloted a separate pump is many times used to supply the pilot fluid with a maximum pressure of approximately 200PSI. Another method is to use a pressure reducing valve connected from the main pump line to supply oil to the "X" port. By using a lower pressure to shift the spool, shock and wear on the valve is reduced.

In the cutaway shown in Figure 3, the pilot plug is located in an internal passage inside the main spool housing. If there is no pilot line connected to the "X" port then the main spool will not move when the pilot valve spool is shifted. This means that the valve must be converted from an externally to internally piloted valve. This is done by removing the pilot valve and removing the threaded plug. The plugs are usually metric so metric allen head wrenches will be required. Removing the plug hydraulically connects the oil at the "P" port of the main spool, through the valve body, to the "P" port of the pilot valve. The other plug found in the valve is in the drain line (Figure 4). When the pilot valve directs pilot pressure to one end of the main spool, the oil on the opposite side of the spool must be ported through the pilot valve and back to tank. When the pipe plug is installed, the drain flow is blocked to the "T" port passage of the main valve. Therefore, the oil must flow out of the "Y" port and back to tank.

In Figure 5, the location of the drain plug is shown inside the main spool housing. Valves are externally drained when high flow surges exist in the system return line. Any backpressure in the return line can affect the shifting of the main spool. If there is no line connected from the "Y" port of the valve back to the tank, then the main spool will not shift. To convert the valve from an external to an internal pilot, the drain line plug must be taken out. The pilot valve must be removed to access the drain plug (Figure 5). The oil that exhausts out of one end of the main spool can then flow through the pilot valve and back into the internal tank passage of the main spool.

With both plugs removed (Figure 6), the valve is configured for an internally piloted and drained valve. Most of these type proportional valves that are used in the industry are of this configuration. The OEM will remove the plugs before installing the valves in the system. In most cases, the part number is NOT changed. So, when the mechanic or electrician changes the valve, many times the plugs are not removed and the machine will not operate.

Another problem can occur when sending the original valve back for repair. When the valve is received at the repair shop, if the part number designates that the valve is externally pilot and drained, the pipe plugs are installed back in the valve. To prevent this from occurring, there are two options: First, order the valve with the part number that indicates the valve is internally piloted and drained (plugs removed). The second option is to make sure everyone in the maintenance department knows to remove the pipe plugs prior to installation. The first option is better.

• In-plant Troubleshooting
• Leakage Problems
• Pressure Settings
• Shock Problems
• Hydraulic Reliability Assessments
• Hydraulic Troubleshooting Manual Development
• Startup Consulting and Recommendations
• Heat Problems
• Repeated Component Failures
• Speed Problems

1/2 c. ketchup
2 tbsp. cider vinegar
2 tbsp. maple syrup
2 tbsp. grated onion
2 tsp. prepared mustard
1/2 tsp garlic salt or 1 tsp fresh crushed garlic
1/2 tsp. ground thyme
1/4 tsp. cayenne pepper
Mix it all together and brush it on the meat in the last 10 minutes or so on the grill

The suction line of a pump must be sized such that the fluid velocity in the suction line is kept between 2 and 5 feet per second. Low fluid velocity is rarely a problem in hydraulic machines. While it is true that low fluid velocity can cause deposits, fouling and corrosion, pipes and hoses would have to be considerably too large for this to become an issue. But high fluid velocity causes heat and turbulence issues in pressure and return lines and, in a pump suction line, high fluid velocity can cause cavitation.

Why is this? Because fluid velocity affects pressure. The lower the velocity, the higher the pressure. In a pump suction line, pressure is below that of atmospheric pressure (14.7 PSI at sea level, or 1 Bar). Hydraulic pumps are positive displacement devices, i.e., a very specific amount of fluid is delivered with each turn of the shaft. This is because of the extremely tight tolerances of the pump (usually no more than .0002-.0003"). Thus, if the pump is trying to deliver more oil than it can get into its suction line, the suction pressure will drop VERY low. Air molecules actually collapse and implode when this is extreme. The violent implosions cause serious damage in time. The steady, high pitched whining sound this process makes is the defining characteristic of cavitation.

But what does the size of the suction line have to do with the fluid velocity? Fluid velocity is inversely proportional to the diameter of the pipe or hose. In other words, fluid velocity is equal to the rate of flow divided by the surface area of a cross section of the suction line. Attendees of our Hydraulic Reliability and Troubleshooting Workshop may recognize that as the formula used to determine cylinder speed, which of course would be equal to the fluid velocity inside the cylinder.

OK, so that explains why the suction port is big, but why should the pressure port be small? One reason is because it doesn't have to be big. Another is because it allows for more material around the port to withstand the greater force against it. This keeps the pump from cracking under the stress of high pressure at the port.