A steel processing plant uncoils 8-ft wide stainless steel coil stock and runs it through a polishing machine. It prepares the steel for use by manufacturers of appliances, such as refrigerators. The processing plant rewinds the finished product, adds protective paper between the coils, then ships them to the customer.
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| The circuit shows a polisher machine at a steel processing plant, where reject problems were becoming a problem. |
The steel processor upgraded its 1980 vintage mill late last year. Part of the upgrade was to replace an old hydraulic system with proportional control and state-of-the-art technology for polishing flat steel stock. Once the processor was up and running, a few problems cropped up, most of which were solved with minor fixes — except for one. The hydraulic schematic shows the polishing circuit believed to be causing reject problems when a short jolt or bump was experienced during the continuous positioning of the mechanically driven polishing heads.
The circuit design incorporated a single cylinder with a self-relieving, pressure-reducing proportional valve to provide pressure to the rod side of a single-acting cylinder. When operators wanted the cylinder to fully retract, they provided a maximum signal to the valve, producing enough pressure to raise the piston rod. When they wanted the piston rod to extend, they sent a signal to the proportional valve to lower the rod-end pressure by allowing the weight of the polishing heads to extend the cylinder. The proportional signal also provided a counterbalance force, which acted against the polisher heads’ weight, thus applying the correct force needed for producing the correct finish on the steel.
When the operators determined that the surface finish of the coiled steel called for higher or lower pressure on the polishing heads, they made a slight adjustment to the proportional signal to the valve. About half the time the adjustment made the necessary correction without quality problems. Other times, however, a pressure bump seemed to create a defect in the coil. They noticed it usually occurred when their adjustment added more polisher head force against the coil stock.
Maintenance people replaced the proportional valve with a new one, but that had no effect. They could not find any mechanical binding, so they called in an electrical engineer. He felt it was a hydraulic problem because the electrical signal to the valve was clean and consistent in both the increasing and decreasing mode.
The problem did not seem to occur with increasing pressure, but reared its ugly head when pressure decreased. The operator turned the problem back over to the machine repair group, which did not know what to do next. Do you?
Robert J. Sheaf Jr., is the founder of Certified Fluid Consultants (CFC) and President of CFC-Solar Inc. CFC-Solar provides technical training, consulting, and field services to any industry using fluid power technology. Visit www.cfc-solar.com for more information.
Individuals that work on mobile equipment often require hydraulic and electrical knowledge beyond that necessary for maintaining equipment in fixed factory settings.
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Industries such as energy, mining, construction, rail, waste management, agriculture, and oil and gas are loaded with mobile equipment. If you are faced with finding professional training for your fleet or unique mobile equipment, CFC Solar may be able to help. CFC-Solar is one of the most respected names in the mobile industrial training market with a history of developing training programs that teach individuals how to maintain today’s technically advanced mobile equipment. In addition to product specific training, CFC-Solar has standard training classes developed for the mobile industrial industry that include:
• Level 1 Mobile Hydraulics – In Depth Fundamentals
• Level 2 Mobile Hydraulics – Advanced Maintenance
• Level 3 Mobile Hydraulics – Design and Sizing
• Troubleshooting Mobile Systems using Schematics
• Hydrostatic Closed Loop Systems
• Level 1 Mobile Electrical – Fundamentals
• Level 2 Mobile Electrical – Multiplex Systems
CFC-Solar is also one of the primary training firms in the world that provide Fluid Power Society Mobile Hydraulic Certification. For more information on our services, contact tsheaf@cfc-solar.com or visit www.cfc-solar.com.
Think you know the answer? We’ll post this problem at fluidpowertalk.blogspot.com, and you can submit solutions there, or e-mail mary.gannon@penton.com with your solutions for a chance at a $50 gift card — We will randomly select a winner from all correct answers.
The correct answer will be printed on Dec. 1 at fluidpowertalk.blogspot.com and will be featured in the next issue of “Troubleshooting Challenge.”
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Last month’s issue presented a problem at a major zoo, where a size D03 valve was ripped from its base by an elephant. The replacement valve could be shifted electrically, but not manually. Surprisingly, the solution was a simple one.
After some investigation at the zoo, Dale Schaefer, a consultant and instructor of CFC-Solar, discovered that the directional valve had been mounted 180° from the correct position. Remounting the valve solved the problem.
D03 bolt patterns are not symmetrical, and some valves have a locating pin next to the pressure port to prevent the valves from being mounted 180° from the correct position. The pattern is a rectangle with two bolts closer together on the small end of the rectangle. However, several manufacturers supply modules that can be rotated to move their function of the screw-in cartridge to a different port or to reverse the bypass check. To accomplish this, the mounting bolt holes are enlarged to accommodate the bolt hole variances, and the locating pins are omitted, removed, or shipped loose for field installation.
Because the modules have larger holes, the mounting bolts can deflect just enough to allow the directional valve to be mounted in the reversed, 180°, position. Consequently, the pressure and tank ports become reversed, with the valve’s pressure port connected to tank and its tank port connected to supply pressure.
Newer wet-pin type solenoids feature a movable armature assembly inside a core tube that a removable coil fits over. The armatures of the solenoid assembly are internally connected to the tank port. When the tank port is pressurized, both armatures have the same hydraulic pressure applied to both ends of each armature, creating a balance between them. However, the manual push pin has an area opposite the area where finger force is applied. Therefore, the higher the tank pressure, the higher the force holding and forcing the push pin out. Applying system pressure to this area makes it almost impossible to shift the armature manually. But because the armatures are hydraulically balanced, the electrical solenoids work fine.
Another concern is the pressure rating of the core tubes. Some are only rated to 1000 psi, whereas others can handle 3000 to 4500 psi. If pressurized beyond its rated maximum, the core tube can leak, expand, and even rupture, which would blow the coil assembly off the valve body.
This is not an uncommon problem; we find this condition is not discovered during startup and debugging or is often just ignored because the electrical solenoids do their job.