Many wood mills use hydraulic positioners like the one in the picture below. Sometimes they try to increase the speed of these cylinders by replacing the prop valves mounted on the cylinder (see pictures) with valves with higher flow rates.
The Task:A technician wants to switch a size D05 valve with a flow rate of 14 gpm (15 l/min) to a D05 valve with a flow rate of 22 gpm (85 l/min). For those familiar with Bosch Rexroth, this would be like switching a Bosch Rexroth 4WRLE10V-55M valve to a 4WRLE10V-85L valve, like the one pictured below. Both valves are a size D05 (NG10). Therefore, both will easily mount on the same sub-plate. Greater speeds are achieved and high response times are maintained.
The Problem:After the 22 gpm valve is installed, the cylinder begins to experience creep, drift, unwanted movement, or oscillations at the zero command. The system shuts down. Technicians are scratching their heads. Maybe something is wrong in the electronics? Maybe the LVDT is not correctly calibrated? They call in an electrician or a programmer to troubleshoot. They call the supplier for help.
Because the higher flow valve fits on the same sub-plate of the lower flow valve, no one realizes the problem is in the hydraulic circuit, not the electronics.
The Fix:If you increase the flow capacity of the prop valve, you may also need to increase the flow capacity of the return line too.
According to the Fluid Power Designers’ Lightning Reference Handbook (if you don't have one, get one), hydraulic fluid return lines should be schedule 40 pipe and sized to maintain flow between 10 and 15 Ft/Sec. However, closer to 15 Ft/Sec is preferable when operating proportional valves. If we look at page 109 in the reference handbook, a ¾” diameter schedule 40 return line has a max flow capacity of 16.6 gpm at 10 Ft/Sec. This is sufficient for the 14 gpm valve, but falls short for the 22 gpm valve. The 22 gpm will need a 1” schedule 40 return line with a max flow capacity of 26 gpm at 10 Ft/Sec.
The Technical Details:High response proportional valves are highly sensitive to return line back pressure. Why? Because backpressure spikes may cause the valves to appear to oscillate due to the backpressure forces pushing against the spool in the opposite direction the spool is travelling.
To understand this, we must remember that if there is no pressure drop across an opening, there can be no flow through it. If there is no flow, there is no pressure drop.
As the spool is pushed through the valve by the magnetic force of the coil, backpressures from the return line push back on the spool in the opposite direction. These opposing forces equalize and the minimum pressure drop required to maintain flow is lost. In Bosch Rexroth 4WRLE valves, the minimum pressure drop required to maintain flow over the spool landings is just 5 BAR, or about 73 psi.
The oscillation the electricians detect and the cavitation the technician can see is the LVDT compensating for the backpressure forces repeatedly pushing the spool out of the position being requested by the command signal.
If you have switched out your proportional valves recently, it is a good idea to go back and ensure your return lines are still properly sized.
I hope you found this article helpful. Feel free to email me at
THyatt@valin.com with any questions.
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