HYDAC Qld cuts through to the crux of the matter when a manufacturer approaches it to supply a hydraulic proportional valve for a straddle carrier hydraulic steering arrangement system.
A straddle carrier or straddle truck is a freight-carrying vehicle that carries its load underneath by "straddling" it rather than carrying it on top like a conventional truck, HYDAC Automation and Control Engineer Christopher Michaels explains.
The straddle carrier can be advantageous in a shipping container yard when compared to a truck as it does not require forklifts or cranes for unloading, and the design allows for multiple shipping containers to be picked up and moved simultaneously.
Phasing cylinder reliable way to accomplish hydraulic steering with drawback
The customer’s hydraulics steering arrangement at the time was made of phasing cylinders, which allow for reliable harmony between both steering cylinders.
“It's where the full bore side of one cylinder is connected to the annulus side of the other cylinder. What this basically ensures is that when one cylinder is moved just one millimetre in one direction, the other cylinder moves the exact same amount in the other,” Mr Michaels says.
“And you can maybe visualise that when these cylinders are connected to a steering knuckle of a car or any sort of mobile machine and extended or retracted, you get a proportional translation to the angle of the wheel."
“So phasing cylinders are a reliable way of accomplishing hydraulic steering but one of the drawbacks is that you can’t deliver tight Ackermann steering. One way to accomplish this is to have independent flow to each cylinder, allowing for direct control of cylinder extension and retraction.”
Ackermann steering is a geometric method to avoid tyre slip that was invented by German carriage builder Georg Lankensperger in Munich in 1817, Mr Michaels highlights.
“To accomplish true Ackermann steering, the turning radii of each wheel are required to be different angles depending on the degree of the turn. And it’s here that phasing cylinders are limited.”
In view of this, the customer wanted independent flow to each cylinder, allowing for direct control of cylinder extension and retraction.
Mr Michaels points out that the customer was specifically looking for a CETOP proportional flow control valve capable of 120L/min.
“A CETOP valve capable of 120L/min can be very expensive and in that a CETOP valve is designed for an industrial machine whereas a straddle carrier is a mobile machine. HYDAC immediately brought attention to the LX6 valve,” Mr Michaels says.
“An added benefit of the LX6 valve is better flow control resolution due to its 10mm of spool travel compared to the 7mm that is usually found in mobile proportional flow control valves.”
In conjunction with the customer, HYDAC designed the LX6 to work with the desired flow rates, pressures, and requirements of the system, which was then integrated onto the straddle carrier.
“The manufacturer has designed a whole new control system to be able to independently control each cylinder to accomplish true Ackermann steering and a tighter steering circle.”
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