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SSI's Pri-Max waste reducer uses hydraulics for virtually all functions. Inset photo shows two tandem-mounted piston pumps, left, and vane pump, right, all driven by a 700-hp diesel engine through multi-pump drive box. The piston pumps drive all primary and auxiliary functions except a hydraulic fan drive, which is powered by the vane pump. |
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Mention the word "shredder," and most people think of a paper shredding machine sitting atop a wastebasket. But ask about shredders at SSI Shredding Systems Inc. (SSI), Wilsonville, Oreg., and you'll be introduced to a whole family of powerful machines that tear through tons of paper, tires, and other waste materials per hour — and those are just the small models.
For really big jobs, such as demolished buildings, tree limbs and trunks, appliances, and even cars, the answer is a primary waste reducer, such as SSI's Pri-Max line. SSI's largest model, the PR-6000T is a track driven unit powered by a 700-hp Caterpillar diesel engine. This machine can process more than 150 tons per hour of construction and demolition waste, and even larger amounts of less-difficult municipal waste.
Not surprisingly, SSI must quickly manufacture a number of waste reducers to aid in cleaning up much of the devastation left behind by Hurricane Katrina. Parker Hannifin's Hydraulic Pump Div., Marysville, Ohio, is cooperating by accelerating production schedules of its P30 Gold Cup piston pumps, which are used in SSI's waste reducers. The normal 14-week lead time has been slashed to two weeks.
Waste reduction technology
"The difference between shredding and waste reduction applications really has more to do with the desired size of the processed material and required throughput than the size of the machine," explained SSI's Mark Fowler, senior project engineer. "With a shredder, you get relatively small pieces of material, whereas with a primary waste reducer, you get fairly big pieces. But, when you're talking about demolished buildings, even big pieces take up a lot less room in a landfill than the unprocessed debris would."
In either case, the operating principles are very similar: a series of rotating cutters pulls debris through small openings to shear it and reduce its bulk. In a shredder, the cutter blades overlap, typically clearing each other by 0.010 in. In a primary waste reducer, the cutters pass between fixed anvils, usually clearing by 3 to 4 in.
Hydraulic muscle at both ends
"As you might expect," Fowler notes, "we have to deal with very high shock loads, and that's why all of our Pri-Max machines use closed-loop hydrostatic drives for the cutter shafts. We have standardized on the Parker-Denison Gold Cup series of axial-piston pumps for the drive application because they offer both the capacity and the control features we need to reliably power these machines."
The pump has a maximum pressure of 5000 psi and maximum theoretical displacement of just under 224 gpm at 2100 rpm. It feeds two pairs of hydraulic motors, each pair mounted to either end of the cutter shafts.
The shaft-mounted motors, models CA210 and CA140, from Hagglunds Drives, Columbus, Ohio, have a hollow output shaft, allowing them to be mounted directly onto the shafts. A torque arm secures each motor housing to the reducer's framework to hold the motor housing stationary.
As the cutters shear waste material apart, torque increases; therefore, so does hydraulic pressure. "A normal duty cycle includes one or two shifts of continuous operation with the system running at 2000 to 3000 psi about 60% of the time," Fowler explained. "The rest of the time, though, we run at higher pressures — 3000 to 5000 psi in response to more resistant materials.
"When the reducer pulls in something really tough, the torque requirement and pressure naturally rise. The control system monitors this and reacts appropriately. Up to a point, the system simply sacrifices rotational speed for higher torque. However, if the object is too resistant, the control simply reverses the direction of rotation to lift the object and relieve the stoppage.
"Obviously, the ability to monitor and control the torque load via system pressure is a convenient way to apply extra torque when it is needed and to protect the machine from potential damage. And the ability to instantly reverse the direction of rotation (to clear jammed material) is simply not practical with anything but a hydrostatic drive," Fowler continued.
Power and control
Fowler revealed, "The displacement control valves and mechanisms are all integral with the Gold Cup pumps. That means we don't need extra manifolds and valves. The Gold Cup pumps (and shaftmounted motors) make for a very compact and efficient installation."
He further explained, "We do torque limiting, but not by using Denison's hydraulic torque limiter. Instead, we take advantage of their electrohydraulic stroker (9A) control option. We connect a pressure transducer to the V port on the pump. This always sees the higher of the pressures on the two sides of the closed loop.
"The transducer sends a 4- to 20-mA signal to our PLC, where the torque limiting decisions are made. The PLC sends a 125- to 325-mA signal back to the coils on the pump's integral proportional servovalves, which control the pump's displacement. This is done directly in over-the-road or trackdriven Pri-Max units. For stationary units, Denison's Proportional Valve Dual Driver is sandwiched between the PLC and the servovalves.
"We do it this way because we already use the 9A controls to allow reducing shaft speed when processing material that is prone to becoming airborne. We also use it in maintenance mode to provide very slow shaft rotation. The 9A control is already there, so it's more cost-effective to electronically limit torque than to add another pump option."
Keeping it cool and clean
"Actually," explained Mike Achterman, of Hydra-Power Systems, a Parker-Denison distributor in the Portland area, "the PR-6000T uses hydraulics for a lot more than simply powering the cutter shafts. For track-driven models, output from the Gold Cup pumps powers the track drive. They also use a hydraulic fan drive to manage the diesel engine's thermal load." Controling the temperature of diesel engines is especially important because efficiency is maximum and emissions are minimum within a narrow range of operating temperature.
Achterman added, "Powering the cooling fans hydraulically gives SSI the freedom to put the radiator package virtually anywhere on the vehicle. It also gives them a great deal of control over how the system operates.
"For example, they can easily program fan speed to match the actual thermal load regardless of engine speed. You can't do that with a belt-driven cooling fan without using a lot of maintenance-prone hardware in the system."
"They can also reverse the airflow through the radiator to blow out any debris that may collect there and impede airflow. That's important in a setting like a landfill where the environment includes both dust and lots of lightweight
debris that is easily sucked into a radiator. It's actually pre-programmed as an automatic system function on the PR-6000T."
A Parker-Denison T6CM pump driven through a pump drive box powers the hydraulic fan drive. Driven through the same gearbox is a Gold Cup pump, which
powers the waste reducer drive and a Parker-Denison P3 with load sensing to provide power for conveyor and auxiliary drives.
For more information on Parker Denison Pumps, call (937) 644-4564.