What is in this article?:
- Fluid power turns trash into cash
- Clean fluid helps convert garbage into energy
This article explores the hydraulic design and control of several machines used in the waste & recycling industry.
To most of us, recycling means lugging a bin of cans, bottles, plastics, paper, or cardboard — or any combination of these — to the curb for pickup by our community's recycling personnel or to a recycling center. Residential materials, however, are just the tip of the recycling iceberg. Industry generates huge amounts of byproducts that once were discarded but now are recycled — in essence, turning trash into cash. In the construction industry, for example, clearing land to make way for a building can produce a mountain of surplus soil that can be transported to an area where it is needed. Brush and branches can be run through a chipper-shredder to produce mulch for subsequent landscaping, but tree stumps call for a different process. Here, stump grinders have emerged as a specialized tool to quickly transform tree stumps into shredded mulch for landscaping. As you might expect, stump grinders rely heavily on hydraulics for the high torque and power needed to reduce wood in its hardest and toughest form to a pile of splinters.
For people involved directly in the industry, recycling is becoming more competitive and more high-tech. Many high-production machines are being developed to process industrial and municipal waste into a useable commodity. Some machines, like stump grinders, excel in performing one specialized function more effectively than anything else. Others are developed to perform numerous sequential operations automatically to reduce or eliminate the labor and human intervention otherwise required in processing. To demonstrate the innovative uses of hydraulics in the recycling industry, we will describe several such machines. Chances are, your job does not involve designing machines for recycling, but you may be able to apply some of the principles described here into the machines you do design.
Torque tackles tough tires
Unless you plan on using them as planters in your front yard, worn-out tires are among the most difficult products to recycle. This is because the engineering and materials that make tires able to stand up to years of torturous conditions on the road also make them nearly impossible to break down for subsequent recycling processes.
For most recycling processes, tires are shred into chips, but what makes this so difficult is having to cut through soft, impact-resistant rubber that contains hard, strong, steel reinforcement. High-speed shredding is not the answer, because high speed would generate heat. This would soften the rubber and make it almost impossible to cut the tires. However, low-speed shredding requires tremendous torque.
To transmit low-speed, high-torque power for shredding, machines could use a gear box to reduce speed from an electric motor and multiply torque. This often proves impractical, because the gear box would be heavy and bulky, which would increase structural requirements. Also, shock loading often occurs when shredding blades suddenly strike steel reinforcement. Moreover, tires get jammed between shredding blades, which makes it necessary to quickly reverse direction of the blades to free the jam. Having to quickly and frequently reverse rotation would prove especially challenging for an electromechanical drive because of its high inertia.
Hydraulic technology rises to the occasion by offering drive components that are much less bulky and more well suited to the demands of high-torque shredding. For example, one company's twin-roll tire shredders are driven directly by a pair of radial-piston motors through a hydraulic system provided by Hagglunds Drives Inc., a division of Bosch Rexroth.
Each motor has a displacement of 1084 in.3/rev and can generate up to 66,840 lb-ft of torque. The motors are rated 14,390 lb-ft/1000 psi, and can handle 5000 psi, so their maximum torque rating is actually 71,390 lb-ft. Severe jam ups that could damage gear drives simply cause these motors to stall. Stall conditions increase pressure in the hydraulic system, which triggers valves to automatically reverse rotation of the drive to free the jam up.
One particular challenge of the application was mounting. The radial-piston motor produces high torque in an envelope that consumes relatively little axial space. Butthe short center distance between shredder blades and the large diameter of the radial-piston motors prevents mounting them side by side in this application. The short center distance also prevented using flange-mounted motors, and foot mounting would have required an extensive framework. Instead, each motor is mounted directly to a shaft at opposite sides of the shredding chamber and is kept from rotating by a torque arm. This arrangement provides a compact, simple, and easy-to-install drive system.
Hydraulics drives clarifiers
Just about any municipal water treatment plant uses clarifiers to remove suspended solids from effluent. A clarifier consists of a round basin — usually 65- to 80-ft in diameter — with an arm that slowly rotates around the center to gently push the sludge that collects at the bottom of the basin into a drain. From there, the sludge undergoes a dewatering processes and typically is burned or transported for disposal in a landfill. As landfill space continues to dwindle, however, this sludge increasingly is used as compost. Its value as compost helps offset costs for transportation to a final destination - which is not the case when it is incinerated.
The heart of the clarifier is the drive. Conventional clarifier drives consist of an electric motor turning the rotating rake through a belt or chain drive and gearing to reduce speed and increase torque. Drives built by DBS Mfg. Inc., Forest Park, Ga., however, use gearing in combination with hydraulics to transmit power. Andrew Bain, director of engineering at DBS, explained that hydraulic drives are more reliable, last longer, and require less maintenance than chain or belt drives. "Especially when greater loads and precise torque control is required, hydraulics is the best engineering solution. Our drive units are designed to operate 24 hours a day for maany years, and it's not uncommon for them to last 30 years or more."
As with many hydraulic systems, DBS drives have the pump mounted submerged in the reservoir. However, Bain said DBS takes this concept a step further by submerging essentially all of the hydraulic components in the reservoir: valves, tubing, fittings — even the low-speed, high-torque motor. "Submerging everything in the reservoir eliminates leakage problems almost completely," reveals Bain. "If the fitting connecting the pump outlet line to the valve inlet port leaks, the fluid leaks harmlessly within the reservoir."
Bain continued that speed and torque control are important for clarifier drives. "The rake typically rotates at only 0.06 rpm. If you go faster, you'll stir up the sludge blanket and prevent particles from separating out of the water. If the rake moves too slowly, too much sludge collects, causing too thick or too dense a sludge blanket. Too thick or dense a sludge blanket increases the torque required to direct material into the drain, which could cause a malfunction. With hydraulics, speed is proportional to flow, and torque is proportional to pressure, so you can easily control torque and speed by regulating pressure and flow." Incorporating a switch to monitor system pressure allows appropriate action to be taken automatically — such as sounding an alarm or shutting down the drive if pressure reaches the cutoff threshold.
When precise speed control is also important, pump displacement provides a means of regulating flow, and, therefore, speed. This generally is accomplished with a variable-displacement pump. Another design variation is to use a fixed-displacement pump and provide a hydraulically actuated jack to raise the rakes if the sludge blanket becomes too thick. This would cause the rake to plow only the upper portion of the sludge blanket toward the drain. In this arrangement, rotational speed of the rake would remain constant, but elevation would be controlled by pressure. For other applications, speed, torque, and elevation all can be controlled to optimize process requirements.
Crushing a negative icon
Probably nothing conjures up as strong an impression of toxic waste as abandoned 55-gal steel drums - even if the drums are empty. Companies often find it difficult — or at least expensive — to dispose of empty steel drums. And if they find a landfill that will accept the steel drums, companies find it costly to transport the drums, not because they are heavy, but because they take up so much space. High transportation charges also make empty drums less practical to sell as scrap. Enter the Model 7000 drum crusher from Drumbeaters of America Inc., Elburn, Ill.
The Model 7000 drum crusher uses a 6-in. bore hydraulic cylinder to reduce drums to as little as one-eighth their original size, which makes it much more economical and practical to recycle steel drums as scrap. The standard in-plant drum crusher uses a 5-, 7.5-, or 10-hp electric motor to drive a pump that delivers 12, 15, or 20 gpm, respectively. The difference between each size is cycle time; the 5-hp size requires 70 sec to crush a drum, and the 7.5- and 10-hp sizes require 55 and 38 sec, respectively. With a relief valve setting of 3000 psi, all sizes produce a crushing force of up to 85,000 lb. An optional model with a 7-in. bore cylinder delivers up to 115,000 lb at 3000 psi.
MacMillan Hydraulic Engineering Corp., Skokie, Ill., builds the hydraulic power units (HPUs) and supplies many of the hydraulic components to Drumbeaters. Robert MacMillin, president of MacMillin Hydraulic Engineering, says that users want a machine that crushes drums quickly, is reliable, and will last a long time. Also, they are not willing to spend a lot of money for a machine that basically processes scrap. Therefore, he says he chose to use a high-low pump that operates at high volume and low pressure when no or little load is encountered. Once fluid pressure increases due to resistance from the load, the pump shifts to a low-volume, high-pressure mode.
The simplicity of the hydraulic system using manually actuated valves provides years of reliable operation. To add longevity, the hydraulic cylinder is designed to extend to only within 3 in. of the bottom of the crushing chamber. MacMillin explained that preventing the cylinder from bottoming out extends its life by eliminating end-of-stroke impact that otherwise would result.
The Model 7000 crusher easily can be moved from one location to another by a fork lift. If necessary, the HPU can be separated from the crushing chamber via quick-acting couplings or mounted remote from the crushing chamber when space is tight. For users that prefer to bring the crusher to the drums, Drumbeaters offers a trailer-mounted transportable unit. With this configuration, the hydraulic pump is driven by a gasoline or diesel engine.
Drumbeaters points out that their crushers are used for more than compressing drums. An optional triple-rinse system accessory removes fluid from drums and rinses the drum in accordance with federal regulation #40 CFR, Part 261.7. Their crushers can also be used to squeeze fluids out of filter media or for compressing waste materials into bales.
Rotating-case motor powers grinder
Hydraulic motors for most applications have a stationary housing with a rotating shaft. Sometimes, however, an application is best served by a motor with a stationary shaft and rotating housing. Such is the case with the cutter drive on wood waste grinders manufactured by Packer Industries Inc., Mableton, Ga.
To maximize reliability, engineers at Packer Industries wanted the cutter drive of their wood waste grinder to be driven directly by a high-torque motor, and without mechanical couplings or auxiliary bearings. All of these requirements were met by specifying a model SMA radial-piston motor from Rotary Power Inc., Columbus, Ohio.
The inherent design of radial-piston motors produces low-speed, high-torque power — in this case, 300 hp from a motor with a displacement of 122 in.3/revolution, rated for 5000 psi maximum continuous pressure and maximum speed of 380 rpm. Because the SMA motors are designed to accommodate high external loading, they are mounted inside cutter drums and need no external bearing supports. Pressurized fluid is routed to and from the motor through its stationary shaft. Rotating unions inside the motor route fluid from the stationary shaft to the working chambers.
Another advantage of Packer's wood waste grinder is an ergonomically designed cab. Hydraulic functions and components are controlled electronically, so the operator is not subjected to the noise and heat associated with pilot-or direct-operated valves. Pump displacement is controlled electronically from inside the cab — a feature that not only provides operator comfort and convenience, but adds to productivity.