Typical refuse trucks rely on a single-section, fixeddisplacement pump that produces flow proportional to engine speed. Such pumps usually require revving the engine to at least 1,200 rpm to generate enough flow for hydraulic cylinders that grab, lift, pack, and eject trash.
A new hydraulic pump system from Denison Hydraulics, Marysville, Ohio, changes all this and brings a hush to garbage collecting. The Oasis (Operate at standard idle speed) system includes a high-efficiency double-vane pump, unloader valves, an inlet recirculation block, and an electronic control module.
National Waste Services, a Chicago-based division of Allied Waste, recently tested the system in a Leach Packmaster commercial rear-loader. The main goals were to reduce engine noise associated with operating the truck's hydraulics while simultaneously reducing cycle time to improve productivity.
To reduce engine speed and maintain or increase hydraulic flow necessitates a larger pump. But higher flow can generate more heat, and that leads to premature fluid breakdown and component failure. Thus, at high engine speeds during acceleration and travel, the system must dissipate heat due to excess hydraulic flow.
The Oasis system solves the problem with a Denison T7DDS double-vane pump. It provides significantly greater flow than a gear pump when engine speed is only 600 to 700 rpm, and features better low-speed efficiency. This means less internal pump leakage generating less system heat. Typical gear pumps running at 600 to 700 rpm and 2,000 psig have volumetric efficiencies of about 70%. Under the same conditions, the Oasis pump operates at about 90% volumetric efficiency.
The double pump also lets one pumping section recirculate directly to the pump's low-pressure inlet when engine speed rises above a preset level. Recirculating flow reduces total output flow and, subsequently, heat, and controls the pump's inlet fluid velocity. At a second predetermined engine speed, the second pump section dumps directly back to the reservoir, again at low pressure, providing a fluid-polishing loop for the system.
In addition, Leach engineers replaced the original control valves with larger, high-flow versions that offer less restrictions and generate less heat.
This flow-control method runs contrary to most truck operators' experience revving the engine actually slows hydraulic functions. Because speeding the engine is counterproductive, the operator leaves it at idle to maximize output. And while diesel engines generally produce-rated horsepower at speed well above idle, the Packmaster's Mack E700 engine, rated at 280 to 300 hp, provides enough torque to power the hydraulics at 650-rpm idle speed.
Although a double-vane pump costs more than a single gear pump, the test vehicle boasts a 22-sec sweep/pack cycle time while the engine runs at 650 rpm. This is 8 to 10 sec faster than the gear-pump version running at 1,300 to 1,400 rpm. In addition to faster cycles and quieter operation, National Waste officials anticipate better fuel economy, less engine wear, and longterm maintenance benefits.