High-production shredding applications produce tremendous shock in hydraulic systems. Hydraulic transmission design, shaft speed, and operating torque are just the beginning of the decision-making process when selecting a pump and motor. There are 12 technical questions that need to be addressed between the shredding design team and the hydraulic pump and/or hydraulic motor manufacturer.
Reducing material – appliances, concrete, tires, etc. – into smaller particles for recycling re-use is the ultimate goal of a shredding system. Hydraulic technology is a natural fit for both systems with gearboxes and those that utilize high-torque, low-speed motors as a direct drive.
In operation, high-production shredders experience system shock when the shredding cutters jam and reverse direction. Since some cutters can cut in both directions, it may only take a fraction of a revolution of the cutter shaft to create what becomes a ‘violent event’ prior to the reversal cycle. High-dynamic loads with system pressure spikes prior to, or during the reversal of, are common with in-feed systems for hammer mills.
These ‘violent events’ can cause instant pressure spikes from 700 to 5,000 psi within milliseconds (ms). Every pump manufacturer has an approach for rugged, heavy-duty shredding applications, as well as how the pressure spikes will be accounted for. A common approach is across a relief valve. In Parker’s Gold Cup Pumps (Figure 1), it is the valve-block technology (Figure 2) that cuts off pressure spikes.
Before designing a system, it is important to have a technical discussion between the shredder team and the hydraulic pump/motor manufacturer. There are 12 questions that need to be taken into consideration. This post touches on the first three questions.
Understanding the cooling capacity often dictates whether or not an additional pump is required or if it is necessary to increase the pump flow of an existing charge or boost pump. For system cooling, an important consideration is an ambient temperature where the machine will be located and operating along with the temperature of the cooling medium (likely water). It is important to know how much charge or replenish pump flow is required to exchange and cool the fluid. If there is an inadequate heat exchanger or cooling fan, it may be required to pull additional fluid out of the system to cool it. As a rule-of-thumb, cooling capacity should be sized at about 25 to 30 percent of full system flow.
Overall pump efficiency is a combination of mechanical and volumetric efficiency. Understanding fluid and mechanical losses are important to the system. Designers have to ensure that the amount of horsepower needed to do the job at the speed desired can be achieved. For example, as the machine becomes loaded and system pressure rises, volumetric efficiencies can change, resulting in slower machine operation.
Knowing the operating conditions where the pump and motor will provide the highest efficiency is critical in system design. The key is to size components to be able to run at or near where one can achieve the greatest output torque or horsepower. For example, if one runs a pump at low displacement where the swashplate angle on the pump is at a low angle, the pump is less efficient. Pump manufacturer catalogs include efficiency curves that show the efficiency at different pressures. The overall efficiency of a typical hydraulic pump will improve as the system pressure increases, but may also have a tendency to have a slight drop off as it approaches maximum pump pressure ratings. Operating the pump where it is most efficient will make system-operating conditions efficient as well. In general, there is a trade-off when it comes to efficiency and life: Max Flow (Q) x Max Pressure (P) = Max Efficiency (%) = Minimum Life.
Article by David C. Ebert, product manager, Parker Hannifin Corporation.
This whitepaper was originally published in www.fluidpowerworld.com, volume 2; number 1; February 2015
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