By Dave Ebert, Product Manager, Gold Cup Pumps & Motors, Hydraulic Pump Div., Parker Hannifin Corp.
High-production shredding applications reduce material—appliances, concrete, tires, and so on—into smaller particles for recycling and re-use. Hydraulics is a natural fit for both systems with gearboxes and those that use high-torque, low-speed motors as a direct drive.
In operation, these shredders can experience extreme shock in the system when the shredding cutters jam and reverse direction. Some cutters can cut in both directions, so 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 are also a daily part of in-feed systems for hammer mills.
These violent events can cause instant pressure spikes from 700 to 5,000 psi within milliseconds. Every pump manufacturer has its own approach for rugged, severe-duty shredding applications, as well 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 the pressure spike.
In system design, it is very important to have an in-depth technical discussion between the shredder team and hydraulic pump/motor manufacturer. The following 12 questions need to be thoroughly discussed.
1. What is the system cooling capacity?
Understanding the cooling capacity often dictates whether or not an additional pump is required or if it is necessary to increase pump flow of an existing charge or boost pump. For system cooling, an important consideration is the 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 you have an inadequate heat exchanger or fan for cooling, it may not be sufficient, and you 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% of full system flow.
2. What is the pump efficiency for a given duty cycle?
Overall pump efficiency is a combination of mechanical and volumetric efficiency. Understanding fluid and mechanical losses is important to the system. Designers must ensure that the horsepower needed 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.
3. What are the optimum efficiency parameters for the pump and motor?
Knowing the operating conditions where the pump and motor will provide the highest efficiency is extremely important. Size components to run at or near where you can achieve the greatest output torque or horsepower. If you run a pump at a very low displacement and where the swash plate angle on the pump is at a low angle, the pump is less efficient. Pump manufacturer catalogs include efficiency curves that show where you can see the efficiency at different pressures. 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 you approach maximum pressure ratings of the pump. Operating the pump where it is most efficient will make the operating conditions of the system efficient as well. As a general statement there is a trade off when it comes to efficiency and life:
Maximum flow (Q) x Maximum pressure (P) = Maximum efficiency (%) = Maximum life.
4. What are the flow and pressure curves for the shuttle valve?
The shuttle valve plays an important role in cooling the loop of the hydrostatic drive. Knowing the shuttle valve flow characteristics will help in understanding how the boost or replenish pressure may be influenced as a result of backpressure. Exceptionally high boost flow can saturate the hot oil shuttle valve, which will increase the minimum boost pressure within the system. Always check to ensure the shuttle valve is capable of handling the cooling flow requirements of the system. If necessary, large externally mounted shuttle valves are an option.
5. What is the pump’s life expectancy and maintenance schedules?
The service life of a pump or motor is calculated based on the expected B-10 bearing life for all of the operating parameters—pressure, fluid viscosity, pump displacement, and so on. However, more often than not, the life of the pump or motor is limited, or reduced, due to the lack of proper maintenance. Fluid cleanliness is critical to any hydraulic component. Well-maintained systems can easily add years to the machine life. In terms of cleanliness, at a minimum, follow the pump manufacturer’s catalog values for fluid particle contamination and water content levels.
6. What is the response time of the compensator?
How quickly a pump can react to limit the pressure in the system is an extremely important selection consideration. Reducing the pressure overshoot in the system can significantly extend overall machine component life. The Gold Cup Pump Compensator, for example, responds fast enough that cross port relief valves are not required, as in typical hydrostatic pumps from other manufacturers. This eliminates pressure spikes and additional head in the system, making it an ideal choice for applications with high dynamic loads. Basically the compensator serves as a “clipping” device to limit pressure overshoot.
7. What is the ability to manually set the compensator and replenishing pressure?
Compensator adjustments are used to limit system pressure. It is also the point where a system can stall. If you have a load that reaches the compensator setting—a primary reduction shredder, for example, it may just bring in the material and stall the motor to the point where it sits and holds at the set pressure. It will not be able to crush through the material. The compensator set point should be selected to allow material to be processed at the forces required, without exceeding structural limitations of the machine. In addition to clearing and processing material more efficiently, this is why shredders have a reversing feature. If plugged, and a stalled condition occurs, the system may be programmed to automatically go into reverse to unplug the material.
The compensator setting on the Gold Cup pump is easily accessible on the valve block should it be necessary to make a one-time adjustment. A shredder manufacturer may set the compensator and then add a locking device to prevent tampering to ensure that the machine structure won’t be overstressed or compromised.
Replenish or boost pressure of the system is typically a factory setting. To modify this setting, it may be necessary to shut the system down, remove and adjust the pilot relief valve, and reinstall, prior to restarting the system.
In actuality, compensator and replenish pressure should not need to be adjusted after the OEM manufacturer has tested the machine during its break-in period. The replenish pressure of some pump manufacturers is preset at the factory and typically does not require further adjustment.
8. What are the approaches to incorporating a shuttle valve—mounting to the pump or is it done remotely?
As previously mentioned, the hot oil shuttle valve is an important component for cooling the hydrostatic loop. However, placement within the system is shredder design preference. The hot oil shuttle valve can be mounted on the pump or remotely as a stand-alone component. Direct mounting eliminates the need for additional hoses and plumbing reducing potential leak points. This provides a cleaner and more simplified look for the system, and it can be factory set. Shuttles are often an integrated option for hydraulic motors; however, these are used as flushing valves, not to be confused with the hot oil shuttles designed to remove heat from the closed circuit system.
9. What is the pump’s response time going on-stroke as well as off-stroke?
The on- and off-stroke time to control pump displacement is a key design consideration to obtain desired machine speeds and increased productivity levels. Gold Cup pumps, for example, have servostems with the appropriate control orifices for the majority of shredding applications. However, these orifices can be easily changed to different sizes to achieve various stroke speeds. The majority of the time, pumps leave the factory with standard control orifices. But, if a machine designer needs a different response for the rapid back and forth motion of a shredder, an orifice change can help in shaving off seconds within a minute on stroking back and forth. Depending on the application, this can make a significant difference in terms of dollars in shredding product.
10. What are the fluid types and analysis for maximum life and performance?
Typical shredder applications use a mineral-based fluid. However, if the machine is running in a hazardous or safe environment, it is possible that fire-resistant or environmentally friendly fluids will be required. Some pump manufacturers publish pump life catalog recommendations based on fluid viscosity. As an example, for optimum pump life, it is best to operate with a fluid viscosity around 140 SUS or 30 cSt. But, getting into that range may depend on where the shredder is going to run. What is the ambient temperature? What is the fluid cooling requirement, and how are you going to cool the machine? The choice of a heavier or thinner grade fluid may depend on the climate.
11. What non-catalog options are available for improved performance?
When dealing with extreme system pressure spikes, one approach with the Gold Cup pump is to attach a length of hose, to act like an accumulator, on the valve block vent port (Figure 3). This is more cost effective than having to add a crossover relief. In this approach, it is a bit too fast for the pump to compensate and control the pressure spike for the compensator to handle the 700- to 5,000-psi spike over 30 msec. Instead, the rapid increase in pressure swells the hose connected to the valve block. This initiates flow in the valve block that activates the compensator prior to the 5,000-psi setting to delay the pressure spike a little bit longer. Instead of 20 to 30 msec, it is extended to 50 to 60 msec and starts de-stroking the pump prior to reaching the compensator setting. This approach cuts off the pressure spike, saving energy and increasing shredder life by eliminating successive fatigue loading on system components.
When dealing with extreme system pressure spikes, one approach with the Gold Cup pump is to attach a length of hose, to act like an accumulator, on the valve block vent port (Figure 3). This is more cost effective than having to add a crossover relief. In this approach, it is a bit too fast for the pump to compensate and control the pressure spike for the compensator to handle the 700- to 5,000-psi spike over 30 msec. Instead, the rapid increase in pressure swells the hose connected to the valve block. This initiates flow in the valve block that activates the compensator prior to the 5,000-psi setting to delay the pressure spike a little bit longer. Instead of 20 to 30 msec, it is extended to 50 to 60 msec and starts de-stroking the pump prior to reaching the compensator setting. This approach cuts off the pressure spike, saving energy and increasing shredder life by eliminating successive fatigue loading on system components.
Vertical pump mounting is another option where the electric motor sits on top and the pump is mounted underneath for space savings. In this instance, a 5-psi case drain check is added to the pump to keep the case full of oil and the bearing lubricated.
A 65-psi bypass check valve incorporated within the hot shuttle oil circuit is a good solution when a machine runs in the neutral position for a long period of time. The problem is that the hot oil shuttle valve is not shifted in either direction and no oil is forced out to provide cooling. Nor is there any fluid filtration, and in long time periods, the oil in the loop can heat up. Use of the bypass check valve provides loop cooling when in the neutral position.
12. What are the control options for either flow response, flow accuracy, or both?
To gain faster stroking response, the pump valve block can be easily modified with a few different poppets and springs. Also, to achieve higher response for faster stroking, servo shoes and servostems with larger control orifices can be added to get more fluid flow and a higher stroking response. In combination with the items just mentioned, the Gold Cup will accommodate dual controls operating in parallel for yet even faster response.
To gain faster stroking response, the pump valve block can be easily modified with a few different poppets and springs. Also, to achieve higher response for faster stroking, servo shoes and servostems with larger control orifices can be added to get more fluid flow and a higher stroking response. In combination with the items just mentioned, the Gold Cup will accommodate dual controls operating in parallel for yet even faster response.
Conclusion
Successful design and application of the hydraulic solution for high-demand shredding applications is one of the toughest problems to solve. Dynamic loads and violent high system shock are the norm. In fact, shredding is by far one of the hardest applications on hydraulic components, and quite possibly above the demanding requirements seen in mining and oil and gas applications. Knowing the answers to the questions above will help ensure maximum performance and long machine life.
Parker Hannifin Corp.
www.parker.com
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