November 1, 2013
Most stampers don’t think twice about running a hydraulic press like it has been run for the last 20 years. By doing so, they are losing out on dramatic cost savings associated with a possible energy-saving retrofit that’s centered around a smart variable-speed pump drive.
Stamping presses that form and bend metal under pressure are themselves under pressure to improve throughput, process cost control, and energy efficiency.
Many stamping presses have been in operation for 20 to 25 years and typically operate under punishing conditions. Cycle times for these presses are between 100 milliseconds and several seconds, and utilization rates range from 3,600 to 6,000 hours per year.
That much operation consumes a lot of energy, which can significantly affect stamping press productivity and profitability. In the last decade, electricity prices have increased an average of 25 percent and higher in certain U.S. markets and up to 50 percent in Europe. If the hydraulic systems driving your stamping press operate with less than optimum efficiency, you could be losing out on an opportunity to reduce production costs. Fortunately, you can realize multiple benefits by considering the value of a hydraulics retrofit.
Advanced hydraulics and intelligent drive systems are now available to provide greater energy efficiency and improve stamping press productivity and product quality. Retrofits using new technologies provide:
While all of these benefits ultimately contribute to a positive return on investment (ROI), the main cost savings result from energy reductions.
The primary reason to consider a retrofit is to improve energy efficiency. Even on standard machines, energy costs can represent 20 to 30 percent of total life cost—and a much higher share with energy-intensive applications.
In a typical hydraulic stamping press operation, a majority of the energy consumed is to generate the force to stroke the ram. Conventional approaches utilize a variable-displacement hydraulic pump driven by an electric motor running at constant RPM. The hydraulic pressure requirements are regulated by a pump control, such as a pressure compensator, or by additional hydraulic flow and pressure control valves downstream of the pump.
With a constant-RPM design, the motor is always running at rated nominal speed, even if the machine is operating at part load or idle. Some motor horsepower is always being wasted. Simultaneously, internal hydraulic pump and valve leakage generates heat in the hydraulic fluid, which must be cooled to maintain optimum operating conditions. The cooling process also results in an additional energy demand on the system.
With the latest technology, however, it is possible to obtain significant energy savings with a retrofit that replaces a constant-speed electric motor coupled to a variable-displacement or fixed-displacement pump with a “smart” pump system. The smart pump system also can eliminate the need for downstream pressure and flow control valves in some machines.
What is a smart pump? It is a pump with intelligence to adjust flow and system pressure based on the process demand. This is accomplished by varying the drive speed and, in some cases, the swivel angle of a variable-displacement pump. In a variable-speed drive system (see Figure 1), the hydraulic system’s flow requirements are controlled using an electronic variable-frequency drive (VFD) coupled to either a conventional asynchronous or synchronous servomotor. A pressure transducer provides a signal to control the hydraulic pressure. The combination of VFDs and variable-speed pumps allows the system to operate at the pump’s and motor’s optimal efficiency points. This reduces energy losses directly at the source.
A minimal configuration, consisting of a fixed-displacement pump and VFD, delivers a flow rate proportional to the motor’s drive speed. The closed-loop control is located in the VFD and reduces the drive speed to match the load conditions. Additionally, the variable-speed pump drive can be used to perform intelligent axis functions.
A properly integrated variable-speed pump drive can cut stamping press energy consumption dramatically (see Figure 2). Using an on-demand control, the system can adjust the pump pressure and flow to the hydraulic actuator without the need for additional control valves. Consequently, the average input power is reduced over the entire machine cycle.
Moreover, by eliminating valve-induced pressure drops that generate heat, the temperature rise in the hydraulic oil can often be minimized, reducing or eliminating the additional energy required for the cooling system.
Additional improvements may be realized when using variable-speed drives. The machine cycle can be smoother, minimizing maintenance and downtime. This also can extend the operational life of the press. Reducing the pump drive speed can lower noise levels of the hydraulic power unit by 10 to 20 dBA.
Each stamping press operation is unique, and the potential energy savings for systems vary. The important variable is the competitive pressure your operation faces in controlling costs and improving ROI. Here are five factors to weigh when considering a retrofit to a variable-speed drive hydraulic system for your stamping press.
1. Account for hydraulic system energy consumption. Improper evaluation of pumps, motors, and controls can have a significant negative impact on performance, reliability, and efficiency. It is critical to assess your current stamping operation in terms of cycle times, stroke action, and force to calculate the energy needed to generate the required flow and pressure.
It is also valuable to assess the level of control needed to deliver the desired throughput, including the position and force required to fabricate parts precisely to your customer’s specifications.
For many applications, today’s variable-speed hydraulic pump systems are designed to be competitive replacements for constant-speed motors with fixed- or variable-displacement pumps. The disadvantage with conventional systems is that motor speed cannot be reduced for partial load. Energy can therefore be wasted through a significant portion of the overall cycle.
In many cases, it is more efficient to achieve flow and pressure control by regulating pump speed and stroke than by using control valves. Because the energy is available as high-pressure hydraulic fluid, much of the energy is released in the form of heat as the fluid passes through a control valve, from a high-pressure state to a low-pressure state. Control valve operation results in doubling energy waste. Energy is consumed when pressurizing the fluid, and then the energy is lost in the pressure drop that occurs through normal valve operation. This generates heat that will require additional energy for cooling. Additionally, dissipating the heat will require large heat exchangers that are operationally complex and expensive.
A variable-speed drive’s intelligent adjustment of motor drive speed can be used to meet the precise demand and avoid inefficient energy waste. As an example, a machine retrofitted with a variable-speed drive using a synchronous servomotor, or a variable-speed pressure and flow rate control system with an asynchronous motor, delivers only the required flow rate by adjusting the speed and displacement of the pump.
A pressure transducer is used to measure the hydraulic pressure and adjusts the pump speed accordingly. No excess flow is generated, and less efficient throttling control utilizing proportional valves can be eliminated.
2. Consider modernizing and simplifying the hydraulic system. Because older stamping presses use fixed-displacement pumps with relief valves, variable-displacement pumps with proportional valves, or older load-sensing directional control valve technologies, the value of a retrofit can be determined by asking several age-related questions:
Retrofitting to a variable-speed system can simplify the hydraulic system, head off future component replacements, and greatly reduce maintenance costs.
3. Evaluate the hydraulic system’s environmental requirements. It is often assumed that noise and heat are the price to pay for a work environment needed to harness the power delivered by a hydraulic stamping press. Consider heat issues. It is worth assessing system fluid temperatures in regard to the double penalty of generating excessive pressure and/or flow, and the energy needed to remove that excess heat. It is also important to factor in the expense of cooling capacity and oversized oil reservoirs.
One of the main sources of machine noise in a hydraulic press is the hydraulic pump. The sound pressure level depends on the pump’s rotational speed and operating pressure. Higher speeds will produce greater noise. Beyond pump noise, older control system designs and controllers may introduce high levels of hydraulic “shock” during the press motion cycle, which can generate sound levels as high as 80 dBA and can stress piping, valves, and seals.
A variable-speed pump can reduce heat load dramatically in the system. When throttling losses (pressure drops) are eliminated using pump speed and stroke control, rather than throttling valves, heat transferred into the fluid is reduced, resulting in drastically decreased or eliminated cooling requirements.
Finally, variable-speed pump control results not only in lower average pump speeds, but also smoother accelerations and decelerations with controlled transitions between force and position control. The result is reduction in average noise emissions.
4. Assess and optimize drive and pump sizes. Typical hydraulic drive systems are oversized to deliver peak pressure and flow rather than what is optimal for real-world applications. It’s not uncommon for motors to be 50 percent larger than the actual stamping press process requirements. Oversizing is intended to compensate for inefficiencies in the hydraulic circuit caused by situations such as pressure drops and leakage flows.
For the electric motor driving the pump, proper sizing requires assessing dwell times and operation at partial and full loads to determine the actual required drive power. To determine optimal sizing of the drive components, simulation tools can be used to investigate dynamic stamping cycle variables, including pressures, flows, forces, and cylinder motion.
Hydraulic and control system engineering experts equipped with advanced simulation tools can provide designs incorporating variable-speed drives to retrofit a stamping press and ideally match demanding cycle requirements. For example, variable-speed pump drives, used in conjunction with an energy-on-demand system design, can optimize the use of controllers, variable-speed pumps, and motors to deliver just the precise energy needed. In addition, optimal motor sizing coupled with variable-speed pump drives can help a complete system fit into a smaller footprint when compared to conventional designs.
5. Improve controls for better operation and longer equipment life. Shocks transmitted through a hydraulic system can result in mechanical stress and physical wear on the press frame, fittings, pipes, connections, valves, and manifolds. This can have a negative impact on equipment life, increase downtime, and present the need for more frequent maintenance. If these conditions prevail on your stamping press, the press controls regulating pressure and flow, as well as upper-level control, should be evaluated. These physical shocks, even if intermittent, may indicate that the legacy control system may be hampering performance and energy efficiency of your stamping press.
If these shocks are present in your current press, consider retrofitting the existing controls with a state-of-the-art motion control system specifically engineered to take full advantage of modern electrohydraulics and variable-speed pump drives. These latest-generation systems provide intelligent, high-performance control of variable-speed pump systems as well as systems controlled with traditional proportional valves.
As an example, a modern hydraulic controller platform offers advanced control software packages tailored to the unique demands of hydraulic system properties (see Figure 3). Advanced software compensation for factors such as fluid compressibility and nonlinear system dynamics can provide optimal control. Hydraulic-specific algorithms for proportional valve and variable-speed pump drives allow for smooth transitions between position control and force control, generation of smooth motion trajectories, and multiaxis synchronization. These capabilities reduce system shocks and reduce the impact on tooling.
Dramatically improving the energy efficiency and performance of hydraulic stamping presses is now possible with high-performance, intelligent pumps that can deliver the required speed and flow without the energy inefficiency of control valves. An analysis and comparison of existing hydraulic power units and related components against a variable-speed pump drive system typically reveals energy usage and the environmental impact of heat and noise can be reduced.
1. Control hydraulic fluid temperature. Many hydraulic system failures are caused by poor fluid conditions. Excess temperatures can directly affect viscosity and degrade fluid if insufficient cooling capacity is not available. Proper system design, pump sizing, and controls will reduce excess flow and pressure that generate heat.
2. Maintain fittings, piping, seals, and valves. Regular maintenance inspections can prevent unexpected downtime and production breakdowns.
3. Monitor pump condition. Although pumps are the heart of hydraulic stamping presses, they often are overlooked until a breakdown occurs. Advanced condition monitoring systems can provide early warning of potential problems.
4. Listen to acoustic conditions. Acoustic emission monitoring equipment is available that masks background noise and vibration while picking up sounds that may indicate an impending failure. Hydraulics expertise may be required to interpret data and identify solutions to the acoustic, vibration, and other performance issues in the hydraulic system.
5. Employ controllers with advanced algorithms. Using best-in-class controllers with open- and closed-loop control algorithms specifically tailored for electrohydraulic systems can provide precision and consistency to increase press throughput and product quality.
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