When it comes to framing members and other structural elements for cars and trucks, hydroforming is where it’s at. Hydroforming starts with a tube — generally carbon steel — that has been pre-bent into a three-dimensional shape that will fit into a closed die. Fluid is then pumped into the tube, high pressure is applied to the fluid, and the pressure applies uniform stress to the inner surface of the tube. The tube undergoes plastic deformation, expands, and assumes the shape of the die’s interior surface. This is no light-duty task, considering that tubing typically has an OD of 1/2 to 6 in. with a wall thickness of 0.020 to 0.120 in. Pressure is then relieved, the fluid drained, the formed part ejected, and preparations are made for the next workpiece.
| Bending machine uses enclosed pump-motor assembly mainly for quiet operation but also benefits from positive side effects. |
To pre-bend tubing so it will fit into a die, many OEMs rely on tube-bending machines designed and built by Eagle Precision Technologies Inc., Brantford, Ontario. Eagle Precision also has a line of end forming and muffler-assembly machines. Their end-forming machines perform such tasks as forming a flared or tapered tube end or expanding the ID at the end of one tube so it will mate with another tube of the same size. The muffler assembly machines combine bending, end forming, roll forming and joining, pressing, and cutting (not necessarily in that order) to form mufflers from steel tubing sheet. All three types of machines are used extensively for making exhaust system parts for cars — both at the OEM level and for the huge exhaust aftermarket.
But Eagle’s machines certainly are not limited to just automotive applications or to working with tubing that has a round cross section. At the low end of the scale are machines for bending, end forming, and cutting of 1/2-in. diameter tubing that may become hydraulic tubing assemblies for aircraft, marine vessels, off-highway equipment, or similar end uses. The finished assemblies may instead use square tubing to serve as a single-piece frame for a kitchen chair or fabricated into a heavy-duty assembly for office chairs. Or high-grade steel tubing may be used for precise heat exchanger tubes in refrigeration, furnaces, chemical processing, and other demanding applications. In the ever-growing health and fitness industry, manufacturers of treadmills and other exercise equipment use Eagle Precision’s machines for producing strong but lightweight tubular frames made from a variety of materials and cross-sectional shapes, even extruded shapes.
Jim Sabine, engineering manager at Eagle Precision, reveals, “We use our own CNC control based on a VME bus computer system and write our own software. Bending is a high-level program: you enter the physical parameters of the tube you’re starting with, the basic shape of the tube you want, and the controller takes over from there. The software then factors in the tubing material’s properties, size, and thickness, and executes the bend at optimum speed. “For some materials, such as Inconnel, bending must occur slowly to avoid ripping them. However, because metals work harden as they bend, you can’t go too slow. Plain carbon steel, on the other hand, can be bent at much higher rates. That’s why we have to use servovalves: we must generate extremely high forces at high speed, yet maintain positional accuracy within thousandths of an inch.”
|Pictured above are typical parts and assemblies made using tube-bending machines manufactured by Eagle Precision.|
Eagle Precision’s benders operate at approximately 2250 psi, relief valves are set at 2700 psi, and all components are rated for at least 3000 psi. Sabine continues, “We generally specify tandem pumps, because servovalve circuits demand higher pressure than is needed for other functions on the machine — typically clamping functions, which operate at 1500 to 1800 psi. Therefore, one pump is dedicated to the servo system. Sabine explains that using a dedicated pump to isolate the servovalve circuit eliminates wide pressure fluctuations in the rest of hydraulic system. Pressure can be set manually, but usually is set automatically by the computer control.
For a typical part, Eagle Precision’s machines may produce as many as ten bends on a single length of tubing. Jim Overbeeke, vice president, sales and marketing, explains that “Electromechanical functions are more cost effective at lower power levels, but a wide range of power requirements exists where cost differences between hydraulics and electrics is not important. Although in high power ranges where precise control is required, hydraulics certainly has a cost advantage.”
“Based on their awareness of potential leakage, some customers don’t want hydraulics on a machine,” offers Sabine. “However, a lot of clamping occurs to hold a length of tubing in place during bending. Hydraulics is by far the most practical method for performing these clamping operations, so most machines have at least some hydraulics on them. Once a machine contains some hydraulics, you can add more hydraulic functions fairly cost effectively simply by increasing the size of the existing power unit and providing the necessary actuators, valves, and plumbing.
“With electromechanical functions, you have to provide a motor, starter, gearbox, actuator, and associated components for each individual function. It’s almost like starting from scratch every time you add functionality. It’s not only more expensive, but the components are so much larger that it can be difficult to find enough space to mount everything.”
“Many of our customers would prefer an all-electric machine rather than one with electrics and hydraulics,” admits Overbeeke. “In fact, on our 11/2-in. machines, many customers are willing to pay $5000 Canadian ($3700) more for a machine that is all electric over one that is electric and hydraulic.” Sabine explains that “Much of this additional cost is tied up in a 12-hp servomotor and large, zero-backlash speed reducer. Compare this to a hydraulic servomotor or cylinder driven by a servovalve. Customers are willing to pay the premium, though, because a total machine costs over $100,000 ($74,000). So their justification is that it adds maybe 5% to the total cost. Once you get beyond 12 to 15 hp, though, the cost of an all-electric machine skyrockets, which makes it almost impossible to pay 10% or more for the all-electric machine.”
Both Sabine and Overbeeke identify noise as a major criticism by customers about hydraulics. Ironically, though, new technology now permits hydraulic systems to operate much quieter than electromechanical systems. A case in point: enclosed motor-pump assemblies, referred to as the Integrated Motor Pump (IMP) by its manufacturer, Vickers Inc., Maumee, Ohio.
Philip Swisher, director, venture development at the Vickers Advanced Technology Center, Rochester Hills, Mich., explains, “Electric motors — whether driving a hydraulic pump or electromechanical system — generate considerable noise. A motor’s cooling fan is a major contributor to noise. With an electromechanical system, add to this the noise generated by a shaft coupling, bearings, and gearbox, chain drive, or belt drive, and you’re looking at investing some serious money in sound enclosures. It should not come as a surprise that this complicates maintenance due to the additional components that must be removed and replaced to get to the motor and speed reducer.”
Swisher reveals that in the IMP, the electric motor and hydraulic pump are surrounded and cooled by hydraulic fluid and encased in a sound-dampening enclosure. Furthermore, the motor is not air cooled, but cooled by the hydraulic oil that circulates around and through it. He explains, “Air is not a particularly good heat transfer medium. Actually, it’s a better insulator than a heat conductor. This means you have to move a tremendous amount of air through a motor to provide the same amount of cooling provided by a relatively low flow of hydraulic fluid. With our oil-cooled motor, the fluid picks up heat and carries it away much more effectively. Ultimately, the more effective cooling means that the motor can be made much smaller than an air-cooled motor with the same power rating.”
To get a handle on the magnitude of the noise reduction at Eagle Precision, Sabine points out that a conventional 40-hp power unit mounted on the machine structure generated about 88 to 92 dBA of noise 3 ft from the source. Now, with a 60-hp IMP running at the same pressure and flow as the conventional power unit, noise has dropped to 78 to 80 dBA. “We attribute most of this to the IMP. However, we mount the IMP differently than we did a conventional power unit. We also installed a pulsation dampener and put a 90° hose-and-tubing assembly at the pump outlet line. While these practices have some effect, the end result is a 10-dB reduction in noise, which makes a huge difference.”
To illustrate, Sabine mentioned that after adopting these techniques for their machines, the hydraulic power unit no longer is the greatest source of noise. “Our return line filtration is accomplished in a kidney loop tapped into the reservoir. A circulating pump routes fluid from the reservoir into a filter, through a heat exchanger, and back into the reservoir. Interestingly, the motor on the kidney loop’s circulating pump makes more noise than the power unit. A conventional power unit makes so much noise that you can’t even hear the circulating pump. On machines using the IMP, we might even start looking for quieter circulating pumps.”
Quiet operation sells
“Any of these machines is basically a horizontal press,” continues Sabine, “so we must ward off competition by continually developing ways of improving our products and services. We believe we have an edge over the competition right now with the quiet package we offer.” Overbeeke adds, “Our quiet package using the IMP is less expensive and more effective than an equivalent package from our competitors.
“Quieter operation of machines has become an issue not only in North America, but especially in Europe, where our machines are manufactured and marketed through our facility in England. The health and safety requirements over there seem to be more stringent than in North America. Even so, many customers are beginning to incorporate specifications for noise as a basic application requirement.”
“Without the IMP, we’d have to install expensive sound suppression components in conjunction with a conventional power unit,” reveals Sabine. “On the average, this can cost as much as $20,000 Canadian ($14,600 U. S.) to bring noise levels down to what they are with the IMP package. A 20-hp Uframe motor itself kicks out 70 dB or more. Add to this the pump, coupling, and other components, and you’re well into the mid 90-dB range.”
Installation and maintenance
Once engineers at Eagle Precision decided to incorporate the IMP as a key component of their quiet package, concern shifted to how customers would react to this unconventional setup. Greg Young, technical sales representative for Berendsen Fluid Power, Stoney Creek, Ontario, shares his observations. “From a maintenance standpoint, the IMP is easier to set up than a conventional pump and motor because it comes assembled, tested, and mounts as a single, self-contained unit, so you don’t have to bother aligning the motor-topump coupling. It’s also a neater package, especially with tandem or triple pump assemblies.”
As one might expect, concerns went beyond installation. Technicians, especially at automotive plants, wanted reassurance that the IMP would not become a maintenance nightmare if it or its internal pump had to be replaced. Young continues, “At first, technicians were apprehensive because they hadn’t dealt with a liquidcooled motor, and they didn’t know what kind of mounting hardware they might encounter. But after they have changed out a pump, they realize that most of the hardware inside the enclosure is simpler than that found in a conventional pump-motor assembly. What’s more, many find it easier to work on because they don’t have to deal with coupling guards and precise alignment.” Because the IMP is manufactured as a unit, all components are designed fit together. One might conclude, then, that the extra time spent on draining and removing the enclosure would be at least partially offset by the shorter time required to replace the pump.
Sabine says he is not aware of any problems with having to replace a pump or motor. “We’ve had them in the field for about three years and are not aware of any motor failures. Replacing the pump element follows roughly the same procedure as replacing a conventional pump. Our experience has been that the oil-cooled motor of the IMP seems less likely to burn out than an air-cooled motor is. The pump is no more prone to failure than a standard pump. In fact, the IMP uses the same vane and piston pumps that can be installed conventionally.”
Sabine adds that one modification they made was to install a shutoff valve at the pump inlet. “Because we mount the pump with a flooded suction, once we remove the IMP, having the inlet line to the pump open would drain all the fluid from the reservoir. Therefore, we install a shutoff valve between the reservoir and IMP. The only precaution is that the valve get opened before starting the IMP.
“Another modification was to install a pressure gage near the pump outlet. With a conventional power unit, you can see what direction the pump and motor are rotating. But because both are enclosed in the IMP, you can’t see what direction they are turning. Consequently, we have to read the pressure gage at startup to ensure that the motor was wired for the correct direction of rotation.” Young adds that the IMP’s commissioning guide recommends using a phase meter to check direction of rotation.
“But all this attention on changing the pump should not be construed as us experiencing problems. To the contrary, pump failures we’re aware of have been caused either by a lack of maintenance or by a failure elsewhere in the hydraulic system that caused large particles to be drawn into the pump.”
Incorporating the Integrated Motor Pump into their machines caused engineers at Eagle Precision to rethink some of their design practices. “For example,” explains Sabine, “we used to suspend a standard pump-and-motor assembly upside-down from equipment framework to make the machine somewhat portable. In automotive plants and other applications, a bending machine gets moved periodically from location to another. The machines, therefore, must be self-contained and readily moveable. When we first started using the IMPs, we could not mount them upside-down. Therefore, to incorporate the IMP into our existing design, we had to mount it rightside-up on its own base.
“Current versions of the IMP no longer have to be mounted rightside-up, so we could mount them upside-down in the original location. We found, though, that mounting the power unit on its own base reduces noise — whether we use a conventional pump-motor or the IMP. This is because the section of 12-in. square tubing framework that supported the pump and motor actually amplified the noise emanating from the assembly. Consequently, we continue to mount all IMPs on a base that is attached to the machine’s framework, and we install vibration dampeners to isolate the assembly from the rest of the machine. Plus, mounting the power unit on its own base makes maintenance easier.”
Efficiency is another issue, especially with automotive plants, because many require that electric motors be recognized as energy efficient. Electric motor manufacturers have relied on voluntary efforts to conduct efficiency tests and publish their own spcecifications. However, even when new standards take effect later this year, the liquid- cooled electric motor that drives the IMP will be exempt.* In fact, conventional efficiency tests do not apply because the motor is designed specifically to drive the hydraulic pump.
“When we specify an IMP in these instances,” offers Sabine, “the customer generally calls and asks for energy efficiency documentation. We then explain that the IMP uses a special-purpose motor, so standard efficiency guidelines do not apply. They accept it, but generally we have to correspond back and forth a few times to satisfy a customer’s requirements. Once standards, like EPACT, take effect, we hope things will go a little smoother. Official guidelines should make it easier for these plants to accept an IMP because it will be recognized as exempt from efficiency ratings for general-purpose motors.”
“As far as I know, we are the only company offering the quiet package using the IMP on a tube bender,” says Sabine. “We do not yet offer the IMP in the quiet package for our end formers, primarily because the IMP has only recently become available in the smaller sizes needed for end formers. We have begun testing end formers using these smaller (7.5 kW) IMPs.”
Another advantage to the IMP can be a much smaller envelope. Eagle Enterprises did not find the IMP to be significantly smaller than a conventional pump-motor combination. Young attributed this, however, to the fact that Eagle Precision’s machines needed slightly more hydraulic power than what a 15 kW IMP could deliver. Therefore, they specified an IMP that can transmit up to 30 kW and had it configured to deliver 20 kW.
“IMPs are actually 30% to 40% smaller than a conventional pump-motor assembly. IMPs in the higher power ranges tend to be 40% smaller, and those in the lower range tend to be about 30% smaller,” adds Young. “IMPs are currently available in 7.5-, 15-, 22–, 45-, 75-, and 92-kW sizes.”
*The Energy Policy Act of 1992 (EPACT) takes effect this October. EPACT defines minimum energy efficiency requirements for perhaps half of all industrial electric motors sold for general-purpose use. For a free copy of an article discussing EPACT and electric motor efficiency (published in the March 1997 issue of POWER TRANSMISSION DESIGN, a sister publication), Circle 440 on a reader service card.*