|This open-bed, portable milling machine boasts a 14 ft2 working area and relies on Nutron’s ball-piston motors in combination with Mico hydraulic spring brakes for precise speed contol over a wide range of speed and ripple-free performance at extremely low speeds.|
| Photo: Climax Portable Machine Tools, Inc. |
Despite their many advantages, hydraulic servodrives often are not considered for machine tool applications. The primary reason is the continuing increase in performance of electronic servodrives and controls combined with relatively low increases in cost. A hydraulic servodrive, on the other hand, often is considered unrefined technology that is difficult to integrate with modern electronic controls. However, when designers become aware of the real performance capabilities of hydraulic servodrives, they can recognize clear-cut advantages of using hydraulics over electromechanical drives in certain applications. Consequently, integrating electronic and hydraulic servodrives alongside each other within the same machine yields optimum performance by matching the benefits of the appropriate technology to the specific task. The portable milling machine shown in the accompanying photos demonstrates this concept.
To recognize when an application is better served by a hydraulic servodrive over an electronic one, or vice versa, a designer must understand the following points:
Power characteristics — Unless there are specific advantages to using hydraulics for low-torque, high-speed applications, then the electronic drive is probably the most suitable choice. Low-speed, high-torque electronic drives (servomotor and speed reducer combination) are a viable choice for many applications, but when higher power ranges are involved, hydraulic servodrives can offer greater economy and performance. Generally when 3 hp or greater is needed, the hydraulic drive delivers higher torque per dollar.
Speed control — Both types of drives can run at a constant speed, and closing the control loop with an encoder or other feedback device, there will be little speed change even under a varying loads. However, certain hydraulic motors with a pressure-compensated meter-out flow control can maintain constant speed within 1% under varying load conditions and without electronic feedback. These motors will run without speed reducers at 1 rpm and below with smooth, pulsation- free rotation.
Stall torque — Both electronic and hydraulic drives can produce a consistent stall torque. The electronic motor requires a torque-limiting control circuit to prevent motor damage. A hydraulic motor with standard pressure control can maintain stall torque for long periods without damage. A hydraulic motor that exhibits no volume change during the 360° of rotation displays constant stall torque that, again, can be held for long periods without damage.
Rapid change in speed or direction —Speed and direction can be changed with both types of drives, but a directdrive hydraulic motor can change speed or be reversed extremely rapidly without the need for ramp control.
|Fluctuations in stall torque relative to shaft position indicate torque ripple produced by various hydraulic motor designs. Note how this condition varies with pressure for most designs. |
Space and weight limitations — Electric drives are usually the clear choice for high-speed, high-torque applications requiring relatively low power requirements. As power rating increases, hydraulic motors become lighter and more compact than electric motors. For low-speed, hightorque applications, the electronic drive requires a speed reducer. A hydraulic motor drives the load directly, so a hydraulic drive will be lighter and more compact.
Environment — The hydraulic drive is of immense benefit in explosive or otherwise hazardous locations and generally requires no special protection or certification. Both types of drives require external protection for wet and corrosive environments, but the hydraulic drive does not require any special protection from the ingress of moisture, dust, and dirt.
Installation — Mounting of either drive involves about the same amount of work, and both require power supplies and control equipment. There is perceived to be more work to install hydraulic pipework, but this perception ignores electronic wiring that requires bracketing, ducting, or conduit.
Many of these favorable characteristics have always made hydraulic servodrives the logical choice performance wise for many machine tool applications — even though electronic drives found favor based on antiquated misconceptions. However, performance improvements and enhanced features offered by manufacturers is causing hydraulic servodrives to recapture many machine-tool applications that had formerly won out to electronics.
Submitted by Peter Chalk, applications manager, Nutron Motor Co., Inc., Eliot, Maine. 207/439-5272.