Katie Hahn
Tol-O-Matic Inc.Hamel, Minn.
A thorough evaluation of motion parameters is a requirement for correct sizing of any motion actuator. Such parameters include the load support system, force requirements, and limiting factors. The typical first step is to specify the stroke length, load weight, speed, thrust force, resolution, repeatability, actuator orientation, moment loads, and duty cycle. This is the basic information needed to select the actuator, and is also the basis for calculations of motor speed and torque.
Motor selection for high-performance motion control today tends to be a process of settling on dc brushless servo, microstepping, or dc brush technology. Brushless servosystems tend to excel at delivering high torque (typically to 45 lb-in. continuous, 140 lb-in. peak) and high speeds, to 6,000 rpm. They are smooth and quiet, and provide high positioning resolution (to 4,000 counts/rev), good torque control, and are ideal for short, repetitive moves. They are good for handling multiple axes and applications involving interpolation and are maintenance-free.
Microsteppers can provide high resolution, to 50,800 steps/rev, without a position feedback transducer. These are considered medium cost systems and are good for torque requirements below about 35 lb-in. Though their motion is not as smooth as a servomotor, it is superior to that of full and half-stepping motors.
Finally, brush dc systems are best for lower torque needs (to 32 lb-in. continuous, 48 lb-in. peak) and speeds below about 2,500 rpm. Such systems can run as servos, with position or speed feedback, or open loop. They are considered low to medium-cost systems and can typically provide resolutions of 1,000 counts/rev.
Servo or stepper motors typically operate in conjunction with a mechanical actuator in any positioning system. Two of the most widely applied actuators are belt drives and ball screws. Belt-drive actuators are generally applied where speed and stroke length needs exceed what a screw actuator can provide. Screw actuators provide more resolution and better repeatability than belt drives. Thus, they are the method of choice to provide short strokes and to handle high move-repetition rates.
Load support and guidance systems come in a variety of forms. For consistent carrier tracking, high-quality actuators employ recirculating bearings on ground steel shafts. Where there is a possibility of heavy loads or a need to handle appreciable moments, recirculating bearings may sit in special guide rails to minimize carrier friction.
Screw-type actuators generally are available in either cylindrical or slide-type body styles. Cylindrical models tend to be most cost effective for ordinary moment load and linear accuracy requirements. There are also special models built to handle heavier loads with higher accuracy. Slide-type units can incorporate bearings providing lower friction, high load and moment-load capacity, improved torsional stability, and greater linear-path accuracy over the life of the actuator.
Screw selection involves considerations such as accuracy needs, acceptable backlash, and smoothness requirements. Many general-purpose screws are unhardened and use engineered resin nuts. Pitch accuracy ratings of 0.005 in./ft are typical. Use of an Acme thread with a solid nut may reduce back driving in vertical applications.
Applications demanding more precision are more likely to employ a precision rolled ball screw. Used with ball nuts, this hardened screw has a pitch-accuracy rating of 0.003 in./ft. It can provide high thrust and longer life in continuous-duty applications.
The selection of a ball nut depends on the amount of backlash that is acceptable. Similarly, factors such as the amount of screw pitch and thrust, speed, and stroke requirements are the typical inputs used for selection purposes with screw-capability curves.
The typical method of picking a mechanical actuator is to select a screw size or belt drive and then see if it can handle the anticipated loads and speeds. If not, move to the next size and repeat the process. Key tradeoffs to be aware of for screw actuators include pitch versus lead. Lower pitch (higher lead) provides more linear speed, yet requires more drive torque. Larger bodies provide more load-support capability and handle larger-diameter screws. Larger screw diameters permit higher rotational speeds, more thrust, and allow use of longer screws.
Applications where substantial moments come into play require consideration of how the cylinder tube, carrier, or supports might deflect. As a guide, manufacturers typically chart load deflections for recommended support spacing.