The following diagrams show linear movement according to three most common kinematic schemes of portal mechanism used in modern machinery manufacturing: ball screw, gear rack (rack & pinion) and linear drive (direct drive).
Ball screw unit
- Components subject to mechanical wear
- Components subject to mechanical wear
- Components subject to mechanical wear
- Components free from mechanical wear
Comparison between ball screw unit and direct drive
The structure of the ball screw unit possesses the big number of mechanical joints which lead to wear and consequently to lower processing accuracy. In its turn, the linear drive consists of just two parts which interact only at the electromagnetic level. The parts do not contact mechanically and consequently the mechanical wear of the linear drive is absent.
However, the coordinate system uses linear guides and linear bearings which are mechanical parts and are subject to wear either. They are used in the linear drives, ball screw units and rack pinion systems. Therefore, the statement that the linear drives are not subject to wear may be partly considered true or a marketing trick.
It’s worth mentioning that the ball screw systems for long travels show additional problems due to additional mechanical tension having appeared as a result of screw deflection. Such systems demand precise engineering calculations and high quality components. To sum up, some manufacturers may provide unreliable coordinate systems since they don’t possess satisfying engineering facilities.
Features comparison of rack & pinion with direct drive
Unlike the ball screw, the rack & pinion do not show tension and high inertial load to the motor. Nevertheless, it still demands high quality production of the coordinate system: high precision installation of guides, rack & pinion, gear box and motor. Decent quality of the machine’s bed and close fitting let the rack &pinion based on the hardened gear pinion ensure long and stabile machine operation.
However, errors and tolerances can result in backlash and fast mechanical wear of the drive. As in the previous case, some manufacturers may provide unreliable coordinate systems since they don’t possess satisfying engineering facilities.
Let’s consider the control processes in each case
Since there is smaller number of stages to convert electrical energy into linear motion, the linear drive shows higher efficiency than the ball screw drive or pinion rack. Besides this the linear drive doesn’t have mechanical backlash due to absence of mechanical joints.
However, the linear drive is not only part of the coordinate system, and basically it can’t guarantee backlash absence in the system. The coordinate system includes linear bearings which move along the linear guides which in their turn may have backlash.
Apart from this linear drive feedback (magnet or optical scale) may show the hysteresis effect, i.e. a particular dead area if motion direction is changed. This may affect the positioning accuracy in the same way as the mechanical backlash.
To sum up, absent backlash in the coordinate system based on the linear drive may be considered as a marketing half-truth.
Comparison of drive types
Coordinate systems based on ball screw unit and pinion rack are subject to slightly higher wear than linear drive since a bigger number of interacting mechanical parts. Nevertheless, linear drive does not guarantee full absence of wear, since coordinate system still possesses some mechanical parts.
System’s mechanical backlash affects directly the repeatable positioning accuracy. Due to bigger number of mechanical joints coordinate systems based on ball screw unit and pinion rack have slightly larger backlash values. However, the coordinate system based on the linear drive still has the backlash which appears at the linear bearings.
Long travel accuracy
Regarding coordinate system based on linear drive the long travel accuracy is defined by feedback accuracy of position (magnet or optical scale), and accuracy of portal installation (right angle) and tracking system. Concerning coordinate systems based on ball screw and rack & pinion the accuracy is defined by machine bed quality and guides’ installation. The calibration of the coordinate system using a laser interferometer is required in both cases in order to compensate deviations of the long travels. After it the long travel accuracy remains almost the same not depending on the used type of drive. Application of the linear drive is preferable since it is simpler and doesn’t require facilities to ensure high quality production and calibration. However, application of the linear drive itself doesn’t guarantee the long travel accuracy.
The coordinate system based on the ball screw unit demonstrates quite low dynamic parameters due to technological issues (heavy weight of the ball screw unit and, as a consequence, high inertial load). Nevertheless, the dynamic parameters can be enhanced due to increased motor power.
The coordinate system based on the rack & pinion possesses high dynamic characteristics limited by motor’s inertial load and rotor.
The coordinate system based on the linear drive show high dynamic characteristics limited by load and motor response.
Particular dynamic parameters of the coordinate system depend on the applied drive and load weight (portal). Weak linear drive can’t provide the proper dynamics and then can’t even be compared to the system of rack & pinion and servo drive. At the same time, powerful motor installed with ball screw unit is able to ensure satisfying dynamic parameters of the coordinate system.
Since the system based on the linear drive possesses smaller number of mechanical parts and joints it ensures the highest level of reliability. However it’s worth mentioning that a drive is not the only component of the laser cutting machine that affects the reliability, therefore, it is the general reliability of the system that shall be estimated.
Accuracy of travel along the contour
Accuracy of travel along the contour hardly depends on the drive type and it is defined by the rigidity of coordinate system structure (incl. portal), as well as by the frequency response of the coordinate system (which in its turn depends on structural features of the portal, head mounting and other components).
Accuracy of travel along the contour is also concerned with dynamic parameters of the coordinate system. Since the powerful drive is installed, the high dynamics at free running can be reached (with the switched off tool); if the machine construction is not rigid enough, the significant decrease of acceleration and deceleration values is required to provide the accuracy.
Having equal technical parameters, the linear drive is more expensive than the ball screw or the gear rack. The reason is that the direct drive requires long powerful neodymium “magnet way” along which the drive is moving. At the same time regular servo drive has smaller amount of magnets and they are located on the rotor, which rotates many times during working travel of coordinate.
The table can be scrolled left / right
|Coordinate system based on the bass screw unit||Coordinate system based on the rack & pinion||Linear drive|
|Subject to wear. Heavy wear if low quality manufacturing.||Subject to wear. Heavy wear if low quality manufacturing.||Subject to wear. The wear is slightly less than other types of coordinate systems.|
|Presence of carriage and drive backlash which affects the accuracy.||Presence of carriage and drive backlash which affects the accuracy.||Presence of carriage and drive backlash which affects the accuracy. Presence of hysteresis effect at feedback, which affects the accuracy. Repeatable accuracy is slightly higher than others.|
|Reduced precision at long travels.||Reduced precision at long travels.||High precision at any travels.|
|Average dynamics.||High traveling dynamics (if applying high performance drive).||High dynamics (if applying high performance drive).|
|High reliability while maintaining proper quality of manufacturing.||High reliability while maintaining proper quality of manufacturing.||High reliability.|
|Average cost.||Average cost.||High cost.|
The linear drive has some undeniable advantages, which are not a universal remedy though. In fact the linear drive itself insignificantly prolongs equipment’s life time, increases accuracy of coordinate system and traveling dynamics. The linear drive’s advantages can be fully estimated unless and until its high power, well-thought construction of the coordinate system (high rigidity and frequency response) and high quality of manufacturing, in other words applying the combined approach to the equipment.
Assuming high manufacturing quality, conventional drives (ball screw and rack pinion) will be slightly less reliable than the direct drive machines (as long as the rest of components of the compared machines are equally reliable). Nevertheless, the linear drive is the simplest and most effective solution for reaching the required level of reliability under unsatisfying technical and engineering capabilities; which is the reason of its fast-growing popularity.