Linear Drive

Kimla The Future of CNC Machines

No one else has this kind of accuracy in the industry! NO moving parts, No motors or Gears. Nothing to wear out.

Currently, only about 1% of CNC machines use linear drives, which makes their production costs very high (prices of such machines start from a million dollars), and this becomes a barrier to their spread – and the circle is closed. Importantly, the prices of linear motors, unlike rotary motors, will always be proportional to the length of the axis travel, as there must be a magnetic path along the entire path of the motor over which the forcer, i.e. the core assembly with coils, must travel. How it’s working

The linear motor and the principle of its operation are easiest to imagine as the development of a rotary motor with permanent magnets. In a rotary motor, the core with the coils creates a rotating magnetic field, and the magnets on the rotor surface interact with this field, turning the rotor. If we imagine that we develop such a rotary motor like a carpet, we get an image of a linear motor. Instead of a rotating magnetic field, a shifting magnetic field will be created, and in place of magnets placed on the rotor surface, we have magnets located on the surface of the magnetic path. In order for the motor to function, the magnetic path must be along the entire length of the axis travel – and this is the main reason why these types of drives are more expensive. Linear motors were known from the beginning of the development of electric machines, however, the magnets used at that time had a relatively low magnetic energy density and very large and heavy linear motors were required to obtain useful forces. The situation changed with the discovery of neodymium magnets – in the 80s of the last century. Neodymium magnets are many times stronger than the previously used ferrite magnets, which allowed for a significant reduction in the size of the magnets in the magnetic path, but their initially high price effectively discouraged manufacturers from using them. However, since there are few neodymium magnets in rotary motors, they quickly replaced ferrite magnets in this application. Currently, servo drives are no longer produced using ferrite magnets, because the advantage of neodymium magnets is so large that it more than compensates for their higher price.


Limited transmission capabilities
Linear drives have a huge advantage over traditional methods of axis drive of machine tools. The use of rotary motors to drive linear machine axes requires the use of a gear that converts the rotary motion of the motor into linear motion of the axis. It may be a toothed belt, a rack or a ball screw, but each of these solutions has limited accuracy, efficiency and life. The simplest and cheapest way to convert a rotary motion to a linear one is a toothed belt driven by a toothed wheel located on the motor axis together with the idler at the opposite end of the machine’s linear motion. The linear axis is driven directly by a section of the belt running in a straight line between the drive and idler pulleys. Unfortunately, the belt is flexible and stretches even with a small load. Such solutions are therefore only used in simple machines, e.g. cutting plotters or printers for moving the head. The accuracy of such drives, as a rule, does not exceed 1 mm. In heavier machines, and at the same time requiring movement over long distances, drives using toothed bars are used – similarly to the drive of entrance sliding gates. The toothed wheel is meshed with the toothed rack and during rotation it moves or slides along it. The gear wheel must not be too small due to the shape of the teeth, therefore a relatively large torque is required, which cannot be produced directly by a motor. Accordingly, in practice, a torque-increasing gear is used between the motor and the pinion mating with the rack. Although this type of drive is much stiffer than a toothed belt, it is the tooth backlash and torsional stresses that generate reverse play, which is further increased in operation due to frictional wear. Such drives allow to achieve an accuracy of 0.1 mm. They can be increased by using helical gears (so-called helical gears), but the achieved accuracy does not exceed 0.05 mm. Ball screw gears offer further possibilities of increasing the accuracy. They work by screwing the bolt into the nut – balls are inserted between the bolt and the nut to minimize play and friction, just like in a rolling bearing. After passing through the cap, these balls are directed through special channels to its other side, circulating in a closed circuit. This solution allows for an accuracy of 0.01 mm, however, such drives are unsuitable for use over long distances due to the sagging of the unsupported bolt, resulting in vibration and vibration at faster feeds. All these types of transmission have one thing in common: the axis position is measured indirectly by means of an encoder placed on the axis of the servo motor. As a result, displacements of the working element in the range of backlash or stresses in the drives cannot be detected and corrected. In such a situation, after changing the direction of movement, the servo drive performs a certain rotation which does not result in the displacement of the working element. Only after resetting the backlash after changing the direction of rotation and tensioning the drive system, the rotation of the motor begins to translate into linear movement of the tool. The values ​​of backlash depending on the type of transmission are in the range of 0.01–0,


Advantages of linear drives
All of the above Linear drives are free from imperfections. There is no backlash phenomenon in them, because the position measurement is carried out with the help of rulers measuring directly the linear displacements of the axis, and the movement is caused in a non-contact way by a magnetic field that cannot be worn out. There are also no restrictions on travel length due to sagging bolts, or flexibility with gear stress. These types of drives easily achieve accuracy of 0.001 mm. As no mechanical parts in contact with each other are used to transmit power, there is no wear and tear and the machine can be as accurate as new after 10 years of operation. No limitation of the servo motor rotational speed means that the machines can be much faster, and the elimination of the moment of inertia of the rotating elements allows for greater accelerations. These are undeniable advantages of linear motors compared to classic drives, but there are still a number of myths regarding the use of linear drives in CNC machine tools.



Myth 1: Linear drives use more energy
According to the principle of conservation of energy, energy does not disappear or come out of nowhere – if we are going to do work, energy must be brought into the system. If this work is to be, for example, milling with a rotary tool with a cutting force of 100 N and a feed speed of 0.2 m / s, then the power needed to just move the tool in the material will be P = F * V, i.e. 100 * 0.2 = 20 W. Surprisingly little compared to the power of axis drive motors in machine tools. Why, then, these kilowatts of power in the feed drive motors? Let’s try to move the table of the turned off CNC machine by hand. In most cases it turns out to be impossible, which means that the resistance to movement of the gears is relatively high. If we assume that the force needed to move the axis of a medium-sized machine tool is at least 1000 N, it turns out that just moving the unloaded axis requires 200 W. There are still rolling resistances of the guides, but these do not generate significant resistance and it can be assumed that the value of 100 N will be exaggerated anyway. In total, such a system requires 240 W of mechanical power. As you can see, most of the power used for the axis travel drives is the power needed to compensate for the losses in the gears themselves. In linear drives there are simply no such gears, so the power consumed by the axis with a linear drive will be the sum of the resistance to tool movement and the rolling resistance of the trolleys on the guides – that is 40 W. The efficiency of the motors themselves remains. This term means the ratio of the mechanical output power to the electrical power supplying the motor. Both rotary servo drives and linear motors are based on the same principle of operation: they are Permanent Magnet Synchronous Motor (PMSM), and their efficiency exceeds 90%. There is therefore no rational reason to believe that linear motors consume more energy, even more – the above analysis shows that it is exactly the opposite and that linear motors are clearly more economical. The difference is so great that even when running at higher speeds and higher accelerations, linear drives consume less energy than conventional drives. In addition, in laser cutters, where there is no tool resistance, almost all the energy used to accelerate the axis can be recovered during braking and transferred to the accelerating axis. This technology was used by Kimla and allowed for a further reduction in energy consumption by up to 70%. even more – the above analysis shows that it is exactly the opposite and that linear motors are clearly more economical. The difference is so great that even when running at higher speeds and higher accelerations, linear drives consume less energy than conventional drives. In addition, in laser cutters, where there is no tool resistance, almost all the energy used to accelerate the axis can be recovered during braking and transferred to the accelerating axis. This technology was used by Kimla and allowed for a further reduction in energy consumption by up to 70%. even more – the above analysis shows that it is exactly the opposite and that linear motors are clearly more economical. The difference is so great that even when running at higher speeds and higher accelerations, linear drives consume less energy than conventional drives. In addition, in laser cutters, where there is no tool resistance, almost all the energy used to accelerate the axis can be recovered during braking and transferred to the accelerating axis. This technology was used by Kimla and allowed for a further reduction in energy consumption by up to 70%. In addition, in laser cutters, where there is no tool resistance, almost all the energy used to accelerate the axis can be recovered during braking and transferred to the accelerating axis. This technology was used by Kimla and allowed for a further reduction in energy consumption by up to 70%. In addition, in laser cutters, where there is no tool resistance, almost all the energy used to accelerate the axis can be recovered during braking and transferred to the accelerating axis. This technology was used by Kimla and allowed for a further reduction in energy consumption by up to 70%.


Myth 2: Linear drives are expensive to operate and repair.
Linear drives, unlike rotary drives, work without contact – they have no contact surfaces that would wear out during operation. Non-contact operation completely eliminates this process as the axis is moved directly by a magnetic field that is not subject to wear. It should be emphasized that the most expensive element in linear drives is the magnetic path. However, this one consists of many segments bolted to the machine tool body, so even in the event of mechanical damage, it is enough to replace one damaged segment, the cost of which is a fraction of the price of the entire drive. The forcer, i.e. the core with windings, is most often cheaper than a classic rotary engine.


Myth 3: Linear drives get dirty and need frequent cleaning
Linear drives have a large number of magnets that can attract ferromagnetic particles. However, due to the fact that they always work in a shield, they have sufficient protection to protect the magnets from contamination. Indeed, linear drives do require cleaning from time to time, but these are simple steps, performed by the operator, and their possible neglect will not cause any problem for several years, because the gap between the magnets and the forcer is approx. 1 mm – in practice, this is how much dirt would accumulate for over 10 years. At the same time, the use of linear drives avoids the need to lubricate and clean ball or gear drives, not to mention the need to replace them in the event of wear. The development of drives in CNC machine tools can be compared to the development of railways, where initially the steam locomotives were very complex and inefficient because the linear motion of the steam engine piston had to be converted into wheel rotation and wheel rotation into train linear movement. After the appearance of electric drives, the locomotives were significantly simplified, getting rid of pistons and connecting rods, and replacing steam engines with electric ones. Eventually, the era of magnetic-powered trains is approaching, which can be compared with modern linear drives on CNC machine tools.