Flat Linear vs Shaft Motors
Why are shaft (shaft-cylinder) motors not called "linear"?
Why do many of those who use shaft motors in their wire EDMs stubbornly avoid the word “linear”?
For example, the Japanese company M, which was the first to use shaft motors in its wire EDMs.
Why do leading companies stop using shaft motors in their machines after years of production?
Shaft-cylinder motors were developed to replace pneumatic, hydraulic and ball-screw drives in robotic manipulators, stackers, assembly platforms, as well as medical and special equipment. Electric Discharge Machines are the first known application of shaft-type drives in machine-tools in general. Shaft-cylinder engines are manufactured by a number of specialised companies. It wasn’t until after 2010 that machine-tool makers began to purchase and employ shaft motors into their machine-tools. And some of them have started already refusing shaft-motor use! WHY?
What is the discrepancies between shaft motors and true (flat) linear motors?
FLAT (PLANAR) LINEAR MOTORS vs
Shaft motors are manufactured by a number of companies, such as the Japanese JMC Hillstone jointly with Nippon Pulse Company (NPC). Production – since 2005. Other manufacturers of shaft LM: LinMot, PBA Systems, Orientalmotor, Parker, Ametek, Delta, etc.
Besides the name “shaft motors”, these motors are known as “tubular” and “cylindrical” motors.
It should be noted that the employees of the Japanese machine tool company, which was the first to use such motors in its EDMs in 2010, do not call them LINEAR – only SHAFT MOTOR!!!
Shaft motors were developed in order to replace pneumatic, hydraulic and ball-screw drives in robot manipulators, stackers, assembly platforms, as well as in medical and special equipment.
Wire-cut electric discharge machines are the first known application of shaft motors in machine tools in general.
Shaft motors (SM) have coreless coils and, as a result, insufficient thrust. Only small and medium-sized models of wire-cut EDM can be equipped with such motors.
As for die-sinking electric discharge machines, such motors are unapplicable. An underpowered shaft-cylinder motor with coreless EM coils merely will not lift a heavy electrode!
Large scale production of EDMs with flat (planer) linear motors started up in 1998
Until 2000, only die-sinking type linear EDMs were produced and only with Z axis linear drive while XY drives remained conventional. Since 2000, wire-cut linear EDMs with LM on XY axes and die-sinking machines with LM on XYZ axes have appeared.
Since 2001, all Sodick EDMs are built employing LMs on all axes:
- die-sinking EDMs – on axes XY & Z
- wire-cut EDMs – on axes XY & UV
Sodick flat linear motors are the company’s entirely in-house product from the R&D stage, as well as Sodick’s own production – up to the rare-earth NdFeB magnets.
Sodick LMs are basically just 2 parts: flat panels of permanent magnets and electromagnetic (EM) coils with (mostly) ferromagnetic core that are separated by a constant gap of 0.4 mm and are installed parallel to the plane of movement. Another part is an optical scale with resolution of 10 (or 5) nanometers. Sodick LMs can be called “planer-parallel”, but the more common term is flat linear motors.
Sodick machine-tools with flat linear motors are unified mechatronic systems: LMs are created for machine tools “individually”, and machines, in turn, are created for these LMs and the corresponding loads.
A shaft motor cannot be integrated in a die-sinking EDM!
A weak shaft motor is simply underpowered to lift a heavy electrode!
The main advantage of shaft motors (SM):
- A shaft motor can be easily embedded instead of a ball
screw drive into existing devices (machine-tools).
But this is, in fact, the only merit!
- thrust deficit (ЕМ coil are coreless!)
- heat sink problems (or its absence!)
- thrust is generated at a distance from the plane of movements – at any movement the motor pulls one side of the table down and the other – up
- multidirectional runouts of the magnetic shaft and
dynamic asymmetry of the gap (thrust vector
“dances” chaotically from the direction of the feed!);
- flimsy design (the shaft is attached only at the ends in
tension and requires retensioning intermittently).
Main advantages of Sodick flat LMs:
- reliability and longevity –
fully proved by over 20 years of successful operation;
- thrust is generated in a plane
close to the linear guides of the carriages;
- highest dynamic precision throughout the entire operation
(the thrust vector coincides to the greatest extent with
the feed direction);
- high power and thrust due to core EM coils design;
- perfect heat sink – the block of electromagnetic coils is
attached with the entire plane to massive structural
elements with high heat conductivity;
- extra rigid structure;
- unalterably constant gap = 0.4 mm;
- flat linear motors cannot be embedded into a conventional machine-tools “in place of ball screws”; flat LMs are “individual” and “specific” for each machine-tool, which, in turn, is developed for these LMs and the corresponding loads. Sodick linear machine-tools are designed for their linear motors and Sodick linear motors are designed for their linear machines.
Sodick machine-tools with flat linear motors are unified mechatronic systems
Shaft twists and dances in shaft-cylinder motors during their operation
A rather thin shaft (30 mm ±) is inevitably distorted and contorted due to changing strong magnetic fields and under its own mass. As a result, the shaft, with any movement of the coils along it, “twists” in different directions, impairing, as a result, the gap. The gap width in shaft motors is an undefined value, actually “dancing”.
Rigid powerful flat linear Sodick motors - proven by over two decades in operation
Both the permanent magnet panels and the EM coil assemblies of Sodick planar linear motors are rigidly mounted on massive machine structures, which completely eliminates any deformation of the parts of linear motors and machine tools.
Just try bending a cast-iron frame or column!
Or a massive table!
The gap between magnets and coils is always constant – 0.4 mm.
One of the reasons for consistently high accuracy throughout the long life of the machine.
Motors such as shaft-cylinder have been known for a long time. Suffice it to recall the solenoid from the school lessons of physics . The shaft-cylinder motor is, in fact, a solenoid with an elongated prefabricated core with separate ring permanent magnets and controlled ring core-less electromagnet coils.
Shaft Motor as a rule is built into a machine-tool in place of a conventioinal ball screw. In the same way as the ball screw was off-center in the old machine, the shaft motor is also off-center in the new machine-tool with shaft motors.
A thin magnetic shaft slightly thicker than the index finger is easily deformed, multidirectional lateral runouts occur, fatally affecting the machine accuracy. There are at least two reasons for the “dancings”:
longitudinal waves caused by compressive and tensile forces, which are generated by the inhomogeneity in density of magnetic fields of the shaft motor magnets and coils;
- deviations of the parameters of specfic magnets on the shaft, as well as the heterogeneity of magnetic parameters of different parts of a magnet – there are no two completely identical magnets!
In a working shaft motor, a thin shaft bends in different directions, as if “dancing”, and the gap between the EM coils and ring magnets changes continuously and in different directions. Such “twists” of the magnetic shaft give rise to variable multidirectional lateral loads on the guides. It is known that the guides are designed for vertical loads, but wear out quickly and lose accuracy if the loads are lateral.
In order for a thin magnetic shaft to distort less, manufacturers of rod motors prescribe to fix the magnetic rod with tension wedges (!) in the supports on the bed at the machine tool by manufacturers. How much pressure is enough? How often will the user of a machine-tool with shaft motors have to “re-tighten” the shaft already in the working machine? And “how much” will it cost?
The danger of chaotic dances and twists of the shaft increases many times when the frequency of such oscillations coincides with the natural resonant frequency of the structure … In any machine there are many resonant regions that depend on physical characteristics and temperature changes. There are plenty of situations!
Sodick began developing linear motors (LM) in the early 90s in an atmosphere of complete secrecy. The company had already suffered having a sad occurrence: the original idea of the wear-free EDM process was “borrowed” from the company’s founder Mr. Furukawa.
The developers have examined and tested a lot of LM designs at the stands. Designs with magnet panels and a block of EM coils perpendicular to the plane of movement were considered as well among others, similar to constructions that some companies tried to produce years later (and the production “successfully” discontinued!). Among others, designs with ring magnets, similar to the newfangled shaft-cylinder LMs. All tested LM configurations and projects had been rejected due to their imperfections and shortcomings, and only the flat (planer-parallel) LM construction turned out to be ideal for machine tools, but with one proviso (condition): for drives with such an LD design, it is necessary to re-create the entire machine-tool. In point of fact,
Sodick linear machine with planar linear motors is a unified mechatronic system.
A machine being created anew is a big expense, but still … cheap is rarely good – the best fish swim near the bottom!
As for the choice of flat LM construction, the exactness of it is confirmed by the experience of other machine tool manufacturers: almost all successful machine-tools with LMs from the world’s leading manufacturers (non-EDM) use flat LMs – there is no other time-tested alternative!
The inter-attraction force between the panel of permanent magnets and the block of electromagnetic coils is about 6 times greater than the thrust created in operation of the LM in the feed direction. However, if the machine is initially designed for the installation of such a LM, the problem is fixed by itself: the rigidity of cast structures is much higher than the forces generated in LM operation, and the load falls on the guides, which are just designed for these loads.
Sodick machines use THK SSR guideways (caged ball technology), designed for use primarily in precision measuring machines. These guideways can withstand movements 100 times more than the distance from the Earth to the Moon and back.
Loads on the guideways are only vertical or in the direction perpendicular to the plane of the LM. There are no side loads during operation of planar LMs. And this ensures that the original positioning accuracy is retained for many years. In practice, the accuracy is retained on the machine-tools, produced 20 years ago and more!
Shaft motors have core-less ring coils and for this reason exhibit chronic thrust deficit. It is known that a core-less coil generates a magnetic field orders of magnitude less than ferromagnetic core EM coil. Note that in shaft LM, the magnetic field utilisation factor is somewhat higher (due to the ring magnets and the tubular design) – approximately 2 times. But this only slightly compensates for the losses from the lack of ferromagnetic cores. Due to the lack of thrust, shaft LM can not be used in die-sinking EDMs and large wire-cut EDMs. Lack of thrust in shaft LM gives rise to problems with the smoothness of feeds at small increments, when feeds are worked out with a discreteness of the order of a micron when cutting, for example, a small module gear. In such cases, the shaft LM behaves like an under-powered overloaded old truck, which, moving uphill, goes in jerks – lacking is power reserve!
Sodick linear machine-tools use (mostly) ferromagnetic core flat linear motors. Magnetic cores amplify magnetic fields and thrust by orders of magnitude.
Sodick uses core-less LMs only in nano-precision machine-tools with a feed resolution of less than 10 nm, but installs two or more such linear motors on each feed axis. We are talking about machines such as EXC100L wire-cut EDM, AE05 EDM, SuperNANO 100 machining center and AZ series special machine tools.
All other machines are equipped with linear motors with ferromagnetic core coils, which provide – together with Sodick Motion Control “M4-LINK” or older K-SMC control system – both high precision and practically excessive thrust with superb smoothness of feeds.