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-CYLINDER MOTORS
Shaft (Cylinder / Tubular) Motor
Flat (planar) Linear Motor
Shaft motors—also referred to as tubular or cylindrical linear motors—are produced by several manufacturers, including the Japanese company JMC Hillstone in collaboration with Nippon Pulse Company (NPC), with production beginning in 2005. Other notable shaft linear motor (shaft LM) manufacturers include LinMot, PBA Systems, Oriental Motor, Parker, Ametek, and Delta.
It is worth noting that the engineers at the Japanese EDM manufacturer who first adopted shaft motors in 2010 explicitly avoid calling them linear motors. Instead, they refer to them only as shaft motors.
Shaft motors were originally developed as a replacement for pneumatic, hydraulic, and ball-screw drives in applications such as robotic manipulators, stacking systems, assembly platforms, and specialized medical equipment.
Interestingly, wire-cut EDM machines represent the first recorded use of shaft motors in metal-cutting machine tools.
Shaft motors (SMs) feature coreless electromagnetic coils, which inherently limit their thrust. This restricts their use to small and mid-sized wire-cut EDM machines.
In die-sinking EDMs, these motors are entirely unsuitable—a coreless shaft-cylinder motor simply does not generate enough force to move or support a heavy electrode during machining.
Large-scale production of EDM machines equipped with flat (planar) linear motors began in 1998.
The pioneer in linear motor EDM technology was Sodick — a global leader and innovator in the EDM industry.
Presently Wire EDM with flat linear motors are manufactured also by Excetek.
A flat linear motor consists of only two main components: flat panels of permanent magnets and electromagnetic (EM) coils, typically with a ferromagnetic core. These components are arranged parallel to the plane of movement and separated by a constant air gap, which typically ranges from 0.1 mm to 1.0 mm — 0.4 mm in the case of Sodick machines.
An additional component is an optical scale, usually with a resolution of about 10 nanometers.
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 (Cylinder-Tubular) Motors (SM):
- A shaft (or tubular, or cylinder) motor can be easily embedded instead of a ball
screw drive into existing devices (machine-tools).
But this is, in fact, the only merit of shaft motors!
The main disadvantages of Shaft (Cylinder-Tubular) Motors (SM):
- 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 Flat Linear Motors:
- 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
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 motors - proven by almost 3 decades in operation
Both the permanent magnet panels and the EM coil assemblies of flat 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.
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.
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!
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 shaft motors prescribe to fix the magnetic shaft 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!
Illustration from the service manual of the manufacturer of shaft-cylinder motors:
The manufacturer proposes to regulate and eliminate the shaft deflections with the help of wedges. Such is the “hi-tech”!
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!
Flat linear Motors use (mostly) ferromagnetic core flat linear motors. Magnetic cores amplify magnetic fields and thrust by orders of magnitude.
The machines equipped with flat linear motors with ferromagnetic core coils provide both high precision and practically excessive thrust with superb smoothness of feeds.
Flat linear motors are sometimes referred to as planar-parallel, but the term flat linear motor is more widely used.