HOLOGRAPHIC NANOLENGTH METER WITH SLIDE BEARING
Humanity is currently attempting to exploit nanoscale objects. These cognitive processes concern all fields of activity, from intellectual to metrology, instrumentation, aerospace, robotics, and in all high-tech fields of engineering, science, including ecology. An important result of such processes, in particular, is the creation of the Russian holographic nano length meter with a sliding bearing with a record resolution of 10 nm and higher.
A plain bearing (PB) is an assembly that interacts between two moving parts, one of which is stationary. The bearing is mounted with its sliding element on the surface of one of the units and will move along it, replicating its characteristics with the least distortion, transmitting this motion to the other unit.
A PB has certain requirements, including low friction, minimum runout, reliability, stiffness, constant parameters over time and independence from external conditions, durability, temperature stability, minimum wear, etc.
A known device is the "lubricated flat plain bearing for a linear rail guide" . It is a single-coordinate linear bearing containing a sliding element (SE) in the form of a carriage with grooves for grease supply and rollers located on its opposite sides corresponding to the two sides of the linear rail guide. The guide contains recesses on both sides on which the SE bearings roll. Each groove of the linear guidance system acts as a bearing surface. There is grease between the carriage and the rail. The dirty grease gradually seeps outwards via a rubber assembly located on each opposite side of the SE, which makes contact with the corresponding grooves of the linear rail guide.
The SE carriage slides along the centreline of the linear rail, being in contact with it by means of a lubricant. The rollers, located on the sides of the SE, fit into the corresponding recesses of the guideway. The design reduces friction and noise, but the accuracy of movement that this plain bearing provides is limited by the sum of the following factors:
the use of a grease in which dust particles accumulate, changing its characteristics over time and depending on the ambient temperature;
the influence of temperature on the steel parts of the slide bearing.
There is also a linear PB , containing an SE in the form of a guideway. The guideway contains rail hollows (grooves) to which correspond longitudinal ridges (convexities) on the opposite sides of the SE. Each groove of the guideway, which acts as a bearing surface, is shaped like a gothic arch, and the corresponding opposite convexity of the SE is shaped like a circular arch in cross-section. There is grease between the carriage and the rail. The carriage slides along the centreline of the rail, being in contact with it. The longitudinal ridges on the sides of the SE fit into the corresponding arch hollows of the rail. The design reduces friction and noise. However, the SE device has the following disadvantages:
- the complex configuration of the mating surfaces of the longitudinal grooves of the rail guide and the ridges on the slide carriage leads to difficulties in manufacturing and fitting the two respective surfaces, especially for large dimensions, which in turn reduces the accuracy of the movement provided by the bearing;
- the presence of lubricants reduces the accuracy of the device due to a variation in the clearance due to the ingress of contaminants from the environment into the lubricant as a result of friction between the contact surfaces;
- the possibility of moving the carriage in only one axis;
- temperature dependence of the metal parts of the slide bearing.
Holographic Length Meter with plain bearing (HLMPB)
The HLMPB enables increased accuracy of movement and allows it to move on any plane and in any trajectory. It serves as an intermediate link between two nodes of the same or different mechanisms, transmitting the movement of one node relative to the other with the least amount of loss and distortion.
This is ensured by the factors identified in the device:
- reduction of dynamic errors;
- reduced influence of dust particles;
- reduced influence of temperature on the bearing units due to the possibility of using materials with a low thermal expansion coefficient;
- increased resistance to corrosion, as the bearing units are made of high hardness materials;
- possibility of non-lubricated operation due to design features of the SE containing grooves, as well as due to the use of SE materials with micro-grooves;
- no conductivity with selection of appropriate materials;
- possibility of plane movement in any trajectory.
Fig.1a, b, c shows the construction of the PB with SE unit, where a is the bottom view of the sliding bearing, b and c are the cross-sections of the sliding bearing.
ES 1 is executed from firm ceramics on basis Si3N4 and represents a cylinder, with big 2 and smaller diameter 3 in which the cavity 10 is executed, and through ball 5 is connected to a platform 4, thus the axis of an aperture of ball 5 coincides with axes of apertures in SE and in a platform 4. The SE 1 and platform 4 are mounted in the housing 6. Through the hole in the platform 4, the ball and the SE a strong kevlar thread 12 is passed. One end of the thread 12 is secured in the retainer 8 with the locking screw 9, and the other end is secured to the SE on the surface A side with the retainer 13.
The larger diameter SE cylinder has a slot 18. In the housing ring 6, at the location of slot 18 there is a pin 17 rigidly connected with it. The slot 18 and the pin 17 are located at an angle of 450 with respect to the orthogonal grid of grooves 11 and are designed to limit the angular rotation of the SE cylinder. In its lower part, the cylindrical body 6 has a shoulder 19 which encloses the larger diameter cylinder 3 of the SE and the retainer 13. There is a spring 7 between the lock 13 and the platform 4, which provides the thread tension on the ball 5 and maintains a constant value of the gap between the platform 4 and SE 1. The value of interference can be easily adjusted before operation by changing the length of thread 12 and locking it with the locking screw 9. The lightly polished surface plane of the SE comes into contact with the surface on which the slide bearing will travel. The ball is fixed by welding or gluing 14 in the spherical hole 15 made in the lower plane of the platform. This provides a clearance between the platform and the sliding element. Another clearance 16, between platform 4 and SE 1, is also provided by the difference in their outer diameters. Ball 5 must be adjusted before operation by changing the length of thread 12 and securing it with locking screw 9. This design ensures that the platform, ball and sliding element are connected without gaps in the system axis. The slot 18 serves for the passage of a pin installed to limit the rotation of SE 1 around the system axis. The platform has a hole 19 for fixing in an external device, where the sliding bearing will function.
This combination of attributes in the HLMPB allows for:
- increase the accuracy of the mechanical unit’s movement on the bearing surface;
- reduce the influence of temperature;
- operate without lubrication;
- be corrosion resistant;
- eliminate electrical conductivity,
- to move it flat on any trajectory.
Operation of the HLMPB device
Prior to operation, a PB is assembled from the assemblies shown in Fig.1a, including platform 4, ball 5 (rigidly fixed with upper hemisphere 8 to platform 4 by bonding), SE 1, thread 12 and housing 10. The thread 12 passes through the platform 4, the ball 5 and SE 1, is stretched and fixed at the bottom 13 of the SE and at the top of the platform 8 with a certain tension so that there is no gap between the SE and the ball after which it is tightened in the assembly 8 by means of the screw 9. The bearing is then installed in an external device (in this case the DG-30 length gauge) where it will further function between two units of this device, one of which is stationary (see Fig.4 - DG-30 rod (unit 3) as a "bearing surface") and rigidly connected to the plain bearing through the fixing hole in the plain bearing platform 19, the other is stationary (see Fig.4, unit 2).
The SE 1 of the sliding bearing is mounted with the plane A containing grooves 11 (Fig.1b) on the supporting surface 2 (Fig.4), on which it will move (slide), coming into contact with it. Figure 2 shows a view of the SE from the side of the metal plate under investigation. The bearing is located between two parts (units): the fixed length gauge and the movable outer unit 2 (Fig.4), transmitting, from one of them to the other, motion without any distortion. In other words, it must have as little effect on the motion process as possible, meeting the following basic requirements: low friction, minimum run-out, durability, temperature stability, minimum wear, rigidity, reliability, no accumulation of static electricity, etc. When moving on the bearing surface, the SE is positioned in such a way that it can then be moved at an angle of 450 in the direction of the slot 18. For this purpose, the axis of slot 18 must coincide with the direction of movement. This direction will best allow the SE to pick up foreign bodies, such as dust, from the guide rail to ensure the greatest possible accuracy of movement.
The size of the screw 17 is smaller than the width of the slot 18 and the length is smaller than the depth of the slot, allowing the sliding element to rotate about its axis at a small angle (about 1–20) and oscillate relative to the supporting surface. These movements allow the SE to self-align with the supporting surface and thereby monitor the true deviation of this surface from flatness. The grooves 11 have two functions: "skimming" the dust particles in the grooves and reducing the contact area of the sliding element with the bearing surface. Motion without lubrication occurs due to the fact that at least one of the two surfaces of the SE or bearing surface, on which the bearing slides, has microchannels. In this particular case, when using a gabrodiabase platform on the surface where the investigated plate 2 is located, which has a fine crystal structure, due to which its polished in-plane surface turns out to be covered with grooves 0.5–1 microns deep and up to several microns wide, in which air is located. Thus, an air cushion is formed under each pad on the A – SE side, which prevents the sliding element from sticking to the supporting surface, and the sliding element has the additional possibility to move on this given supporting surface.
In order to determine the error introduced by the sliding bearing during its practical application in the HLMPB system, a high-precision digital holographic longimeter DG-30 with 0.01 µm resolution was used, which, through its stem, was rigidly connected to the platform 4 of the sliding bearing through hole 19. The study determined the flatness of the plate surface 2 (Fig.4), which was moved during the test on the bearing surface of the gabrodiabase platform. At that deviation from flatness of gabrodiabase platform surface on which investigated slab 2 was mounted (±0.02 µm/300 mm) and an error of holographic long meter DG-30 (0.01 µm/30 mm) in total must be less than an expected error value of the PB including its important parameter - run-out. The system works in the following way: during the experiment the holographic long meter together with the PB is immovable relative to the investigated moving plate located on a gabrodiabase platform.
At the same time the DG-30 holographic length meter allows to trace the deviations from the flatness of the surface of the investigated slab. It can be seen on Fig.5 that the error of all elements used in the experiment: length gauge DG-30 with PB, investigated slab and gabrodiabase platform was less than ±0.045 µm (90 nm) at the length of 180 mm.
The design of the plain bearing therefore allows to:
improve accuracy of movement of two mechanical units in relation to each other by means of a plain bearing by:
averaging surface irregularities over the area of the A - SE plane;
to position the sliding element on the bearing surface, by means of a ball and the contact of the sliding element with the entire plane to one of the surfaces;
to minimise runout;
to move the sliding elements without lubrication.
As the HLMPB must slip on a supporting surface, these surfaces (HLMPB and supporting surface) must be selected so as to avoid sticking between them, e.g. ceramic (sliding element) and diabase (supporting surface). Diabase has a grained structure even after careful polishing, and there are air grooves between the grains, or alternatively, select a grained surface at the sliding element. Quite large sets of hard polished materials are available for this purpose.
collect the dust particles encountered along the way in the grooves of the sliding element, so that the dust particles do not fall between the planes and do not introduce an additional error.
The sliding element is more resistant to corrosion due to the possibility of using low-corrosive materials such as:
solid ceramics based on Si3N4 or
bronze-fluoroplastic antifriction material modified with fulleroid nano-modifiers, which leads to a 2÷2.5-fold increase in the lifetime.
The temperature dependence of the sliding element is considerably lower than that of its metal counterparts and prototypes.
The above-mentioned materials have a low expansion coefficient, which allows the sliding element to be used at high temperatures up to +250°C and higher and makes it applicable in hydraulic turbines, in power engineering, transport and aircraft engineering. They have:
They also make it possible to eliminate electrical conductivity, which increases its range of application in variable electromagnetic fields, to move it along a plane along any trajectory, to use it in combination with the more complex support surface on which it travels, transferring the non-planar shape of its lower end, on which grooves are made, and the shape of the support surface, while simultaneously adhering to it and maintaining high precision movement. This will increase the functionality of the plain bearing.
PEER REVIEW INFO
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Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.