For the spatial fixation of shafts and axles and the perception of loads (axial, radial), the bearing assemblies serve. Quality of manufacturing and installation of bearings influences working capacity and durability of machines.
What is the difference between a plain bearing and a rolling bearing? The kind of friction within the bearing assembly. The sliding friction bearing directly contacts the shaft or axle, their surfaces are mutually mated. In the rolling bearing, these surfaces do not mate with each other, they are separated by a separator \u0026 ndash; balls, rollers, needles.
Bearings of sliding, the photos of which are located below, are radial, thrust (such bearings are called thrust bearings), radial-thrust. These characteristics indicate the characteristic direction of the loads in the bearing assembly relative to the shaft center line.
Sliding bearing, range of application
The use of plain bearings is justified in cases where it is required to ensure the operation of high-speed shafts, since rolling bearings under certain conditions are not sufficiently durable; when precision is required for the installation of shafts and axles, especially high-speed ones; if the standard rolling bearings of the corresponding sizes are not developed.
Another situation in which a plain bearing is preferable to a rolling bearing is that it is necessary to detach the bearing housing during assembly or disassembly (during installation or repair), for example, installing the crankshaft journals in bearing supports. Sometimes the operation of the bearing assembly must take place in water or an aggressive environment, so that the risk of corrosion makes it impossible to use rolling bearings. In addition to the above, there are other situations, for example, the economic benefit of using simpler sliding bearings instead of rolling bearings, in particular for low-speed schemes of non-compliant mechanisms.
In general, the sliding bearing is not as much in demand as a rolling bearing.
Construction and materials
The sliding bearing is a assembled body and a liner, i.e. its structural design is simpler than a rolling bearing. The enclosure can be integral or disassembled. In the latter case, both parts are fastened with studs or bolts. The insert is made in the form of a sleeve. In the non-removable bearing, the insert can be made in the form of two separate halves, the upper and lower halves. The sleeve of the slide bearing is pressed into the body. Although the non-removable bearing is simpler in design, the detachable version is much more convenient for installation.
If the shaft is susceptible to great deformation or exact mounting of the mechanism is impossible, self-aligning bearings are used. In other words, a spherical plain bearing is required.
Structural materials: cast iron for the body (grades 12-28 and 18-36), bronze, cast iron and plastic for liners. Babbits and lead-bearing bronzes, light antifriction materials are mounted on a steel, bronze or cast iron base. Applied and cast iron or bronze liners with babbittovogo filling. There are also wooden liners and even bushes made of particle board!
Some materials allow producing liners that can work without additional lubrication.
The geometry of the working surfaces of sliding bearings can be different. Cylindrical, conical, planar or spherical forms are applicable under appropriate conditions, the same shape must be the mating surface of the shaft. Tapered and spherical bearings are infrequent \u0026 ndash; The first ones are convenient for small loads in the conditions of a systematic adjustment of the gap. Second \u0026 ndash; self-aligning \u0026 ndash; They are able to work under conditions of a shaft skewing in the bearing assembly.
Requirements for sliding bearings
The sliding bearing must meet certain requirements.
First, the materials and design of the assembly must provide a minimum of friction losses and shaft wear.
Secondly, the strength and rigidity of the bearing assembly should be sufficient for continuous operation under the conditions of the existing loads.
Thirdly, assembly, installation and maintenance of bearing units should be as simple as possible.
Fourthly, the dimensions of the working (contact) surfaces of the bearing must be sufficient to create conditions for efficient heat removal and to perceive the pressure arising during operation without squeezing out the lubricant.
Friction \u0026 ndash; the enemy is a sliding bearing. In addition to wear on the working surfaces, increased friction can cause a severe overheating of the unit.
The main means of fighting friction along with the choice of the optimal clearance, accurate mounting and finishing of rubbing surfaces is lubrication.
Grease for sliding bearings can be different, it can be solid or liquid, gaseous or thick (consistent). Unique mechanisms work even with bearings on the magnetic cushion, that is, the role of the lubricant is played by the magnetic field! But more often in the technique for lubrication of bearing units mineral oils are used in the liquid state.
As greases, also widely used greases are used solidolols. Everyone who has encountered the exploitation of cars or agricultural machinery, this kind of lubricants is well known.
In a high-temperature working environment, liquid or grease lubricants are difficult, almost impossible to hold in the bearing \u0026 ndash; they flow out. In this case, talc, mica, graphite and other types of solid lubricants come to the rescue.
Liquid lubricant is fed into the working area of the bearing by means of special systems that operate in one of the following modes: individual or centralized lubrication, periodic or continuous lubrication, no forced delivery pressure or working with such pressure. Periodic individual lubrication is provided by oilers with a rotatable lid, there are press greases, cap oiler. Continuous individual lubrication requires the use of a wick or drop oiler.
A high efficiency of the bearing can be achieved by creating a hydrodynamic lubrication system when the shaft exposed to external forces eccentrically rotates in the liner, dragging the grease into the gap that has arisen. As a result, an oil wedge with hydrodynamic pressure is formed, providing a fluid type of friction.
Types of friction
The thickness of the oil layer determines the operating mode of the bearing: boundary, semi-dry, semi-liquid or liquid friction.
With a boundary or semi-dry mode, the thickness of the lubricant layer is so small that this layer loses the properties of the liquid. In the semi-fluid or liquid mode, the working surfaces of the bearing and shaft are separated by a layer of lubricant that covers the unevenness of the treated surface. For nodes with a boundary friction regime, we apply a simplified calculation for the mean pressure (p) or for another criterion, the product (pv).
Fluid friction mode for the operation of a sliding bearing is most favorable. It contributes to achieving high wear resistance. In this case, the calculation is based on the theory of lubrication (hydrodynamic aspect).
The conditional boundary between friction types is considered to be the Sommerfeld number:
[S0 ] \u0026 ndash; the Sommerfeld number;
P \u0026 ndash; average bearing pressure;
Ψ \u0026 ndash; relative diametral clearance, the ratio of the actual clearance to the diameter of the bearing size of the shaft in the bearing;
μ \u0026 ndash; the viscosity of the oil is dynamic;
ώ, rad / s \u0026 ndash; bearing angular velocity.
These numbers are defined for different types and models of bearings and are found according to the corresponding tables. The actual value of S0. then it is compared with the tabular and concludes:
For S0 ≥ [S0 half-liquid friction.
For S0 \u0026 lt; [S0 ] friction liquid.
The first criterion of calculation requires satisfaction of the following equation:
where [p] \u0026 ndash; table normative value of the maximum permissible average bearing pressure;
P \u0026 ndash; calculated mean pressure.
This check reflects the wear resistance of the bearing.
Another criterion requiring the fulfillment of the condition:
where v \u0026 ndash; sliding speed, m / s,
reflects thermal tension.
The values of p and pv do not reflect the influence of a number of important factors (surface quality, wear, etc.) on the performance of the calculation object, which forces the engineers to classify the calculation itself as an approximation.
The values of [p] and [pv] are given in the directories, since they are average for different types of bearings.
The moment of frictional force is calculated by the formula:
where f \u0026 ndash; table coefficient of friction, is chosen taking into account the working conditions.
Calculation of heat dissipation is calculated as follows:
Here the velocity v is also selected from the tables.
The speed at which the phenomenon of the transition of boundary friction to semi-fluid occurs arises from the formula of Vogelpol:
where P \u0026 ndash; bearing load, N;
μ \u0026 ndash; viscosity of oil (dynamic), N * s / m 2;
V = πd 2 l / 4 \u0026 ndash; working volume of the bearing, m 3;
with \u0026 ndash; constant coefficient, assigned depending on the materials:
- cast iron \u0026 ndash; gray 1 \u0026 hellip; 2;
- for bronze and babbitt the values are 2 \u0026 hellip; 3 and 2.5 \u0026 hellip; 4 respectively.
The upper values are for self-aligning bearings.
Expressing P in terms of p, we transform the formula of Vogelpol:
Comparing the slip velocity v, the coefficient of friction f, the slip velocity v2 coefficient of friction f2. we give the formula for the coefficient of friction in the bearing:
The indices without the index correspond to the calculated regime, the index 1 belongs to the regime of transition from the process of boundary friction to the semi-liquid state, index 2 is assigned to the parameters adopted for the transition from semi-liquid to liquid friction. In turn, the coefficient of friction f2 is determined by the Faltz formula:
Calculation under conditions of liquid friction
For this case, the calculation of the sliding bearings is based on the Reynolds formula:
where μ \u0026 ndash; viscosity of oil, N * s / m 2;
hm \u0026 ndash; gap in the section where the maximum pressure is observed, mm;
h \u0026 ndash; gap, defined in an arbitrary section, mm;
v \u0026 ndash; speed, m / s.
For a sliding bearing, this equation should be converted using polar coordinates:
where p \u0026 ndash; pressure in the bearing, is determined in an arbitrary section at an angle φ to the center line;
μ \u0026 ndash; viscosity (dynamic);
ψ = Δ / d \u0026 ndash; relative clearance in the coupling of the shaft and the bearing;
\u0026 chi; = e /? delta? \u0026 ndash; the relative eccentricity;
\u0026 lt; / RTI \u0026 gt; \u0026 ndash; radial clearance.
From this equation, the formula for determining the hydrodynamic load-carrying capacity is obtained.
where Φρ \u0026 ndash; the coefficient of tension, the dimensionless function, is determined from the tables.
Knowing that the viscosity of the oil is determined by the formula:
we can derive the equation:
Friction coefficient in bearing:
where the relation Φt / Fr is determined from the tables of directories.
The heat release value is determined by the formula:
where d has a dimension in m; P \u0026 ndash; in H; \u0026 omega; \u0026 ndash; in rad./s.
For the thermal calculation of the bearing (and for the selection of the lubrication system), it is necessary to know the consumption of the lubricant. It is determined by measuring the liquid flowing through the gaps in the ends of the zones - loaded and unloaded. Even such measurements are made for oil extruded through grooves designed for lubrication. Then the obtained values are divided by the time during which the process of selection of the leaked lubricant took place.
Per second, the flow rate is:
\u0026 ndash; dimensionless coefficient.
In this formula: q1 \u0026 ndash; table lubricant consumption factor through gaps in the ends of the loaded zone;
\u0026 ndash; coefficient of lubricant consumption in the ends of the unloaded zone;
Here? \u0026 ndash; dimensionless coefficient, tabular value;
- pe \u0026 ndash; pressure in the system of forced lubricant supply;
- coefficient that takes into account the intensity of oil flow through the grooves for lubrication:
- θ \u0026 ndash; dimensionless coefficient, tabular value;
Dimensions a and b are calculated by the formulas:
a ≈ 0.05d + (3 → 5) mm;
b ≈ (0.20 0.25) d
The equation of the heat balance of the bearing assembly is:
where W is determined by the formula given above and marked with the sign (*).
The amount of heat transferred during operation by the bearing lubrication
where with \u0026 ndash; Specific heat of oil, J / m 3 * deg .;
Q \u0026 ndash; oil consumption, m 3 / s;
t1 and t2 \u0026 ndash; temperature of the lubricant (index 1 at the inlet and index 2 at the outlet of the bearing).
The amount of heat released into the surrounding space by a massive bearing housing:
where k \u0026 ndash; table heat transfer coefficient, W / m2 deg .; its average values are in the 9 \u0026 divide; 16 W / m 2 degree;
F \u0026 ndash; air-washed bearing surface, m 2;
tM \u0026 ndash; the average temperature of the lubricant in the working area;
tB \u0026 ndash; air temperature.
The oil temperature in the bearing load zone is not known in advance, as a rule, so they are set by the values of some average temperature of the lubricant. Because of this, the hydrodynamic calculation of the bearing has to be carried out by the iteration method (successive approximations).
The same method of iteration determines the optimal values of ψ, the viscosity of the oil μ. Solutions must satisfy the condition:
The thrust bearings, which are a kind of bearing that perceive the axial load, operate in the state of only boundary or semi-liquid friction.
Manufacture of plain bearings
The slide bearing factory, as a rule, is a specialized enterprise. Are produced as ready-made knots, but separately inserts. The factories also carry out repairs of failed or worn bearings. The capacity includes mechanical and thermal workshops, assembly lines, which often operate in automatic mode. There is also equipment for spraying coatings on friction surfaces in order to improve wear resistance and reduce the friction coefficient of the bearing assembly. The sliding bearings, photos of which are given in the article, serve as an excellent illustration of the high technical level of production of these products.
Despite the fact that sliding bearings are not as common as rolling bearings, they confidently occupy their niche, meet the requirements for them, and the technology of their production is constantly being improved.