For the spatial fixation of shafts and axles and the perception of loads (axial, radial) are bearing units. The quality of manufacture and installation of bearings affects the performance and durability of machines.
What is the difference between a sliding bearing and a rolling bearing? A kind of friction inside the bearing assembly. The sliding friction bearing is in direct contact with 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.
Sliding bearings, the photos of which are placed below, are radial, thrust (such bearings are called thrust bearings), radial-thrust. These characteristics indicate the characteristic direction of loads in the bearing assembly relative to the axial line of the shaft.
Sliding bearing, scope
The use of sliding bearings is justified in cases when it is necessary to ensure the operation of high-speed shafts, since rolling bearings in some modes are not sufficiently durable; when the accuracy of the installation of shafts and axles, especially high-speed; unless standard rolling bearings of the appropriate sizes have been developed.
Another situation is when a plain bearing is preferable to a rolling bearing: the possibility of bearing housing separation during assembly or disassembly (during assembly or repair) is required, for example, installation of crankshaft journals in bearing supports. Sometimes the operation of the bearing assembly must occur in water or a corrosive environment, as a result of which the threat of corrosion makes it impossible to use rolling bearings. In addition to these, there are other situations, for example, the economic benefit of using simpler bearings to replace rolling bearings, in particular, for low-speed circuits of non-responsible mechanisms.
In general, the sliding bearing is not as often claimed as a rolling bearing.
Construction and materials
The sliding bearing is a assembled body and liner, that is, its structural scheme is simpler compared to rolling bearings. The body can be solid or detachable. In the latter case, both its parts are fastened with studs or bolts. The liner is made in the form of a sleeve. In the one-piece bearing liner can be made in the form of two separate halves, the top and bottom. The sleeve of the sliding bearing is pressed into the housing. Although the one-piece bearing is simpler in design, the split version is much more convenient for installation.
If the shaft is subject to large deformations or the exact assembly of the mechanism is impossible, self-aligning bearings are used. In other words, a spherical plain bearing is required.
Construction materials: cast iron for the hull (grades SCH 12-28 and SCH 18-36), bronze, cast iron and plastic for inserts. Babbits and lead bronzes, light anti-friction materials are mounted on steel, bronze or cast iron base. Cast iron or bronze liners with babbit fill are also used. There are also wooden liners and even sleeves made of chipboard!
Some materials allow you to make liners that can work without additional lubrication.
The geometry of the working surfaces of the bearings may be different. Cylindrical, conical, flat or spherical shapes are applicable in appropriate conditions, the same shape should be the mating surface of the shaft. Tapered and spherical bearings are used infrequently \u0026 ndash; The first ones are convenient for small loads under the conditions of systematic clearance adjustment. Second \u0026 ndash; self-installing \u0026 ndash; able to work in conditions of shaft skew in the bearing assembly.
Requirements for bearings
The sliding bearing must meet certain requirements.
First, the materials and construction of the assembly must ensure a minimum of friction losses and shaft wear.
Secondly, the strength and stiffness of the bearing assembly must be sufficient for long-term operation under conditions of actual loads.
Thirdly, the assembly, installation and maintenance of bearing assemblies should be as simple as possible.
Fourth, the dimensions of the working (contact) surfaces of the bearing should be sufficient to create the conditions for effective heat dissipation and perception of the pressure that occurs when the work is performed without squeezing out the lubricant.
Friction \u0026 ndash; enemy bearing slip. In addition to the wear of working surfaces, increased friction can cause severe overheating of the assembly.
The main means of dealing with friction, along with the choice of the optimal gap, accurate installation and finishing of rubbing surfaces is lubricant.
Lubricants for sliding bearings can be different; they can be solid or liquid, gaseous or thick (consistent). Unique mechanisms even work with bearings on a magnetic cushion, that is, the magnetic field plays the role of lubricant! But most often in the technique for lubrication of bearing units used mineral oil in a liquid state.
As grease, also widespread lubricants applied grease. Anyone who has experienced car or farm machinery operation is familiar with this type of lubricant.
With a high-temperature working environment, liquid or grease lubricants are difficult, almost impossible to keep in a bearing - they flow out. In this case, talc, mica, graphite and other types of solid lubricants come to the rescue.
Liquid lubricant is supplied to the working area of the bearing using special systems that operate in one of the following modes: individual or centralized lubrication, periodic or continuous lubricant supply, no forced supply pressure or work with such pressure. Periodic individual lubrication is provided by the nipples with a swivel cover; there are press grease fittings and cap grease nipples. Continuous individual lubrication requires the use of wick or drip nipples.
High efficiency of the bearing can be achieved by creating a system of hydrodynamic lubrication, when the shaft exposed to external forces rotates eccentrically in the liner, dragging the lubricant into the resulting gap. As a result of this, an oil wedge with hydrodynamic pressure is formed, which provides liquid friction type.
Types of friction
The thickness of the oil layer determines the mode of operation of the bearing: boundary, semi-dry, semi-liquid or liquid friction.
In the boundary or semi-dry mode, the thickness of the lubricant layer is so small that this layer loses its fluid properties. In semi-liquid 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 friction boundary mode, we apply a simplified calculation for the average pressure (p) or according to another criterion, the product (pv).
Liquid friction mode for sliding bearing operation is most favorable. It contributes to the achievement of high wear resistance. In this case, the calculation is based on the theory of lubrication (hydrodynamic aspect).
The conventional boundary between the types of friction is considered to be the Sommerfeld number:
[S0 ] \u0026 ndash; Sommerfeld number;
P \u0026 ndash; average bearing pressure;
Ψ \u0026 ndash; relative diametral clearance, the ratio of the actual size of the gap to the diameter of the mounting shaft size in the bearing;
μ \u0026 ndash; dynamic oil viscosity;
рад, rad / sec \u0026 ndash; angular velocity of the bearing.
These numbers are defined for different types and models of bearings and are located on the appropriate tables. Calculate the actual value of S0. then it is compared with tabular and the conclusion is made:
At s0 ≥ [S0 ] semi-liquid friction.
At s0 \u0026 lt; [S0 ] liquid friction.
The first calculation criterion requires satisfaction of the following equality:
where [p] \u0026 ndash; tabular standard value of the maximum permissible average pressure in the bearing;
P \u0026 ndash; calculated value of average pressure.
This check reflects the degree of wear resistance of the bearing.
Another criterion requiring the condition:
where v \u0026 ndash; sliding speed, m / s,
reflects thermal stress.
The values of p and pv do not reflect the influence of a number of important factors (surface quality, degree of wear, etc.) on the performance of the object of calculation, which forces the engineers to consider the calculation itself as approximate.
The values of [p] and [pv] are given in reference books, since they are average for different types of bearings.
The moment of friction force is calculated by the formula:
where f \u0026 ndash; tabular coefficient of friction, is selected taking into account the working conditions.
The calculation of heat dissipation is calculated as:
Here, the speed v is also selected according to the tables.
The speed at which the phenomenon of transition of friction to semi-liquid occurs, is determined by the Vogelpol formula:
where P \u0026 ndash; bearing load, N;
µ \u0026 ndash; oil viscosity (dynamic), N * s / m 2;
V = πd 2 l / 4 \u0026 ndash; working volume of the bearing, m 3;
s \u0026 ndash; constant coefficient assigned depending on the materials:
- cast iron \u0026 ndash; gray 1 \u0026 hellip; 2;
- for bronze and babbit, the values are 2 \u0026 3 and 2.5 \u0026 hellip; 4, respectively.
The upper values are for self-aligning bearings.
Expressing P through p, we transform the formula of Vogelpol:
Comparing the sliding velocity v, the friction coefficient f, the sliding velocity v2 friction coefficient f2. we give the formula of the coefficient of friction in the bearing:
The indicators without an index correspond to the design mode, index 1 belongs to the transition mode from the boundary friction process to the semi-liquid state, index 2 is assigned to the indicators adopted for the transition from semi-liquid to liquid friction. In turn, the friction coefficient f2 is determined by the Fold formula:
Calculation in the conditions of fluid friction
For this case, the calculation of the bearings is based on the Reynolds formula:
where µ \u0026 ndash; oil viscosity, N * s / m 2;
hm \u0026 ndash; the gap in the cross section, where there is a maximum pressure, mm;
h \u0026 ndash; the gap defined in an arbitrary section, mm;
v \u0026 ndash; speed, m / s.
For a plain bearing, this equation should be transformed using the polar coordinates:
where p \u0026 ndash; pressure in the bearing, is determined in an arbitrary section at an angle ϕ to the line of centers;
µ \u0026 ndash; viscosity (dynamic);
ψ = ∆ / d \u0026 ndash; relative clearance in the coupling of the shaft and bearing;
\u0026 chi; = e / δ; \u0026 ndash; relative eccentricity value;
δ \u0026 ndash; radial clearance.
From this equation the formula for determining the hydrodynamic load capacity is obtained.
where is fr \u0026 ndash; tension factor, dimensionless function, is determined by the tables.
Knowing that the viscosity of the oil is determined by the formula:
you can derive the equation:
Bearing friction coefficient:
where is the ratio ft / Fr determined by the tables of reference books.
The amount of heat is determined by the formula:
where d has a dimension in m; P \u0026 ndash; in H; \u0026 omega; \u0026 ndash; in glad./s.
For the thermal calculation of the bearing (and for the selection of the lubrication system) it is necessary to know the lubricant consumption. It is determined by measuring the liquid flowing out through the gaps at the ends of the zones - loaded and unloaded -. Even such measurements are made for oil squeezed out through the grooves intended for lubrication. Then, the obtained values are divided by the time during which the leaked lubricant is sampled.
For a second consumption is:
\u0026 ndash; dimensionless coefficient.
In this formula: q1 \u0026 ndash; tabular coefficient of lubricant consumption through the gaps in the ends of the loaded area;
\u0026 ndash; coefficient of lubricant consumption at the ends of the unloaded zone;
Here, \u0026 beta; \u0026 ndash; dimensionless coefficient, tabular value;
- pe \u0026 ndash; pressure in the forced lubrication system;
- coefficient taking into account the intensity of the oil outflow through the grooves for lubrication:
- θ \u0026 ndash; dimensionless coefficient, tabular value;
The sizes a and b are calculated by the formulas:
a ≈ 0.05 d + (3 × 5) mm;
b ≈ (0.20 ≤ 0.25) d
The heat balance equation for a bearing assembly is:
where W is determined by the formula above and marked by the sign (*).
Amount of heat transferred when the bearing is lubricated
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; lubricant temperature (index 1 at the inlet and index 2 at the outlet of the bearing).
The amount of heat discharged into the surrounding space by a massive bearing housing:
where k \u0026 ndash; Tabular heat transfer coefficient, W / m2 deg .; its average values are in the area of 9 \u0026 divide; 16 W / m 2 grad .;
F \u0026 ndash; air-washed bearing surface, m 2;
tM \u0026 ndash; average lubrication temperature 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, and therefore is set by the values of a certain average lubricant temperature. Because of this, the hydrodynamic calculation of the bearing has to be done by iteration (successive approximations).
The same iteration method determines the optimal values of ψ, oil viscosity µ. Solutions must satisfy the condition:
Thrust bearings, which are a kind of bearings that perceive axial load, work in a state of only boundary or semi-fluid friction.
The plant of sliding bearings, as a rule, is a specialized enterprise. Produced as ready-made units, yet separately liners. Plants also carry out repairs of damaged or worn bearings. As part of the capacity are mechanical and thermal shops, assembly lines, which often operate in automatic mode. Equipment is also available for spraying coatings on rubbing surfaces in order to increase wear resistance and reduce the coefficient of friction of the bearing assembly. Sliding bearings, the 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 their production technology is constantly being improved.