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Guide to Use Spherical Bearings

Guide to Use Spherical Bearings

Radial fixing of the bearings (coupling)

 

Between the spherical surface of the internal ring and that of the external ring a movement takes place which is defined as “sliding”, and based on this the quality, is proportional and the surface treatment of the product is done. Imprecise sliding movements on the shaft or in the seating can provoke damage and breakage to parts of the machinery. It is therefore important that the elements of the mounting structure are proportional to the of ring bearings.

In order to obtain a friction coefficient superior to that of the bearing, one can use coupling features, which permit maximum friction of the rings taking advantage of the total load capacity.

Should the need to protect the rings from eventual undesirable sliding movements arise, it is possible to use axial blocking devices.

 

The choice of coupling fixtures should be done with particular care, a contraction of the external ring provoked by the interference between itself and the housing could in fact occur causing a dilation of the internal ring due to interference between the shaft and the opening of the bearing. 

The elastic deformations mentioned above, can lead to a diminished clearance in the spherical joint. The bearing clearance and the osculation of the sliding surface must always maintain a reciprocal balance between them. The fol lowing tables ( table 2 and 3) l i s t recommended coupling values which are more suitable for obtaining the above mentioned balance.

 

 

The following tables list (table 4 and 5) the relative values expressed for possible shifting with regards to the housing opening and the shaft.

 

 

Quality of the shaft and the housing opening

 

The spherical plain bearings are normally subjected to loads which are much greater, for this reason the housing seats, the shafts and the plain bearings themselves must be configured so that the forces

which are transmitted do not cause permanent deformation such as to compromise their use (C/P < or = 1,2). The accuracies of the form of the seats are comprised in the tolerance of the different couplings indicated. As for the surface quality, the values for roughness reported and expressed in μbelow are valid.

 

Housing bore: from Rz 10 to Rz 16

Shaft: ≤ Rz 10

 

Coupling conditions

 

Dispersion factors can be found when assembling components with di erent tolerance " elds. During

the project phase it is difficult to calculate the theoretic values; one can none the less keep

the average values in mind of possible and real coupling conditions.

Modern technology consents a project to be constructed, (spherical plain bearings, shafts andseat) with and average tolerance which is always more precise. The table indicated below (table 6 and

7), are based on data on ISO housings with reference to the tolerance DIN, considering always that also the dimensions are in the average tolerances.

 

During the functioning and because of the loads, the spherical plain bearings are subjected to constant elastic deformations that provoke micro-movements between the coupling parts.

Independently of the coupling fixtures, when over exposed, a gradual axial movement of the rings of the bearing can be provoked; this can be resolved by blocking the rings on the axial.

The seat of the mobile supports must be projected in such a way that the axial movement occurs between the shaft and the bore of the bearing in which it is  mounted. In the case of the spherical plain bearings with external rings split axially, the same will be dilated from the axial force applied, which provokes the fixture in the seat of reference.

By using the security rings, the plain bearings can be fixed axially. Should it not be possible to get grooves on the shaft it is opportune to provide for the blockage of the bearing using rings or distance holders, which consent, if placed in line with bearing rings to act with axial pre-loading on the frontal surface of the rings of the bearings.

The spherical plain bearings can be considered in every respect, components of elevated precision. This is because from the experience of the ISB® producers, always along side the major constructors, has emerged in the majority of cases, that the mounting of spherical plain bearings which are not suitable can lead to machines which are stopped and or broken, often expensively. This is why the product must absolutely not be manipulated, cleaned with unsuitable liquids or even simply touched with naked hands. Remember, in fact, that the sweat from hands can be corrosive and create rusty spots on the surface of the rings, compromising as such the correct use. The precise application of the spherical plain bearing is the primary condition for good functioning. The sharp edges of the side of the shaft mounting and the housing should berounded o! an angle of between 10° and 20° which should allow appreciable results in terms of the life of the product (fig. 2).

 

In the presence of fixed coupling and particularly burdensome conditions, it is consented to lightly oil the surface of the shaft of the tree and of the housing seat. Remember that for maintenance-free supports, the mounting should take place without lubricating. When spherical plains are used with an external ring cut, it is indicated that the split is placed at 90° (fig.3) with respect to the principle load, making sure that the bores of lubricant are placed in the area of the load; this should distribute uniformly the lubricating liquid in the zone where the load is greatest. During the phase of mounting the spherical plain bearing the use of the appropriate tools is indicated, for example: a compass for mounting, hydraulic press and tub. It is always best to avoid direct hits on the frontal sides, as this could possibly cause minute damage and malfunctioning.

Should it not be possible to use the ordinary tools, it is possible to mount using heating and cooling techniques, but it is always advisable to seek assistance from ISB® producers because the incorrect application of these techniques could compromise definitively the use of the spherical plain bearing.

The dismounting phase of the spherical plain bearing can be extremely delicate thanks to the conformation of the product acting on the ring to dismount, the resistance due to the coupling of the other ring, creates a blocking effect. To overcome this problem, it would be already indicated, in the project phase to foresee on the seat of the bores threads for screws for dismounting or profiling on the shaft to use instruments for dismounting.

In order to obtain a good functioning  and correct wear of the spherical plain bearings it is fundamental to use protection from eventual external contaminating agents (dust, humidity etc…) applying an adequate amount. Different criteria exist for the choice of protection:

space, radial movement , bouncing angle, environmental conditions etc. A possible setting which is very simple and effective could be given from the layer of grease used for lubrication. With constant maintenance, this type of setting gives excellent results in particular functioning conditions.

In the case of internal applications, the common hold 2RS, consisting of polyurethane, mounted bilateral is sufficient . There are “special holds” which are adapted for applications in an

ambient with extreme conditions (up to +200 °C).

Externally it is possible to use a simple hold in elastic polyurethane, particularly indicated for the spherical radial bearings. It is also possible to use V rings, particularly indicated in the presence of significant bouncing movements; rings in plastic with steel armature and rings with antidust lips are supplementary.

In a general sense, the lubrication of the bearings is particularly important, because it leads to a reduction in friction, protects from corrosive external agents, and acts as a separator from the sliding surface.

The choice of lubricant to use is determined by different factors, for example: load, direction of the load, angle of oscillation, speed and ambient condition.

For standard applications, common lubricants that can be found on the commercial market that are anticorrosive, resistant to pressure and based on lithium soap with solid lubricants and EP added are indicated.

These solid additives, mixed with the lubricant, allow the perfect separation between the sliding surfaces, even in cases of elevated surface pressure, avoiding, in addition, breakage of the bearing.

Lubricants with about 3% of MoS2 are suitable, or solid additives containing calcium and zinc phosphate combined.

For spherical plain bearings with iron/bronze coupling, lubricants with a base of lithium soap, anticorrosive, hydrorepellant and of normal consistency, but without MoS2 or other solid lubricants are particularly indicated.

The initial phase of lubrication is very important; one must pay a lot of attention to the technical conditions in which one is working in order to obtain a uniform consumption and an optimal yield from a spherical plain bearing over time.

For spherical plain bearings with steel/steel couplings, a regular lubrication is required; through this activity, residual used lubricant is eliminated and substituted with new lubricant, abrasive residue impurities are expulsed. The frequency as well as the intervals of lubrication must be evaluated carefully confronting different factors, such as: load, speed, conditions of use etc. because a lubrication which is too frequent can lead to malfunctioning for the spherical plain bearing reducing its lifetime.

Maintenance-free spherical plain bearings must not be lubricated; which is why they are not supplied with relubrication devices. The running process is facilitated during the running period when the PTFE particles are transferred from the external ring to the surface of the internal ring, this smoothes the roughness that could be present of the surface of the internal ring.

If maintenance-free spherical plain bearings on the other hand were lubricated, this transfer and smoothing e ect would be eliminated considering the scarce adhesive capacity of PTFE particles on lubricated surfaces. All of the above mentioned concepts are valid for rod ends as well.

 

Friction

 

The friction phenomenon which is created when the 2 surfaces slide against each other, can depend

on 2 factors, for example: coupling, the load, the velocity, the lubrication conditions and the quality

of the material used in the surface of the 2 sliding surfaces. During the life of the spherical plain

bearing, the friction values are not always equal. In fact, in the initial and final phases the friction

values are higher while friction values on a well run spherical plain bearing are noticeably lower. It is

however a good idea, for security reasons, to always  use the highest values as a reference.

Reported below (table 8), it is possible to verify the reference coefficients of friction for spherical plain

bearings. Should the friction coefficients surpass the values indicated, greater wear and tear would be

generated, increasing the temperature and eventual inconvenience of the functioning.

 

 

The sliding coupling is an element which directly influences the temperature exercise level obtainable. In the following table (table 9), the temperature values one can reach with spherical plain bearings is reported for different executions.

 

high temperatures are reached, the life of the spherical plain bearing diminishes and proportionally, so does the efficiency of the seal. In cases where the application of spherical plain bearings (with seal) is not possible, spherical plain bearings followed by external seals or the use of lubricants, with shielding, that guarantee good thermal resistance would be indicated. If the heat should extend through the internal ring, this could lead to an expansion of the same, therefore necessitating the use of spherical plain bearings with wider clearances, for example C3. The types of materials with which the seals are constructed (table 10), guarantee different results in terms of thermal resistance, for example:

 

 

LOAD

 

The applicable load is surely one of the principle  elements which influence the duration and the

choice of the spherical plain bearing. Together with the load, other elements must be considered, such as the movement, the duration and the maintenance if foreseen. The choice of the dimension of the bearing varies when the load as well as the direction and sliding coupling vary. Based on all of the

above mentioned elements, the best choice can be carried out on which spherical plain bearing to use. Once the load value is determined, determine the theoretic duration, knowing that the load acts in radial sense for radial spherical plain bearings and in axial sense for axial spherical plain bearings and that during the functioning, the direction and entity remain unvaried.

It i s possible the bearings are sollicited contemporaneously in both radial and axial direction, it is therefore a good idea to indicate P value in the duration formula which will be calculated as follows:

 

P = “X” • Fr

P = “Y” • Fa

considering,

• P: dynamic load kN equivalent

• Fr: radial load kN

• Fa: axial load kN

• “X”: axial load movement factor referred to radial

spherical plain bearings

• “Y”: radial load movement factor referring to axial

spherical plain bearings

 

DINAMIC  LOAD C 

The “C dynamic load” is defined the load applied on  spherical plain bearings or heads subjected to dynamic solicitations, in the presence as such of oscillating movements of bouncing or rotation. The action of the load will be in a radial sense for radial spherical plain bearings and at angular contact for the rod ends, while purely axial and centered for axial spherical plain bearings. Every movement on the sliding surface generates wear and tear and this should be kept in mind. The indications of the load coefficients are often tied to the producer and as such could become di" cult to make comparisons with other products of the same category.

 

The “C0 static load” is defined as the load maximum  applicable on the spherical plain bearings or rod ends in the presence of:

• small settling movements

• added loads dues to shocks

• static situations

At ambient temperature the static load must not  influence the functioning of the product in so as

not to provoke breakage or damage on the sliding surface. To obtain a good duration of the exercise,

it is fundamental that the load is adapted to the functioning conditions. The static load, determines

the surface pressure on the spherical plain bearing. In order to determine such pressure, one can adopt the following formula:

 

 

MOVEMENT OF THE SPHERICAL PLAIN BEARINGS

 

The spherical plain bearings have been projected  to resist heavy loads with oscillating, bouncing and rotating type movements. The oscillating movement is translated as movement of the two rings of the bearing, one with respect to the other around the axial of the bearing itself. The width of this movement is expressed by the angle of oscillation B. The bouncing movement, foresees the internal ring moves in a transversal sense  with respect to the external ring. The value B, that is the bouncing angle express the separation of the axes relative to the rings. In the rotating movement the angle B= 180° so that the back and forth movement corresponds to about double the angle Band A.  The principal movements are visible from (fig. 4) and according to the type and the number directly influence the duration of the spherical plain bearing.