1. INTRODUCTION

The rod ends and spherical bearings are mechanical junctions which, because of their internal geometry, are particularly suited for the transmission of static and dynamic forces, together with rotary, oscillatory and tipping movements. They are standard products which are produced in accordance with ISO 12240-4 Series K and ISO 12240-1 Series K respectively. The project takes into account the ever more stringent requirements of the market which calls for a reduced clearance, remaining constant with time, and completely maintenance free operation.

2. CHOICE OF ROD ENDS AND SPHERICAL BEARINGS

IIn order to make the appropriate choice of rod ends or spherical bearings it is important to know what their real application will be. The type of load applied, static or dynamic, will be decisive for the correct dimensioning of the rod end or the spherical bearing.

Static Load

The static load means the maximum radial or axial load applied to the rod ends or spherical bearings in a static way, without oscillations or rotations, at room temperature, that does not cause permanent deformations or does not endanger its functionality. The values of the maximum acceptable load were obtained by calculations and then checked on considerable samplings taken from our mass production.

Radial Static Load

The maximum acceptable radial static load for rod ends and spherical bearings is the one indicated on the dimensional table.

Axial Static Load

For the rod ends the maximum axial static load should not exceed the axial holding load of the joint on the head (table 1). Furthermore, it should be calculated basing on the length of engagement of the thread for the male type and on the male resistance connected to the female type. For the spherical bearings the maximum acceptable axial static load is the one indicated in the dimensional tables.

Axial holding load of the rod end

In case of dynamic load (load with relative movement between the inner and outer ring) it is necessary to carry out the following tests

a) Permissible radial load on the rod end

According to the type of load, it is indispensable to check the permissible stress. Considering the same nominal load, dynamic loads produce greater stress on the rod end than constant loads; it is therefore necessary to introduce the coefficient Kf in the calculation of the permissible radial stress:

b) Permissible axial load on the rod end

Normally there are no dynamic axial loads, so the conditions to be observed are the ones given in the paragraph on static axial loads on page 9.

c) Equivalent load allowed

If both types of load, axial and radial, are applied to rod ends or spherical bearings, the equivalent load P will be calculated by the formula:

d) Specific pressure p allowed:

The specific pressure is the one detected between the inner ring and the surface on which it slides. Check that the specific pressure allowed is included within the limits indicated on table 2

e) Sliding speed allowed:

The average sliding speed is the one detected between the inner ring and the surface on which it slides. Check that the sliding speed allowed is included within the limits indicated on table 2

f) Permissible pv factor:

Check that the permissible sliding speed falls within the limits indicated in table 2. It is important to point out that the above safety coefficients have been calculated on the basis of the standard use of the units. Where heavier use is made (with strong pulsating loads or compound stresses) or where the personal safety is at risk, it is essential to employ greater safety factors to reduce stress and where necessary practical tests corresponding to the real use should be carried out, in order to analyse the actual durability. In case of these special applications we can take no responsibility for the product since we are unable to foresee the actual use to which it will be put.

g) Operating temperature

The operating temperatures of rod ends and spherical bearings are the following ones: Remark: in case of high temperatures, the life of the rod end/spherical bearing will be reduced.

3. CRITERIA OF USE

Rod ends and spherical bearings are products, that do not need any kind of maintenance before or during operation. As precision components they maintain their performances unchanged only if the following criteria are observed: › keep them in their original packaging and stored them in a suitable place; › during assembly avoid that foreign bodies might get between the inner and the outer ring; › do not stress the outer ring during assembly/disassembly and do not put it in contact with any other parts outside the joint itself during working; › avoid shocks or damages during assembly.

Use of spherical bearings

It is essential that the assembly is made by following precise rules: › To make easier the assembly, the end of the shaft and the one of the housing should present a chamfer with an angle between 10° and 20°; › It is recommended to assemble the joint with a suitable bush (or tube) which sticks to the entire surface of the outer ring (fig.1); however, direct blows to the spherical bearings should be avoided. › In assembling the housing and the shaft, it is of primary importance to pay careful attention to the precision of the tolerances and to errors of form. The chamfers of the shaft r1 and of the housing r2 should be inferior to those ones of the joint indicated on the dimensional tables (r1s and r2s respectively) (fig.1). The choice of the coupling should not be made by chance , since a too much high interference may cancel the radial play between the outer and the inner ring, considerably increasing the friction and so compromising the spherical bearing’s life. On the other hand, a loose coupling would cause a deformation of the outer ring, reducing its duration.
Therefore, the tolerances of the below mentioned table should be strictly followed:

4. TOLERANCES

External Support

h, h1 height from the flat side of the shank to the centre of the hole in the inner ring
d3 screw thread in the shank

Outer ring of the rod end

D nominal external diameter
∆dmp deviation of the medium diameter of the hole from the nominal value
C nominal width
∆cs variation of the nominal width C

Inner ring

d nominal diameter of the hole
B nominal width
∆dmp deviation of the medium diameter of the hole from the nominal value
∆B variation in the nominal width B

Rod ends DIN ISO 12240-4 K Series

Rod ends similar to DIN ISO 12240-4 K Series

Other tolerances according to DIN ISO 12240-1 K Series

Spherical bearings DIN ISO 12240-1 K Series

Spherical bearings similar to DIN ISO 12240-1 K Series

Other tolerances according to DIN ISO 12240-1 K Series

5. CLEARANCE OF THE JOINT

The clearance of the joint indicates both the radial and axial movement of the inner ring as regards the outer ring. It is measured by applying a load of ± 10 daN (see picture)

RADIAL CLEARANCE
Rod end

Spherical bearing

NOTE WELL: the axial clearance is aprox. 3-5 times the radial clearance The radial clearance of the spherical bearing may vary according to the interference of the spherical bearing into its body. Therefore it is assembled with an oversize clearance so as to obtain the correct one.

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