Technical Information

Bearing Load ratings are bearing specific data, derived from the characteristics of the materials used. They are used when selecting Spherical Plain Bearings or Rod Ends for a particular load, but may have to be reduced in adverse operating conditions.

Static Load Rating C_o [kN]

C_o indicates the maximum permissible static load which a Rod End at its weakest cross section can withstand without developing permanent distortion. The C_o values listed in the tables of this brochure have been calcula ted by using the appropiate material specifications and have been tested on a number of Rod Ends during tensile tests carried out at ambient temperature. 80% of the yield strength resulting from the tests have been used so that a safety factor of 1.25 is included.

The static load C_o is also used for establishing the maximum axial load which is limited by an additional bending stress principally due to the method of faste ning of the insert. Following are maximum axial values (deformation) which have been established by pressure testing:

Dynamic Load rating C [kN]

This rating is used to establish the working life of Spherical Plain Bearings or Rod Ends when under dynamic load conditions. That is to say when they oscil late, rotate or pivot under load. The values listed in the table result from multiplying the maximum surface pres sure pmax admissible in gliding movements by the pro jected bearing surface. Aproj, whereby a specific load tating is established for each type of Rod End. The established standard values for maximum surface load for various combinations of antio friction material have been listed in table 1 which allows for movement when oscillation. Information: Depending on the material strength of the Rod End housing (eg. pages 34 and 35) the static load might be lower than the dynamic load. For this the procedure stated on page 23 has to be observed.

For applications with threshold or/and alternating loads, the dynamic load rating of the rod end housing needs to be considered separately.

Table 1: Maximum surface pressures

Abbreviations: St = Steel, Ms = Brass, Bz = Bronze, TBz = Woven Bronze Fabric, TNy = Woven Nylon

The loads affecting a Spherical Plain Bearing can vary.

They can be:

• intermittent, constant or variable (illustration 1)
• static or dynamic

Attention: For Rod Ends with male thread factors choose fB = 0,35 when load changeable.

Forces when under static load

Radial only (F_r) or radial and axial (F_a) forces arise and there is no movement between the ball and the insert

Forces when under dynamic load

Radial or radial and axial forces arise, when the Ball pivots at angle a, oscillates at angle ß or rotates relative to the Insert.

In the case of a constant load F_r, F_a a dynamically equivalent bearing load Fe can be established in accordance with formula (2).

The axial factor Y in table 2 is dependent on the load ratio.

In the case of a variable load (picture 4), formula (4) can be used to calculate a mean dynamic bearing load F_m from the individual load levels F_i and the appropriate time factor t_i.

The selection is usually made step by step, repeated if necessary, by comparing:

1. the load ratio involved with the normal minimum values for that ratio;
2. the forces affecting the bearing and the maximum permitted load of the bearing proposed;
3. the maximum surface pressure and the surface pressure on the proposed bearings;
4. the maximum glide speed and the glide speed involved of the bearing proposed;
5. the specific performance of the bearing involved with the published catalogue limits.

Re 1:

he load ratio (C/F) is a value for a specific use of a bearing according to formula (5).:

(5) (C/F)_exist ≤ (C/F)_min

The common minimum values for (C/F) for different antifriction surfaces as listed in table 3, can be used to establish the required dynamic load rating C in accordance with formula (5a) by changing formula (5). By this means a suitable bearing size can be selected from the tables of this catalogue..

(5a) C_reg≥ (C/F)_min * F_vorh [kN]

Re 2:

When the existing force affecting the bearing is a static load, it can be used as is for a comparison. When it is a dynamic load, it can be calculated by using formula (2), (3) or (4).

When a Rod End is mounted with a locking nut or retransfer with two nuts, the additional tensile stress at the male thread or the connecting rod has to be taken into consideration.

However the static or dynamic load must always be smaller than the maximum permitted load, which is calculated from the static load rating C_o using formula (6).This might have to be further reduced by the load factor f_B (picture1) and the temperature factor f_T (table4).

(6) F_{r, max} = C_O × f_B × f_T [kN]

(6a) F_{a, max} = a × F_{r, max}

If no bearing size is given in the application the required static load rating can be established by changing formula (6) and a Rod End can be selected from the tables accordingly.

(7) C_{o, req} ≥ F_exist / (f_B · f_T) [kN]

Re 3:

The load on a mating surface can be worked out by using formula (8). It must be less than the standard value for surface load according to the antifriction combination of materials, selected as listed in table (1).

(8) p_exist = p_max / (C/F)_exist [N/mm²]

Re 4:

The existing average glide speed v_m is calculated according to formula (9) using the frequency of rotation of the crank K and the glide distance of the Spherical Plain Bearing G. (At one rotation of K it corresponds to the double arc b between the centres 1 and 2 in Picture 5 and thus to the double maximum oscillating angle ß).

(9) V_{m, exist} = 2 × b × f = (d_k × ß × f) / (1000 × 57.3 × 60) [m/s]

Diameter of ball d_k [mm] and f [1/min]

In case where the bearing rotates fully ß needs to be substituted by 180°. The slip speed has to be less than the speed permissible listed in table 5.

Re 5:

The product p.v can be defined as a specific bearing performance PL (see formula 10). Thus, an estimated value for the heat build-up per mm2 of the Spherical Plain Bearing surface ist available, mainly dependent on the antifriction material combination, the lubrication/cooling applied and the surface pressure and glide speed. By increasing temperate the allowable surface

pressure of maintenance free bearings is decreasing (picture 1 and 4).

(10) P_{L, exist} = p_exist × V_exist

[(N·m)/(mm²·s) = W/mm²]

After the selection of the bearing the following is valid:

In the case of a static load it is not necessary to calculate the working life. The permissible limit set at 80% of the breaking point allows the forces to act indefinitely.

In the case of dynamic loads calculating the bearing life is problematic. There are many, sometimes interdependent influences, that cannot always be taken into consideration.Therefore, a calculation of the bearing life can only be approximate. As an approximation the bearing has an increased life proportional to its load rating and also when used at a moderate speed.

Additional influences can be taken into account by making use of the factors in formula (11).

(11) G_h ≈ 3 × f_L × f_T × f_G × f_V × \left( \frac{(C/F)}{V_m} \right )_exist

f_L = Direction of load to table 7

f_T = Temperature factor to table 4

f_G = Glide factor to table 8

f_V = Relubrication factor to table 9

C/F = Load ratio

V_m = Mean glide speed [m/s]

The direction of load factor indicates whether the direction of load is uni-directional, constant, variable or oscillating.

The slip factor f_G takes into account the materials used on the mating surfaces of a bearing. As a result the only distinction that can be made is between being maintenance free (not lubricated) and where lubrication is necessary.

The relubrication factor fv takes into account the extension of the bearing life Gh when regularly lubricated. The greater the surface pressure pexist the more often the bearing has to be relubricated. If the bearing is only lubricated on commissioning as in the case of bearings with PTFE liners, f_v = 1 has be inserted.

Lubrication intervals are dependent on load conditions and therefore have to be set by the operator.