Design and safety guidelines

The chapter Technical principles brings together the essential guidelines on the subjects of friction, rating life and the adjacent construction.

Dimensioning

In the case of rod ends, the permissible static load of the housing and shank must always be checked. In calculation of the rating life of the rod end, the decisive factor is the rating life of the spherical plain bearing fitted in the housing.

The direction of load and the type of load determine the design of the rod end and the sliding contact surface of the spherical plain bearing.

Permissible loading of rod ends

The permissible load depends on the type of load.

Pulsating or alternating loads subject the material of the rod ends to greater stress than constant loads. For calculation, these operating conditions must be taken into consideration by means of the load factor f_b , see table.

Loading of rod end

The following applies:

The permissible load on the rod end P_per is calculated as follows:

Load factors

Basic static load rating

In this case, the basic static load rating C_0r indicates the load carrying capacity of the housing and shank under static tensile load. At room temperature, it includes a safety factor of at least 1,2 in relation to the yield strength of the material used for the housing and shank. The basic load rating describes the maximum permissible static tensile load when utilising 83% of the material yield strength at the most highly stressed cross-section.

Basic dynamic load rating

The basic dynamic load rating C_r refers to the spherical plain bearing fitted and is a parameter for calculating the bearing rating life. It is dependent on the sliding contact surface and has a significant influence on the rating life of the rod ends.

Calculation example for radial rod end GAR25-UK

The rating life of the radial rod end is calculated on the basis of the sliding layer PTFE composite .

Given data

The given data for calculation of the rating life are as follows:

• linkage bearing arrangement of a transfer unit
• unilateral combined load.

Operating parameters

Bearing load	Fr	=	16 000 N
	Fa	=	1 200 N
Swivel angle	β	=	27°
Swivel frequency	f	=	12 min–1
Operating temperature	ϑ	=	+100 °C

Bearing data

Radial rod end			=	GAR25-UK
	Basic dynamic load rating	Cr	=	51 100 N
	Basic static load rating of housing	C0r	=	105 000 N
	Sphere diameter	dK	=	35,5 mm
	Sliding material	PTFE composite

Required

• Bearing with the required rating life L_h ≧ 6 000 h.

Checking of permissible loads

Combined load

For calculation of the combined load, the factor X must be determined from the diagram for radial spherical plain bearings using the ratio F_a/F_r = 1 200 N / 16 000 N = 0,075:

Specific bearing load

The specific bearing load must be calculated with the aid of the specific load parameter K and checked for validity:

Sliding velocity in swivel motion

The sliding velocity must be calculated with the aid of the sphere diameter d_K and the swivel angle β and checked for validity:

Specific frictional energy pv

The specific frictional energy pv must be checked for validity:

Permissible load on the rod end

The load carrying capacity of the rod end must be considered separately from that of the spherical plain bearing. Calculation of the load carrying capacity required should be carried out using the load factor f_b, see table.

The rod end GAR25-UK with a housing having a basic load rating C_0r = 105 000 N is suitable.

Determining the rating life equation

For calculation of the rating life, the valid rating life equation must be selected and then subjected to correction.

Selection of the valid rating life equation

For maintenance-free plain bearings, the following applies:

Correction factors, as a function of bearing type

The correction factors necessary for the plain bearing material PTFE composite  must be selected from the matrix and used to correct the rating life equation appropriately.

Rating life equation following correction

Calculation of rating life

The values for the correction factors in the corrected rating life equation must be taken from the diagrams. The specific plain bearing factor K_L = 1 000.

Correction factors

Rating life L_h

The rating life is calculated as follows:

Result

The selected rod end GAR25-UK fulfils the requirement for a rating life L_h ≧ 6 000 h.