Design and safety guidelines

The chapter Technical principles brings together the essential guidelines on the subjects of operating clearance, the adjacent construction and mounting and dismounting.

Friction

The friction behaviour is dependent on the sliding contact surface and changes during the operating life. Calculation of the bearing frictional torque and the typical coefficients of friction are given in the chapter Technical principles.

Principles of lubrication

Spherical plain bearings with a steel/steel sliding contact surface requiring maintenance are given a special surface treatment and are provided with MoS₂. Nevertheless, the quality of maintenance has a considerable influence on the function and wear of spherical plain bearings.

Functions of the lubricant

The functions of the lubricant are as follows, ➤ Figure :

• A lubricant film is formed on the contact surfaces that is sufficiently capable of supporting loads and thus preventing wear and premature fatigue

  

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• When grease lubrication is used, the bearing is additionally sealed against the entry of both solid and fluid contaminants

  

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• It gives protection against corrosion

  

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Functions of the lubricant

Formation of a lubricant film capable of supporting loads ·

Sealing of the bearing against external contaminants in the case of grease lubrication ·

Protection against corrosion

Selection of lubricant

The lubricant is selected as a function of the sliding contact surface in the bearing.

The following criteria must be considered:

• the load
• the load direction
• the swivel angle
• the sliding velocity
• the ambient temperature
• the environmental conditions.

Steel/steel sliding contact surface

For standard applications, conventional, corrosion-inhibiting, high-pressure greases with a lithium soap base, EP additives and solid lubricant additives are suitable.

The composition of suitable lubricants is as follows:

• a proportion of approx. 3% MoS₂
• solid additives based on calcium and zinc phosphate compounds. Even under high contact pressure, these additives separate the sliding surfaces from each other.

Steel/bronze sliding contact surface

Conventional, corrosion-inhibiting, water-repellent lithium soap greases of normal consistency are suitable.

Running-in phase

The running-in phase has a significant influence on the later wear behaviour of the bearing. Correct lubrication is therefore of particular importance at this point.

During running-in, the surfaces of the contact zones undergo smoothing and elastic moulding. This gives additional load-bearing areas and reduces the strain on the material.

Guidelines on greasing

During the running-in phase, the pressure in the bearing is particularly high. The spherical plain bearings are therefore coated with Durotect M and treated with molybdenum disulphide MoS₂. The running-in wear phase proceeds all the more favourably the more MoS₂ is embedded in the porous-crystalline Durotect M coating.

This process is at its most effective with the following sequence:

• The bearing is subjected to about ten swivel motions under load without additional greasing.
• The bearing is then given its initial greasing.

If this is not possible, the initial greasing must be metered carefully in order to avoid flushing an excessive quantity of MoS₂ out of the bearing.

Relubrication

During relubrication, old grease is replaced by fresh grease. At the same time, the grease flushes wear debris and contaminants out of the bearing.

Relubrication conditions

The grease used for relubrication must be the same as that used in initial greasing.

If other greases are used, the miscibility and compatibility of the greases must be checked.

The following conditions should be used for relubrication:

• The bearing should still be warm from operation.
• It should always be carried out before the bearing comes to rest.
• Relubrication should continue until a fresh collar of grease appears at the seal gaps. Old grease must be allowed to leave the bearing unhindered.

Design of bearing arrangements

In the design of the adjacent construction, the guidelines in the chapter Technical principles must be observed.

The housing locating washer of an axial spherical plain bearing is exposed to an expansion force under load, which leads to tangential stresses and thus to an expansion of the diameter of the housing locating washer. This reduces the load carrying capacity of the bearing. An adequately dimensioned, closed adjacent construction partially counteracts this situation and enables full utilisation of the load carrying capacity of the axial spherical plain bearing. However, if this requirement is not met, the maximum static axial load must remain limited to 1,2  ·  C.

Predimensioning

In the case of spherical plain bearings requiring maintenance, predimensioning can be carried out.

Dimensioning and rating life

The dimensioning of spherical plain bearings requiring maintenance is brought together in the chapter Technical principles.

Depending on whether the bearing is subjected to dynamic or static load, the following must be checked:

• static load safety factor S₀
• maximum permissible specific bearing load p
• maximum permissible sliding velocity v
• maximum specific frictional energy pv.

Calculation example for radial spherical plain bearing GE50-DO

The rating life of the radial spherical plain bearing is calculated on the basis of the steel/steel sliding contact surface.

Given data

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

• pivot of a linkage rod
• axial and radial alternating load.

Operating parameters

Bearing load	Fr	=	25 000 N
	Fa	=	5 000 N
Swivel angle	β	=	35°
Swivel frequency	f	=	6 min–1
Relubrication interval	lW	=	16 h
Operating temperature	ϑmin	=	–20 °C
	ϑmax	=	+60 °C

Bearing data

Radial spherical plain bearing			=	GE50-DO
	Basic dynamic load rating	Cr	=	157 000 N
	Sphere diameter	dK	=	66 mm
	Sliding contact surface	Steel/steel

Required

• Bearing with the required rating life L_h ≧ 10 000 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 = 5 000 N / 25 000 N = 0,2:

Specific bearing load

The specific bearing load must be calculated with the aid of the specific load parameter K:

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:

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 plain bearings requiring maintenance, the following applies:

Correction factors, as a function of bearing type

The correction factors necessary for the steel/steel sliding contact surface 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 = 30.

Correction factors

Rating life L_h

The rating life is calculated as follows:

Rating life L_hN with periodic relubrication

The rating life can be increased by means of periodic relubrication as a function of the relubrication interval.

The necessary relubrication interval and preferred relubrication interval must be calculated and checked for validity:

Based on a relubrication frequency L_h/l_W = 467 h/16 h = 29,19, a correction factor f_NH = 4,7 is determined. Based on a swivel angle β = 35°, the correction factor f_Nβ = 5,6.

Result

The selected radial spherical plain bearing GE50-DO fulfils the requirement for a rating life L_h ≧ 10 000 h.