Details for sliding bearings
In relation to radially-moved bearings, the running smoothness of the sliding partners is ensured by the bearing free play between shaft and bearing box.
In relation to axially-moved bearings, the bearing free play is determined by two axial areas assigned to each other by a mutually shared shaft. The sum of the opposed sliding area and distances determines the bearing free play.
State-of-the-art constructions place ever bigger demands on common sliding materials. Many customers expect a maintenance-free bearing, even if operating conditions are complicated.
Furthermore, constantly rising pressure to maintain low production costs demands an enduring availability of our products, as to ensure the allowance of the continuous use of the engines and plants deployed, as such, any compromise on the reliability of our products is unacceptable.
As a result of the differing modes of utilizing sliding bearings, even the most complex aims can be achieved.
Due to their material and the damping effect of the hydrofoil, some sliding bearings are relatively insensitive to impacts, vibrations and shocks. Sliding bearings run silently, are robust, and are to a great extent corrosion resistant and further, rarely need additional sealing.
Bushings are also available in split versions, which serve their requisite purpose in certain constructions.
For a few types of sliding bearings, a heavy starting torque is inevitable.
None-maintenance-free sliding bearings require constant and sufficient maintenance and lubricant supply.
The degree of the effectiveness of sliding bearings is generally lower than that of antifriction bearings.
PV – value
The pv-value has a substantial impact on the length of use. The pv-value is the product of specific bearing load (p) and speed (v).
The length of use decreases with rising pv-value.
The friction-value depends on the following impacts:
Choice of material matching
Surface roughness of counter surfaces
Specific bearing load
Type of lubrication
For low rotational frequency, for pivoting and axial movement, for shock loading and dirt strain.
For all-purpose applications, for agricultural engines, construction machinery and vehicle construction.
For applications calling for simple design and low price level.
For high- or low-temperature environments and for special corrosion resistance.
For long-time applications requiring long durability, e.g. water and steam turbines, generators, rotary pumps, heavy journal bearings and the like, also for applications requiring wear-resistant running as is the case in the liquid friction sector.
The major features of the appropriate sliding bearings can be seen from our selection list.
Abrasion and length of use of sliding bearings depend on:
- Specific bearing load
- Sliding speed
- Material and surface finish of sliding partners
- Load zone allocation
- Operation period
Bronzed, stainless steel and plastic sliding bearings are corrosion resistant. Steel and compound sliding bearings should be stored in dry environment at low air humidity and should be taken out of package only immediately before mounting.
For production reasons, wrapped bearings show a certain ovality and an open butt joint, as quoted in the valid standards.
The perimeter of those bearings is measured in such way that they show sufficient interference fit and roundness after mounting into a housing drill.
Basically, sliding bearings should be moulded with the help of a mandrel. Should a wrapped bearing’s perimeter exceed 50 mm, it is advisable to use an additional mounting ring.
To simplify the mounting of a sliding bearing, a bevel of 15° – 30° at the housing is essential.
Flanged bushings should be provided with an additional bevel of 1.0 x 45° (bigger bushings 1.5 to 2.0 x 45°) so as to make sure the flange bears completely and planar on the housing’s surface.
To ensure proper mounting of the bolt, the shaft extensions must be bevelled and chamfered. While assembling the bearing, sharp angles can cause damage to the sliding surface and degrade the operation of the bearing.
Metallic or plastic bushings, thrust washers, strips or special parts can not only be affixed by bolting together, pinning or soldering but also by bonding. This does not apply to oil-soaked sinter bearings.
To choose the ideal lubricant, it is advisable to talk to an adequate manufacturer.
Besides press-fit assembly, our sliding bearings, thrust washers and strips can be fixed by means of screwing, pinning or glueing.
Especially our compound plastic bearings are often additionally secured by glueing into the housing. This is applicable if the bearing is confronted with high
- Oil lubrication
Oil lubrication is preferable for high rotational frequency and strains, but also applicable for low rotation speed. The type of lubrication oil depends on the particular application. By increasing the adhesion and smoothing of the sliding surfaces, additives of molybdenum sulfide or graphite can improve the lubrication properties.
- Grease lubrication
Grease lubrication is to be applied in case of low rotational frequency, pendulousness and impact loading, or if float friction is not within reach. High-class sliding bearing fat should be used exclusively. Lubrications with solid additives exceeding a proportion of 2 % are not recommendable, as they are likely to cause wastage ahead of time.
- Dry-film lubrication
Dry-film lubricant like molybdenum sulfide (MoS2) or graphite come to use at high temperatures, dry-running operation or one-time lubrication.
- Dry bearings without lubrication
These bearings mostly consist of compound material with PTFE or graphite bronze coating. Massive bronzed bushings with solid-lubricant pockets in their slide are as well maintenance-free. Full-plastic sliding bearings increasingly come to use in environments requiring high-class dry bearings. Limits of application of dry bearings are usually set by specific heat conductivity and heat strain.
Cylindrical and flanged bushings are normally mounted with a force-fitting mandrel.
To ease mounting, we recommend to lubricate the bearing carrier face with grease or oil.
When mounting bigger wrapped bushings, a mounting ring is recommended, whereby the bushing gets pre-centred. Thus, canting is prevented when force-fitting.
The butt joint at wrapped sliding bearings which are exposed to high strains has to be aligned 90° opposite to the working resistance direction.
In the running in process, lubricant or parts of the sliding layer are transferred to the counter surface. Thereby, surface roughness on the contact surface are filled and smoothed out. Herefrom results a sliding pairing, which shows a low friction resistance. Not until this procedure has been finished, can a positive sliding behavior then be developed.