Load/RPM sliding bearings
The fact boxes about the sliding bearings’ different values describes what the material itself can handle. “Max static load” describes what the bearing can handle in static (no motion). At motion, the max load decreases. When calculating load and motion, a so-called PV-value is used. The PV-value is calculated like this: N/mm2xm/sec. To calculate the load area in a sliding bearing, you calculate the projected bearing area.
Guide for bushings
Calculation examples for bearing area:
BK1 sliding bearing BK1 2020 has an inner diameter of 20 mm, length 20 mm. Calculated 2×2 cm = Projected bearing area 4 cm2.
As e.g. plain bearing BK1 (steel/PTFE) can take a maximum static load of 250N/mm2 (2500 kg/cm2), it gives a maximum static load: 2500 kg x 4 dm2= 10,000 kg/slide bearing.
This is the product of specifik load P and sliding velocity V. The PV-value is important when choosing a sliding bearing as well as for the bearings life span. The longer life span desired, the lower PV-value. Different types of loads affect the life span of a sliding bearing, e.g. rotation, pendulous or axial motion. A high ambient temperature reduces the loadability and allowed PV-value.
Choosing a shaft
For the wear to occur on the “right” place, the shaft hardness must be greater than the material on the sliding surface. The surface finish is also of great importance for the life span of sliding bearings. The finer the surface finish is on the shaft, the lower friction and therefor longer life span on the sliding bearing. Friction adds heat which contributes to increased wear.
Installation of sliding bearings
Installation with a suitible mandrel is appropriate. A sliding bearing must always be press fitted according to the prescribed tolerances of the bearing seat. It is very rare that a sliding bearing need to be glued. Plastic bearings are often somewhat oversized when not installed, but is pressed togheter both externally and internally when installed in a recommended housing. Slotted sliding bearings are installed with the slit outside the load area. The slit is eliminated when the bearing is installed in its seat.
Lubrication is a way of decreasing the friction between two materials. Different sliding bearings require different lubrication intervals and have different lubrication requirements. Plastic bushings often require an initial lubrication, but regular lubrication increase the life span considerably. Bronze bushings like FB090 and lathed bronze bearings can be lubricated with grease or oil. Grease is used for periodic lubrication, while oil should be used in continuous lubrication. Regular lubrication also prevents corrosion. Lubrication free sliding bearings BK1 must not be lubricated, doing so can result in particles from the PFTE surface mixes with fat accumulations which in turn can result in abnormaly high wear to bearings and shafts. Sliding bearings with internal acetal coating is called “low lubricating” and only requirce an inital lubrication. We recommend regular relubrication to increase the life span.
Choosing a sliding bearing
This require knowledge about load, rpm, type of motion, lubrication requirements, material requirements etc. Sometimes its only about stabilizing a shaft and eliminate noise.
Stainless or not
Firstly, you should think about if you need a stainless bearing or not. Already, some options are out. If it is a diry environment with high loads, a greasable bronze bushing (the grease push out the dirt) is recommended. If a lubrication free bearing is chosen, it should be sealed. As a greasable bushing, FB090 with grease pockets is a good choice. FB090 is often used in construction and forestry machines where moist and though environments are common.
Some bearing points are exposed to greater forces than others in construction and forestry machines. At high dynamic strains on e.g. a bucket, a harder bearing material is required. In these cases, hardened steel bushings CX or BGFR is chosen. These require a harder shaft than normal and continous lubrication.
Small forces/high rpm
When the forces are small and the rpm is higher than 2 m/sec, you should take a look at sintered bronze options. Sintered bronze bearings is a highly cost efficient bearing that is uses as an “allround bushing”. This bushing contains an oil that benefits the shaft at sliding velocities from 10 m/min (0,2/sec). At this velocity, sintered bronze bushings have their greatest loadability. They are used as axial and radially moving bearings. These bearings can be post processed to desired dimensions, but the following need to be considerd; tools used, cutting velocity and tip radius. If the post processing is done incorrectly, there is a chance that the oil-suppling pores gets obstructed and cannot lubricate the shaft.
Plastic/nylon bearings is a cheap and simple type of bearing, however with precision-adaptation like the other bearings. At small forces, low rpm and when anti-corrosion is a requirement, is this a good choice. Nylon (polyamide) absorb relatively much moisture (8-10%). At high humidity this should be considered when choosing the shaft tolerance. Polyamide bushings are lubrication free, but wear and friction is reduced at initial and regular lubrication. If the bearing is placed in a high-humidity environment and the shaft tolerance is specific, we have polyamide bearings with added molybdenum disulphide grease. The benefits of this is; insignificant moisture absorbtion and lower friction (longer life span).
Slotted steel bushing with PTFE
BK1 is the top seller which is used extensively. It is a space-saving and lubrication free bearing and manage relatively high loads, rpm and temperatures. However, the rpm should be kept at a margin because the life span decreases by high friction heat against the PTFE surface. These sliding bearings are made out of rolled metal strip with a sintered bronze layer (0,25-0,35 mm) and coated with a PTFE+Pb layer (0,01-0,03 mm), the outside is zinc plated. We have frames made of bronze and stainless material if a more corrosion resistant material is required.
Slotted steel bushing with acetal layer
BK2 is manufactured as an alternative to the lubrication free variant mentioned above. BK2 is often used in constructions where lubriation is a requirement due to contamination and corrosion protection. German manufactures call this sliding bearing as “Wartungsam”, which means low lubrication. It is a cheaper alternative t0 bronze bushing FB090, but a possible disadvantage with a steel frame is if the construction is located outdoor.
This information text can be made twice the lenght with more details about what to consider, but we chose to briefly describe our different alternatives. We will gladly send you construction information if you request it. Your are welcome to contact us for free consultation regarding which sliding bearing that fits your construction.