Tuesday, May 24, 2011

The various forms of Sliding Bearings

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The application of PTFE in load bearings is not new. Amongst its many other attributes, PTFE also has an excellent compressive strength, allowing it to absorb pressures of up to 200 Kgf/cm2 (2900 psi). This is approximately double the compressive strength of neoprene (the material used in most elastomeric bearings) meaning that PTFE bearing pads can be much smaller and manage the same load.
In addition to the load bearing capacity, PTFE also exhibits a low coefficient of friction (the lowest of any known solid) – which only goes lower with the addition of more pressure and is exceptionally low when PTFE slides against polished stainless steel (the lowest between any two known solids).
This combination of load bearing strength and low-friction makes PTFE the preferred material for sliding bearings – where both load bearing and sliding movement are required to create an effective bearing assembly.
The use of sliding bearings is fairly widespread. Some of the areas we have supplied to are:
  1. Oil and gas pipelines
  2. Waterways and water pipelines
  3. Conveyor systems (both indoors and outdoors)
  4. Boiler plants
  5. Minor bridges
  6. Power plants
The compact size and overall effectiveness of the bearing makes it an ideal choice in lower load applications (under 100 Tonnes). Furthermore, the simplicity of slide bearing design ensures that as long as the basic design specifications are adhered to, there exists a lot of latitude as to the exact dimensions and form of the bearing. This is useful for clients, who would prefer to design their structures independently and have the bearing modified to suit their overall design.
It must be pointed out that in India, there is no official rulebook for the design of sliding bearings. For the most part one refers to standards such as BS:5400 and AASHTO – taking care to cross check against the IRC:83 (the Indian code book for POT-PTFE bearings) to ensure that the material specifications match.
As a manufacturer of these bearings, this does add a lot of flavour to the task of design. Very rarely do two separate projects look for the same bearing design – there are always nuances and specific constraints against which the bearing must be altered to accommodate the client’s requirements. And although the constraints may be somewhat common – the method of accommodating them can vary significantly.
Movement
In many cases, the bearing requires a sliding movement in only one direction. This results in the requirement of guides. Our experience with guides is that as long as there is negligible horizontal load on the bearing (under 2 tonnes), any of the two following guiding elements can be used.

- Bracketed guides – these are normally two guide plates welded/ bolted to the side of the top or bottom plate
- Dowel guides – guide pins can be used either at the center of the plate or on the sides
In case the load is higher than 5 Tonnes, a centre dowel guide is always preferable. Some designs may also specify a guide that is monolithic with the top plate. While this is the definitely better from a load bearing stand point – it is often expensive, as the plate needs to be either cast or machined out of a much thicker plate.
In any case, as the horizontal load increases beyond 10-15 Tonnes, it becomes viable – both technically and commercially – to look at POT-PTFE bearings.


Rotation (lateral)
Rotation along the horizontal axis (perpendicular to the direction of the vertical load) is not a common requirement.
It is most easily achieved by employing a circular dowel pin at the centre of the bearing around which the top plate can rotate.
In case the load is high, you could also look at a hybrid POT bearing – where a PTFE disc is used in place of the elastomer and a polished stainless steel sheet is affixed on the piston to allow for rotational sliding movement.


Rotation (vertical)
Vertical rotation (around the direction of the vertical load) is most easily achieved by employing an elastomeric pad along with PTFE. In most design specifications, there is a stainless steel sheet required in between the PTFE and the elastomer.
In more heavy-duty applications, a fully reinforced elastomeric bearing may be employed. The bearing is affixed (either by bonding or during vulcanizing) to the base plate housing the PTFE.


However – as discussed earlier – the lower compressive strength of elastomeric bearing material (such as neoprene) would require the size of the PTFE bearing to be defined by the size of the elastomeric bearing required. In some cases, where space is a constraint, designers opt for spherical bearings to accommodate the vertical rotation.
The benefit of a spherical bearing is that it can be compact and that the radius can be changed to match the extent of the rotation required. In contrast, to accommodate higher rotation in an elastomeric bearing, the thickness of the bearing would need to be increased – making it more expensive and bulky.

On the other hand, the smoothness of the rotation provided by an elastomeric bearing (which is effectively using it’s elasticity to accommodate the rotation) is compromised in a spherical bearing. Although in most spherical bearings a PTFE-SS match is created to allow for smooth rotation – it will perform slightly less effectively than an elastomeric bearing. Ultimately, this is a trade-off that the designer will need to assess depending on the requirement of the project.
Arc bearing
Arc bearings are normally used in pipelines, as the bearing needs to take the curved shape of the pipe. The most common arc type bearings we have come across employ two sets of PTFE-Neoprene pads, which have been heated and bent to form the required radius needed to match the pipe. One set of PTFE-Neoprene is bonded with the pipe, such that the PTFE layer faces downwards. The second set is bonded to the concrete base, such that the PTFE surface faces upwards. When the pipe is lowered on to the concrete base, the PTFE layers mate, such that there is sliding along the length of the pipe. Also, due to the neoprene layers – there is rotation allowable.
This bearing can also be made using stainless steel to replace one of the PTFE layers. However, bending the stainless steel to match the radius of the pipe is more expensive than bending PTFE (which can be done using heat and a cheap metal die). Furthermore, it is likely that there would be slight variations on-site in the radii of the pipe and the concrete support. In this case, the stainless steel may develop kinks/ irregularities on the surface once the load is applied whereas PTFE, being much more pliable, will accommodate the same quite easily.


Two-way bearing
Our experience with this type of bearing has been mainly in the erection of conveyor systems. Often, along with the vertical load exerted on the bearing, there is some amount of horizontal load (along with restricted horizontal sliding movement in one direction) and some upward load. Usually, these loads are very small – within 2 Tonnes – so a complex or heavy-duty solution becomes wasteful
The concept of a low-cost, but effective bearing has let us to consider 2 alternate designs as shown below.
The simple design would employ side guides to form a bracket around the lower plate – allowing sliding movement in one direction and ensuring any uplift is contained. However, as the guides are welded, their strength is limited to within 2 Tonnes at most.
In case the uplift load is higher than 2-3 Tonnes, one would need to look at the second design – where a bolting arrangement allows the total load to go much higher. The second arrangement is altogether more elegant and compact – but comes at a much higher cost, owing to the extensive fabrication required and the extra thickness on the top plate needed to accommodate the guide-cum-anchor pin.


Although rocker bearings are usually stand-alone metal bearings, we have seen them used along with a PTFE sliding arrangement to give a rocking-cum-sliding arrangement.
The base plate housing the PTFE is usually the top plate of the rocker bearing.


Conclusion
We have described here only some of the bearing types and features that can be designed, based on the requirement of a specific project. Considering that projects take many forms and the constraints they may present could be very unpredictable, the above list could only be a fraction of the complete set of sliding bearings that can be envisaged. However, our experience in this field suggests that these are the primary features which are required of a given bearing and that ultimately, most bearings would be a combination of the above design forms.

3 comments:

  1. That would be really nice if you list more of all the kinds of bearings you have there.
    white metal bearings

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  2. Steel bearings are usually preferred in industrial usage; let it be chrome or stainless steel, specifically due to their last longing ability. babbit bearings

    ReplyDelete