Monday, July 4, 2011

Cantilever Load Considerations for PTFE Sliding Bearings

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As a manufacturer of sliding bearings in India, one of the challenges we face is that there does not exist an official book of guidelines specifically for this type of bearing. The closest we have is the IRC:83 – which is the code book for POT-PTFE bearings, and which details the specifications of the materials to be used and also the testing parameters for POT bearings. As such the IRC:83 does provide some guidelines – but does not address the finer design requirements for sliding bearings. Consequently, during design and testing of the bearings, customers rely on the Quality Assurance Plan (QAP) we provide them, with the option to question, refute and even modify the requirements as they see fit.

Although the BS:5400 and AASHTO standards do make specific recommendations for PTFE sliding bearings, customers are not always willing to accept their parameters as they may sometimes vary from those specified in the IRC – in places where the codes overlap. This can result in technical stand-offs, between the customer and manufacturer, as each tries to convince the other regarding a certain process or parameter, without any official rule book, to back up their view point.

Among the more interesting technical debates we have had recently has been regarding the thickness of the top plate (upper sole plate) in bearings with high movement requirements.

Given a specified vertical load, a PTFE sliding bearing will be required to have a PTFE pad with side determined by the compressive strength of PTFE (usually taken at 100-200 Kgs per sq. cm). This side in turn determines the side of the lower sole plate. Give these two dimensions, the movement of the bearing (specified usually by the client/ contractor) will give us the side of the Stainless Steel sliding element, which in turn will give us the side of the upper sole plate.

Our designing of PTFE sliding bearings began with taking standard designs for specified loads and movements (such as C&P) and recommending them to clients. As we became better versed with PTFE sliding bearings, we started designing from scratch – using the parameters of movement, load and rotation to design customized PTFE bearings for specific project requirements. As many of the original standard drawings recommended sole plates of 12-15mm thickness for bearings up to loads of 50-65 Tonnes, we too continued with this recommendation. After all, the constraint of a bearing in taking a vertical load depends purely on the PTFE – as when compared with steel, PTFE has a much lower compressive strength (100-200 Kg per sq. cm versus steel’s 14000-15000).

It came as a surprise to us therefore when a client expressed concern over the top plate thickness – citing that the bending moment caused by the vertical load would be in excess of what the top plate could accommodate, given the overhang of the top plate over the bottom. This led us to revisit a lot of the standard designs – only to find that for similar load-movement parameters, other bearing designs did not recommend a thicker sole plate that our customer was insisting on.

We therefore went back to theory to gauge exactly how much of a bending moment would be caused by our given load and assess whether there was a case for changing our top plate design.

The formula for calculating the bending moment is as follows:

M= (P x L2)/2

Where:

P = Pressure (Kg/cm2)
L = Length of overhang (cm)
M = Maximum Bending Moment (Kg)

Once we know M, we divide by the Maximum Allowable Bending Stress (S) of the material (1650Kg/cm for steel) – to derive the thickness of the steel plate required.

In the example in question, the value of M obtained was 3941 Kgs, which when divided by S gave a thickness of 3.78cm – or 38mm.

This was a shocking revelation, as our recommendation was for 15mm – clearly insufficient for the load in question. Nonetheless, we wanted to dig deeper to find out how most standard bearing designs only called for a 12-15mm thickness – when the theory clearly showed that this was inadequate.

In most of our discussions with consultants and industry experts, the value of 38mm was ratified and we were told that this was in fact the thickness needed. They were unable to explain, however, why the standard designs did not tally with the theoretical calculations. Our question was finally answered when we spoke with a contractor who studied the drawings and design details and confirmed that while a thickness of 38mm was indeed required, the thickness of the insert plate also needed to be taken in to account.

The insert plate is installed at site and is simply a 25-30mm thick steel plate grouted/ cast along with the concrete substructure and/or superstructure. The bearing is welded or bolted to this plate and the load on the bearing is transferred through the plate as well. Thus, a bending moment will act through the layers of both the upper sole plates and the insert plates – meaning that even with a sole plate thickness of 15mm, the total thickness through which the load acts is 40mm – which is more than sufficient in our example.

We went back to our client with this, but were informed that they were not using an insert plate in this particular project and that the sole plate thickness needed to be at least 40mm as per our calculations.

The exercise was an eye-opener, because we had never before been questioned on the bending moment for sliding bearings, nor had we come across any design calculations for this in the various codebooks. However it is a critical point in the design of a PTFE sliding bearing – as there is always an overhang of top plate over the bottom. As a rule, one needs to verify that an insert plate is being used and of what thickness it is. Adding this thickness to the sole plate thickness, the designer needs to verify whether the bending moment is being accommodated. If not, the thickness of the upper sole plate must be re-worked.

1 comment:

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