Friday, August 30, 2013

PTFE Sliding Bearings: Calculating Coefficient of Friction

PTFE is a preferred material in sliding bearings for three very specific reasons:
  1. Load bearing capacity
  2. Weather ability (due to its overall chemical inertness)
  3. Low coefficient of friction
The first two factors are well accepted and easily tested. The vertical load on a bearing is simply tested by placing the bearing under a hydraulic press of suitable capacity, applying 1.25 times the rated load of the bearing and holding this load for a period of 1 hour to observe any adverse impact on the bearing material. In our own experience, it is very rare that pure PTFE would fail in this instance, since:
  1. Pure PTFE genuinely does have a very high load capacity and even in the event of over-loading, would tend to deform rather than break down
  2. The design load for most bearings incorporates a safety factor of up to 60% – implying that while PTFE may be able to withstand a load of up to 40Mpa, it is designed with a load of only 16Mpa and is thus well within its own capacity to take the load applied
Weather ability is difficult to test, as this is a long-term guarantee that the material can stay in outdoor conditions without experiencing any degradation in properties. However, most clients are happy to take this assurance at face value – as long as they can satisfy themselves that the material being used is in-fact pure PTFE. Its should be mentioned here that in the event that reprocessed or recycled PTFE is used in sliding bearings (a gross violation of quality norms, but one that may occur all the same, especially if the bearing manufacturer is buying their PTFE from a third-party and is therefore not directly in control of the quality), there will definitely be a failure of the bearing after installation.
As mentioned in earlier articles, we have witnessed many deviations from expected performance when presented with reprocessed PTFE. Amongst these is the tendency of the material to become brittle and even crumble after being kept outside for a prolonged period (usually over a few months). No doubt, a similar effect would be experience by a material used in a bearing that is installed on a bridge or flyover – with the result that the bearing may fail after only a year of service.
Coefficient of friction
The problem with the coefficient of friction is that most people are not fully familiar with what it implies. So let us start with defining it and then look at the misassumptions surrounding it.
The coefficient of friction between two planes is defined as the ratio of the force needed to move one plane over the other divided by the force pushing the two planes together.
So in the case of a block resting on a table, coefficient of friction between the block and table would simply be the force needed to slide the block across the table, dived by the weight of the block itself.
Coefficient of Friction PTFE (1)
Since the coefficient is a ratio of two forces, it does not have any units. A common mistake clients make is to ask us to define the unit we have considered for the coefficient of friction.
In the case of the PTFE sliding bearing, the coefficient of friction being considered is that between PTFE and polished stainless steel. Here again, a mistake is often made asking what the coefficient of friction of PTFE is. There is no such thing as a stand-alone value for coefficient of friction for any material. The coefficient between PTFE and polished stainless steel will no doubt be much lower that between PTFE and concrete. In other words, it would take more force to move a slab of PTFE across a concrete surface than it would to move the same slab across a polished stainless steel surface. Thus, when we talk about a coefficient of 0.04 between PTFE and stainless steel (the commonly accepted value for bearing manufacturers), we are saying that for a 1Kg PTFE block to slide across a polished stainless steel surface, it would require only 40 Grams of horizontal force. (Note: we are aware here that Kgs and Grams are units of mass and not force, but seeing as these are ratios of force, the values in Kgs/Grams against the values in Newtons would yield the same results).
Measuring coefficient of friction
We have already described that the value for the coefficient is derived by dividing the horizontal force over the vertical force. However, in practice, this is less straightforward. We would like to look at some of the methods that are used around the world to check these values as pertaining specifically to PTFE bearings, before describing what we feel is the most straightforward and easily implemented method.
  1. Two-press method
    It is not possible to test the bearings simply by applying a vertical load and seeing at what horizontal load the bearing slides. This is because the bearing plate on which the horizontal load is applied also has another surface, which would be in contact with the vertical press and therefore be subject to friction from the press itself (which is likely to be very high).
    Therefore, the accepted method is to place 2 bearings, back-to-back (shown below) and exert load on the centre.
    This process is technically sound, but practically not always feasible. For starters, the bearing shape itself may not lend itself to being places back to back. It may have welded guides attached to it or be of an unusual shape. In addition to this, the process in expensive – requiring two hydraulic presses.
  2. Load indicator method (laboratory)
    In this method, the stainless steel plate is placed horizontally with the PTFE plate on top. The PTFE plate is connected to a steel wire which is in-turn connected to a load indicator. The load indicator has a motor, which causes it to move upwards very slowly. As the indicator moves up and the wire gets tight, the load reading starts to show the horizontal load being applied. Once the PTFE plate starts to move, the reading is recorded and divided by the weight of the PTFE plate to give the coefficient of friction.
    This method is possibly the most accurate, as load indicators can offer values in grams. However, it again suffers from the issue that if the bearing plates are not totally flat or are too big for the equipment, they cannot be accurately tested. Furthermore, it is debatable whether such accuracy is needed in the realm of sliding bearings
  3. Poly Fluoro method (inclined plane)
    In an attempt to find a quick, repeatable, logical and universally applicable method to check the coefficient of friction, our method follows the rather simple process of gauging the angle of incline.
    For starters, we do not believe that getting an accurate value of the coefficient of friction would add any value to the product. If we can confirm that the plates slide at a coefficient of friction set to 0.04, then it does not matter whether the coefficient is in-fact 0.03 or 0.036, as the product has met its required specification.
    Coefficient of Friction PTFE (2)
    The diagrams  show that when the planes are inclined, the coefficient of friction takes the value of the tangent of the angle of inclination. This allows us to easily check the coefficient of friction, as we simply set the ratio of Y to X to equal 0.04 and check if the PTFE plate slides down the plane, when placed on the SS plate. Again – note here that only in the event that the PTFE plate does not slide, can we conclude that the coefficient is greater than 0.04 (and hence outside tolerance). Whether the plate slides slowly or very fast, is of no consequence – as either way it confirms that the coefficient is at least 0.04.
    Coefficient of Friction PTFE (3)
    In the event that the client specifies a higher or lower coefficient, the same method can be employed, by simply changing the value of Y. So we first assess what the value of Y should be by multiplying the length of the bearing plate by 0.04 and then use a calibrated slip gauge to prop up the bearing on one side (preferably on a flat bed) so that the angle is attained.
    The method employed here is useful to check PTFE bearings as it can be applied to any design and multiple bearings can be checked from one lot if needed, without the hassle of making separate fixtures and modifications. Furthermore, it can be checked even before the bearing is assembled so as to confirm that the PTFEmaterial being employed meets the parameters.
Static vs Dynamic coefficients
It must be mentioned that both static and dynamic coefficients of friction are relevant metrics and that the inclined plane method only measures the static coefficient. However, this is of no concern for PTFE bearings as PTFE is known to exhibit nearly identical values for both static and dynamic coefficients (a property unique to PTFE and not universally applicable).
We believe that the method described above is a more effective way to check for a very important parameter that bearings are based on.