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Coefficient of friction of elastomer on steel

  1. May 15, 2015 #1
    I need to measure the co-efficient of friction of rubber (elastomer) seal on steel. How can I do this by experimentally ?

    I appreciate your any kinds of help regarding this issue. Thanks in advance.
     
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  3. May 15, 2015 #2

    berkeman

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    Have you seen the simple tilting an incline test for friction (try Googling it)? Do you have access to the rubber block and a steel sheet to use for the incline? :smile:
     
  4. May 16, 2015 #3
    Thank you for your reply.

    My situation is different. I have to measure friction co-efficient of elastomer rubber and steel contact. This rubber is used in the pipe for sealing purpose. Rubber dimensions are OD: 102.5mm; ID: 47 mm ; Thickness: 31.5 mm the pipe size is 4 inch dia. Need to measure the pressure and friction inside the pipe wall and rubber.
     
  5. May 16, 2015 #4

    Baluncore

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    I think you may be trying to find the force needed for disassembly rather than the coefficient of friction.
    Unless you know the force holding the steel against the elastomer there is no point knowing the coefficient of friction.

    The contents of the pipe may be a lubricant. Many seals work by the application of unbalanced hydraulic forces to cause distortion of elastomeric components such as O-rings. The hydraulic pressure increases the contact force of the rubber onto the steel which then holds the parts together with a force proportional to the hydraulic pressure that is trying to push the components apart.

    Can you please attach a drawing of the pipe and seal section to your next post. What pressure is expected.
     
  6. May 16, 2015 #5
    I have to apply 30 KN force on the elastomer and then it will deformed. When the elastomer seal contact width the pipe wall then I need to measure the contact pressure and friction co-efficient of elastomer steel pipe seal.
     

    Attached Files:

  7. May 16, 2015 #6
  8. May 16, 2015 #7
  9. May 16, 2015 #8

    Baluncore

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    In the diagram it looks like fluid can travel outside the elastomer and so also present pressure below the seal. Is that the case? You give seal OD as 102.5 mm. Pipe ID is 4” = 101.6 mm. There is 0.9 mm crush on the OD of the seal. That is a quite different situation to the diagram.

    Consider a seal that fills the area between the shaft and pipe. The seal might rest on the base plate or have an independent reservoir of fluid between the seal and the base. Hydraulic force will apply pressure to the exposed annulus face of the elastomer. The elastomer will respond by maintaining it's volume while transmitting the face pressure to all other contact surfaces.

    The force of 30kN is applied to the area of the seal face annulus.
    Area = Pi * ( 50.82 – 23.52) = 6372. mm2
    That represents a pressure of 30 kN against 6372. mm2
    That pressure is transmitted to the outer contact of the elastomer with the steel.

    Knowing the area of the friction contact is (Pi * 101.6mm * 31.5mm) = 10054. mm2
    You can work out the contact force of the elastomer against the steel pipe ID.

    Contact force, F = 30kN * 10054. / 6372. = 30 kN * 1.5778 = 47.335 kN.
    Is this the “friction force” you require?

    Without movement of the steel to elastomer contact we cannot know the coefficient of friction.
    Is the hydraulic fluid a lubricant?



    This assumed that the length of the elastomer was 31.5mm when installed in the pipe.
     
  10. May 16, 2015 #9
    Thanks .

    Actually, I have machined the pipe wall thickness and made a gap 3mm in between the elastomer seal and pipe wall. Then I have added compression load to 30 KN and then the seal deformed and contacted the pipe wall. I measure the contact pressure by some pressure strip but I got some lower pressure value. Now I want to calculate the co-efficient of friction of elastomer seal and inside the pipe wall. I am planning to add some hydraulic pressure on the elastomer to do the same experiment.
     
  11. May 16, 2015 #10

    Baluncore

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    The simple rules to follow are:
    1. The elastomer has a fixed volume, independent of pressure.
    2. The pressure is the same throughout the elastomer and on all the surfaces.

    If there is a gap between the seal and the inside surface of the pipe, then the force must be applied mechanically to the face of the seal. Hydraulic fluid must not pass the seal. The area of the annulus is that of the mechanical contact with the elastomer, not the diameter of the pipe.

    You should calculate the initial volume of the elastomer. A 3mm gap will significantly reduce the axial length of the elastomer contact with the steel wall. 3mm on radius is 6mm on diameter.
     
  12. May 16, 2015 #11
    Thank you very much.
     
  13. May 23, 2015 #12
    I want to develop a set up for the measurement of co-efficient of friction of elastomer rubber in contact with steel. I have attached a drawing for information. Here, I will apply vertical load through universal testing machine and then I want to apply another tensile load horizontally. But I dont understand how can I apply this horizontal tensile load with this set up.

    I always appreciate your suggestions.Thanks in advance.
     

    Attached Files:

  14. May 23, 2015 #13

    Baluncore

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    I see no advantage in performing the test under a force of 10 kN. Why such loads? When under pressure the elastomer behaves like a fluid and will be extruded through any gaps in the housing. The elastomer does not need to be thick. Only a surface coating is required.

    The test could be done on an inclined plane. http://en.wikipedia.org/wiki/Inclined_plane#Inclined_plane_with_friction
     
  15. May 23, 2015 #14
    In my project in real world, the elastomer seal will take much higher load even 1000 bar. Now I want to test the friction according to pressure. I will control the vertical load before extrusion of elastomer and in this case, how I will give the horizontal load on the steel. It will be helpful for me if I get any idea.
     
  16. May 23, 2015 #15

    Baluncore

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    There are several ways of solving the elastomer containment and anti-extrusion features. If you are studying the friction coefficient of elastomer against steel under different confining pressures with different lubricants then there is probably a reason why you are doing the experiments. The testing should be conducted on specimens of similar geometry and size to the real world application.

    What is the application and size of the product you are engineering?
    What lubrication / fluid will be used in the real world application?
    You will need a two axis Universal Testing Machine. What make and model of UTM do you have available?
     
  17. May 23, 2015 #16
    You are right. I want to perform it with different confining pressures with different lubricants. The application of the product is in pipeline welding test. The size of the product is 4inch diameter with 31mm thickness. I do not have two axis universal testing machine.
     
  18. May 23, 2015 #17

    Baluncore

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    Sorry about asking six more questions but you will need to test the design important parameters. If possible you should test the device itself.

    1. Will high pressure fluid, (such as water), be confined inside the pipe by the seal, while being used to test the porosity and integrity of the weld? Will you use a visible dye or a fluorescent chemical? A pinhole in a weld will produce a fine jet of fluid that will cause physical injury with a subsequent infection to anyone who encounters it. If oil is used as the testing fluid the resultant infection will probably be gangrene. You should surround the weld while it is under pressure test with a steel clam shell to reduce the risk of injury to the inspectors.

    2. Was your design with the 3mm radial clearance designed to pass the irregular internal surface of the finished weld after testing?

    3. Seals come in different types. Some must slide while expanding. Others less often, must grip with static friction and lock in place under pressure. Am I correct that you want to measure static friction while the seal is locked in place?

    4. There are ways of using mechanically expanded deep cups to static seal a pipeline. You appear to be considering only a cylindrical plug without any cup. Would you consider cup seals?

    5. Will the weld testing device have one seal, behind the welded joint, with pressure applied from the open end of the pipeline, (where another seal is needed)? Or will the testing system have two seals, close on each side of the welded joint, with pressure applied through a small diameter, long tube? If you select two seals on the same tension rod, the force on the two seals will cancel and friction will be less important. It will then be a case of expanding the elastomer against the pipeline wall rather than relying on static friction to hold it in place.

    6. Your 3mm radial clearance, (6mm gap on diameter), presents a significant but tractable design challenge. Hydraulics systems with significantly smaller gaps prevent seal extrusion by employing a 'back-up ring' or an 'anti-extrusion device' Google those and similar terms.

    The experimental testing jig for a single axis UTM should be based on the design of the real world device.
    I have a few ideas for design of the UTM jig, but we need to further restrict the possible device configurations.
     
  19. May 23, 2015 #18
    1. yes, water will be used to test the weld. No.
    2. yes
    3. Actually, I want to measure both static and kinetic friction.
    4. I am using only cylindrical .
    5. only one seal
    6. I cant use back up ring or anti extrusion device because of complexity.
     
  20. May 24, 2015 #19

    Baluncore

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    If your elastomer is soft enough to seal the 3mm gap on the high pressure side, then it will be extruded on the low pressure side. A backup ring and/or a harder grade elastomer on the LP side is going to be essential to contain the elastomer.

    How will you physically expand the seal to take up the 3mm annular gap? That will require a circumferential change of about 20mm.
    Why will you not consider a cup profile on the HP side of the seal so as to use the confined fluid pressure to prevent leakage and help lock the seal in place?

    The UTM indicator can measure the force needed to overcome the static friction of the elastomer in the jig.
    A UTM can measure the static friction, but how can it measure the sliding friction without chatter?
    The second axis will need to be the hydraulic pressure applied to the seal.
     
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