Steel-Rubber friction

[droletk]

hello
I work for a steel structure constructor. We are currently trying to improve our loading method on train and the main idea of the project is to find a material who can provide a high coefficient of friction with steel. For now, we are using hardwood block with a COF of 0.2. I know I can use rubber to improve the friction between materials but I don't have any data on the COF to base my calculations. I'd like to know if you can provide me some information about friction between Steel-Rubber and Steel-Harwood. I know that friction vary a lot with, temperature, roughness....but I just need a conservative reference value. Also, If you have better idea than rubber I could really use your help. And if you want to know little about the project here are the main point : we useally load big girder about 56 tons and the train compagny force us to multiply the weight of the load by 3. So, we need to strap or to hold the objet with structural items like strap, rods and cable. To reduce the number of holding item we want to optimize the friction between steel and an other material.


Thanks a lot, sorry for my english my main language is french.

[Gordon]

Hi Droletk

to the Surface Engineering Forum.

As you have said friction can vary significantly depending on many factors. To be honest, I don't think a third party coefficient of friction value (if you could find one) would be reliable for your calculations. Why not do a scale down tests for yourself. You can then at least evaluate the effects of say dirt, moisture oils etc. that might be a factor in real life. Would altering the texture of the steel face be a viable option (grit blasting or coating)?

[droletk]

Thanks Gordon,

I did some test to find a coefficient of friction that I can use but I have an other question concerning the surface of contact between the beam and the girder. Every girder we have to load are approximately about 90 to 120 feet and we only use 6 to 8 hardwood block to support them. I'd like to know if I can use the total weight of the girder in example : 16.666% of the weight on each block if I have 6 blocks, or I need to find a relation between the surface of the block/girder and the weight that he can support?

[Gordon]

Hi Droletk

I was hoping someone else with experience (far more than mine ) of this type of problem would take up the challenge of helping . From what I understand the maximum friction force (force which enables object to remain in place) is roughly equivalent to the perpendicular force holding surfaces together x coefficient of fiction value. When an applied force opposing the fictional force exceeds the maximum, movement occurs. During movement the coefficient of friction and the maximum friction force can become much lower. This relationship does not consider surface area of contact directly.The dimensionless COF value basically is the fudge factor that contains the effects of the materials, environment and surface area contact etc.

So assuming I'm right and your COF value is reasonable, you will take the total weight (force) x COF to get your max fictional force. I don't think you can use number of blocks or contact area in this relationship (that is part of the COF value).

I certainly would not feel confident just using this simplified approach to solving your problem as I'm sure the situation is far more complex. Seek the advice of an expert or test results in real life conditions.