Correct Tool Steel for Cam and Roller 2

[JohnStef]

Hello
I am looking for some guidance on material selection for a rolling contact machine assembly.

We are in the midst of a tool rebuild, there is a cam/roller sup-assembly that typically wears. The Roller is 5.0" dia 4140 Rc 52/56, the cam is oval shaped 12.0" x 15.0" with an 2.0" offset bore, the same mat'l is used here as well, 4140 Rc 52/56. The length of contact is 6.0". The RPM is 240

I have had some exposure to sliding metal contact, such as Progressive Die design. The rule there is to have dissimilar metals when there is sliding contact. It was almost a knee jerk reaction to use A2 for the Die steel and D2 for the punch. And in a different application of a machine assembly, A10 (graphitic tool steel) was used for roller contact. A10 is excellent for sliding metal on metal contact.
It seems like the same rule would apply here.

From what I have been able to glean from the internet, differing Rc is an advantage as well as differing microstructure (metal choice)
What I am asking for are basic metal selection for sliding metal contact applications.

 

[sreid]

A cam and roller implies rolling contact. You then mention sliding contact, two very different things. Is there lubrication? Is there mis-alignment at the 6 in contact?

 

[dvd]

see track capacity factor on page 12 (pdf page 14) from McGill Camrol literature

McGill used to have an in-house expert on the topic. You might try calling your cam follower manufacturer and ask to speak with an applications engineer.

Coincidentally, see page 12 of the Osborn LoadRunner literature for track capacity factor discussion as well

 

[tbuelna]

The first thing to do is determine the root cause of this "wear". Is it due to excessive surface contact stress levels? Is it due to relative sliding at the roller/cam surface contact? Is it due to high local contact stresses caused by misalignment between the roller/cam surfaces? Is it due to lack of adequate lubrication? Is the wear localized or does it occur all over the cam/roller surfaces?

The relative hardness between your cam and roller is likely not a problem assuming there is rolling contact. With rolling contact, loss of surface material can be the result of mechanical adhesion between the contacting surfaces, or it could be the result of surface micro-pitting/spalling. Mechanical adhesion can be addressed by improved lubrication, better surface finish, or reduced contact stress levels. Micro-pitting or spalling can be addressed by reducing the peak surface contact stress levels. The L/D ratio of your roller is >1.0, so adding some crown to its face width can help minimize high local contact stress from edge loading. Modifying the cam profile to increase its minimum radius of curvature can help reduce peak contact stress levels.

 

[sreid]

also note that this a a pretty big cam follower and is probably driving a lot of mass at 4 Hz with a 2 inch stroke.

 

[romke]

what type of wear do you experience? depending on the type of wear experienced, different steps might be taken to prevent premature failure in future.

a well designed roller/cam contact eventually should show a form of pitting after prolonged use as a result of frequent load changes and the stresses that come with it. if the wear pattern looks more like scratching the cause might well indicate a lack of lubrication or use of a lubricant with a unsuitable viscosity or insufficient antiwear properties.

the assumption that in cam/roller contacts two more or less identical materials are best used is correct - due to the type of line contact (that will deform slightly to a kind of a elliptic shape under load) the loads are such that use of a softer material for one of the two components is out of the question. in most roller/follower contacts however there usually also is a sliding component in the contact area which may call for additional measures in terms of material treatment and make designing a optimized tribologic system (comprising both hardware components and a lubricant) far more difficult.