Friction Heating.....Thermal Seizure Question

[smyth13]

I have a question concerning the potential for frictional heat generation to cause the seizure of a shaft within a bore?

My issue involves a punch becoming stuck within a bore during a compression punching operation. I believe the issue is a lubrication issue where the frictional coefficient becomes high thus creating a frictional force in excess of the applied load causing mechanical seizure, or, perhaps the frictional heat generates enough thermal expansion in the punch, eliminating the clearance.

I have been trying to relate the thermal expansion of the punch with the "flash" temperature created during the frictional sliding (proposed by Blok, etc). I am having a hard time gaining confidence in my results. Ultimately I am looking for some general guidance in regards to this issue and whether it is commonly experienced. In addition, I would like to determine a "minimum" frictional coefficient needed to cause the seizure of the punch. Thanks for the help.

 

[IRstuff]

While expansion might be an aggravator, it has to be something beyond that, like galling or actual welding, to result in a "seizure."

TTFN

FAQ731-376

 

[smyth13]

Thank you for the reply. That is my feeling exactly. Although the clearance is relatively small (~0.009"-0.020"), the amount of temperature change needed to "fully" close this clearance gap by thermal expansion alone does not appear to be possible due to the effects of frictional heat generation alone in this environment (room temperature).

I guess my question boils down to whether this frictional heat generation has the potential to alter the frictional coefficient enough in the localized contact area to prevent the motion of the punch.

FYI...the operation in question is a rotary pharma tablet punch. The punch diameters appear to have some "burnished" areas indicating a temperature change. In addition there are some apparant wear issues, although their origin can't be confirmed due to the removal process. Thanks again.

 

[gruntguru]

The seizure of plunger in bore is a well known phenomenon in IC engines. I have watched an engine fail on a couple of occasions during dynamometer testing. The sequence of events is: 1. Excessive heat input to piston causes the diameter to grow. 2. At some point he local oil film thickness drops below a safe value and lubrication breaks down. 3. This results in localised heating which adds additional heat to the piston. 4. The cycle proceeds exponentially resulting in 5. Severe scuffing, galling, loss of ring-seal, blow-by, oil consumption, misfire etc.

 

[ione]

Thermal expansion is not the main issue in piston seizure. Proper lubricating oil film should always act to prevent one bare surface touching the other. If the oil film fails, the metal surfaces of the cylinder and piston will effectively touch each other, and if the film is not quickly restored a metal exchange between cylinder and piston indeed takes place (seizure).

You can have this read further to different types of seizure. A visual inspection of the damaged surface could give you valid inputs of sources/causes which may have led to seizure.

http://www.groupk.com/tec-seizures.htm

 

[gruntguru]

If thermal expansion exceeds design limits, it will dictate. Oil film cannot resist the pressures generated when the plunger no longer fits the bore.