Why pins are used in small head of connecting rod???

[namihabib]

A journal bearing is used at the big end of connecting rod (crankshaft side) but not at the small end . on the contrary a pin is used. why is that? why can't we use journal bearing at the small end too.
I am considering a huge reciprocating compressor/pump

 

[JJPellin]

All of our reciprocating compressors use a journal bearing at the crosshead end. All of these are double acting designs. For a single acting machine, there will never be a full load reversal on the con rod. The crosshead pin bushing does not make a full rotation, but only rocks back and forth by a relatively small angle. A journal bearing loaded in only one direction and only rotated a few degrees will not get enough lubrication into the load zone and will fail. That is probably why some applications use needle bearings. If it is designed to experience a load reversal (compression, tension, compression, tension . . .) a journal bearing design could work.

Johnny Pellin

 

[namihabib]

1. do you use forced lubrication?
2. we have a double acting reciprocating compressor having bush & pin. My boss told me that bearings ain't used as they are Babbitted & can't take impact load .

 

[JJPellin]

I may have misinterpreted your question. The pin you refer to is necessary to be able to assemble and disassemble the con rod to crosshead connection. That pin runs through an oil lubricated bushing that acts as a journal bearing. Smaller engines may use a roller bearing at this location. But, all of the ones I have worked on use a removable pin with a bushing for a bearing.

Johnny Pellin

 

[namihabib]

question persists.
Why can't we use journal bearings?
is it due to improper oil wedge formation?

 

[JJPellin]

What do you mean by journal bearings? As I noted, you can use a hydrodynamic bearing as long as you are sure of rod reversal. The inability to form a stable oil wedge is a limiting factor.

Johnny Pellin

 

[JJPellin]

Babbitt is not needed or recommended. The loads and forces would tend to overload or fatique babbitt. Why do you want to use babbitt?

Johnny Pellin