Patent examiners are people and are capable of making mistakes. One potential reason for missing the '892 patent is that the search was limited to classifications, and the search did not include any of the classifications of the '892 patent.
The '892 patent was classified as:
- 30/292; 30/293; 30/294; and 83/883.
To determine what classification fields were searched, we can look at a pdf version of the '136 patent under the Field of Classification Search heading or look at the prosecution details in Public PAIR.
In this case, the following fields were searched:
- 33/27.03; 33/32.1; 33/32.2; 33/32.3; 33/33; 33/41.1; 33/42;
- 30/280; and 30/28.
The fields searched are available on Public PAIR under the document code:
SRFW (Search info including classification, databases, and other search related notes).
Often, additional details can also be found under document code:
SRNT (Examiner’s Search Strategy and Results).
There are no documents coded with SRNT, so it is not clear whether a keyword search was not performed, or just not documented. The examiner may have believed that it would be difficult to accurately describe the subject matter with a text query and elected to only do a classified search. This is described in the Manual of Patent Examining Procedure under section 904.02:
“Examiners will recognize that it is sometimes difficult to express search needs accurately in textual terms. This occurs often, though not exclusively, in mechanical arts where, for example, spatial relationships or shapes of mechanical components constitute important aspects of the claimed invention.”
That said, it is hard to say how considering the '892 patent would have affected the prosecution of the '136 patent. While they are similar, there are elements in the '136 patent that are not taught in the earlier one.