Have they always made it public or was there a time when the voting results were kept private, only allowing us to see the results of the Congress a whole instead?
http://memory.loc.gov/ammem/amlaw/lwhbsb.html
Each House shall keep a journal of its proceedings, and from time to time publish the same, excepting such parts as may in their judgment require secrecy; and the yeas and nays of the members of either House, on any question, shall, at the desire of one-fifth of those present, be entered on the journal. [constitution]
... The printed version published by order of the Senate is presented here in electronic form.
The Journal should be seen as the minutes of floor action. It notes the matters considered by the Senate and the votes and other actions taken. It does not record the actual debates, which can be consulted through the "Link to date-related documents" in the full text transcription of the Journal.
http://memory.loc.gov/ammem/amlaw/lwsj.html
so no and the answer is in the constitution. for example http://memory.loc.gov/cgi-bin/ampage?collId=llsj&fileName=003/llsj003.db&recNum=294&itemLink=r?ammem/hlaw:@field(DOCID+@lit(sj003390)):%230030286&linkText=1
this page includes an 1803 vote which transcribes the yeas and neys in a simple text format.
it's more of a link dump than an answer but the links take you to a site for all house and senate minutes up to 1873 (and with a classic horribly nineties cover page). so not only have they always had public records you can go look at them!
there might be some national security exceptions (especially more recently) that i'm not thinking of but a general answer is they have always been available.