How to use crawler with Rcrawler for automatic download in r?

I am not too familiar with Rcrawler, but I'd encourage you to start with by working through examples posted online. Here's one using rvest.
And if you're stuck on using RCrawler, just google for "RCrawler examples".

If you have some problematic code you'd like feedback on, be sure to post a reproducible example. This will make it easier for people to help you out.