Since it specifically mentions 'rouleau' it is a 'breast beam' which rotates, as distinct from other breast beams which are fixed (and are not usually of circular cross-section). Some looms have both a roller (to ensure smooth movement of the threads) and a fixed beam (above the threads) on which the operator can lean when reaching into the loom with touching the threads.

Glossary - Glimakra-USA - [ Traducir esta página ]Beam, breast. Beam at the front of the loom over which the cloth passes before it winds onto the ... A protective board that is attached to the breast beam. ...
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