The necessity of frequent adjustment of the slack in railway brakes in order to maintain the brake piston travel within safe limits has long been an onerous item for the Railways, and the need of reliable automatic slack adjustment has grown more and more urgent. The Slack Adjuster should be “Double Acting’. This term means that the adjuster is capable not only of automatically taking up excess slack as produced, e.g by brake shoe wear, but also of automatically paying out the required amount when the slack gets too small.
Slack Adjuster not only incorporates the Double Acting and the Rapid Acting features, but also another new and improvement, namely what could be called the “Verificative Action”.
This means that upon meeting heavy resistance early in the brake application movement, such as is caused by too small slack but may also be due to other causes. e.g frozen brake rigging or ice coating on the brake shoes, the Slack Adjuster does not pay out slack immediately, but only indexes the amount of slack to be paid out. If the slack really is too small, this indexing still remains at the release of the brake, and at the next application the Slack Adjuster will then the piston travel finally reaches its normal length, and no false pay-out will then occur. Thus, the slack adjuster first “Verifies” the need for a pay-out before actually performing it.