Self-adjusting spring-loaded wedges, supplied by the Franklin corporation, were designed to maintain zero clearance between axleboxes and horn guides, thereby preventing the shock loads or "knocks" that were once commonly heard from run-down locomotives as the clearances between axle boxes and hornguides were taken up with reversal of piston thrust.
In Clause 5.23 of the 5AT Specifications, Dave Wardale wrote as follows:
"The driving and coupled wheel axleboxes shall be provided with wedges as follows: driving axlebox; parallel guides at the rear and spring-loaded wedges at the front: leading coupled axlebox; fixed wedges at the rear and spring-loaded wedges at the front: trailing coupled axlebox; spring-loaded wedges at both front and rear (to allow for mainframe expansion between the driving and trailing coupled axles due to radiation from the firebox). The position of the fixed wedges shall accord with the exact leading coupling rod lengths between bearing centres, and shall be lockable. The spring loading of the wedges shall be of the Franklin lever type, mounted transversely on frame cross-stays, as necessary to clear the underhung springs."
The 5AT axlebox and hornguide arrangement is illustrated below from a CAD arrangement drawing by Richard Coleby.