This system (shown in the off position) has an air chamber power unit threaded into the wedge bore of the plunger
housing. The socket in the end of the diaphragm push rod connects the air chamber to the wedge rod. The wedge
retracting spring acts as the return spring for both the wedge and the diaphragm. A pair of rollers are held in
corresponding slots in the inner ends of the plungers. The unslotted portions of the inner ends of the plungers are resting
on abutments in the plunger housing. The outer ends of the plungers are engaged with and supporting the brake shoes.
On a double-actuated brake, each of the two actuating systems has one anchor (solid) plunger and one adjustable
plunger. On a single actuated brake the one actuating system would have two adjustable plungers. All of the plungers
are retained in the housings and the roller slots are kept in proper alignment by means of guide screws which engage
slots in the side of the plungers. A hydraulic brake would have a hydraulic cylinder threaded into the plunger housing (in
place of the air chamber). The hydraulic piston would connect with the wedge rod.
When the brake is actuated, the air chamber pushes the wedge head deeper in between the rollers. This spreads the
rollers and plungers apart and pushes the brake shoes outward. Initially all the plungers are lifted off of the plunger
abutments and momentarily suspended. As the shoes (linings) contact the drums, the drum drags the shoes and the
suspended plungers around with it. This causes the plunger at the trailing end of each shoe to reseat on its abutment
and thus absorb and transfer the brake torque to the brake support. When the brake is released, the wedge spring
returns the wedge and diaphragm to the off position. At the same time, the shoe return springs push the raised plungers
back to their abutments.
The Fail-Safe unit is a spring powered brake actuator that assembles piggy-back on the air chamber. When 70 psi or
more air pressure is applied against the piston, the spring will be held in a compressed position. When the air pressure is
removed, the spring will push the piston against the diaphragm plate and apply the brake.
The Stopmaster II Wedge brake now incorporates a series of design changes based on the original Stopmaster brake.
Major design improvements have been made in four general areas of the brake: (1) The automatic adjusting assembly;
(2) The adjusting pawl assembly; (3) The non-pressure half of the air chamber; and (4) The brake shoe and drum.
The automatic adjusting assembly for the Stopmaster II employs a limited travel adjusting bolt which cannot over adjust
or cock when the brake lining has been extremely worn.
This is accomplished with the addition of a snap ring at the foot of the bolt and a redesigned adjusting sleeve (actuator)
The adjusting sleeve has been counterbored on the I.D. to accept the redesigned adjusting bolt. Thus, when the lining is
worn, the counterbored shoulder of the adjusting sleeve stops the bolt from advancing further. Consequently, the bolt is
prevented from over adjusting.