Duo Servo Brake Dialog Box¶

This Brake Designer dialog is used to edit the physical parameters of a duo-servo drum brake, in which the primary shoe drives the secondary shoe through a link, producing a self-energizing (servo) action. The dialog includes a schematic preview drawing of the brake, updated to reflect the current dimensions. (The previous manual titled this topic "Due Servo Brake"; the correct name is Duo Servo.)

Wheel Cylinder¶

Piston Diameter (in) Diameter of the wheel cylinder piston.

Wheel Cyl. Dist (in) Distance from the brake center to the wheel cylinder actuation point.

Push-out Pressure (psi) Pressure required to overcome force associated with return springs and begin developing brake torque.

Shoe/Drum¶

Drum Diameter (in) Inside diameter of the brake drum.

Drum Lining Width (in) Width of the lining material.

Drum Thickness (in) Thickness of the drum wall (used for calculating the thermal mass of the drum).

Lining (Primary and Secondary columns)¶

The following parameters are entered separately for the primary (Pri) and secondary (Sec) shoes. Parameters that apply to only one shoe of a duo-servo brake are disabled for the other shoe.

Lining Arc Length (in) Length of the lining material along the arc of contact with the drum.

Lining Alph 1 (deg) (labeled Lining Alph 1 in the dialog — a UI typo for Lining Alpha 1) Angle locating the leading edge of the lining relative to the shoe actuation axis. (Entered for the secondary shoe only.)

Lining Beta (deg) Angle locating the lining relative to the shoe abutment. (Entered for the primary shoe only.)

Lining Thickness (in) Thickness of the lining material.

Abutment Dist (in) Distance from the brake center to the primary shoe abutment (the link between the shoes). (Entered for the primary shoe only.)

Abutment Width/2 (in) Half-width of the shoe abutment. (Entered for the primary shoe only.)

Abutment Angle (deg) Angle of the abutment contact surface. (Entered for the primary shoe only.)

Abutment Friction Coefficient of friction at the abutment contact. (Entered for the primary shoe only.)

Shoe Pin Radius (in) Radial distance from the center of the brake to the anchor pin for the pinned (secondary) shoe. (Entered for the secondary shoe only.)

Shoe Block Length (in) Length of the shoe abutment block. (Entered for the secondary shoe only.)

Mechanical Efficiency Fraction of the actuation force actually delivered to the shoes after mechanical losses.

Buttons and Results¶

Friction ... Opens the Lining Friction Properties dialog, used to edit the lining friction coefficient as a function of lining temperature for the primary and secondary linings.

Material ... Opens the Brake Designer Material Properties dialog, used to edit the drum and lining material thermal properties.

Apply Recomputes the results below from the current physical parameters.

Brake Factor (read-only) Calculated ratio of total drum drag force to actuation force for the brake.

Actuation Factor (read-only) Calculated actuation force per unit of system pressure.

Brake Torque Ratio (in-lb/psi) (read-only) Calculated wheel brake torque per unit of system pressure. On pressing OK, this value replaces the Brake Torque Ratio in the Brake Assembly dialog.

Removed options¶

Preview button — removed; the schematic preview drawing is now always displayed in the dialog.

Source topic: DueServoBrkDlg.htm — updated from source code (HVEINV-64, Physics) 2026-07-05.

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