13. Controls and Indicators

13.1 General Controls

All controls shall be marked, indexed, sized, and located so that the control position can be readily identified. Controls shall have fixed guide marks if pre-setting of the controls is required. Controls located adjacent to their associated displays shall be so positioned that operation of the control should not obscure the display. Controls shall be so connected in the circuit that the controlled characteristics; (e.g., sensitivity, volume, or voltage) increase with clockwise rotation of the control as seen from the operating position. In general, movement of a control forward, clockwise, to the right, or up, shall turn the equipment on, cause the quantity to increase or cause the equipment to move foreword, clockwise, to the right or up.

13.2 Operating Controls

Controls necessary for the operation of the equipment shall be readily accessible, and unless otherwise specified shall be located on the front panel of the unit.

13.3 Adjustment Controls

Adjustment controls that are required for periodic alignment or calibration shall be mounted behind covered openings, such as access doors, on the surfaces of the equipment accessible when installed. When not adjustable by hand, controls shall be designed to accept a common screwdriver blade tip. Controls which infrequently require adjustment need not be accessible from the operating panel, but shall be readily accessible for servicing when the equipment is opened for maintenance purposes.

13.4 Stops

Mechanical stops shall be provided for all adjustable controls, except controls designed for unlimited rotation. Where flexible control shafts are employed, or where stops integral to the adjustable control of the mechanism could be damaged by excessive torque, stops shall be provided on the driving end of the shaft.

13.5 Locking Devices

Control locking devices shall be capable of retaining the controls in any given setting within the range of control. The locking and unlocking action shall be easily and quickly accomplished, and shall not affect the setting of the control. When in the unlocked position, the locking devices shall not interfere with the normal operation of the control. Where vernier controls are used, the locking devices shall operate on both main and vernier controls if necessary to prevent damage.

13.6 Non-Turn Devices

All non-turning controls and bodies or cases of turning controls shall be equipped with a positive device to prevent their turning in the panel or assembly on which they are mounted.

13.7 Control Knobs and Handles

Control knobs and handles shall have high impact strength and shall be firmly secured to the control shafts by use of setscrews, nuts, etc., wherever that type of fastener is applicable. Plastic knobs and handles shall have metal inserts for setscrews and shall not warp or crack.

13.8 Stability

All controls shall be so designed that setting, position, or adjustment of any control shall not be altered when the equipment is subjected to the service conditions specified in the detail equipment specification.

13.9 Arrangement and Location

Controls shall be arranged to facilitate smooth and rapid operation. All controls which have sequential relations, which are related to a particular function or operation, or which are operated together shall be grouped together along with their associated displays. Controls shall be conveniently located with respect to associated visual displays. Controls shall be of such size and so spaced that the manipulation of a given control does not interfere with the setting of an adjacent control. Adjustment controls with required test points shall be grouped and so marked as to provide for simplicity and ease of maintenance.

13.10 Mechanical Operation

Infrequently required controls shall be screwdriver adjusted. Play and backlash in controls shall be held to a minimum commensurate with intended operational functions and shall not cause poor contact or inaccurate setting. Controls shall operate freely and smoothly without binding, scraping, or cutting. Controls may be lubricated when lubrication does not interfere with operation and is specified in the detail equipment specification.

13.11 Shafts and Couplings

Shafts subject to removal may have their couplings secured by two setscrews 90 degrees to 120 degrees apart. Flexible couplings may be used for controls where the use of rigid couplings would interfere with the satisfactory operation or mounting of such controls.

13.12 Tuning Dial Mechanisms

13.12.1 Dial

The division marking and lettering on tuning dials shall be suitably etched. Dial markings shall be legible at a distance of 0.6 meter from any point within a solid angle of 60 degrees; defined by a surface of revolution about a line through the center of the dial and perpendicular to the panel. Minimum space between characters shall be one stroke width. The width of the lubber line or pointer tip shall not exceed the width of the graduation marks. Except for digital tuning indicators, for which only one calibration number shall be seen, dials shall be marked so that at least two calibration numbers on each band can be seen at any dial setting.

13.12.2 Balance and Friction

Weighted turning knobs shall be counterbalanced. Friction in turning dial mechanism shall allow smooth and easy adjustment of the operating knob over the entire operating range of the mechanism, but shall have sufficient resistance or shall incorporate a positive locking device to maintain the setting under all specified service conditions. Friction shall be achieved through dry or elastic resistance rather than by fluid resistance.