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Ashcroft A4A multi-coiled

Bourdon tube

pressures and special applications. See Section 7.1 for

additional information.

The Bourdon tube used in the Ashcroft A4A Gauge is a

"Unitized" single or multi-coil element which is approximate-

ly three times the length of tubes used in competitive

gauges. The greater length results in a much lower deflec-

tion rate, minimizes stress and virtually eliminates hystere-

sis, creep and fatigue effect.

The unitized tube design eliminates soldered, welded or

threaded joints which further provides for controlled distribu-

tion of stress during the tube forming operations. Internal

traps in which foreign matter may accumulate are also elimi-

nated. The interior of the Ashcroft A4A tube may be easily

cleaned by positive flow through flushing through the stan-

dard tip flushing (bleeder cap) connector.

Heat Treatment

All Ashcroft A4A Bourdon tubes are heat treated in high

vacuum to maintain internal cleanliness and optimize

mechanical properties.

Ultra-High Pressure Gauges

Ashcroft laboratories pioneered and continues to lead in the

development of precision Bourdon tubes for ultra-high pres-

sure gauges in ranges to 100,000 psi.

External Bleeder Cap

In order to achieve optimal accuracy the Bourdon tube

should be bled prior to initiating operation. If the gauge is to

be used on a gaseous media oil trapped in the Bourdon

tube should be purged from the tube. If the gauge is to be

used in liquid service trapped gas should be removed from

the Bourdon tube. Both activities are facilitated by the bleed-

er cap (flushing connector) which is an integral part of the

Bourdon tube. The bleeder cap is accessed by removing

the blow out disc from the back of the gauge.

Pressure Connection

Pressure connections may be either bottom or back loca-

tions. A complete listing of the available connections is pro-

vided in the specifications section (7.0) of this manual.

Tip Piece Assembly

The tip piece assembly is pivotally mounted on the free end

of the Bourdon tube. It comprises a quadrant and clamp

arrangement to permit adjustment of its angular position,

and a quadrant slide upon which the connecting link is piv-

oted. It is used to establish the angular relationship of the

connecting link and the sector slide which controls linearity

in the pointer movement.

Connecting Link

The Connecting Link transmits motion from the tip piece

assembly to the sector slide of the movement.

2.3.3

MOVEMENT ASSEMBL Y

Integral Movement

The solid front case provides a single rigid support for all

moving parts of the gauge, maintaining critical alignment

under all conditions. No significant calibration error will

occur in the Ashcroft A4A gauge, as a result of distortion or

shifting of supporting parts or components during the instal-

lation process.

Miniature Stainless Steel Ball Bearings

Friction and wear are minimized and sensitivity improved at

the pinion shaft by precision miniature stainless steel ball

bearings. Smooth action at this critical point in the move-

ment assures an immediate, accurate response to minute

pressure changes. This makes the Ashcroft A4A gauge the

most sensitive instrument of its type.

Gear Sector and Drive

The gear sector drives the pointer pinion directly in the

Ashcroft Model A4A precision pressure gauge. The pointer

pinion is spring loaded against backlash by a hair spring.

For maximum accuracy it is important that the original tooth

relationship be maintained throughout the system. If, for any

reason, the gears are to be disengaged they should first be

marked to assure correct re-engagement.

Hair Spring

The hair spring takes up backlash in the gearing and main-

tains the smooth sensitive action of the movement. This

spring has been pre-set to approximately 2 turns counter

clockwise with zero pressure applied. Lack of hair spring

tension will result in erratic pointer motion. Should this

occur, the gears may be disengaged, observing the precau-

tions noted above, the hair spring tightened, and the gears

re-engaged.

Sector Slide (course span adjustment)

The sector slide is mounted on the gear sector. It permits

adjustment of the ratio of pointer travel to Bourdon tube

motion by altering the multiplication through the "Micro-

Slide" adjustment. This adjustment controls the pointer trav-

el or range of the gauge (see figure 5.2).

Micro Slide Adjustment (fine span adjustment)

The micro slide adjustment provides convenient and simpli-

fied development in calibration methods. When required,

precise recalibration adjustment may be made in the total

effective length of the sector slide. The micrometer screw

attached to the slide may be adjusted to 0.0005" when

extremely minute corrections are necessary. The pre-loaded

adjustment spring stabilizes the calibration setting against

accidental shock (see figure 5.2).